Version-check is_docket_available() to avoid transitive pydocket crash (#3807)

src/fastmcp/server/dependencies.py

@@ -8,6 +8,7 @@ CurrentWorker) and background task execution require fastmcp[tasks].
 from __future__ import annotations
 
 import contextlib
+import importlib.metadata
 import inspect
 import json
 import logging
@@ -30,6 +31,7 @@ from mcp.server.auth.provider import (
     AccessToken as _SDKAccessToken,
 )
 from mcp.server.lowlevel.server import request_ctx
+from packaging.version import Version
 from starlette.requests import Request
 from uncalled_for import Dependency, get_dependency_parameters
 from uncalled_for.resolution import _Depends
@@ -420,15 +422,34 @@ def _get_sync_redis(url: str) -> Any:
 _DOCKET_AVAILABLE: bool | None = None
 
 
+_MIN_DOCKET_VERSION = Version("0.18.0")
+
+
 def is_docket_available() -> bool:
-    """Check if pydocket is installed."""
+    """Check if a compatible pydocket (>= 0.18.0) is installed and importable.
+
+    Three things have to be true for fastmcp's task features to work:
+      1. pydocket distribution metadata is discoverable
+      2. its version is at least ``_MIN_DOCKET_VERSION`` (older versions are
+         missing symbols like ``docket.dependencies.current_execution``,
+         which fastmcp imports on the request hot path)
+      3. the package actually imports — guards against broken/partial
+         installs where metadata exists but ``import docket`` blows up
+
+    Any of those failing means we treat docket as unavailable and fall back
+    to the no-tasks code paths instead of crashing deep inside a request.
+    """
     global _DOCKET_AVAILABLE
     if _DOCKET_AVAILABLE is None:
         try:
-            import docket  # noqa: F401
+            installed = Version(importlib.metadata.version("pydocket"))
+            if installed < _MIN_DOCKET_VERSION:
+                _DOCKET_AVAILABLE = False
+            else:
+                import docket  # noqa: F401
 
-            _DOCKET_AVAILABLE = True
-        except ImportError:
+                _DOCKET_AVAILABLE = True
+        except (importlib.metadata.PackageNotFoundError, ImportError):
             _DOCKET_AVAILABLE = False
     return _DOCKET_AVAILABLE
 
@@ -440,12 +461,27 @@ def require_docket(feature: str) -> None:
         feature: Description of what requires docket (e.g., "`task=True`",
                  "CurrentDocket()"). Will be included in the error message.
     """
-    if not is_docket_available():
-        raise ImportError(
-            f"FastMCP background tasks require the `tasks` extra. "
-            f"Install with: pip install 'fastmcp[tasks]'. "
-            f"(Triggered by {feature})"
+    if is_docket_available():
+        return
+
+    try:
+        installed = importlib.metadata.version("pydocket")
+    except importlib.metadata.PackageNotFoundError:
+        installed = None
+
+    if installed is None:
+        detail = (
+            "FastMCP background tasks require the `tasks` extra. "
+            "Install with: pip install 'fastmcp[tasks]'."
         )
+    else:
+        detail = (
+            f"FastMCP background tasks require pydocket>={_MIN_DOCKET_VERSION}, "
+            f"but pydocket {installed} is installed (likely pulled in by another "
+            f"package). Upgrade with: pip install -U 'pydocket>={_MIN_DOCKET_VERSION}'."
+        )
+
+    raise ImportError(f"{detail} (Triggered by {feature})")
 
 
 # Import Progress separately — it's docket-specific, not part of uncalled-for

src/fastmcp/server/tasks/capabilities.py

@@ -1,7 +1,5 @@
 """SEP-1686 task capabilities declaration."""
 
-from importlib.util import find_spec
-
 from mcp.types import (
     ServerTasksCapability,
     ServerTasksRequestsCapability,
@@ -12,23 +10,27 @@ from mcp.types import (
 )
 
 
-def _is_docket_available() -> bool:
-    """Check if pydocket is installed (local to avoid circular import)."""
-    return find_spec("docket") is not None
-
-
 def get_task_capabilities() -> ServerTasksCapability | None:
     """Return the SEP-1686 task capabilities.
 
     Returns task capabilities as a first-class ServerCapabilities field,
     declaring support for list, cancel, and request operations per SEP-1686.
 
-    Returns None if pydocket is not installed (no task support).
+    Returns None if a compatible pydocket is not installed (no task support).
+    Uses the canonical ``is_docket_available()`` check so that capability
+    advertisement and handler registration stay in sync — otherwise a server
+    with an old transitive pydocket would advertise task support and then
+    return "method not found" when clients invoked it.
 
     Note: prompts/resources are passed via extra_data since the SDK types
     don't include them yet (FastMCP supports them ahead of the spec).
     """
-    if not _is_docket_available():
+    # Function-local import to avoid a circular import at module load time:
+    # fastmcp.server.tasks.__init__ pulls in this module, and dependencies
+    # transitively reaches back into fastmcp.server.tasks.keys.
+    from fastmcp.server.dependencies import is_docket_available
+
+    if not is_docket_available():
         return None
 
     return ServerTasksCapability(

tests/server/tasks/test_task_capabilities.py

@@ -42,3 +42,28 @@ async def test_client_uses_task_capable_session():
         assert client.initialize_result is not None
         # Session should be a ClientSession (task-capable init uses standard session)
         assert type(client.session).__name__ == "ClientSession"
+
+
+def test_capabilities_hidden_when_pydocket_too_old(monkeypatch):
+    """Capability advertisement and handler registration must agree.
+
+    If ``is_docket_available()`` returns False (e.g. an old transitive
+    pydocket), the server skips registering task handlers — so it must
+    also stop advertising task capabilities, or clients would discover
+    task support and then hit "method not found" at runtime.
+    """
+    import importlib.metadata
+
+    from fastmcp.server import dependencies
+
+    original_version = importlib.metadata.version
+
+    def fake_version(name: str) -> str:
+        if name == "pydocket":
+            return "0.16.6"
+        return original_version(name)
+
+    monkeypatch.setattr(dependencies, "_DOCKET_AVAILABLE", None)
+    monkeypatch.setattr(importlib.metadata, "version", fake_version)
+
+    assert get_task_capabilities() is None

tests/server/test_dependencies.py

@@ -757,12 +757,109 @@ class TestDependencyInjection:
 
         assert is_docket_available() is True
 
+    def test_is_docket_available_false_when_pydocket_too_old(self, monkeypatch):
+        """``is_docket_available()`` must treat pre-0.18.0 pydocket as unavailable.
+
+        Older pydocket versions (e.g. 0.16.x, pulled in transitively by
+        packages like prefect) import cleanly but lack the APIs fastmcp
+        uses (``docket.dependencies.current_execution``, etc.). Without a
+        version floor, the check would report available and then crash at
+        runtime. Simulate by forcing ``importlib.metadata`` to report an
+        old version.
+        """
+        import importlib.metadata
+
+        from fastmcp.server import dependencies
+
+        original_version = importlib.metadata.version
+
+        def fake_version(name: str) -> str:
+            if name == "pydocket":
+                return "0.16.6"
+            return original_version(name)
+
+        monkeypatch.setattr(dependencies, "_DOCKET_AVAILABLE", None)
+        monkeypatch.setattr(importlib.metadata, "version", fake_version)
+
+        assert dependencies.is_docket_available() is False
+        # The wrapper that actually failed in #3803 must now return None
+        # instead of raising ImportError on the inner import.
+        assert dependencies.get_task_context() is None
+
+    def test_is_docket_available_false_when_pydocket_not_installed(self, monkeypatch):
+        """``is_docket_available()`` returns False when pydocket is absent."""
+        import importlib.metadata
+
+        from fastmcp.server import dependencies
+
+        original_version = importlib.metadata.version
+
+        def fake_version(name: str) -> str:
+            if name == "pydocket":
+                raise importlib.metadata.PackageNotFoundError(name)
+            return original_version(name)
+
+        monkeypatch.setattr(dependencies, "_DOCKET_AVAILABLE", None)
+        monkeypatch.setattr(importlib.metadata, "version", fake_version)
+
+        assert dependencies.is_docket_available() is False
+
+    def test_is_docket_available_false_when_import_broken(self, monkeypatch):
+        """Metadata says installed but ``import docket`` fails — treat as unavailable.
+
+        Catches the broken/partial-install case where ``importlib.metadata``
+        still reports a usable version but the package itself isn't actually
+        importable (corrupted wheel, sys.path weirdness, etc.). Without the
+        import probe, fastmcp would later crash on its first ``from docket
+        ...`` instead of falling back gracefully.
+        """
+        import builtins
+
+        from fastmcp.server import dependencies
+
+        original_import = builtins.__import__
+
+        def fake_import(name, *args, **kwargs):
+            if name == "docket" or name.startswith("docket."):
+                raise ImportError("simulated broken docket install")
+            return original_import(name, *args, **kwargs)
+
+        monkeypatch.setattr(dependencies, "_DOCKET_AVAILABLE", None)
+        monkeypatch.setattr(builtins, "__import__", fake_import)
+
+        assert dependencies.is_docket_available() is False
+
     def test_require_docket_passes_when_installed(self):
         """Test require_docket doesn't raise when docket is installed."""
         from fastmcp.server.dependencies import require_docket
 
         require_docket("test feature")
 
+    def test_require_docket_error_mentions_version_when_too_old(self, monkeypatch):
+        """``require_docket()`` distinguishes "missing" from "too old".
+
+        When pydocket is installed but pinned below the floor, the install
+        instructions in the error must point at upgrading pydocket — not at
+        installing the ``tasks`` extra (which the resolver will treat as a
+        no-op as long as the lower pin is held by another package).
+        """
+        import importlib.metadata
+
+        from fastmcp.server import dependencies
+
+        original_version = importlib.metadata.version
+
+        def fake_version(name: str) -> str:
+            if name == "pydocket":
+                return "0.16.6"
+            return original_version(name)
+
+        monkeypatch.setattr(dependencies, "_DOCKET_AVAILABLE", None)
+        monkeypatch.setattr(importlib.metadata, "version", fake_version)
+
+        with pytest.raises(ImportError, match="pydocket 0.16.6 is installed"):
+            dependencies.require_docket("CurrentDocket()")
+
     def test_dependency_class_exists(self):
         """Test Dependency and Depends are importable from fastmcp."""
         from fastmcp.dependencies import Dependency, Depends