fix(mcp): invalidate tools cache on config content + path, not just newer mtime (#4124)

* fix(mcp): invalidate tools cache on config content + path, not just newer mtime

The MCP tools cache invalidated only on a strict extensions-config mtime `>`
comparison and tracked no resolved config path, so `_is_cache_stale()` missed:

- content changes with an unchanged mtime (same-second edits; object-store /
  network mounts that do not bump mtime);
- content changes with a backward mtime (git checkout, cp -p / backup restore,
  tar / rsync preserving timestamps);
- a resolved-path switch to a different config file with mtime <= the recorded
  value (structurally invisible — no path was tracked at all).

On multi-worker (uvicorn/gunicorn) or stale-mtime deployments this leaves the
LangGraph-embedded runtime and every non-writer worker serving stale MCP tools
after `PUT /api/mcp/config`, breaking the module's documented promise that
changes made through the Gateway API are reflected in the embedded runtime.

Record the resolved config path and a `(mtime, size, sha256)` content signature
at initialization and invalidate when the path OR the signature differs (`!=`),
mirroring `config/app_config.py::get_app_config()` so the two runtime-editable
config files share one content-based staleness signal. The per-call stat was
already paid, and the small-file sha256 matches the cost app_config pays per
request. The "config missing / not yet initialized" no-op behavior and the
cache reset endpoint are preserved.

Adds backend/tests/test_mcp_cache.py covering all three failure modes plus
unchanged-file and forward-edit sanity cases. Also folds in an incidental
backend/AGENTS.md doc-sync: the restart-required field list was missing
`scheduler` and `run_ownership`, both present in
reload_boundary.py::STARTUP_ONLY_FIELDS.

* test(mcp): pin cache staleness contracts raised in review

Two review observations on the content-signature cache fix, both raised as
non-blocking design questions rather than bugs:

- Whether relying on mtime+size alone (skipping the sha256) could ever be
  "optimized" back in, reopening the narrow same-second / identical-length
  swap gap the signature was built to close.
- Whether the extensions config being deleted entirely after a successful
  init leaves the cache in a defined state, since current_signature flips
  to None and _is_cache_stale() returns False.

Neither is a behavior change: both were already the intended contract,
preserved verbatim from the pre-fix mtime-only code (which also returned
False once the file could no longer be stat-ed). Record the reasoning
inline and add regression tests that pin each contract so a future change
cannot alter either silently:

- test_same_mtime_same_size_swap_is_stale: a same-length server-name swap
  that leaves mtime AND size unchanged (the precise scenario from review,
  sharper than the existing same-mtime test, which also changes size) is
  still caught only because the sha256 is computed unconditionally.
- test_config_deleted_after_init_is_not_stale: deleting the config file
  after init keeps the cache serving its last-known-good MCP tools instead
  of invalidating into an unconfigured state.

Both new tests were confirmed to fail against a deliberately reintroduced
version of the regression they guard (hash short-circuit / removed
None-guard), then confirmed to pass against the real code.

backend/AGENTS.md

@@ -271,7 +271,7 @@ Setup: Copy `config.example.yaml` to `config.yaml` in the **project root** direc
 
 **Config Hot-Reload Boundary**: Gateway dependencies route through `get_app_config()` on every request, so per-run fields like `models[*].max_tokens`, `summarization.*`, `title.*`, `memory.*`, `subagents.*`, `tools[*]`, and the agent system prompt pick up `config.yaml` edits on the next message. `AppConfig` is intentionally **not** cached on `app.state` — `lifespan()` keeps a local `startup_config` variable for one-shot bootstrap work and passes it to `langgraph_runtime(app, startup_config)`.
 
-Infrastructure fields are **restart-required**. The authoritative list lives in `packages/harness/deerflow/config/reload_boundary.py::STARTUP_ONLY_FIELDS` and is mirrored by the standardised `"startup-only:"` prefix on the corresponding `Field(description=...)` in `AppConfig`, so IDE hover on those fields surfaces the reason inline (no need to context-switch into this table). Currently registered: `database`, `checkpointer`, `run_events`, `stream_bridge`, `sandbox`, `log_level`, `logging`, `channels`, `channel_connections`. Adding a new restart-required field requires updating the registry; drift is pinned by `tests/test_reload_boundary.py`.
+Infrastructure fields are **restart-required**. The authoritative list lives in `packages/harness/deerflow/config/reload_boundary.py::STARTUP_ONLY_FIELDS` and is mirrored by the standardised `"startup-only:"` prefix on the corresponding `Field(description=...)` in `AppConfig`, so IDE hover on those fields surfaces the reason inline (no need to context-switch into this table). Currently registered: `database`, `checkpointer`, `run_events`, `stream_bridge`, `sandbox`, `log_level`, `logging`, `channels`, `channel_connections`, `scheduler`, `run_ownership`. Adding a new restart-required field requires updating the registry; drift is pinned by `tests/test_reload_boundary.py`.
 
 **Persistence backend resolution**: the unified `database` section selects the
 Gateway's LangGraph checkpointer, LangGraph Store, and DeerFlow SQL repositories.
@@ -397,7 +397,7 @@ Proxied through nginx: `/api/langgraph/*` → Gateway LangGraph-compatible runti
 
 `get_available_tools(groups, include_mcp, model_name, subagent_enabled)` assembles:
 1. **Config-defined tools** - Resolved from `config.yaml` via `resolve_variable()`
-2. **MCP tools** - From enabled MCP servers (lazy initialized, cached with mtime invalidation)
+2. **MCP tools** - From enabled MCP servers (lazy initialized, cached with resolved-path + content-signature invalidation)
 3. **Built-in tools**:
    - `present_files` - Make output files visible to user (only `/mnt/user-data/outputs`)
    - `ask_clarification` - Request clarification (intercepted by ClarificationMiddleware, which preserves text fallback and adds `artifact.human_input` for Web UI Human Input Cards)
@@ -429,7 +429,7 @@ Additional providers also live here (`boxlite`, `brave`, `browserless`, `crawl4a
 
 - Uses `langchain-mcp-adapters` `MultiServerMCPClient` for multi-server management
 - **Lazy initialization**: Tools loaded on first use via `get_cached_mcp_tools()`
-- **Cache invalidation**: Detects config file changes via mtime comparison
+- **Cache invalidation**: Detects extensions-config changes by comparing the resolved config path and a `(mtime, size, sha256)` content signature against the values recorded at initialization (mirrors `config/app_config.py::get_app_config()`), not a strict mtime `>` comparison. This catches same-second edits, mtime that stays put or moves backward (`git checkout`, `cp -p` / backup restore, `tar` / `rsync`, object-store / network mounts), and a switch to a different config file with an equal-or-older mtime
 - **Transports**: stdio (command-based), SSE, HTTP
 - **OAuth (HTTP/SSE)**: Supports token endpoint flows (`client_credentials`, `refresh_token`) with automatic token refresh + Authorization header injection
 - **Routing hints**: `extensions_config.json -> mcpServers.<server>.routing` and
@@ -442,7 +442,7 @@ Additional providers also live here (`boxlite`, `brave`, `browserless`, `crawl4a
   add a parallel routing middleware for PR1-style preference hints.
 - **Stdio file outputs**: Persistent stdio sessions are scoped by `user_id:thread_id`. For stdio transports only, DeerFlow pins the subprocess default `cwd` to the thread workspace and `TMPDIR`/`TMP`/`TEMP` to `workspace/.mcp/tmp/`, unless the operator explicitly configured `cwd` or temp env values. SSE/HTTP transports skip this filesystem prep entirely.
 - **Stdio path translation**: MCP-returned local file references are not copied. If a `ResourceLink` or conservative free-text path resolves to an existing file inside the thread's mounted user-data tree, it is translated deterministically to `/mnt/user-data/...`; paths outside that tree remain unchanged.
-- **Runtime updates**: Gateway API saves to extensions_config.json; the Gateway-embedded runtime detects changes via mtime
+- **Runtime updates**: Gateway API saves to extensions_config.json; the Gateway-embedded runtime detects changes via the resolved-path + content-signature check above, so multi-worker / stale-mtime deployments still pick up an added/removed MCP server without a restart (the `PUT /api/mcp/config` reset only clears the cache in its own worker)
 
 ### Skills System (`packages/harness/deerflow/skills/`)
 

backend/packages/harness/deerflow/mcp/cache.py

@@ -1,8 +1,9 @@
 """Cache for MCP tools to avoid repeated loading."""
 
 import asyncio
+import hashlib
 import logging
-import os
+from pathlib import Path
 
 from langchain_core.tools import BaseTool
 
@@ -11,43 +12,101 @@ logger = logging.getLogger(__name__)
 _mcp_tools_cache: list[BaseTool] | None = None
 _cache_initialized = False
 _initialization_lock = asyncio.Lock()
-_config_mtime: float | None = None  # Track config file modification time
+
+# Cache-invalidation key for the resolved extensions config file. We track the
+# resolved path *and* a ``(mtime, size, sha256)`` content signature — mirroring
+# ``deerflow.config.app_config`` for the sibling runtime-editable config file —
+# rather than only the mtime. A strict mtime ``>`` comparison misses same-second
+# edits and mtime that stays put or moves backward (object-store / network
+# mounts, ``git checkout``, ``cp -p`` / backup restore, ``tar`` / ``rsync`` that
+# preserve timestamps), and tracking no path at all makes a switch to a
+# different config file with an equal-or-older mtime structurally invisible.
+_ConfigSignature = tuple[float | None, int | None, str | None]
+_config_path: Path | None = None  # Resolved extensions config path at init time
+_config_signature: _ConfigSignature | None = None  # (mtime, size, sha256) at init time
 
 
-def _get_config_mtime() -> float | None:
-    """Get the modification time of the extensions config file.
-
-    Returns:
-        The modification time as a float, or None if the file doesn't exist.
-    """
+def _resolve_config_path() -> Path | None:
+    """Resolve the extensions config file path, or ``None`` when unconfigured."""
     from deerflow.config.extensions_config import ExtensionsConfig
 
-    config_path = ExtensionsConfig.resolve_config_path()
-    if config_path and config_path.exists():
-        return os.path.getmtime(config_path)
-    return None
+    return ExtensionsConfig.resolve_config_path()
+
+
+def _get_config_signature(config_path: Path) -> _ConfigSignature | None:
+    """Get cache metadata for the extensions config file, including a content digest.
+
+    Mirrors ``deerflow.config.app_config._get_config_signature`` so both
+    runtime-editable config files (``config.yaml`` and ``extensions_config.json``)
+    share the same content-based staleness signal. Returns ``None`` when the
+    file cannot be stat-ed (e.g. it does not exist).
+    """
+    try:
+        stat_result = config_path.stat()
+    except OSError:
+        return None
+
+    # Always hash the full file here rather than short-circuiting when
+    # mtime/size already match a previously recorded signature: swapping in a
+    # different MCP server config of identical byte length within the same
+    # second leaves mtime *and* size unchanged, so only the sha256 catches
+    # that swap. Skipping the hash on an mtime/size match would reopen the
+    # narrow gap this signature was built to close.
+    digest = hashlib.sha256()
+    try:
+        with config_path.open("rb") as f:
+            for chunk in iter(lambda: f.read(1024 * 1024), b""):
+                digest.update(chunk)
+    except OSError:
+        return (stat_result.st_mtime, stat_result.st_size, None)
+
+    return (stat_result.st_mtime, stat_result.st_size, digest.hexdigest())
+
+
+def _current_config_state() -> tuple[Path | None, _ConfigSignature | None]:
+    """Return the currently resolved extensions config path and its signature."""
+    config_path = _resolve_config_path()
+    if config_path is None:
+        return None, None
+    return config_path, _get_config_signature(config_path)
 
 
 def _is_cache_stale() -> bool:
     """Check if the cache is stale due to config file changes.
 
+    The cache is stale when the resolved extensions config path changed, or when
+    the ``(mtime, size, sha256)`` content signature differs from the one recorded
+    at initialization. Using content equality (``!=``) instead of a strict mtime
+    ``>`` comparison detects same-second edits and backward mtime moves, and
+    tracking the resolved path detects a switch to a different config file.
+
     Returns:
         True if the cache should be invalidated, False otherwise.
     """
-    global _config_mtime
-
     if not _cache_initialized:
         return False  # Not initialized yet, not stale
 
-    current_mtime = _get_config_mtime()
+    current_path, current_signature = _current_config_state()
 
-    # If we couldn't get mtime before or now, assume not stale
-    if _config_mtime is None or current_mtime is None:
+    # Preserve the original "config missing / not yet recorded" behavior: if
+    # there was no readable config when the cache was populated, or there is
+    # none now, do not invalidate. This also covers the config being deleted
+    # entirely after a successful init (current_signature flips to None): the
+    # cache intentionally keeps serving its last-known-good MCP tools rather
+    # than invalidating into an unconfigured state, matching the pre-fix
+    # mtime-only contract (which also returned False once the file could no
+    # longer be stat-ed). Treat this as a deliberate fail-soft choice, not an
+    # oversight — a future change that wants "config deleted" to tear down
+    # MCP tools needs its own explicit signal here, not an inferred one.
+    if _config_signature is None or current_signature is None:
         return False
 
-    # If the config file has been modified since we cached, it's stale
-    if current_mtime > _config_mtime:
-        logger.info(f"MCP config file has been modified (mtime: {_config_mtime} -> {current_mtime}), cache is stale")
+    if current_path != _config_path:
+        logger.info("MCP config path changed (%s -> %s), cache is stale", _config_path, current_path)
+        return True
+
+    if current_signature != _config_signature:
+        logger.info("MCP config content changed (signature %s -> %s), cache is stale", _config_signature, current_signature)
         return True
 
     return False
@@ -61,7 +120,7 @@ async def initialize_mcp_tools() -> list[BaseTool]:
     Returns:
         List of LangChain tools from all enabled MCP servers.
     """
-    global _mcp_tools_cache, _cache_initialized, _config_mtime
+    global _mcp_tools_cache, _cache_initialized, _config_path, _config_signature
 
     async with _initialization_lock:
         if _cache_initialized:
@@ -73,8 +132,8 @@ async def initialize_mcp_tools() -> list[BaseTool]:
         logger.info("Initializing MCP tools...")
         _mcp_tools_cache = await get_mcp_tools()
         _cache_initialized = True
-        _config_mtime = _get_config_mtime()  # Record config file mtime
-        logger.info(f"MCP tools initialized: {len(_mcp_tools_cache)} tool(s) loaded (config mtime: {_config_mtime})")
+        _config_path, _config_signature = _current_config_state()  # Record config path + content signature
+        logger.info("MCP tools initialized: %d tool(s) loaded (config path: %s)", len(_mcp_tools_cache), _config_path)
 
         return _mcp_tools_cache
 
@@ -136,10 +195,11 @@ def reset_mcp_tools_cache() -> None:
     Also closes all persistent MCP sessions so they are recreated on
     the next tool load.
     """
-    global _mcp_tools_cache, _cache_initialized, _config_mtime
+    global _mcp_tools_cache, _cache_initialized, _config_path, _config_signature
     _mcp_tools_cache = None
     _cache_initialized = False
-    _config_mtime = None
+    _config_path = None
+    _config_signature = None
 
     # Close persistent sessions – they will be recreated by the next
     # get_mcp_tools() call with the (possibly updated) connection config.

backend/tests/test_mcp_cache.py

@@ -0,0 +1,245 @@
+"""Tests for MCP tools cache staleness detection (``deerflow.mcp.cache``).
+
+Regression coverage for the content-signature invalidation fix. The cache used
+to invalidate on a strict extensions-config *mtime* ``>`` comparison and tracked
+no resolved path, so it missed three real edit patterns that leave stale MCP
+tools serving in the LangGraph-embedded runtime and every non-writer worker:
+
+1. content change with an unchanged mtime (same-second edit; object-store /
+   network mounts that do not bump mtime),
+2. content change with a backward mtime (``git checkout``, ``cp -p`` / backup
+   restore, ``tar`` / ``rsync`` preserving timestamps),
+3. a resolved-path switch to a different config file whose mtime is <= the one
+   recorded at initialization.
+
+The fix mirrors ``deerflow.config.app_config``'s ``(path, (mtime, size,
+sha256))`` detection so both runtime-editable config files share one staleness
+signal. These tests fail on the pre-fix code (cases 1-3 return ``False``) and
+pass afterwards.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+from pathlib import Path
+
+import pytest
+
+import deerflow.mcp.cache as cache_module
+from deerflow.config.extensions_config import ExtensionsConfig
+
+_MISSING = object()
+
+# Module globals that hold cache state. Snapshotted and restored around every
+# test so an initialized cache — or an asyncio lock bound to a closed loop —
+# cannot leak between tests. ``_config_mtime`` is the pre-fix global name and is
+# tracked too so the same fixture works when the source fix is reverted.
+_TRACKED_GLOBALS = (
+    "_mcp_tools_cache",
+    "_cache_initialized",
+    "_config_path",
+    "_config_signature",
+    "_config_mtime",
+    "_initialization_lock",
+)
+
+
+def _write_extensions_config(path: Path, servers: dict) -> None:
+    path.write_text(json.dumps({"mcpServers": servers, "skills": {}}), encoding="utf-8")
+
+
+def _server(command: str = "npx") -> dict:
+    return {"enabled": True, "type": "stdio", "command": command}
+
+
+@pytest.fixture()
+def cache_globals():
+    """Snapshot/restore ``deerflow.mcp.cache`` module globals and reset the lock."""
+    saved = {name: getattr(cache_module, name, _MISSING) for name in _TRACKED_GLOBALS}
+
+    cache_module._mcp_tools_cache = None
+    cache_module._cache_initialized = False
+    for name in ("_config_path", "_config_signature", "_config_mtime"):
+        if hasattr(cache_module, name):
+            setattr(cache_module, name, None)
+    # asyncio.Lock binds to the first event loop it is awaited on, so each test
+    # (which drives initialize_mcp_tools via a fresh asyncio.run) needs its own.
+    cache_module._initialization_lock = asyncio.Lock()
+
+    try:
+        yield
+    finally:
+        for name, value in saved.items():
+            if value is _MISSING:
+                if hasattr(cache_module, name):
+                    delattr(cache_module, name)
+            else:
+                setattr(cache_module, name, value)
+
+
+def _initialize_against(monkeypatch, config_path: Path) -> None:
+    """Populate the cache against ``config_path`` via the real init entry point.
+
+    ``initialize_mcp_tools()`` records the resolved config path + content
+    signature after loading tools; the tool load itself is stubbed so this stays
+    a cache-state unit test with no real MCP servers.
+    """
+    monkeypatch.setenv("DEER_FLOW_EXTENSIONS_CONFIG_PATH", str(config_path))
+
+    async def _fake_get_mcp_tools():
+        return []
+
+    monkeypatch.setattr("deerflow.mcp.tools.get_mcp_tools", _fake_get_mcp_tools)
+    asyncio.run(cache_module.initialize_mcp_tools())
+    assert cache_module._cache_initialized is True
+
+
+def test_not_stale_before_initialization(cache_globals):
+    """An uninitialized cache is never stale (preserved behavior)."""
+    assert cache_module._cache_initialized is False
+    assert cache_module._is_cache_stale() is False
+
+
+def test_initialize_records_path_and_signature(cache_globals, monkeypatch, tmp_path):
+    """initialize_mcp_tools records the resolved path and a full content signature."""
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+
+    _initialize_against(monkeypatch, cfg)
+
+    assert cache_module._config_path == cfg
+    assert cache_module._config_signature is not None
+    mtime, size, digest = cache_module._config_signature
+    assert mtime == cfg.stat().st_mtime
+    assert size == cfg.stat().st_size
+    assert isinstance(digest, str) and len(digest) == 64  # sha256 hexdigest
+
+
+def test_same_mtime_content_change_is_stale(cache_globals, monkeypatch, tmp_path):
+    """Failure mode 1: content rewritten, mtime forced to stay identical."""
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+    recorded_mtime = cfg.stat().st_mtime
+
+    _write_extensions_config(cfg, {"srv1": _server(), "srv2": _server("uvx")})
+    os.utime(cfg, (recorded_mtime, recorded_mtime))
+    assert cfg.stat().st_mtime == recorded_mtime  # guard: mtime truly unchanged
+
+    assert cache_module._is_cache_stale() is True
+
+
+def test_backward_mtime_content_change_is_stale(cache_globals, monkeypatch, tmp_path):
+    """Failure mode 2: content rewritten, mtime moved backward."""
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+    recorded_mtime = cfg.stat().st_mtime
+
+    _write_extensions_config(cfg, {"different": _server()})
+    older = recorded_mtime - 100
+    os.utime(cfg, (older, older))
+    assert cfg.stat().st_mtime < recorded_mtime  # guard: mtime went backward
+
+    assert cache_module._is_cache_stale() is True
+
+
+def test_config_path_switch_is_stale(cache_globals, monkeypatch, tmp_path):
+    """Failure mode 3: resolved path switches to a different file, mtime <= recorded."""
+    cfg_a = tmp_path / "extensions_config.json"
+    cfg_b = tmp_path / "other_extensions_config.json"
+    _write_extensions_config(cfg_a, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg_a)
+    recorded_mtime = cfg_a.stat().st_mtime
+
+    _write_extensions_config(cfg_b, {"totally": _server("uvx")})
+    older = recorded_mtime - 50
+    os.utime(cfg_b, (older, older))  # a DIFFERENT file, mtime <= recorded
+
+    # The resolver now points at cfg_b (e.g. DEER_FLOW_EXTENSIONS_CONFIG_PATH
+    # was repointed, or default resolution now finds a different file).
+    monkeypatch.setattr(
+        ExtensionsConfig,
+        "resolve_config_path",
+        classmethod(lambda cls, config_path=None: cfg_b),
+    )
+
+    assert cache_module._is_cache_stale() is True
+
+
+def test_unchanged_file_is_not_stale(cache_globals, monkeypatch, tmp_path):
+    """Sanity: an untouched config file does not trigger a needless reinit."""
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+
+    assert cache_module._is_cache_stale() is False
+
+
+def test_forward_edit_is_stale(cache_globals, monkeypatch, tmp_path):
+    """Sanity: a genuine forward edit is still detected as stale."""
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+    recorded_mtime = cfg.stat().st_mtime
+
+    _write_extensions_config(cfg, {"srv1": _server(), "srv2": _server("uvx")})
+    newer = recorded_mtime + 100
+    os.utime(cfg, (newer, newer))
+
+    assert cache_module._is_cache_stale() is True
+
+
+def test_same_mtime_same_size_swap_is_stale(cache_globals, monkeypatch, tmp_path):
+    """Precise variant of failure mode 1: mtime *and* size both stay unchanged
+    (an equal-length server-name swap), so mtime/size alone are indistinguishable
+    and only the sha256 content digest can catch the change. Guards the content
+    digest itself: a future change that starts short-circuiting the hash
+    whenever mtime/size already match a recorded value must not make this test
+    pass without actually detecting the swap.
+    """
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+    recorded_mtime = cfg.stat().st_mtime
+    recorded_size = cfg.stat().st_size
+
+    _write_extensions_config(cfg, {"srv9": _server()})  # same-length key swap
+    os.utime(cfg, (recorded_mtime, recorded_mtime))
+    assert cfg.stat().st_mtime == recorded_mtime  # guard: mtime truly unchanged
+    assert cfg.stat().st_size == recorded_size  # guard: size truly unchanged too
+
+    assert cache_module._is_cache_stale() is True
+
+
+def test_config_deleted_after_init_is_not_stale(cache_globals, monkeypatch, tmp_path):
+    """Latent edge preserved by design: if the resolved config file is deleted
+    entirely after a successful init, ``current_signature`` becomes ``None`` and
+    the cache does NOT invalidate — it keeps serving its last-known-good MCP
+    tools instead of tearing down into an unconfigured state. This matches the
+    pre-fix mtime-only contract, which also returned ``False`` once the file
+    could no longer be stat-ed, so it is not a regression introduced by the
+    content-signature fix.
+
+    The resolver is monkeypatched to keep pointing at the (now-missing) path,
+    isolating ``_is_cache_stale``'s own stat-failure handling from
+    ``ExtensionsConfig.resolve_config_path``'s separate not-found contract for
+    explicit path/env-var configuration (that function raises
+    ``FileNotFoundError`` in that mode instead of returning ``None`` — a
+    distinct, pre-existing latent issue outside this module's scope).
+    """
+    cfg = tmp_path / "extensions_config.json"
+    _write_extensions_config(cfg, {"srv1": _server()})
+    _initialize_against(monkeypatch, cfg)
+    assert cache_module._config_signature is not None  # guard: had a real signature
+
+    cfg.unlink()  # the config file is deleted entirely, not just edited
+    monkeypatch.setattr(
+        ExtensionsConfig,
+        "resolve_config_path",
+        classmethod(lambda cls, config_path=None: cfg),
+    )
+
+    assert cache_module._is_cache_stale() is False