diff --git a/coordinator/internal/agent/check.go b/coordinator/internal/agent/check.go
index e81741b..f26568e 100644
--- a/coordinator/internal/agent/check.go
+++ b/coordinator/internal/agent/check.go
@@ -85,8 +85,10 @@ func CheckEnvironment(ctx context.Context) CheckReport {
 	out, err := cmd.Output()
 	if err != nil {
 		// The worker executes workloads by spawning scimesh's task runner, so
-		// the package is a hard requirement, not an optimisation.
-		report.Scimesh = CheckItem{Name: "scimesh", OK: false, Detail: "the worker runs workloads through scimesh — install with: pip install scimesh"}
+		// the package is a hard requirement, not an optimisation. The PyPI
+		// name belongs to a different project, so the wizard installs from
+		// SCIMESH_PIP_PACKAGE instead of suggesting a bare pip install.
+		report.Scimesh = CheckItem{Name: "scimesh", OK: false, Detail: "the worker runs workloads through scimesh — install it from your wheel or index (SCIMESH_PIP_PACKAGE)"}
 		return report
 	}
 	report.Scimesh = CheckItem{Name: "scimesh", OK: true, Detail: strings.TrimSpace(string(out))}
diff --git a/coordinator/internal/agent/setupui/server.go b/coordinator/internal/agent/setupui/server.go
index c3584a7..45161f7 100644
--- a/coordinator/internal/agent/setupui/server.go
+++ b/coordinator/internal/agent/setupui/server.go
@@ -450,7 +450,13 @@ func (s *Server) handleInstallRuntime(w http.ResponseWriter, r *http.Request) {
 		pkg = os.Getenv("SCIMESH_PIP_PACKAGE")
 	}
 	if pkg == "" {
-		pkg = "scimesh"
+		// No PyPI default on purpose: the PyPI name "scimesh" belongs to an
+		// unrelated project, so a silent `pip install scimesh` would install
+		// the wrong software.
+		writeJSON(w, http.StatusConflict, map[string]string{
+			"error": "no scimesh source configured: set SCIMESH_PIP_PACKAGE to your wheel, checkout or index, then retry",
+		})
+		return
 	}
 
 	python3, err := exec.LookPath("python3")
diff --git a/coordinator/internal/agent/setupui/server_test.go b/coordinator/internal/agent/setupui/server_test.go
index a122718..ff63217 100644
--- a/coordinator/internal/agent/setupui/server_test.go
+++ b/coordinator/internal/agent/setupui/server_test.go
@@ -311,18 +311,35 @@ func TestInstallRuntimeCreatesVenvAndReportsPython(t *testing.T) {
 		return nil
 	})
 
-	rec, data := postJSON(t, base, "/api/runtime/install", map[string]any{})
+	rec, data := postJSON(t, base, "/api/runtime/install", map[string]any{"scimesh_package": "/wheels/scimesh.whl"})
 	if rec.Code != http.StatusOK || data["ok"] != true {
 		t.Fatalf("install: got %d %v, want 200 ok", rec.Code, data)
 	}
-	if installedPkg != "scimesh" {
-		t.Errorf("package = %q, want the default scimesh", installedPkg)
+	if installedPkg != "/wheels/scimesh.whl" {
+		t.Errorf("package = %q, want the requested wheel", installedPkg)
 	}
 	if !strings.HasSuffix(data["python"].(string), "venv/bin/python") {
 		t.Errorf("python = %v, want the venv python", data["python"])
 	}
 }
 
+func TestInstallRuntimeRequiresASource(t *testing.T) {
+	sup := &fakeSup{}
+	_, base := newTestServerWithInstall(t, sup, func(ctx context.Context, venvPython, pkg string) error {
+		t.Fatal("install must not run without a package source")
+		return nil
+	})
+	// No source anywhere (SCIMESH_PIP_PACKAGE unset, request empty): 409 with
+	// guidance. The PyPI name is another project, so no silent fallback.
+	rec, data := postJSON(t, base, "/api/runtime/install", map[string]any{})
+	if rec.Code != http.StatusConflict {
+		t.Fatalf("install without source: got %d, want 409", rec.Code)
+	}
+	if !strings.Contains(data["error"].(string), "SCIMESH_PIP_PACKAGE") {
+		t.Errorf("error = %v, want a hint about SCIMESH_PIP_PACKAGE", data["error"])
+	}
+}
+
 func TestInstallRuntimeFailureIsExplained(t *testing.T) {
 	sup := &fakeSup{}
 	_, base := newTestServerWithInstall(t, sup, func(ctx context.Context, venvPython, pkg string) error {
diff --git a/install.ps1 b/install.ps1
index 12d2d65..0740280 100644
--- a/install.ps1
+++ b/install.ps1
@@ -97,6 +97,7 @@ if ($Component -eq "coordinator") {
         Write-Host "For a coordinator started with 'coordinator serve', the worker token is"
         Write-Host "in ~\.scimesh\worker.token on that machine. Set SCIMESH_PIP_PACKAGE to"
         Write-Host "install scimesh into a managed venv, or install it yourself:"
-        Write-Host "  pip install scimesh"
+        Write-Host "  set SCIMESH_PIP_PACKAGE=<your wheel or index>"
+        Write-Host "  $Target setup"
     }
 }
diff --git a/install.sh b/install.sh
index 697b997..d0a3d54 100644
--- a/install.sh
+++ b/install.sh
@@ -112,5 +112,5 @@ else
   echo "For a coordinator started with 'coordinator serve', the worker token is"
   echo "in ~/.scimesh/worker.token on that machine. Set SCIMESH_PIP_PACKAGE to"
   echo "install scimesh into a managed venv, or install it yourself:"
-  echo "  pip install scimesh"
+  echo "  SCIMESH_PIP_PACKAGE=<your wheel or index> worker-agent setup"
 fi
diff --git a/mkdocs/index.md b/mkdocs/index.md
index cde3d57..ad92d1b 100644
--- a/mkdocs/index.md
+++ b/mkdocs/index.md
@@ -113,10 +113,12 @@ chmod +x coordinator
   ```
 
   It spawns `python -m scimesh.worker.task`, so the machine needs Python 3
-  with the `scimesh` package (`pip install scimesh`, or let the managed venv
-  do it via `SCIMESH_PIP_PACKAGE`). For a `coordinator serve` instance, the
-  worker token is in `~/.scimesh/worker.token`. On Windows set
-  `SCIMESH_COMPONENT=worker` for `install.ps1`.
+  with the `scimesh` package. The wizard installs it into its own venv; the
+  package must come from your wheel, checkout or index — point
+  `SCIMESH_PIP_PACKAGE` at it (the PyPI name `scimesh` belongs to an unrelated
+  project). For a `coordinator serve` instance, the worker token is in
+  `~/.scimesh/worker.token`. On Windows set `SCIMESH_COMPONENT=worker` for
+  `install.ps1`.
 - **coordinator** needs no external services at all in its default mode:
   `coordinator serve` embeds SQLite (both databases), the userservice, and
   local workers. The `SCIMESH_DB=postgres` engine remains for cluster