diff --git a/src-tauri/src/download.rs b/src-tauri/src/download.rs
index 7093bf3..0873918 100644
--- a/src-tauri/src/download.rs
+++ b/src-tauri/src/download.rs
@@ -73,12 +73,19 @@ pub fn file_hashes(path: &Path) -> io::Result<(String, String)> {
         sha1.update(&buffer[..read]);
         sha256.update(&buffer[..read]);
     }
-    Ok((format!("{:x}", sha1.finalize()), format!("{:x}", sha256.finalize())))
+    Ok((
+        format!("{:x}", sha1.finalize()),
+        format!("{:x}", sha256.finalize()),
+    ))
 }
 
 /// True if `path` already exists, matches `expected_size` (when given) and
 /// `checksum`. Used to skip re-downloading files that are already current.
-pub fn is_current(path: &Path, expected_size: Option<u64>, checksum: &Checksum) -> io::Result<bool> {
+pub fn is_current(
+    path: &Path,
+    expected_size: Option<u64>,
+    checksum: &Checksum,
+) -> io::Result<bool> {
     let metadata = match path.metadata() {
         Ok(metadata) => metadata,
         Err(error) if error.kind() == io::ErrorKind::NotFound => return Ok(false),
@@ -104,6 +111,42 @@ fn temp_path(target: &Path) -> Result<PathBuf, DownloadError> {
     Ok(target.with_file_name(format!(".{file_name}.shacraft.part")))
 }
 
+/// Replaces `target` with a fully-written temporary sibling. Unix rename
+/// replaces an existing file atomically, while Windows rename rejects an
+/// existing destination. The backup dance keeps the old file recoverable if
+/// the second rename fails (for example because antivirus briefly locks it).
+pub(crate) fn replace_file(temporary: &Path, target: &Path) -> io::Result<()> {
+    #[cfg(not(windows))]
+    {
+        fs::rename(temporary, target)
+    }
+    #[cfg(windows)]
+    {
+        if !target.exists() {
+            return fs::rename(temporary, target);
+        }
+        let file_name = target
+            .file_name()
+            .and_then(|name| name.to_str())
+            .ok_or_else(|| {
+                io::Error::new(io::ErrorKind::InvalidInput, "target has no valid filename")
+            })?;
+        let backup = target.with_file_name(format!(".{file_name}.shacraft.backup"));
+        match fs::remove_file(&backup) {
+            Ok(()) => {}
+            Err(error) if error.kind() == io::ErrorKind::NotFound => {}
+            Err(error) => return Err(error),
+        }
+        fs::rename(target, &backup)?;
+        if let Err(error) = fs::rename(temporary, target) {
+            let _ = fs::rename(&backup, target);
+            return Err(error);
+        }
+        let _ = fs::remove_file(backup);
+        Ok(())
+    }
+}
+
 /// Downloads `url` to `target`, verifying size (if known ahead of time) and
 /// `checksum` before atomically renaming the temporary file into place.
 /// `on_progress(downloaded_bytes, total_bytes)` is called after every chunk;
@@ -126,18 +169,28 @@ pub fn download_verified(
     let total = expected_size.or_else(|| response.content_length());
     if let (Some(expected), Some(length)) = (expected_size, response.content_length()) {
         if expected != length {
-            return Err(DownloadError::SizeMismatch { expected, actual: length });
+            return Err(DownloadError::SizeMismatch {
+                expected,
+                actual: length,
+            });
         }
     }
 
     let temporary = temp_path(target)?;
-    let result = write_and_verify(&mut response, &temporary, expected_size, checksum, total, &mut on_progress);
+    let result = write_and_verify(
+        &mut response,
+        &temporary,
+        expected_size,
+        checksum,
+        total,
+        &mut on_progress,
+    );
     if let Err(error) = result {
         let _ = fs::remove_file(&temporary);
         return Err(error);
     }
     let bytes = result.unwrap();
-    fs::rename(&temporary, target).map_err(DownloadError::Io)?;
+    replace_file(&temporary, target).map_err(DownloadError::Io)?;
     Ok(bytes)
 }
 
@@ -160,7 +213,9 @@ fn write_and_verify(
         if read == 0 {
             break;
         }
-        output.write_all(&buffer[..read]).map_err(DownloadError::Io)?;
+        output
+            .write_all(&buffer[..read])
+            .map_err(DownloadError::Io)?;
         sha1.update(&buffer[..read]);
         sha256.update(&buffer[..read]);
         bytes += read as u64;
@@ -170,7 +225,10 @@ fn write_and_verify(
 
     if let Some(expected) = expected_size {
         if bytes != expected {
-            return Err(DownloadError::SizeMismatch { expected, actual: bytes });
+            return Err(DownloadError::SizeMismatch {
+                expected,
+                actual: bytes,
+            });
         }
     }
     let sha1_hex = format!("{:x}", sha1.finalize());
@@ -183,14 +241,20 @@ fn write_and_verify(
 
 #[cfg(test)]
 mod tests {
-    use super::{file_hashes, is_current, Checksum};
-    use std::{fs, process, time::{SystemTime, UNIX_EPOCH}};
+    use super::{file_hashes, is_current, replace_file, Checksum};
+    use std::{
+        fs, process,
+        time::{SystemTime, UNIX_EPOCH},
+    };
 
     fn temp_file(contents: &[u8]) -> std::path::PathBuf {
         let path = std::env::temp_dir().join(format!(
             "shacraft-download-test-{}-{}",
             process::id(),
-            SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_nanos()
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_nanos()
         ));
         fs::write(&path, contents).unwrap();
         path
@@ -201,7 +265,10 @@ mod tests {
         let path = temp_file(b"hello shacraft");
         let (sha1_hex, sha256_hex) = file_hashes(&path).unwrap();
         assert_eq!(sha1_hex, "124b319646ec08b4fb2a2b65bbd21c0431b4eaf4");
-        assert_eq!(sha256_hex, "d34eb8ea6396e8492109813c717f7eefd0437c10ff55a7b11949cfae900c946d");
+        assert_eq!(
+            sha256_hex,
+            "d34eb8ea6396e8492109813c717f7eefd0437c10ff55a7b11949cfae900c946d"
+        );
         fs::remove_file(path).unwrap();
     }
 
@@ -220,4 +287,15 @@ mod tests {
         let path = std::env::temp_dir().join("shacraft-download-test-missing-file-xyz");
         assert!(!is_current(&path, None, &Checksum::Sha256("0".repeat(64))).unwrap());
     }
+
+    #[test]
+    fn replaces_an_existing_file() {
+        let target = temp_file(b"old");
+        let temporary = target.with_extension("replacement");
+        fs::write(&temporary, b"new").unwrap();
+        replace_file(&temporary, &target).unwrap();
+        assert_eq!(fs::read(&target).unwrap(), b"new");
+        assert!(!temporary.exists());
+        fs::remove_file(target).unwrap();
+    }
 }
diff --git a/src-tauri/src/java.rs b/src-tauri/src/java.rs
index a8e68ce..e96e086 100644
--- a/src-tauri/src/java.rs
+++ b/src-tauri/src/java.rs
@@ -2,7 +2,11 @@ use crate::download::ProgressCallback;
 use crate::runtime::{self, RuntimeError};
 use reqwest::blocking::Client;
 use serde::Serialize;
-use std::{env, fmt, path::{Path, PathBuf}, process::Command};
+use std::{
+    env, fmt,
+    path::{Path, PathBuf},
+    process::Command,
+};
 
 #[derive(Clone, Debug, Serialize)]
 #[serde(rename_all = "camelCase")]
@@ -21,9 +25,14 @@ pub enum EnsureJavaError {
 impl fmt::Display for EnsureJavaError {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::Provisioning(error) => write!(formatter, "Cannot install a Java runtime: {error}"),
+            Self::Provisioning(error) => {
+                write!(formatter, "Cannot install a Java runtime: {error}")
+            }
             Self::ProvisionedButUnrecognised(path) => {
-                write!(formatter, "Installed a Java runtime at {path:?}, but it did not report a usable version")
+                write!(
+                    formatter,
+                    "Installed a Java runtime at {path:?}, but it did not report a usable version"
+                )
             }
         }
     }
@@ -38,21 +47,29 @@ pub fn detect() -> Option<JavaInstallation> {
     candidates().into_iter().find_map(check_candidate)
 }
 
-/// Returns a Java runtime with at least `required_major`, preferring
-/// whatever the user already has installed. Only downloads and extracts a
+/// Returns a Java runtime with exactly `required_major`, preferring a matching
+/// installation already on the machine. Newer JVM majors are not assumed to
+/// be compatible with the selected NeoForge/modpack version. Only downloads and extracts a
 /// ShaCraft-managed Eclipse Temurin JRE under `runtime_root` (never touches
 /// the user's own Java) when nothing suitable is already on the machine.
 /// `on_progress` reports real download bytes when a JRE actually needs
 /// fetching; it fires once with `(1, 1)` when an existing Java is reused.
-pub fn ensure_java(client: &Client, runtime_root: &Path, required_major: u8, on_progress: &ProgressCallback) -> Result<JavaInstallation, EnsureJavaError> {
+pub fn ensure_java(
+    client: &Client,
+    runtime_root: &Path,
+    required_major: u8,
+    on_progress: &ProgressCallback,
+) -> Result<JavaInstallation, EnsureJavaError> {
     if let Some(installation) = detect() {
-        if installation.major >= required_major {
+        if installation.major == required_major {
             on_progress(1, 1);
             return Ok(installation);
         }
     }
-    let executable = runtime::ensure_runtime(client, runtime_root, required_major, on_progress).map_err(EnsureJavaError::Provisioning)?;
-    check_candidate(executable.clone()).ok_or(EnsureJavaError::ProvisionedButUnrecognised(executable))
+    let executable = runtime::ensure_runtime(client, runtime_root, required_major, on_progress)
+        .map_err(EnsureJavaError::Provisioning)?;
+    check_candidate(executable.clone())
+        .ok_or(EnsureJavaError::ProvisionedButUnrecognised(executable))
 }
 
 fn candidates() -> Vec<PathBuf> {
diff --git a/src-tauri/src/launch.rs b/src-tauri/src/launch.rs
index a39a118..71ae513 100644
--- a/src-tauri/src/launch.rs
+++ b/src-tauri/src/launch.rs
@@ -69,7 +69,12 @@ fn build_classpath(game_dir: &Path, merged: &MergedVersion, client_jar: &Path) -
         .libraries
         .iter()
         .filter(|library| mojang::rule_allows(&library.rules, &no_features))
-        .filter_map(|library| library.downloads.as_ref().and_then(|downloads| downloads.artifact.as_ref()))
+        .filter_map(|library| {
+            library
+                .downloads
+                .as_ref()
+                .and_then(|downloads| downloads.artifact.as_ref())
+        })
         .map(|artifact| game_dir.join("libraries").join(&artifact.path))
         .collect();
     entries.push(client_jar.to_path_buf());
@@ -78,7 +83,11 @@ fn build_classpath(game_dir: &Path, merged: &MergedVersion, client_jar: &Path) -
     // (for example on gson-2.10.1.jar), so preserve order and keep each path
     // only once.
     let entries = unique_classpath_entries(entries);
-    entries.iter().map(|path| path.display().to_string()).collect::<Vec<_>>().join(classpath_separator())
+    entries
+        .iter()
+        .map(|path| path.display().to_string())
+        .collect::<Vec<_>>()
+        .join(classpath_separator())
 }
 
 /// A persistent-but-not-security-sensitive per-install identifier for the
@@ -104,7 +113,13 @@ static UUID_COUNTER: AtomicU64 = AtomicU64::new(0);
 
 fn random_uuid_v4() -> String {
     let mut hasher = Sha256::new();
-    hasher.update(SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_nanos().to_le_bytes());
+    hasher.update(
+        SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_nanos()
+            .to_le_bytes(),
+    );
     hasher.update(std::process::id().to_le_bytes());
     hasher.update(UUID_COUNTER.fetch_add(1, Ordering::Relaxed).to_le_bytes());
     let stack_marker = 0_u8;
@@ -114,7 +129,10 @@ fn random_uuid_v4() -> String {
     bytes.copy_from_slice(&digest[0..16]);
     bytes[6] = (bytes[6] & 0x0f) | 0x40; // version 4
     bytes[8] = (bytes[8] & 0x3f) | 0x80; // RFC 4122 variant
-    let hex = bytes.iter().map(|byte| format!("{byte:02x}")).collect::<String>();
+    let hex = bytes
+        .iter()
+        .map(|byte| format!("{byte:02x}"))
+        .collect::<String>();
     crate::session::format_uuid_with_dashes(&hex)
 }
 
@@ -129,6 +147,36 @@ fn substitute(template: &str, vars: &HashMap<&str, String>) -> String {
     result
 }
 
+/// Java's argument-file syntax is independent of the platform shell. Keeping
+/// the large JVM/module/classpath portion in an argfile avoids Windows'
+/// 32,767 UTF-16 command-line limit while leaving account tokens out of it.
+fn quote_argfile_argument(argument: &str) -> String {
+    let mut quoted = String::with_capacity(argument.len() + 2);
+    quoted.push('"');
+    for character in argument.chars() {
+        match character {
+            '\\' => quoted.push_str("\\\\"),
+            '"' => quoted.push_str("\\\""),
+            '\n' => quoted.push_str("\\n"),
+            '\r' => quoted.push_str("\\r"),
+            '\t' => quoted.push_str("\\t"),
+            other => quoted.push(other),
+        }
+    }
+    quoted.push('"');
+    quoted
+}
+
+fn write_jvm_argfile(path: &Path, arguments: &[String]) -> io::Result<()> {
+    let mut contents = arguments
+        .iter()
+        .map(|argument| quote_argfile_argument(argument))
+        .collect::<Vec<_>>()
+        .join("\n");
+    contents.push('\n');
+    fs::write(path, contents)
+}
+
 /// Builds the full `java` command line for `request.merged` and spawns it
 /// detached, with stdout/stderr both redirected to `request.log_path`.
 /// Never blocks on the child exiting — the caller decides how to observe
@@ -140,7 +188,8 @@ pub fn launch(request: &LaunchRequest) -> Result<Child, LaunchError> {
     fs::create_dir_all(&natives_dir)?;
     let assets_root = request.game_dir.join("assets");
     let libraries_dir = request.game_dir.join("libraries");
-    let client_jar = mojang::client_jar_path(request.game_dir, &request.merged.client_jar_version_id);
+    let client_jar =
+        mojang::client_jar_path(request.game_dir, &request.merged.client_jar_version_id);
     let classpath = build_classpath(request.game_dir, request.merged, &client_jar);
 
     let mut vars: HashMap<&str, String> = HashMap::new();
@@ -155,7 +204,10 @@ pub fn launch(request: &LaunchRequest) -> Result<Child, LaunchError> {
     vars.insert("assets_root", assets_root.display().to_string());
     vars.insert("assets_index_name", request.merged.asset_index.id.clone());
     vars.insert("auth_uuid", request.identity.uuid());
-    vars.insert("auth_access_token", request.identity.access_token().to_string());
+    vars.insert(
+        "auth_access_token",
+        request.identity.access_token().to_string(),
+    );
     vars.insert("clientid", launcher_client_id(request.game_dir)?);
     vars.insert("auth_xuid", request.identity.xuid().to_string());
     vars.insert("user_type", request.identity.user_type().to_string());
@@ -168,13 +220,24 @@ pub fn launch(request: &LaunchRequest) -> Result<Child, LaunchError> {
     vars.insert("classpath_separator", classpath_separator().to_string());
 
     let no_features = HashMap::new();
-    let jvm_args = mojang::resolve_arguments(&request.merged.jvm_arguments, &no_features);
+    let jvm_args = mojang::resolve_arguments(&request.merged.jvm_arguments, &no_features)
+        .into_iter()
+        .map(|argument| substitute(&argument, &vars))
+        .collect::<Vec<_>>();
     let game_args = mojang::resolve_arguments(&request.merged.game_arguments, &no_features);
 
     let mut command = Command::new(request.java_executable);
-    command.arg(format!("-Xmx{}M", request.memory_mb));
-    for argument in jvm_args {
-        command.arg(substitute(&argument, &vars));
+    let memory_argument = format!("-Xmx{}M", request.memory_mb);
+    if cfg!(windows) {
+        let argfile = request.profile_dir.join(".shacraft-jvm.args");
+        let mut argfile_arguments = Vec::with_capacity(jvm_args.len() + 1);
+        argfile_arguments.push(memory_argument);
+        argfile_arguments.extend(jvm_args);
+        write_jvm_argfile(&argfile, &argfile_arguments)?;
+        command.arg(format!("@{}", argfile.display()));
+    } else {
+        command.arg(memory_argument);
+        command.args(jvm_args);
     }
     command.arg(&request.merged.main_class);
     for argument in game_args {
@@ -199,12 +262,18 @@ mod tests {
         let parts: Vec<&str> = id.split('-').collect();
         assert_eq!(parts.len(), 5);
         assert_eq!(parts[2].chars().next().unwrap(), '4');
-        assert!(matches!(parts[3].chars().next().unwrap(), '8' | '9' | 'a' | 'b'));
+        assert!(matches!(
+            parts[3].chars().next().unwrap(),
+            '8' | '9' | 'a' | 'b'
+        ));
     }
 
     #[test]
     fn client_id_is_persisted_across_calls() {
-        let dir = std::env::temp_dir().join(format!("shacraft-launch-clientid-test-{}", std::process::id()));
+        let dir = std::env::temp_dir().join(format!(
+            "shacraft-launch-clientid-test-{}",
+            std::process::id()
+        ));
         let first = launcher_client_id(&dir).unwrap();
         let second = launcher_client_id(&dir).unwrap();
         assert_eq!(first, second);
@@ -217,12 +286,34 @@ mod tests {
         vars.insert("auth_player_name", "Steve".to_string());
         assert_eq!(substitute("--username", &vars), "--username");
         assert_eq!(substitute("${auth_player_name}", &vars), "Steve");
-        assert_eq!(substitute("-Djava.library.path=${natives_directory}", &vars), "-Djava.library.path=${natives_directory}");
+        assert_eq!(
+            substitute("-Djava.library.path=${natives_directory}", &vars),
+            "-Djava.library.path=${natives_directory}"
+        );
     }
 
     #[test]
     fn classpath_entries_are_unique() {
-        let entries = unique_classpath_entries(vec![PathBuf::from("gson.jar"), PathBuf::from("gson.jar"), PathBuf::from("client.jar")]);
-        assert_eq!(entries, vec![PathBuf::from("gson.jar"), PathBuf::from("client.jar")]);
+        let entries = unique_classpath_entries(vec![
+            PathBuf::from("gson.jar"),
+            PathBuf::from("gson.jar"),
+            PathBuf::from("client.jar"),
+        ]);
+        assert_eq!(
+            entries,
+            vec![PathBuf::from("gson.jar"), PathBuf::from("client.jar")]
+        );
+    }
+
+    #[test]
+    fn quotes_java_argfile_arguments() {
+        assert_eq!(
+            quote_argfile_argument(r#"-Dpath=C:\\Users\\Jane Doe\\game"#),
+            r#""-Dpath=C:\\\\Users\\\\Jane Doe\\\\game""#
+        );
+        assert_eq!(
+            quote_argfile_argument(r#"-Dname="ShaCraft""#),
+            r#""-Dname=\"ShaCraft\"""#
+        );
     }
 }
diff --git a/src-tauri/src/mojang.rs b/src-tauri/src/mojang.rs
index 497bb6c..004cba5 100644
--- a/src-tauri/src/mojang.rs
+++ b/src-tauri/src/mojang.rs
@@ -27,7 +27,8 @@ use std::{
 };
 use url::Url;
 
-const VERSION_MANIFEST_URL: &str = "https://piston-meta.mojang.com/mc/game/version_manifest_v2.json";
+const VERSION_MANIFEST_URL: &str =
+    "https://piston-meta.mojang.com/mc/game/version_manifest_v2.json";
 const MOJANG_HOSTS: [&str; 4] = [
     "piston-meta.mojang.com",
     "piston-data.mojang.com",
@@ -41,7 +42,10 @@ const MOJANG_HOSTS: [&str; 4] = [
 const ASSET_WORKERS: usize = 48;
 
 pub fn is_allowed_host(url: &str) -> bool {
-    Url::parse(url).ok().and_then(|parsed| parsed.host_str().map(|host| MOJANG_HOSTS.contains(&host))).unwrap_or(false)
+    Url::parse(url)
+        .ok()
+        .and_then(|parsed| parsed.host_str().map(|host| MOJANG_HOSTS.contains(&host)))
+        .unwrap_or(false)
 }
 
 /// Library entries in a merged loader profile may point at the loader's
@@ -59,6 +63,7 @@ pub enum MojangError {
     ChecksumMismatch(String),
     DisallowedHost(String),
     MissingField(String),
+    ConflictingLibrary(String),
     Download(DownloadError),
     Io(io::Error),
 }
@@ -70,8 +75,14 @@ impl fmt::Display for MojangError {
             Self::HttpStatus(status) => write!(formatter, "Mojang returned {status}"),
             Self::InvalidJson(error) => write!(formatter, "invalid Mojang JSON: {error}"),
             Self::ChecksumMismatch(context) => write!(formatter, "checksum mismatch for {context}"),
-            Self::DisallowedHost(url) => write!(formatter, "URL is not a recognised Mojang host: {url}"),
+            Self::DisallowedHost(url) => {
+                write!(formatter, "URL is not a recognised Mojang host: {url}")
+            }
             Self::MissingField(field) => write!(formatter, "version JSON is missing {field}"),
+            Self::ConflictingLibrary(path) => write!(
+                formatter,
+                "merged version contains conflicting library entries for {path}"
+            ),
             Self::Download(error) => write!(formatter, "{error}"),
             Self::Io(error) => write!(formatter, "I/O error: {error}"),
         }
@@ -110,7 +121,10 @@ pub fn fetch_version_manifest(client: &Client) -> Result<VersionManifest, Mojang
     fetch_json(client, VERSION_MANIFEST_URL, None)
 }
 
-pub fn find_version<'a>(manifest: &'a VersionManifest, id: &str) -> Option<&'a VersionManifestEntry> {
+pub fn find_version<'a>(
+    manifest: &'a VersionManifest,
+    id: &str,
+) -> Option<&'a VersionManifestEntry> {
     manifest.versions.iter().find(|entry| entry.id == id)
 }
 
@@ -145,7 +159,10 @@ pub struct Arguments {
 #[serde(untagged)]
 pub enum ArgumentValue {
     Plain(String),
-    Conditional { rules: Vec<Rule>, value: StringOrList },
+    Conditional {
+        rules: Vec<Rule>,
+        value: StringOrList,
+    },
 }
 
 #[derive(Debug, Deserialize, Clone)]
@@ -231,7 +248,11 @@ fn current_os_name() -> &'static str {
 }
 
 fn arch_matches(expected: &str) -> bool {
-    let normalized = if expected == "arm64" { "aarch64" } else { expected };
+    let normalized = if expected == "arm64" {
+        "aarch64"
+    } else {
+        expected
+    };
     normalized == std::env::consts::ARCH
 }
 
@@ -250,7 +271,9 @@ fn os_matches(os: &RuleOs) -> bool {
 }
 
 fn features_match(required: &HashMap<String, bool>, active: &HashMap<String, bool>) -> bool {
-    required.iter().all(|(key, value)| active.get(key).copied().unwrap_or(false) == *value)
+    required
+        .iter()
+        .all(|(key, value)| active.get(key).copied().unwrap_or(false) == *value)
 }
 
 /// Evaluates a Mojang-style rule list: no rules means always allowed;
@@ -265,7 +288,10 @@ pub fn rule_allows(rules: &[Rule], active_features: &HashMap<String, bool>) -> b
     let mut allowed = false;
     for rule in rules {
         let os_ok = rule.os.as_ref().is_none_or(os_matches);
-        let features_ok = rule.features.as_ref().is_none_or(|required| features_match(required, active_features));
+        let features_ok = rule
+            .features
+            .as_ref()
+            .is_none_or(|required| features_match(required, active_features));
         if os_ok && features_ok {
             allowed = rule.action == RuleAction::Allow;
         }
@@ -275,7 +301,10 @@ pub fn rule_allows(rules: &[Rule], active_features: &HashMap<String, bool>) -> b
 
 /// Flattens an argument list into plain strings, dropping conditional
 /// entries whose rules don't match this platform/feature set.
-pub fn resolve_arguments(arguments: &[ArgumentValue], active_features: &HashMap<String, bool>) -> Vec<String> {
+pub fn resolve_arguments(
+    arguments: &[ArgumentValue],
+    active_features: &HashMap<String, bool>,
+) -> Vec<String> {
     let mut resolved = Vec::new();
     for argument in arguments {
         match argument {
@@ -293,11 +322,18 @@ pub fn resolve_arguments(arguments: &[ArgumentValue], active_features: &HashMap<
     resolved
 }
 
-pub fn fetch_version_json(client: &Client, entry: &VersionManifestEntry) -> Result<VersionJson, MojangError> {
+pub fn fetch_version_json(
+    client: &Client,
+    entry: &VersionManifestEntry,
+) -> Result<VersionJson, MojangError> {
     fetch_json(client, &entry.url, Some(&entry.sha1))
 }
 
-fn fetch_json<T: DeserializeOwned>(client: &Client, url: &str, expected_sha1: Option<&str>) -> Result<T, MojangError> {
+fn fetch_json<T: DeserializeOwned>(
+    client: &Client,
+    url: &str,
+    expected_sha1: Option<&str>,
+) -> Result<T, MojangError> {
     if !is_allowed_host(url) {
         return Err(MojangError::DisallowedHost(url.to_string()));
     }
@@ -348,8 +384,14 @@ pub struct MergedVersion {
 /// the parent's, and its libraries are appended after the parent's.
 /// `assetIndex`/`downloads.client` always come from the parent, since
 /// modloader profiles don't redeclare them.
-pub fn merge_versions(parent: &VersionJson, child: Option<&VersionJson>) -> Result<MergedVersion, MojangError> {
-    let asset_index = parent.asset_index.clone().ok_or_else(|| MojangError::MissingField("assetIndex".into()))?;
+pub fn merge_versions(
+    parent: &VersionJson,
+    child: Option<&VersionJson>,
+) -> Result<MergedVersion, MojangError> {
+    let asset_index = parent
+        .asset_index
+        .clone()
+        .ok_or_else(|| MojangError::MissingField("assetIndex".into()))?;
     let client = parent
         .downloads
         .as_ref()
@@ -401,34 +443,69 @@ pub fn natives_directory(game_dir: &Path, version_id: &str) -> PathBuf {
 }
 
 pub fn client_jar_path(game_dir: &Path, version_id: &str) -> PathBuf {
-    game_dir.join("versions").join(version_id).join(format!("{version_id}.jar"))
+    game_dir
+        .join("versions")
+        .join(version_id)
+        .join(format!("{version_id}.jar"))
 }
 
-pub fn ensure_client_jar(client: &Client, game_dir: &Path, version_id: &str, download_ref: &DownloadRef) -> Result<PathBuf, MojangError> {
+pub fn ensure_client_jar(
+    client: &Client,
+    game_dir: &Path,
+    version_id: &str,
+    download_ref: &DownloadRef,
+) -> Result<PathBuf, MojangError> {
     if !is_allowed_host(&download_ref.url) {
         return Err(MojangError::DisallowedHost(download_ref.url.clone()));
     }
     let target = client_jar_path(game_dir, version_id);
     let checksum = Checksum::Sha1(download_ref.sha1.clone());
     if !download::is_current(&target, Some(download_ref.size), &checksum)? {
-        download::download_verified(client, &download_ref.url, &target, Some(download_ref.size), &checksum, |_, _| {})?;
+        download::download_verified(
+            client,
+            &download_ref.url,
+            &target,
+            Some(download_ref.size),
+            &checksum,
+            |_, _| {},
+        )?;
     }
     Ok(target)
 }
 
 /// Downloads every rule-allowed library with a `downloads.artifact`,
 /// returning the resulting jar paths in the same order as `libraries`.
-pub fn ensure_libraries(client: &Client, game_dir: &Path, libraries: &[Library], on_progress: &ProgressCallback) -> Result<Vec<PathBuf>, MojangError> {
+pub fn ensure_libraries(
+    client: &Client,
+    game_dir: &Path,
+    libraries: &[Library],
+    on_progress: &ProgressCallback,
+) -> Result<Vec<PathBuf>, MojangError> {
     let mut paths = Vec::new();
     let mut tasks = Vec::new();
+    let mut seen: HashMap<PathBuf, (String, u64, String)> = HashMap::new();
     for library in libraries {
         if !rule_allows(&library.rules, &HashMap::new()) {
             continue;
         }
-        let Some(artifact) = library.downloads.as_ref().and_then(|downloads| downloads.artifact.as_ref()) else {
+        let Some(artifact) = library
+            .downloads
+            .as_ref()
+            .and_then(|downloads| downloads.artifact.as_ref())
+        else {
             continue;
         };
         let target = game_dir.join("libraries").join(&artifact.path);
+        let identity = (artifact.url.clone(), artifact.size, artifact.sha1.clone());
+        if let Some(existing) = seen.get(&target) {
+            if existing != &identity {
+                return Err(MojangError::ConflictingLibrary(
+                    target.display().to_string(),
+                ));
+            }
+            continue;
+        }
+        seen.insert(target.clone(), identity);
         paths.push(target.clone());
         tasks.push(DownloadTask {
             url: artifact.url.clone(),
@@ -452,20 +529,39 @@ pub struct AssetObject {
     pub size: u64,
 }
 
-pub fn ensure_asset_index(client: &Client, game_dir: &Path, asset_index: &AssetIndexRef) -> Result<AssetIndex, MojangError> {
+pub fn ensure_asset_index(
+    client: &Client,
+    game_dir: &Path,
+    asset_index: &AssetIndexRef,
+) -> Result<AssetIndex, MojangError> {
     if !is_allowed_host(&asset_index.url) {
         return Err(MojangError::DisallowedHost(asset_index.url.clone()));
     }
-    let target = game_dir.join("assets").join("indexes").join(format!("{}.json", asset_index.id));
+    let target = game_dir
+        .join("assets")
+        .join("indexes")
+        .join(format!("{}.json", asset_index.id));
     let checksum = Checksum::Sha1(asset_index.sha1.clone());
     if !download::is_current(&target, Some(asset_index.size), &checksum)? {
-        download::download_verified(client, &asset_index.url, &target, Some(asset_index.size), &checksum, |_, _| {})?;
+        download::download_verified(
+            client,
+            &asset_index.url,
+            &target,
+            Some(asset_index.size),
+            &checksum,
+            |_, _| {},
+        )?;
     }
     let bytes = fs::read(&target)?;
     serde_json::from_slice(&bytes).map_err(MojangError::InvalidJson)
 }
 
-pub fn ensure_assets(client: &Client, game_dir: &Path, index: &AssetIndex, on_progress: &ProgressCallback) -> Result<(), MojangError> {
+pub fn ensure_assets(
+    client: &Client,
+    game_dir: &Path,
+    index: &AssetIndex,
+    on_progress: &ProgressCallback,
+) -> Result<(), MojangError> {
     let objects_dir = game_dir.join("assets").join("objects");
     let tasks = index
         .objects
@@ -473,7 +569,10 @@ pub fn ensure_assets(client: &Client, game_dir: &Path, index: &AssetIndex, on_pr
         .map(|object| {
             let prefix = &object.hash[0..2];
             DownloadTask {
-                url: format!("https://resources.download.minecraft.net/{prefix}/{}", object.hash),
+                url: format!(
+                    "https://resources.download.minecraft.net/{prefix}/{}",
+                    object.hash
+                ),
                 target: objects_dir.join(prefix).join(&object.hash),
                 size: object.size,
                 checksum: Checksum::Sha1(object.hash.clone()),
@@ -500,7 +599,14 @@ const MAX_DOWNLOAD_ATTEMPTS: u32 = 5;
 fn download_with_retries(client: &Client, task: &DownloadTask) -> Result<u64, DownloadError> {
     let mut last_error = None;
     for attempt in 1..=MAX_DOWNLOAD_ATTEMPTS {
-        match download::download_verified(client, &task.url, &task.target, Some(task.size), &task.checksum, |_, _| {}) {
+        match download::download_verified(
+            client,
+            &task.url,
+            &task.target,
+            Some(task.size),
+            &task.checksum,
+            |_, _| {},
+        ) {
             Ok(bytes) => return Ok(bytes),
             Err(error) => {
                 last_error = Some(error);
@@ -540,12 +646,16 @@ fn download_many(
                 if first_error.lock().unwrap().is_some() {
                     break;
                 }
-                let Some(task) = queue.lock().unwrap().pop() else { break };
+                let Some(task) = queue.lock().unwrap().pop() else {
+                    break;
+                };
                 if !is_allowed(&task.url) {
                     *first_error.lock().unwrap() = Some(MojangError::DisallowedHost(task.url));
                     continue;
                 }
-                let already_current = download::is_current(&task.target, Some(task.size), &task.checksum).unwrap_or(false);
+                let already_current =
+                    download::is_current(&task.target, Some(task.size), &task.checksum)
+                        .unwrap_or(false);
                 if !already_current {
                     if let Err(error) = download_with_retries(client, &task) {
                         *first_error.lock().unwrap() = Some(MojangError::Download(error));
@@ -571,7 +681,10 @@ mod tests {
     fn rule(action: RuleAction, os_name: Option<&str>) -> Rule {
         Rule {
             action,
-            os: os_name.map(|name| RuleOs { name: Some(name.into()), arch: None }),
+            os: os_name.map(|name| RuleOs {
+                name: Some(name.into()),
+                arch: None,
+            }),
             features: None,
         }
     }
@@ -589,7 +702,11 @@ mod tests {
 
     #[test]
     fn non_matching_os_rule_disallows() {
-        let other = if current_os_name() == "windows" { "linux" } else { "windows" };
+        let other = if current_os_name() == "windows" {
+            "linux"
+        } else {
+            "windows"
+        };
         let rules = vec![rule(RuleAction::Allow, Some(other))];
         assert!(!rule_allows(&rules, &HashMap::new()));
     }
@@ -598,7 +715,11 @@ mod tests {
     fn unsupported_feature_is_excluded_by_default() {
         let mut features = HashMap::new();
         features.insert("is_demo_user".to_string(), true);
-        let rules = vec![Rule { action: RuleAction::Allow, os: None, features: Some(features) }];
+        let rules = vec![Rule {
+            action: RuleAction::Allow,
+            os: None,
+            features: Some(features),
+        }];
         // We never activate optional features, so a rule requiring one
         // must not match even though there's no OS constraint.
         assert!(!rule_allows(&rules, &HashMap::new()));
@@ -619,7 +740,10 @@ mod tests {
             },
         ];
         let resolved = resolve_arguments(&args, &HashMap::new());
-        assert_eq!(resolved, vec!["--username", "${auth_player_name}", "--this-os-only"]);
+        assert_eq!(
+            resolved,
+            vec!["--username", "${auth_player_name}", "--this-os-only"]
+        );
     }
 
     #[test]
@@ -649,18 +773,38 @@ mod tests {
         let merged = merge_versions(&parent, Some(&child)).unwrap();
         assert_eq!(merged.id, "neoforge-21.1.248");
         assert_eq!(merged.client_jar_version_id, "1.21.1");
-        assert_eq!(merged.main_class, "cpw.mods.bootstraplauncher.BootstrapLauncher");
-        assert_eq!(resolve_arguments(&merged.game_arguments, &HashMap::new()), vec!["--parentGame", "--childGame"]);
-        assert_eq!(resolve_arguments(&merged.jvm_arguments, &HashMap::new()), vec!["--parentJvm", "--childJvm"]);
-        assert_eq!(merged.libraries.iter().map(|library| library.name.as_str()).collect::<Vec<_>>(), vec!["parent:lib:1", "child:lib:1"]);
+        assert_eq!(
+            merged.main_class,
+            "cpw.mods.bootstraplauncher.BootstrapLauncher"
+        );
+        assert_eq!(
+            resolve_arguments(&merged.game_arguments, &HashMap::new()),
+            vec!["--parentGame", "--childGame"]
+        );
+        assert_eq!(
+            resolve_arguments(&merged.jvm_arguments, &HashMap::new()),
+            vec!["--parentJvm", "--childJvm"]
+        );
+        assert_eq!(
+            merged
+                .libraries
+                .iter()
+                .map(|library| library.name.as_str())
+                .collect::<Vec<_>>(),
+            vec!["parent:lib:1", "child:lib:1"]
+        );
         assert_eq!(merged.asset_index.id, "17");
     }
 
     #[test]
     fn disallowed_host_is_rejected() {
         assert!(!is_allowed_host("https://example.com/evil.jar"));
-        assert!(is_allowed_host("https://piston-data.mojang.com/v1/objects/x/client.jar"));
-        assert!(is_allowed_library_host("https://maven.neoforged.net/releases/net/neoforged/example.jar"));
+        assert!(is_allowed_host(
+            "https://piston-data.mojang.com/v1/objects/x/client.jar"
+        ));
+        assert!(is_allowed_library_host(
+            "https://maven.neoforged.net/releases/net/neoforged/example.jar"
+        ));
         assert!(!is_allowed_library_host("https://example.com/evil.jar"));
     }
 
@@ -677,7 +821,8 @@ mod tests {
         let version = fetch_version_json(&client, entry).unwrap();
         assert_eq!(version.main_class, "net.minecraft.client.main.Main");
 
-        let game_dir = std::env::temp_dir().join(format!("shacraft-mojang-live-{}", std::process::id()));
+        let game_dir =
+            std::env::temp_dir().join(format!("shacraft-mojang-live-{}", std::process::id()));
 
         let merged = merge_versions(&version, None).unwrap();
         let client_jar = ensure_client_jar(&client, &game_dir, &merged.id, &merged.client).unwrap();
@@ -689,11 +834,20 @@ mod tests {
         let mut small_libraries: Vec<Library> = merged
             .libraries
             .iter()
-            .filter(|library| library.downloads.as_ref().and_then(|downloads| downloads.artifact.as_ref()).is_some_and(|artifact| artifact.size < 200_000))
+            .filter(|library| {
+                library
+                    .downloads
+                    .as_ref()
+                    .and_then(|downloads| downloads.artifact.as_ref())
+                    .is_some_and(|artifact| artifact.size < 200_000)
+            })
             .take(5)
             .cloned()
             .collect();
-        assert!(!small_libraries.is_empty(), "expected at least one small library to sanity-check downloads with");
+        assert!(
+            !small_libraries.is_empty(),
+            "expected at least one small library to sanity-check downloads with"
+        );
         small_libraries.truncate(5);
         let progress: ProgressCallback = Arc::new(|_, _| {});
         let paths = ensure_libraries(&client, &game_dir, &small_libraries, &progress).unwrap();
@@ -703,7 +857,8 @@ mod tests {
 
         // Re-running against already-downloaded files must be a no-op (the
         // `is_current` fast path), not re-download or fail.
-        let client_jar_again = ensure_client_jar(&client, &game_dir, &merged.id, &merged.client).unwrap();
+        let client_jar_again =
+            ensure_client_jar(&client, &game_dir, &merged.id, &merged.client).unwrap();
         assert_eq!(client_jar, client_jar_again);
 
         fs::remove_dir_all(&game_dir).ok();
diff --git a/src-tauri/src/msa.rs b/src-tauri/src/msa.rs
index e215657..362c46e 100644
--- a/src-tauri/src/msa.rs
+++ b/src-tauri/src/msa.rs
@@ -41,7 +41,8 @@ const DEVICE_CODE_URL: &str = "https://login.microsoftonline.com/consumers/oauth
 const TOKEN_URL: &str = "https://login.microsoftonline.com/consumers/oauth2/v2.0/token";
 const XBOX_USER_AUTH_URL: &str = "https://user.auth.xboxlive.com/user/authenticate";
 const XSTS_AUTHORIZE_URL: &str = "https://xsts.auth.xboxlive.com/xsts/authorize";
-const MINECRAFT_LOGIN_URL: &str = "https://api.minecraftservices.com/authentication/login_with_xbox";
+const MINECRAFT_LOGIN_URL: &str =
+    "https://api.minecraftservices.com/authentication/login_with_xbox";
 const MINECRAFT_PROFILE_URL: &str = "https://api.minecraftservices.com/minecraft/profile";
 const ACCOUNT_FILE: &str = "account.json";
 
@@ -111,7 +112,10 @@ pub fn start_device_code(client: &Client) -> Result<DeviceCodeStart, MsaError> {
     }
     let response = client
         .post(DEVICE_CODE_URL)
-        .form(&[("client_id", MSA_CLIENT_ID), ("scope", "XboxLive.signin offline_access")])
+        .form(&[
+            ("client_id", MSA_CLIENT_ID),
+            ("scope", "XboxLive.signin offline_access"),
+        ])
         .send()
         .map_err(MsaError::Network)?;
     if !response.status().is_success() {
@@ -144,7 +148,10 @@ struct TokenResponse {
 /// decline. This is the slow step in the whole login flow — the caller
 /// should already have shown `verification_uri`/`user_code` to the user
 /// before calling this (see `start_device_code`).
-pub fn poll_device_code(client: &Client, start: &DeviceCodeStart) -> Result<MicrosoftTokens, MsaError> {
+pub fn poll_device_code(
+    client: &Client,
+    start: &DeviceCodeStart,
+) -> Result<MicrosoftTokens, MsaError> {
     let deadline = Instant::now() + Duration::from_secs(start.expires_in_seconds);
     let mut interval = Duration::from_secs(start.interval_seconds);
 
@@ -167,10 +174,16 @@ pub fn poll_device_code(client: &Client, start: &DeviceCodeStart) -> Result<Micr
         let body: TokenResponse = response.json().map_err(MsaError::Network)?;
 
         if status.is_success() {
-            let (Some(access_token), Some(refresh_token)) = (body.access_token, body.refresh_token) else {
-                return Err(MsaError::UnexpectedResponse("token response missing access_token/refresh_token".into()));
+            let (Some(access_token), Some(refresh_token)) = (body.access_token, body.refresh_token)
+            else {
+                return Err(MsaError::UnexpectedResponse(
+                    "token response missing access_token/refresh_token".into(),
+                ));
             };
-            return Ok(MicrosoftTokens { access_token, refresh_token });
+            return Ok(MicrosoftTokens {
+                access_token,
+                refresh_token,
+            });
         }
 
         match body.error.as_deref() {
@@ -181,12 +194,19 @@ pub fn poll_device_code(client: &Client, start: &DeviceCodeStart) -> Result<Micr
             }
             Some("authorization_declined") => return Err(MsaError::AuthorizationDeclined),
             Some("expired_token") => return Err(MsaError::AuthorizationExpired),
-            other => return Err(MsaError::UnexpectedResponse(other.unwrap_or("unknown device code error").into())),
+            other => {
+                return Err(MsaError::UnexpectedResponse(
+                    other.unwrap_or("unknown device code error").into(),
+                ))
+            }
         }
     }
 }
 
-pub fn refresh_microsoft_tokens(client: &Client, refresh_token: &str) -> Result<MicrosoftTokens, MsaError> {
+pub fn refresh_microsoft_tokens(
+    client: &Client,
+    refresh_token: &str,
+) -> Result<MicrosoftTokens, MsaError> {
     if !is_configured() {
         return Err(MsaError::NotConfigured);
     }
@@ -205,9 +225,14 @@ pub fn refresh_microsoft_tokens(client: &Client, refresh_token: &str) -> Result<
     }
     let body: TokenResponse = response.json().map_err(MsaError::Network)?;
     let (Some(access_token), Some(refresh_token)) = (body.access_token, body.refresh_token) else {
-        return Err(MsaError::UnexpectedResponse("refresh response missing access_token/refresh_token".into()));
+        return Err(MsaError::UnexpectedResponse(
+            "refresh response missing access_token/refresh_token".into(),
+        ));
     };
-    Ok(MicrosoftTokens { access_token, refresh_token })
+    Ok(MicrosoftTokens {
+        access_token,
+        refresh_token,
+    })
 }
 
 // ---------------------------------------------------------------------
@@ -274,7 +299,10 @@ struct XboxUserHash {
     xid: Option<String>,
 }
 
-fn xbox_live_user_token(client: &Client, microsoft_access_token: &str) -> Result<(String, String), MsaError> {
+fn xbox_live_user_token(
+    client: &Client,
+    microsoft_access_token: &str,
+) -> Result<(String, String), MsaError> {
     let request = XboxUserAuthRequest {
         properties: XboxUserAuthProperties {
             auth_method: "RPS",
@@ -284,22 +312,42 @@ fn xbox_live_user_token(client: &Client, microsoft_access_token: &str) -> Result
         relying_party: "http://auth.xboxlive.com",
         token_type: "JWT",
     };
-    let response = client.post(XBOX_USER_AUTH_URL).json(&request).send().map_err(MsaError::Network)?;
+    let response = client
+        .post(XBOX_USER_AUTH_URL)
+        .json(&request)
+        .send()
+        .map_err(MsaError::Network)?;
     if !response.status().is_success() {
         return Err(MsaError::HttpStatus(response.status()));
     }
     let body: XboxTokenResponse = response.json().map_err(MsaError::Network)?;
-    let uhs = body.display_claims.xui.into_iter().next().map(|claim| claim.uhs).ok_or_else(|| MsaError::UnexpectedResponse("missing uhs".into()))?;
+    let uhs = body
+        .display_claims
+        .xui
+        .into_iter()
+        .next()
+        .map(|claim| claim.uhs)
+        .ok_or_else(|| MsaError::UnexpectedResponse("missing uhs".into()))?;
     Ok((body.token, uhs))
 }
 
-fn xsts_authorize(client: &Client, xbox_live_token: &str) -> Result<(String, String, Option<String>), MsaError> {
+fn xsts_authorize(
+    client: &Client,
+    xbox_live_token: &str,
+) -> Result<(String, String, Option<String>), MsaError> {
     let request = XstsRequest {
-        properties: XstsProperties { sandbox_id: "RETAIL", user_tokens: [xbox_live_token] },
+        properties: XstsProperties {
+            sandbox_id: "RETAIL",
+            user_tokens: [xbox_live_token],
+        },
         relying_party: "rp://api.minecraftservices.com/",
         token_type: "JWT",
     };
-    let response = client.post(XSTS_AUTHORIZE_URL).json(&request).send().map_err(MsaError::Network)?;
+    let response = client
+        .post(XSTS_AUTHORIZE_URL)
+        .json(&request)
+        .send()
+        .map_err(MsaError::Network)?;
     let status = response.status();
     if status.as_u16() == 401 {
         // XErr 2148916233 means the account has no Xbox profile at all
@@ -312,7 +360,12 @@ fn xsts_authorize(client: &Client, xbox_live_token: &str) -> Result<(String, Str
         return Err(MsaError::HttpStatus(status));
     }
     let body: XboxTokenResponse = response.json().map_err(MsaError::Network)?;
-    let claim = body.display_claims.xui.into_iter().next().ok_or_else(|| MsaError::UnexpectedResponse("missing uhs".into()))?;
+    let claim = body
+        .display_claims
+        .xui
+        .into_iter()
+        .next()
+        .ok_or_else(|| MsaError::UnexpectedResponse("missing uhs".into()))?;
     Ok((body.token, claim.uhs, claim.xid))
 }
 
@@ -328,8 +381,14 @@ struct MinecraftLoginResponse {
 }
 
 fn minecraft_login(client: &Client, user_hash: &str, xsts_token: &str) -> Result<String, MsaError> {
-    let request = MinecraftLoginRequest { identity_token: format!("XBL3.0 x={user_hash};{xsts_token}") };
-    let response = client.post(MINECRAFT_LOGIN_URL).json(&request).send().map_err(MsaError::Network)?;
+    let request = MinecraftLoginRequest {
+        identity_token: format!("XBL3.0 x={user_hash};{xsts_token}"),
+    };
+    let response = client
+        .post(MINECRAFT_LOGIN_URL)
+        .json(&request)
+        .send()
+        .map_err(MsaError::Network)?;
     if !response.status().is_success() {
         return Err(MsaError::HttpStatus(response.status()));
     }
@@ -347,7 +406,10 @@ pub struct MinecraftProfile {
 /// Confirms game ownership. A 404 here means the account has no Java
 /// Edition profile — i.e. doesn't own the game — and nothing should
 /// install or launch.
-fn fetch_minecraft_profile(client: &Client, minecraft_access_token: &str) -> Result<MinecraftProfile, MsaError> {
+fn fetch_minecraft_profile(
+    client: &Client,
+    minecraft_access_token: &str,
+) -> Result<MinecraftProfile, MsaError> {
     let response = client
         .get(MINECRAFT_PROFILE_URL)
         .bearer_auth(minecraft_access_token)
@@ -376,15 +438,26 @@ fn complete_login(client: &Client, tokens: MicrosoftTokens) -> Result<LoginResul
     let (xsts_token, user_hash, xuid) = xsts_authorize(client, &xbox_live_token)?;
     let minecraft_access_token = minecraft_login(client, &user_hash, &xsts_token)?;
     let profile = fetch_minecraft_profile(client, &minecraft_access_token)?;
-    Ok(LoginResult { minecraft_access_token, profile, refresh_token: tokens.refresh_token, xuid })
+    Ok(LoginResult {
+        minecraft_access_token,
+        profile,
+        refresh_token: tokens.refresh_token,
+        xuid,
+    })
 }
 
-pub fn login_with_device_code(client: &Client, start: &DeviceCodeStart) -> Result<LoginResult, MsaError> {
+pub fn login_with_device_code(
+    client: &Client,
+    start: &DeviceCodeStart,
+) -> Result<LoginResult, MsaError> {
     let tokens = poll_device_code(client, start)?;
     complete_login(client, tokens)
 }
 
-pub fn login_with_refresh_token(client: &Client, refresh_token: &str) -> Result<LoginResult, MsaError> {
+pub fn login_with_refresh_token(
+    client: &Client,
+    refresh_token: &str,
+) -> Result<LoginResult, MsaError> {
     let tokens = refresh_microsoft_tokens(client, refresh_token)?;
     complete_login(client, tokens)
 }
@@ -401,8 +474,15 @@ struct StoredAccount {
 
 pub fn save_refresh_token(data_dir: &Path, refresh_token: &str) -> io::Result<()> {
     fs::create_dir_all(data_dir)?;
-    let saved_at_unix = SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs();
-    let contents = serde_json::to_vec_pretty(&StoredAccount { refresh_token: refresh_token.to_string(), saved_at_unix }).expect("StoredAccount is serializable");
+    let saved_at_unix = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_secs();
+    let contents = serde_json::to_vec_pretty(&StoredAccount {
+        refresh_token: refresh_token.to_string(),
+        saved_at_unix,
+    })
+    .expect("StoredAccount is serializable");
 
     let target = data_dir.join(ACCOUNT_FILE);
     let temporary = data_dir.join(".account.json.shacraft.part");
@@ -412,7 +492,7 @@ pub fn save_refresh_token(data_dir: &Path, refresh_token: &str) -> io::Result<()
         use std::os::unix::fs::PermissionsExt;
         fs::set_permissions(&temporary, fs::Permissions::from_mode(0o600))?;
     }
-    fs::rename(temporary, target)
+    crate::download::replace_file(&temporary, &target)
 }
 
 pub fn load_refresh_token(data_dir: &Path) -> Option<String> {
@@ -438,7 +518,10 @@ mod tests {
         let dir = std::env::temp_dir().join(format!("shacraft-msa-test-{}", std::process::id()));
         assert!(load_refresh_token(&dir).is_none());
         save_refresh_token(&dir, "super-secret-refresh-token").unwrap();
-        assert_eq!(load_refresh_token(&dir).as_deref(), Some("super-secret-refresh-token"));
+        assert_eq!(
+            load_refresh_token(&dir).as_deref(),
+            Some("super-secret-refresh-token")
+        );
         clear_account(&dir).unwrap();
         assert!(load_refresh_token(&dir).is_none());
         fs::remove_dir_all(&dir).ok();
@@ -448,9 +531,14 @@ mod tests {
     #[test]
     fn stored_account_file_is_not_world_or_group_readable() {
         use std::os::unix::fs::PermissionsExt;
-        let dir = std::env::temp_dir().join(format!("shacraft-msa-perm-test-{}", std::process::id()));
+        let dir =
+            std::env::temp_dir().join(format!("shacraft-msa-perm-test-{}", std::process::id()));
         save_refresh_token(&dir, "secret").unwrap();
-        let mode = fs::metadata(dir.join(ACCOUNT_FILE)).unwrap().permissions().mode() & 0o777;
+        let mode = fs::metadata(dir.join(ACCOUNT_FILE))
+            .unwrap()
+            .permissions()
+            .mode()
+            & 0o777;
         assert_eq!(mode, 0o600);
         fs::remove_dir_all(&dir).ok();
     }
@@ -459,7 +547,10 @@ mod tests {
     fn refuses_to_run_with_placeholder_client_id() {
         assert!(!is_configured());
         let client = Client::builder().build().unwrap();
-        assert!(matches!(start_device_code(&client), Err(MsaError::NotConfigured)));
+        assert!(matches!(
+            start_device_code(&client),
+            Err(MsaError::NotConfigured)
+        ));
     }
 
     /// Live smoke test: requests a real device code from Microsoft and
diff --git a/src-tauri/src/neoforge.rs b/src-tauri/src/neoforge.rs
index a66bac6..436343b 100644
--- a/src-tauri/src/neoforge.rs
+++ b/src-tauri/src/neoforge.rs
@@ -57,21 +57,34 @@ pub enum NeoForgeError {
     Download(DownloadError),
     Io(io::Error),
     InvalidJson(serde_json::Error),
-    InstallerFailed { exit_code: Option<i32>, output_tail: String },
+    InstallerFailed {
+        exit_code: Option<i32>,
+        output_tail: String,
+    },
 }
 
 impl fmt::Display for NeoForgeError {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::DisallowedHost(url) => write!(formatter, "URL is not a recognised NeoForge host: {url}"),
+            Self::DisallowedHost(url) => {
+                write!(formatter, "URL is not a recognised NeoForge host: {url}")
+            }
             Self::Network(error) => write!(formatter, "network error: {error}"),
             Self::HttpStatus(status) => write!(formatter, "maven.neoforged.net returned {status}"),
-            Self::InvalidChecksum(text) => write!(formatter, "unexpected checksum response: {text}"),
+            Self::InvalidChecksum(text) => {
+                write!(formatter, "unexpected checksum response: {text}")
+            }
             Self::Download(error) => write!(formatter, "{error}"),
             Self::Io(error) => write!(formatter, "I/O error: {error}"),
             Self::InvalidJson(error) => write!(formatter, "invalid NeoForge version JSON: {error}"),
-            Self::InstallerFailed { exit_code, output_tail } => {
-                write!(formatter, "NeoForge installer failed (exit {exit_code:?}):\n{output_tail}")
+            Self::InstallerFailed {
+                exit_code,
+                output_tail,
+            } => {
+                write!(
+                    formatter,
+                    "NeoForge installer failed (exit {exit_code:?}):\n{output_tail}"
+                )
             }
         }
     }
@@ -89,7 +102,10 @@ impl From<io::Error> for NeoForgeError {
 }
 
 pub(crate) fn is_allowed_host(url: &str) -> bool {
-    Url::parse(url).ok().and_then(|parsed| parsed.host_str().map(|host| host == NEOFORGE_HOST)).unwrap_or(false)
+    Url::parse(url)
+        .ok()
+        .and_then(|parsed| parsed.host_str().map(|host| host == NEOFORGE_HOST))
+        .unwrap_or(false)
 }
 
 fn installer_jar_url(loader_version: &str) -> String {
@@ -99,18 +115,29 @@ fn installer_jar_url(loader_version: &str) -> String {
 /// Downloads (or reuses a cached, still-valid) NeoForge installer jar,
 /// verified against the `.sha256` sidecar Maven publishes next to every
 /// artifact.
-pub fn ensure_installer(client: &Client, cache_dir: &Path, loader_version: &str) -> Result<PathBuf, NeoForgeError> {
+pub fn ensure_installer(
+    client: &Client,
+    cache_dir: &Path,
+    loader_version: &str,
+) -> Result<PathBuf, NeoForgeError> {
     let jar_url = installer_jar_url(loader_version);
     let checksum_url = format!("{jar_url}.sha256");
     if !is_allowed_host(&jar_url) {
         return Err(NeoForgeError::DisallowedHost(jar_url));
     }
 
-    let response = client.get(&checksum_url).send().map_err(NeoForgeError::Network)?;
+    let response = client
+        .get(&checksum_url)
+        .send()
+        .map_err(NeoForgeError::Network)?;
     if !response.status().is_success() {
         return Err(NeoForgeError::HttpStatus(response.status()));
     }
-    let sha256 = response.text().map_err(NeoForgeError::Network)?.trim().to_ascii_lowercase();
+    let sha256 = response
+        .text()
+        .map_err(NeoForgeError::Network)?
+        .trim()
+        .to_ascii_lowercase();
     if sha256.len() != 64 || !sha256.bytes().all(|byte| byte.is_ascii_hexdigit()) {
         return Err(NeoForgeError::InvalidChecksum(sha256));
     }
@@ -139,6 +166,23 @@ pub fn installed_version_json_path(game_dir: &Path, loader_version: &str) -> Pat
         .join(format!("neoforge-{loader_version}.json"))
 }
 
+fn patched_client_path(game_dir: &Path, loader_version: &str) -> PathBuf {
+    game_dir
+        .join("libraries/net/neoforged/neoforge")
+        .join(loader_version)
+        .join(format!("neoforge-{loader_version}-client.jar"))
+}
+
+fn is_nonempty_file(path: &Path) -> bool {
+    path.metadata()
+        .is_ok_and(|metadata| metadata.is_file() && metadata.len() > 0)
+}
+
+fn installation_complete(game_dir: &Path, loader_version: &str) -> bool {
+    is_nonempty_file(&installed_version_json_path(game_dir, loader_version))
+        && is_nonempty_file(&patched_client_path(game_dir, loader_version))
+}
+
 /// The installer jar bundles its own `install_profile.json`, which lists
 /// exactly which libraries it will download and which processors it will
 /// run to patch the client — the same manifest the installer itself reads.
@@ -161,7 +205,14 @@ fn read_install_profile_counts(installer_path: &Path) -> Option<(u64, u64)> {
 /// installer logging a couple of extra non-library downloads) never exceeds
 /// or exceeds `total` by much. `total_libraries` caps the download half so
 /// those extra lines cannot crowd out the processor half of the bar.
-fn observe_installer_line(line: &str, downloads_done: &AtomicU64, processors_done: &AtomicU64, total_libraries: u64, total: u64, on_progress: &ProgressCallback) {
+fn observe_installer_line(
+    line: &str,
+    downloads_done: &AtomicU64,
+    processors_done: &AtomicU64,
+    total_libraries: u64,
+    total: u64,
+    on_progress: &ProgressCallback,
+) {
     let trimmed = line.trim_start();
     if trimmed.starts_with("Download completed") {
         downloads_done.fetch_add(1, Ordering::Relaxed);
@@ -173,12 +224,19 @@ fn observe_installer_line(line: &str, downloads_done: &AtomicU64, processors_don
     } else {
         return;
     }
-    let current = downloads_done.load(Ordering::Relaxed).min(total_libraries) + processors_done.load(Ordering::Relaxed);
+    let current = downloads_done.load(Ordering::Relaxed).min(total_libraries)
+        + processors_done.load(Ordering::Relaxed);
     on_progress(current.min(total), total);
 }
 
 fn truncate_tail(text: &str) -> String {
-    text.chars().rev().take(4000).collect::<String>().chars().rev().collect()
+    text.chars()
+        .rev()
+        .take(4000)
+        .collect::<String>()
+        .chars()
+        .rev()
+        .collect()
 }
 
 /// Runs the installer with piped output, reporting live progress as its own
@@ -219,7 +277,14 @@ fn run_installer_with_progress(
         let on_progress = Arc::clone(on_progress);
         thread::spawn(move || {
             for line in BufReader::new(stdout).lines().map_while(Result::ok) {
-                observe_installer_line(&line, &downloads_done, &processors_done, total_libraries, total, &on_progress);
+                observe_installer_line(
+                    &line,
+                    &downloads_done,
+                    &processors_done,
+                    total_libraries,
+                    total,
+                    &on_progress,
+                );
                 let mut log = combined_log.lock().unwrap();
                 log.push_str(&line);
                 log.push('\n');
@@ -243,7 +308,10 @@ fn run_installer_with_progress(
     let tail = truncate_tail(&combined_log.lock().unwrap());
 
     if !status.success() {
-        return Err(NeoForgeError::InstallerFailed { exit_code: status.code(), output_tail: tail });
+        return Err(NeoForgeError::InstallerFailed {
+            exit_code: status.code(),
+            output_tail: tail,
+        });
     }
     Ok((status.code(), tail))
 }
@@ -258,19 +326,48 @@ fn run_installer_with_progress(
 /// progress (installer-confirmed library downloads plus patch-processor
 /// steps, read from the installer's own `install_profile.json`) while it
 /// runs; it fires once with `(1, 1)` when already installed.
-pub fn ensure_client_installed(client: &Client, java_executable: &Path, game_dir: &Path, cache_dir: &Path, loader_version: &str, on_progress: &ProgressCallback) -> Result<VersionJson, NeoForgeError> {
+pub fn ensure_client_installed(
+    client: &Client,
+    java_executable: &Path,
+    game_dir: &Path,
+    cache_dir: &Path,
+    loader_version: &str,
+    on_progress: &ProgressCallback,
+) -> Result<VersionJson, NeoForgeError> {
     let version_json_path = installed_version_json_path(game_dir, loader_version);
-    if !version_json_path.exists() {
+    if !installation_complete(game_dir, loader_version) {
         ensure_launcher_profiles_stub(game_dir)?;
         let installer_path = ensure_installer(client, cache_dir, loader_version)?;
 
-        let (total_libraries, total_processors) = read_install_profile_counts(&installer_path).unwrap_or((0, 0));
+        // A leftover version JSON makes some installer versions treat the
+        // profile as already installed even when the patched client was
+        // deleted or quarantined. Remove only that generated marker so the
+        // official installer is forced to rebuild the incomplete profile.
+        match fs::remove_file(&version_json_path) {
+            Ok(()) => {}
+            Err(error) if error.kind() == io::ErrorKind::NotFound => {}
+            Err(error) => return Err(NeoForgeError::Io(error)),
+        }
+
+        let (total_libraries, total_processors) =
+            read_install_profile_counts(&installer_path).unwrap_or((0, 0));
         let total = (total_libraries + total_processors).max(1);
         on_progress(0, total);
 
-        let (exit_code, tail) = run_installer_with_progress(java_executable, &installer_path, game_dir, cache_dir, total_libraries, total, on_progress)?;
-        if !version_json_path.exists() {
-            return Err(NeoForgeError::InstallerFailed { exit_code, output_tail: tail });
+        let (exit_code, tail) = run_installer_with_progress(
+            java_executable,
+            &installer_path,
+            game_dir,
+            cache_dir,
+            total_libraries,
+            total,
+            on_progress,
+        )?;
+        if !installation_complete(game_dir, loader_version) {
+            return Err(NeoForgeError::InstallerFailed {
+                exit_code,
+                output_tail: tail,
+            });
         }
         on_progress(total, total);
     } else {
@@ -296,12 +393,15 @@ mod tests {
     #[test]
     fn rejects_non_neoforge_hosts() {
         assert!(!is_allowed_host("https://example.com/evil.jar"));
-        assert!(is_allowed_host("https://maven.neoforged.net/releases/x.jar"));
+        assert!(is_allowed_host(
+            "https://maven.neoforged.net/releases/x.jar"
+        ));
     }
 
     #[test]
     fn launcher_profiles_stub_is_idempotent() {
-        let dir = std::env::temp_dir().join(format!("shacraft-neoforge-test-{}", std::process::id()));
+        let dir =
+            std::env::temp_dir().join(format!("shacraft-neoforge-test-{}", std::process::id()));
         ensure_launcher_profiles_stub(&dir).unwrap();
         let first = fs::read_to_string(dir.join("launcher_profiles.json")).unwrap();
         fs::write(dir.join("launcher_profiles.json"), "custom-content").unwrap();
@@ -312,6 +412,25 @@ mod tests {
         fs::remove_dir_all(&dir).unwrap();
     }
 
+    #[test]
+    fn incomplete_install_is_not_accepted() {
+        let dir = std::env::temp_dir().join(format!(
+            "shacraft-neoforge-completeness-test-{}",
+            std::process::id()
+        ));
+        let version = "21.1.248";
+        let json = installed_version_json_path(&dir, version);
+        fs::create_dir_all(json.parent().unwrap()).unwrap();
+        fs::write(&json, b"{}").unwrap();
+        assert!(!installation_complete(&dir, version));
+
+        let client = patched_client_path(&dir, version);
+        fs::create_dir_all(client.parent().unwrap()).unwrap();
+        fs::write(&client, b"patched").unwrap();
+        assert!(installation_complete(&dir, version));
+        fs::remove_dir_all(dir).unwrap();
+    }
+
     #[test]
     fn observe_installer_line_counts_downloads_and_processor_headers() {
         let downloads_done = AtomicU64::new(0);
@@ -326,12 +445,47 @@ mod tests {
 
         // A "Downloading library from ..." start line reports nothing by
         // itself; only its "Download completed" confirmation counts.
-        observe_installer_line("Downloading library from https://example/a.jar", &downloads_done, &processors_done, total_libraries, total, &on_progress);
-        observe_installer_line("Download completed: Checksum validated.", &downloads_done, &processors_done, total_libraries, total, &on_progress);
-        observe_installer_line("Download completed: Checksum validated.", &downloads_done, &processors_done, total_libraries, total, &on_progress);
-        observe_installer_line("Processor: net.neoforged.installertools:jarsplitter", &downloads_done, &processors_done, total_libraries, total, &on_progress);
+        observe_installer_line(
+            "Downloading library from https://example/a.jar",
+            &downloads_done,
+            &processors_done,
+            total_libraries,
+            total,
+            &on_progress,
+        );
+        observe_installer_line(
+            "Download completed: Checksum validated.",
+            &downloads_done,
+            &processors_done,
+            total_libraries,
+            total,
+            &on_progress,
+        );
+        observe_installer_line(
+            "Download completed: Checksum validated.",
+            &downloads_done,
+            &processors_done,
+            total_libraries,
+            total,
+            &on_progress,
+        );
+        observe_installer_line(
+            "Processor: net.neoforged.installertools:jarsplitter",
+            &downloads_done,
+            &processors_done,
+            total_libraries,
+            total,
+            &on_progress,
+        );
         // A processor's sub-step lines (three colons) must not double-count.
-        observe_installer_line("Processor: net.neoforged.installertools:jarsplitter: Loading patch files", &downloads_done, &processors_done, total_libraries, total, &on_progress);
+        observe_installer_line(
+            "Processor: net.neoforged.installertools:jarsplitter: Loading patch files",
+            &downloads_done,
+            &processors_done,
+            total_libraries,
+            total,
+            &on_progress,
+        );
 
         assert_eq!(*calls.lock().unwrap(), vec![(1, 3), (2, 3), (3, 3)]);
     }
@@ -350,7 +504,8 @@ mod tests {
         use crate::{java, mojang};
 
         let client = Client::builder().build().unwrap();
-        let root = std::env::temp_dir().join(format!("shacraft-neoforge-pipeline-{}", std::process::id()));
+        let root =
+            std::env::temp_dir().join(format!("shacraft-neoforge-pipeline-{}", std::process::id()));
         let game_dir = root.join("game");
         let cache_dir = root.join("cache");
         fs::create_dir_all(&cache_dir).unwrap();
@@ -360,7 +515,8 @@ mod tests {
         let vanilla = mojang::fetch_version_json(&client, entry).unwrap();
 
         let no_progress: ProgressCallback = Arc::new(|_, _| {});
-        let java_install = java::ensure_java(&client, &root.join("runtime"), 21, &no_progress).unwrap();
+        let java_install =
+            java::ensure_java(&client, &root.join("runtime"), 21, &no_progress).unwrap();
 
         // The installer fetches and patches vanilla itself; we don't
         // pre-download it. It only needs a Java runtime and an empty dir.
@@ -369,25 +525,65 @@ mod tests {
             let progress_calls = Arc::clone(&progress_calls);
             Arc::new(move |current, total| progress_calls.lock().unwrap().push((current, total)))
         };
-        let neoforge_version = ensure_client_installed(&client, Path::new(&java_install.executable), &game_dir, &cache_dir, "21.1.248", &progress).unwrap();
+        let neoforge_version = ensure_client_installed(
+            &client,
+            Path::new(&java_install.executable),
+            &game_dir,
+            &cache_dir,
+            "21.1.248",
+            &progress,
+        )
+        .unwrap();
         let merged = mojang::merge_versions(&vanilla, Some(&neoforge_version)).unwrap();
-        assert_eq!(merged.main_class, "cpw.mods.bootstraplauncher.BootstrapLauncher");
-        assert!(merged.libraries.len() > 100, "expected vanilla (97) + neoforge (47) libraries, got {}", merged.libraries.len());
+        assert_eq!(
+            merged.main_class,
+            "cpw.mods.bootstraplauncher.BootstrapLauncher"
+        );
+        assert!(
+            merged.libraries.len() > 100,
+            "expected vanilla (97) + neoforge (47) libraries, got {}",
+            merged.libraries.len()
+        );
 
-        let patched_client = game_dir.join("libraries/net/neoforged/neoforge/21.1.248/neoforge-21.1.248-client.jar");
-        assert!(patched_client.exists(), "FancyModLoader needs this at runtime even though it is not on the generic classpath");
+        let patched_client =
+            game_dir.join("libraries/net/neoforged/neoforge/21.1.248/neoforge-21.1.248-client.jar");
+        assert!(
+            patched_client.exists(),
+            "FancyModLoader needs this at runtime even though it is not on the generic classpath"
+        );
 
         let calls = progress_calls.lock().unwrap();
-        assert!(calls.len() > 5, "expected many incremental progress calls, got {}", calls.len());
+        assert!(
+            calls.len() > 5,
+            "expected many incremental progress calls, got {}",
+            calls.len()
+        );
         let (last_current, last_total) = *calls.last().unwrap();
-        assert_eq!(last_current, last_total, "progress must reach 100% on success");
-        assert!(calls.windows(2).all(|pair| pair[0].0 <= pair[1].0), "reported progress must never go backwards");
+        assert_eq!(
+            last_current, last_total,
+            "progress must reach 100% on success"
+        );
+        assert!(
+            calls.windows(2).all(|pair| pair[0].0 <= pair[1].0),
+            "reported progress must never go backwards"
+        );
         drop(calls);
 
         // Re-running must skip straight to reading the cached version JSON
         // rather than invoking the installer again.
-        let neoforge_again = ensure_client_installed(&client, Path::new(&java_install.executable), &game_dir, &cache_dir, "21.1.248", &no_progress).unwrap();
-        assert_eq!(neoforge_again.libraries.len(), neoforge_version.libraries.len());
+        let neoforge_again = ensure_client_installed(
+            &client,
+            Path::new(&java_install.executable),
+            &game_dir,
+            &cache_dir,
+            "21.1.248",
+            &no_progress,
+        )
+        .unwrap();
+        assert_eq!(
+            neoforge_again.libraries.len(),
+            neoforge_version.libraries.len()
+        );
 
         fs::remove_dir_all(&root).ok();
     }
diff --git a/src-tauri/src/remote.rs b/src-tauri/src/remote.rs
index f7e4808..40c73aa 100644
--- a/src-tauri/src/remote.rs
+++ b/src-tauri/src/remote.rs
@@ -1,14 +1,17 @@
 use crate::manifest::{self, Manifest};
 use base64::{engine::general_purpose::STANDARD, Engine};
 use ed25519_dalek::{Signature, Verifier, VerifyingKey};
+use reqwest::header::ACCEPT_ENCODING;
 use reqwest::{blocking::Client, redirect::Policy};
 use serde::{Deserialize, Serialize};
-use std::{fmt, time::Duration};
+use std::{fmt, thread, time::Duration};
 
 const AERONAUTICS_MANIFEST: &str =
     "https://shacraft.ru/api/launcher/v2/profiles/aeronautics/signed-manifest";
 const AERONAUTICS_ONLINE: &str = "https://shacraft.ru/api/online/aoc";
 const MANIFEST_PUBLIC_KEY: &str = "2S3FRdZj4Xw5nJpZ3IhqVITBg3nTH9AtGSo1Ew9+qVQ=";
+const MAX_MANIFEST_SIZE: u64 = 2 * 1024 * 1024;
+const MANIFEST_ATTEMPTS: u32 = 3;
 
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -47,7 +50,9 @@ impl fmt::Display for RemoteError {
             Self::Status(status) => write!(formatter, "ShaCraft manifest request failed: {status}"),
             Self::TooLarge => formatter.write_str("ShaCraft manifest is too large"),
             Self::InvalidSignature => formatter.write_str("ShaCraft manifest signature is invalid"),
-            Self::InvalidManifest(error) => write!(formatter, "ShaCraft manifest is invalid: {error}"),
+            Self::InvalidManifest(error) => {
+                write!(formatter, "ShaCraft manifest is invalid: {error}")
+            }
         }
     }
 }
@@ -62,26 +67,63 @@ pub fn fetch_manifest(profile_id: &str) -> Result<Manifest, RemoteError> {
         .redirect(Policy::none())
         .build()
         .map_err(RemoteError::Network)?;
-    let response = client.get(url).send().map_err(RemoteError::Network)?;
-    if !response.status().is_success() {
-        return Err(RemoteError::Status(response.status()));
+    let mut last_network_error = None;
+    let mut source = None;
+    for attempt in 1..=MANIFEST_ATTEMPTS {
+        match client.get(url).header(ACCEPT_ENCODING, "identity").send() {
+            Ok(response) => {
+                if !response.status().is_success() {
+                    return Err(RemoteError::Status(response.status()));
+                }
+                if response
+                    .content_length()
+                    .is_some_and(|size| size > MAX_MANIFEST_SIZE)
+                {
+                    return Err(RemoteError::TooLarge);
+                }
+                match response.bytes() {
+                    Ok(bytes) if bytes.len() as u64 <= MAX_MANIFEST_SIZE => {
+                        source = Some(bytes);
+                        break;
+                    }
+                    Ok(_) => return Err(RemoteError::TooLarge),
+                    Err(error) => last_network_error = Some(error),
+                }
+            }
+            Err(error) => last_network_error = Some(error),
+        }
+        if attempt < MANIFEST_ATTEMPTS {
+            thread::sleep(Duration::from_millis(250 * attempt as u64));
+        }
     }
-    if response.content_length().is_some_and(|size| size > 2 * 1024 * 1024) {
-        return Err(RemoteError::TooLarge);
-    }
-    let source = response.text().map_err(RemoteError::Network)?;
-    let envelope = serde_json::from_str::<SignedManifest>(&source)
+    let source = source.ok_or_else(|| {
+        RemoteError::Network(last_network_error.expect("a network attempt failed"))
+    })?;
+    let envelope = serde_json::from_slice::<SignedManifest>(&source)
         .map_err(|_| RemoteError::InvalidSignature)?;
     if envelope.schema_version != 1 || envelope.key_id != "2026-09-06" {
         return Err(RemoteError::InvalidSignature);
     }
-    let payload = STANDARD.decode(envelope.payload).map_err(|_| RemoteError::InvalidSignature)?;
-    let signature_bytes = STANDARD.decode(envelope.signature).map_err(|_| RemoteError::InvalidSignature)?;
-    let public_key_bytes = STANDARD.decode(MANIFEST_PUBLIC_KEY).expect("embedded public key must be valid");
-    let public_key = VerifyingKey::from_bytes(&public_key_bytes.try_into().expect("embedded public key must be 32 bytes"))
+    let payload = STANDARD
+        .decode(envelope.payload)
+        .map_err(|_| RemoteError::InvalidSignature)?;
+    let signature_bytes = STANDARD
+        .decode(envelope.signature)
+        .map_err(|_| RemoteError::InvalidSignature)?;
+    let public_key_bytes = STANDARD
+        .decode(MANIFEST_PUBLIC_KEY)
         .expect("embedded public key must be valid");
-    let signature = Signature::from_slice(&signature_bytes).map_err(|_| RemoteError::InvalidSignature)?;
-    public_key.verify(&payload, &signature).map_err(|_| RemoteError::InvalidSignature)?;
+    let public_key = VerifyingKey::from_bytes(
+        &public_key_bytes
+            .try_into()
+            .expect("embedded public key must be 32 bytes"),
+    )
+    .expect("embedded public key must be valid");
+    let signature =
+        Signature::from_slice(&signature_bytes).map_err(|_| RemoteError::InvalidSignature)?;
+    public_key
+        .verify(&payload, &signature)
+        .map_err(|_| RemoteError::InvalidSignature)?;
     let payload = String::from_utf8(payload).map_err(|_| RemoteError::InvalidSignature)?;
     manifest::validate_json(&payload).map_err(RemoteError::InvalidManifest)
 }
@@ -100,5 +142,7 @@ pub fn fetch_server_status(profile_id: &str) -> Result<ServerStatus, RemoteError
     if !response.status().is_success() {
         return Err(RemoteError::Status(response.status()));
     }
-    response.json::<ServerStatus>().map_err(RemoteError::Network)
+    response
+        .json::<ServerStatus>()
+        .map_err(RemoteError::Network)
 }
diff --git a/src-tauri/src/settings.rs b/src-tauri/src/settings.rs
index acf5645..e5422ae 100644
--- a/src-tauri/src/settings.rs
+++ b/src-tauri/src/settings.rs
@@ -1,3 +1,4 @@
+use crate::download;
 use serde::{Deserialize, Serialize};
 use std::{fmt, fs, io, path::Path};
 
@@ -38,7 +39,11 @@ fn default_nickname() -> String {
 
 impl Default for LauncherSettings {
     fn default() -> Self {
-        Self { memory_mb: DEFAULT_MEMORY_MB, nickname: DEFAULT_NICKNAME.into(), account_mode: AccountMode::Offline }
+        Self {
+            memory_mb: DEFAULT_MEMORY_MB,
+            nickname: DEFAULT_NICKNAME.into(),
+            account_mode: AccountMode::Offline,
+        }
     }
 }
 
@@ -55,8 +60,13 @@ impl fmt::Display for SettingsError {
         match self {
             Self::Io(error) => write!(formatter, "Cannot access launcher settings: {error}"),
             Self::InvalidJson(error) => write!(formatter, "Cannot read launcher settings: {error}"),
-            Self::InvalidMemory => write!(formatter, "Memory allocation must be between 3 and 12 GiB"),
-            Self::InvalidNickname => write!(formatter, "Nickname must be 3-16 ASCII letters, numbers, or underscores"),
+            Self::InvalidMemory => {
+                write!(formatter, "Memory allocation must be between 3 and 12 GiB")
+            }
+            Self::InvalidNickname => write!(
+                formatter,
+                "Nickname must be 3-16 ASCII letters, numbers, or underscores"
+            ),
         }
     }
 }
@@ -65,7 +75,9 @@ pub fn load(data_dir: &Path) -> Result<LauncherSettings, SettingsError> {
     let path = data_dir.join(SETTINGS_FILE);
     let source = match fs::read_to_string(path) {
         Ok(source) => source,
-        Err(error) if error.kind() == io::ErrorKind::NotFound => return Ok(LauncherSettings::default()),
+        Err(error) if error.kind() == io::ErrorKind::NotFound => {
+            return Ok(LauncherSettings::default())
+        }
         Err(error) => return Err(SettingsError::Io(error)),
     };
     let settings = serde_json::from_str(&source).map_err(SettingsError::InvalidJson)?;
@@ -73,7 +85,10 @@ pub fn load(data_dir: &Path) -> Result<LauncherSettings, SettingsError> {
     Ok(settings)
 }
 
-pub fn save(data_dir: &Path, settings: LauncherSettings) -> Result<LauncherSettings, SettingsError> {
+pub fn save(
+    data_dir: &Path,
+    settings: LauncherSettings,
+) -> Result<LauncherSettings, SettingsError> {
     validate(&settings)?;
     fs::create_dir_all(data_dir).map_err(SettingsError::Io)?;
 
@@ -81,15 +96,22 @@ pub fn save(data_dir: &Path, settings: LauncherSettings) -> Result<LauncherSetti
     let temporary = data_dir.join(".settings.json.shacraft.part");
     let contents = serde_json::to_vec_pretty(&settings).expect("LauncherSettings is serializable");
     fs::write(&temporary, contents).map_err(SettingsError::Io)?;
-    fs::rename(temporary, target).map_err(SettingsError::Io)?;
+    download::replace_file(&temporary, &target).map_err(SettingsError::Io)?;
     Ok(settings)
 }
 
 fn validate(settings: &LauncherSettings) -> Result<(), SettingsError> {
-    if !(MIN_MEMORY_MB..=MAX_MEMORY_MB).contains(&settings.memory_mb) || settings.memory_mb % 1024 != 0 {
+    if !(MIN_MEMORY_MB..=MAX_MEMORY_MB).contains(&settings.memory_mb)
+        || settings.memory_mb % 1024 != 0
+    {
         return Err(SettingsError::InvalidMemory);
     }
-    if !(3..=16).contains(&settings.nickname.len()) || !settings.nickname.bytes().all(|byte| byte.is_ascii_alphanumeric() || byte == b'_') {
+    if !(3..=16).contains(&settings.nickname.len())
+        || !settings
+            .nickname
+            .bytes()
+            .all(|byte| byte.is_ascii_alphanumeric() || byte == b'_')
+    {
         return Err(SettingsError::InvalidNickname);
     }
     Ok(())
@@ -98,13 +120,19 @@ fn validate(settings: &LauncherSettings) -> Result<(), SettingsError> {
 #[cfg(test)]
 mod tests {
     use super::{load, save, AccountMode, LauncherSettings};
-    use std::{fs, process, time::{SystemTime, UNIX_EPOCH}};
+    use std::{
+        fs, process,
+        time::{SystemTime, UNIX_EPOCH},
+    };
 
     fn temporary_directory() -> std::path::PathBuf {
         std::env::temp_dir().join(format!(
             "shacraft-settings-test-{}-{}",
             process::id(),
-            SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_nanos()
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_nanos()
         ))
     }
 
@@ -116,9 +144,20 @@ mod tests {
         assert_eq!(default.nickname, "Emil");
         assert_eq!(default.account_mode, AccountMode::Offline);
 
-        let saved = save(&directory, LauncherSettings { memory_mb: 8 * 1024, nickname: "Emil".into(), account_mode: AccountMode::Microsoft }).unwrap();
+        let saved = save(
+            &directory,
+            LauncherSettings {
+                memory_mb: 8 * 1024,
+                nickname: "Emil".into(),
+                account_mode: AccountMode::Microsoft,
+            },
+        )
+        .unwrap();
         assert_eq!(saved.memory_mb, 8 * 1024);
-        assert_eq!(load(&directory).unwrap().account_mode, AccountMode::Microsoft);
+        assert_eq!(
+            load(&directory).unwrap().account_mode,
+            AccountMode::Microsoft
+        );
 
         fs::remove_dir_all(directory).unwrap();
     }
@@ -126,8 +165,24 @@ mod tests {
     #[test]
     fn rejects_unsafe_memory_values() {
         let directory = temporary_directory();
-        assert!(save(&directory, LauncherSettings { memory_mb: 512, nickname: "Emil".into(), account_mode: AccountMode::Offline }).is_err());
-        assert!(save(&directory, LauncherSettings { memory_mb: 6 * 1024, nickname: "невалидный".into(), account_mode: AccountMode::Offline }).is_err());
+        assert!(save(
+            &directory,
+            LauncherSettings {
+                memory_mb: 512,
+                nickname: "Emil".into(),
+                account_mode: AccountMode::Offline
+            }
+        )
+        .is_err());
+        assert!(save(
+            &directory,
+            LauncherSettings {
+                memory_mb: 6 * 1024,
+                nickname: "невалидный".into(),
+                account_mode: AccountMode::Offline
+            }
+        )
+        .is_err());
     }
 
     #[test]
diff --git a/src-tauri/src/shacraft_account.rs b/src-tauri/src/shacraft_account.rs
index 558e9e7..26d7b03 100644
--- a/src-tauri/src/shacraft_account.rs
+++ b/src-tauri/src/shacraft_account.rs
@@ -72,7 +72,9 @@ impl fmt::Display for AccountError {
             Self::Api(message) => formatter.write_str(message),
             Self::Io(error) => write!(formatter, "Не удалось сохранить сессию: {error}"),
             Self::InvalidSession => formatter.write_str("Сессия ShaCraft истекла — войдите снова"),
-            Self::NoLinkedNickname => formatter.write_str("Сначала привяжите игровой ник к серверу Aeronautics"),
+            Self::NoLinkedNickname => {
+                formatter.write_str("Сначала привяжите игровой ник к серверу Aeronautics")
+            }
         }
     }
 }
@@ -87,9 +89,14 @@ fn client() -> Result<Client, AccountError> {
 
 fn api_error(response: Response) -> AccountError {
     #[derive(Deserialize)]
-    struct ErrorBody { detail: Option<String> }
+    struct ErrorBody {
+        detail: Option<String>,
+    }
     let status = response.status();
-    let detail = response.json::<ErrorBody>().ok().and_then(|body| body.detail);
+    let detail = response
+        .json::<ErrorBody>()
+        .ok()
+        .and_then(|body| body.detail);
     AccountError::Api(detail.unwrap_or_else(|| format!("ShaCraft API: HTTP {status}")))
 }
 
@@ -105,14 +112,19 @@ fn save_session(data_dir: &Path, token: &str) -> Result<(), AccountError> {
     #[cfg(unix)]
     {
         use std::os::unix::fs::PermissionsExt;
-        fs::set_permissions(&temporary, fs::Permissions::from_mode(0o600)).map_err(AccountError::Io)?;
+        fs::set_permissions(&temporary, fs::Permissions::from_mode(0o600))
+            .map_err(AccountError::Io)?;
     }
-    fs::rename(temporary, path).map_err(AccountError::Io)
+    crate::download::replace_file(&temporary, &path).map_err(AccountError::Io)
 }
 
 fn load_session(data_dir: &Path) -> Result<String, AccountError> {
     let token = fs::read_to_string(session_path(data_dir)).map_err(|error| {
-        if error.kind() == io::ErrorKind::NotFound { AccountError::InvalidSession } else { AccountError::Io(error) }
+        if error.kind() == io::ErrorKind::NotFound {
+            AccountError::InvalidSession
+        } else {
+            AccountError::Io(error)
+        }
     })?;
     let token = token.trim();
     if token.len() < 32 || token.bytes().any(|byte| byte.is_ascii_whitespace()) {
@@ -121,32 +133,59 @@ fn load_session(data_dir: &Path) -> Result<String, AccountError> {
     Ok(token.to_owned())
 }
 
-pub fn authenticate(data_dir: &Path, username: &str, password: &str, register: bool) -> Result<LoginResult, AccountError> {
-    let endpoint = if register { "/api/launcher/auth/register" } else { "/api/launcher/auth/login" };
-    let response = client()?.post(format!("{API_ORIGIN}{endpoint}"))
-        .json(&Credentials { username, password }).send().map_err(AccountError::Network)?;
-    if !response.status().is_success() { return Err(api_error(response)); }
-    let payload = response.json::<AuthResponse>().map_err(AccountError::Network)?;
+pub fn authenticate(
+    data_dir: &Path,
+    username: &str,
+    password: &str,
+    register: bool,
+) -> Result<LoginResult, AccountError> {
+    let endpoint = if register {
+        "/api/launcher/auth/register"
+    } else {
+        "/api/launcher/auth/login"
+    };
+    let response = client()?
+        .post(format!("{API_ORIGIN}{endpoint}"))
+        .json(&Credentials { username, password })
+        .send()
+        .map_err(AccountError::Network)?;
+    if !response.status().is_success() {
+        return Err(api_error(response));
+    }
+    let payload = response
+        .json::<AuthResponse>()
+        .map_err(AccountError::Network)?;
     save_session(data_dir, &payload.session_token)?;
-    Ok(LoginResult { account: payload.account, recovery_codes: payload.recovery_codes })
+    Ok(LoginResult {
+        account: payload.account,
+        recovery_codes: payload.recovery_codes,
+    })
 }
 
 pub fn get_account(data_dir: &Path) -> Result<Account, AccountError> {
     let token = load_session(data_dir)?;
-    let response = client()?.get(format!("{API_ORIGIN}/api/launcher/account"))
-        .bearer_auth(token).send().map_err(AccountError::Network)?;
+    let response = client()?
+        .get(format!("{API_ORIGIN}/api/launcher/account"))
+        .bearer_auth(token)
+        .send()
+        .map_err(AccountError::Network)?;
     if response.status() == reqwest::StatusCode::UNAUTHORIZED {
         let _ = fs::remove_file(session_path(data_dir));
         return Err(AccountError::InvalidSession);
     }
-    if !response.status().is_success() { return Err(api_error(response)); }
+    if !response.status().is_success() {
+        return Err(api_error(response));
+    }
     response.json::<Account>().map_err(AccountError::Network)
 }
 
 pub fn logout(data_dir: &Path) -> Result<(), AccountError> {
     if let Ok(token) = load_session(data_dir) {
-        let _ = client()?.post(format!("{API_ORIGIN}/api/launcher/auth/logout"))
-            .bearer_auth(token).json(&serde_json::json!({})).send();
+        let _ = client()?
+            .post(format!("{API_ORIGIN}/api/launcher/auth/logout"))
+            .bearer_auth(token)
+            .json(&serde_json::json!({}))
+            .send();
     }
     match fs::remove_file(session_path(data_dir)) {
         Ok(()) => Ok(()),
@@ -155,25 +194,43 @@ pub fn logout(data_dir: &Path) -> Result<(), AccountError> {
     }
 }
 
-pub fn start_link(data_dir: &Path, server_id: &str, nickname: &str) -> Result<LinkStart, AccountError> {
+pub fn start_link(
+    data_dir: &Path,
+    server_id: &str,
+    nickname: &str,
+) -> Result<LinkStart, AccountError> {
     let token = load_session(data_dir)?;
-    let response = client()?.post(format!("{API_ORIGIN}/api/account/link/start"))
-        .bearer_auth(token).json(&serde_json::json!({"server_id": server_id, "mc_username": nickname}))
-        .send().map_err(AccountError::Network)?;
-    if !response.status().is_success() { return Err(api_error(response)); }
+    let response = client()?
+        .post(format!("{API_ORIGIN}/api/account/link/start"))
+        .bearer_auth(token)
+        .json(&serde_json::json!({"server_id": server_id, "mc_username": nickname}))
+        .send()
+        .map_err(AccountError::Network)?;
+    if !response.status().is_success() {
+        return Err(api_error(response));
+    }
     response.json::<LinkStart>().map_err(AccountError::Network)
 }
 
 pub fn link_status(data_dir: &Path, challenge_id: i64) -> Result<LinkStatus, AccountError> {
     let token = load_session(data_dir)?;
-    let response = client()?.get(format!("{API_ORIGIN}/api/account/link/status/{challenge_id}"))
-        .bearer_auth(token).send().map_err(AccountError::Network)?;
-    if !response.status().is_success() { return Err(api_error(response)); }
+    let response = client()?
+        .get(format!(
+            "{API_ORIGIN}/api/account/link/status/{challenge_id}"
+        ))
+        .bearer_auth(token)
+        .send()
+        .map_err(AccountError::Network)?;
+    if !response.status().is_success() {
+        return Err(api_error(response));
+    }
     response.json::<LinkStatus>().map_err(AccountError::Network)
 }
 
 pub fn aeronautics_nickname(data_dir: &Path) -> Result<String, AccountError> {
-    get_account(data_dir)?.links.into_iter()
+    get_account(data_dir)?
+        .links
+        .into_iter()
         .find(|link| link.server_id == "aoc")
         .map(|link| link.mc_username)
         .ok_or(AccountError::NoLinkedNickname)
@@ -182,13 +239,19 @@ pub fn aeronautics_nickname(data_dir: &Path) -> Result<String, AccountError> {
 #[cfg(test)]
 mod tests {
     use super::{load_session, save_session, session_path};
-    use std::{fs, process, time::{SystemTime, UNIX_EPOCH}};
+    use std::{
+        fs, process,
+        time::{SystemTime, UNIX_EPOCH},
+    };
 
     fn temporary_directory() -> std::path::PathBuf {
         std::env::temp_dir().join(format!(
             "shacraft-account-test-{}-{}",
             process::id(),
-            SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_nanos()
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_nanos()
         ))
     }
 
@@ -208,7 +271,14 @@ mod tests {
         use std::os::unix::fs::PermissionsExt;
         let directory = temporary_directory();
         save_session(&directory, "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFG").unwrap();
-        assert_eq!(fs::metadata(session_path(&directory)).unwrap().permissions().mode() & 0o077, 0);
+        assert_eq!(
+            fs::metadata(session_path(&directory))
+                .unwrap()
+                .permissions()
+                .mode()
+                & 0o077,
+            0
+        );
         fs::remove_dir_all(directory).unwrap();
     }
 }