diff --git a/memwalk/__init__.py b/memwalk/__init__.py
index d431b28..2e572c9 100644
--- a/memwalk/__init__.py
+++ b/memwalk/__init__.py
@@ -1,3 +1,3 @@
 """memwalk — ask AI about any codebase via cached SSM state."""
 
-__version__ = "0.3.0"
+__version__ = "0.4.0"
diff --git a/memwalk/cli.py b/memwalk/cli.py
index 0968d77..647fd0f 100644
--- a/memwalk/cli.py
+++ b/memwalk/cli.py
@@ -77,6 +77,11 @@ def digest(
     cfg = load_config()
     source = Path(path).expanduser().resolve()
 
+    if n_ctx is None:
+        from .gpu import auto_n_ctx
+        chosen = auto_n_ctx()
+        console.print(f"[dim]Adaptive n_ctx = {chosen:,}[/dim]")
+
     if split:
         with console.status(f"Discovering subdirectories in {source}…"):
             results = engine_digest_subdirs(cfg, source, n_ctx=n_ctx,
diff --git a/memwalk/engine.py b/memwalk/engine.py
index 53a8b01..7e3c513 100644
--- a/memwalk/engine.py
+++ b/memwalk/engine.py
@@ -12,6 +12,7 @@ from memba import Session
 
 from . import cache, corpus
 from .config import Config
+from .gpu import auto_n_ctx
 
 # Prompt that frames the ingest call so the assistant turn stored in state
 # is *substantive* (not "noted") — avoids the contextual inertia bug we
@@ -64,7 +65,7 @@ def digest(
     if not source_path.exists() or not source_path.is_dir():
         raise NotADirectoryError(source_path)
 
-    n_ctx = n_ctx or cfg.n_ctx
+    n_ctx = auto_n_ctx(n_ctx if n_ctx else None)
     files = corpus.collect_files(source_path)
     if not files:
         raise RuntimeError(f"No source files found under {source_path}")
diff --git a/memwalk/gpu.py b/memwalk/gpu.py
new file mode 100644
index 0000000..88ba559
--- /dev/null
+++ b/memwalk/gpu.py
@@ -0,0 +1,74 @@
+"""GPU VRAM probing and adaptive n_ctx estimation."""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+
+_NEMOTRON_PROFILE: list[tuple[int, float]] = [
+    (8192,  4.5),
+    (16384, 5.0),
+    (32768, 5.5),
+    (65536, 6.5),
+    (131072, 8.5),
+    (262144, 12.0),
+    (524288, 18.0),
+]
+
+
+def probe_free_vram_mb() -> int | None:
+    """Return free VRAM in MiB, or None if no NVIDIA GPU detected."""
+    if shutil.which("nvidia-smi"):
+        try:
+            out = subprocess.check_output(
+                [
+                    "nvidia-smi",
+                    "--query-gpu=memory.free",
+                    "--format=csv,noheader,nounits",
+                ],
+                text=True,
+                timeout=5,
+            )
+            first_line = out.strip().splitlines()[0].strip()
+            return int(float(first_line))
+        except Exception:
+            pass
+
+    try:
+        import pynvml  # type: ignore[import-untyped]
+        pynvml.nvmlInit()
+        handle = pynvml.nvmlDeviceGetHandleByIndex(0)
+        info = pynvml.nvmlDeviceGetMemoryInfo(handle)
+        pynvml.nvmlShutdown()
+        return info.free // (1024 * 1024)
+    except Exception:
+        pass
+
+    return None
+
+
+def estimate_max_n_ctx(free_vram_mb: int, *, headroom_mb: int = 1536) -> int:
+    """Return the largest n_ctx from the profile that fits in free VRAM."""
+    usable_mb = free_vram_mb - headroom_mb
+    if usable_mb <= 0:
+        return 8192
+
+    best = 8192
+    for n_ctx, needed_gb in _NEMOTRON_PROFILE:
+        needed_mb = int(needed_gb * 1024)
+        if needed_mb <= usable_mb:
+            best = n_ctx
+        else:
+            break
+    return best
+
+
+def auto_n_ctx(preferred: int | None = None) -> int:
+    """Return n_ctx to use: preferred if given, else GPU-adaptive."""
+    if preferred is not None and preferred > 0:
+        return preferred
+
+    free_mb = probe_free_vram_mb()
+    if free_mb is None:
+        return 32768
+    return estimate_max_n_ctx(free_mb)
diff --git a/pyproject.toml b/pyproject.toml
index dfc35e2..35ba8f3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name        = "memwalk"
-version     = "0.3.0"
+version     = "0.4.0"
 description = "Ask AI about any codebase — local, cached, SSM-state-backed exploration via memba + Nemotron"
 readme      = "README.md"
 license     = { text = "MIT" }