fix(server): preserve mounted tool task metadata by pandego · Pull Request #3632 · PrefectHQ/fastmcp

pandego · 2026-03-26T13:10:59Z

Mounted tools were losing execution.taskSupport metadata when exposed through a parent server, even though the child tool advertised task support correctly. That made tools/list disagree with actual mounted task behavior and hid SEP-1686 capability from clients.

This preserves the child tool's execution metadata in the mounted provider wrapper and adds a regression test covering the mounted tools/list path. For example, a child @tool(task=True) now still advertises execution.taskSupport == "optional" after parent.mount(child).

jlowin

Thanks for the bug report and fix — this is a real issue.

The fix works but it's in the wrong place. The to_mcp_tool override you added to FastMCPProviderTool is duplicating the same logic from FunctionTool.to_mcp_tool(), and the two already differ slightly (yours preserves self.execution as a fallback, FunctionTool unconditionally overwrites it). Any future Tool subclass that wraps another tool would hit the same bug and need the same override.

The right fix is to move this into Tool.to_mcp_tool() in src/fastmcp/tools/base.py. There's nothing FunctionTool-specific about the logic — it's purely based on task_config, which lives on the base class. Something like:

def to_mcp_tool(self, **overrides):
    ...
    mcp_tool = MCPTool(...)

    if self.task_config.supports_tasks() and "execution" not in overrides:
        mcp_tool.execution = self.execution or ToolExecution(
            taskSupport=self.task_config.mode
        )

    return mcp_tool

That fixes mounted tools, eliminates the FunctionTool.to_mcp_tool() override entirely, and prevents this class of bug from recurring. The execution=tool.execution addition in wrap() is fine to keep.

Your test is good — it should pass as-is against the base-class fix.

pandego · 2026-03-27T12:55:05Z

Thanks - good call. I moved the task execution metadata handling into Tool.to_mcp_tool(), removed the subclass-specific overrides, kept the mounted-tool regression coverage, and rebased on main.

marvin-context-protocol · 2026-03-27T12:57:24Z

Test Failure Analysis

Summary: The Windows CI job ("Tests: Python 3.10 on windows-latest") failed in attempt 1 of this run when a test in tests/client/test_stdio.py timed out after 5 seconds. This appears unrelated to the PR changes and is consistent with a Windows-specific flakiness in subprocess-based stdio tests. A re-run (attempt 2) is currently in progress.

Root Cause: A test in tests/client/test_stdio.py hit the 5-second pytest timeout on Windows during an AnyIO worker thread operation. The stack trace shows the timeout occurred inside ntpath.realpath called from Python's inspect.findsource — this is part of pytest's failure report formatting, not the test code itself. The underlying test that initially failed likely involves PythonStdioTransport or StdioTransport, which spawns subprocess processes and can be slower on Windows. This is not related to the PR's changes, which only touch tool task metadata handling (fastmcp_provider.py, tools/base.py, tools/function_tool.py).

Suggested Solution: No code changes are required for this failure. The second re-run (attempt 2) is already in progress and should confirm whether this is a one-off flaky timeout or a recurring issue. If it fails again:

Consider marking affected tests in tests/client/test_stdio.py with @pytest.mark.timeout(15) (some tests already use this) or @pytest.mark.integration to give them more time on slow Windows runners.
Alternatively, skip the affected test on Windows using @pytest.mark.skipif(sys.platform == "win32", reason="Flaky on Windows").

Detailed Analysis

Workflow run: #23647569998 (attempt 1, now re-running as attempt 2)
Failed job: "Tests: Python 3.10 on windows-latest" (job ID 68883505244)
Branch: fix/3569-mounted-tasksupport

Log excerpt showing the timeout (attempt 1):

tests\client\test_stdio.py .............. +++++++++++++++++++++++++++++++++++ Timeout +++++++++++++++++++++++++++++++++++
~~~~~~~~~~~~~~~~~~~~~ Stack of AnyIO worker thread (8256) ~~~~~~~~~~~~~~~~~~~~~
  File "ntpath.py", line 715, in realpath
    return path
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The Timeout marker appeared while formatting the failure report in ntpath.realpath, meaning:

A test in test_stdio.py failed (root cause not visible — output was cut off when the formatter itself timed out)
pytest's internal failure reporter then also exceeded the 5s timeout while trying to render the traceback

The PR diff shows no changes to test_stdio.py or any stdio transport code. All other jobs (Ubuntu Python 3.10/3.13, MCP conformance, lowest-direct deps, integration tests) passed successfully.

Related Files

tests/client/test_stdio.py — the test file where the timeout occurred; uses PythonStdioTransport / StdioTransport to spawn subprocess servers
src/fastmcp/client/transports/ — stdio transport implementations; not changed by this PR
PR changes (unaffected):
- src/fastmcp/server/providers/fastmcp_provider.py — adds execution=tool.execution to mounted tool wrapping
- src/fastmcp/tools/base.py — moves task execution metadata to Tool.to_mcp_tool() base class
- src/fastmcp/tools/function_tool.py — removes now-redundant to_mcp_tool override
- tests/server/tasks/test_task_mount.py — adds regression test for mounted task metadata

Analysis by marvin bot (edited — previous comment was for an earlier run's lint failure)

jlowin

Clean fix — moving the execution metadata computation to the base class is the right call, and the test covers the mounted path well.

@tool

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com> Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com> Co-authored-by: Marvin Context Protocol <41898282+Marvin Context Protocol@users.noreply.github.com> Co-authored-by: voidborne-d <voidborne-d@users.noreply.github.com> Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com> Co-authored-by: d 🔹 <258577966+voidborne-d@users.noreply.github.com> Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com> Co-authored-by: nightcityblade <nightcityblade@gmail.com> Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com> Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com> Co-authored-by: Bill Easton <strawgate@users.noreply.github.com> Co-authored-by: Sumanshu Nankana <sumanshunankana@gmail.com> Co-authored-by: Eric Robinson <ericrobinson@indeed.com> Co-authored-by: Martim Santos <martimfasantos@gmail.com> Co-authored-by: d 🔹 <liusway405@gmail.com> Co-authored-by: Matthieu B <66959271+mtthidoteu@users.noreply.github.com> Co-authored-by: Sascha Buehrle <47737812+saschabuehrle@users.noreply.github.com> Co-authored-by: Hakancan <142545736+hkc5@users.noreply.github.com> Co-authored-by: nightcityblade <jackchen@haloailabs.com> Co-authored-by: Matt Hallowell <17804673+mhallo@users.noreply.github.com> Co-authored-by: nate nowack <thrast36@gmail.com> Co-authored-by: Bill Easton <williamseaston@gmail.com> Co-authored-by: Marcus Shu <46469249+shulkx@users.noreply.github.com> Co-authored-by: Rushabh Doshi <radoshi@gmail.com> Co-authored-by: AIKAWA Shigechika <shige@aikawa.jp> Co-authored-by: Jeremy Simon <simonjer805@gmail.com> Co-authored-by: Miguel Miranda Dias <7780875+pandego@users.noreply.github.com> Co-authored-by: Anthony James Padavano <padavano.anthony@gmail.com> Co-authored-by: Mostafa Kamal <hiremostafa@gmail.com> Fix auto-close MRE script posting comment without closing (#3386) Fix WorkOS token scope verification bypass 🤖 Generated with Codex (#3407) Fix initialize McpError fallthrough 🤖 Generated with Codex (#3413) Fix transform arg collisions with passthrough params (#3431) Fix get_* returning None when latest version is disabled (#3439) Fix get_* returning None when latest version is disabled (#3421) Fix server lifespan overlap teardown (#3415) Fix $ref output schema object detection regression (#3420) resolved annotations (#3429) Fix async partial callables rejected by iscoroutinefunction (#3438) Fix async partial callables rejected by iscoroutinefunction (#3423) fix: add version to components (#3458) fix: use intent-based flag for OIDC scope patch in load_access_token (#3465) Fixes #3461 fix: normalize Google scope shorthands and surface valid_scopes (#3477) fix: resolve ty 0.0.23 type-checking errors and bump pin (#3481) fix: shield lifespan teardown from cancellation (#3480) fix: forward custom_route endpoints from mounted servers (#3462) fix updates _get_additional_http_routes() to traverse providers, Fixes #3457 fix: remove hardcoded version from CLI help text (#3456) fix: monty 0.0.8 compatibility, drop external_functions from constructor (#3468) fix: task test teardown hanging 5s per test (#3499) Closes #3498 fix: validate workspace path is a directory before cursor install (#3440) Fixes #3426 fix: handle re.error from malformed URI templates in build_regex (#3501) fix: reject empty/OIDC-only required_scopes in AzureProvider (#3503) fix: restrict $ref resolution to local refs only (SSRF/LFI) (#3502) fix warnings and timeouts (#3504) close upgrade check issue when build passes (#3505) Closes #3484 fix: URL-encode path params to prevent SSRF/path traversal (GHSA-vv7q-7jx5-f767) (#3507) fix: prevent path traversal in skill download (#3493) fix: prefer IdP-granted scopes over client-requested scopes in OAuthProxy (#3492) fix: remove unrelated transform and http.py changes from PR scope fix: remove forced follow_redirects from httpx_client_factory calls (#3496) fix: stop passing follow_redirects to httpx_client_factory fix: restore follow_redirects=True for custom httpx client factories Closes #3509 fix: CSRF double-submit cookie check in consent flow (#3519) fix: validate server names in install commands (#3522) fix: use raw strings for regex in pytest.raises match (#3523) fix: reject refresh tokens used as Bearer access tokens (#3524) fix: route ResourcesAsTools/PromptsAsTools through server middleware (#3495) fix: resolve Pyright "Module is not callable" on @tool, @resource, @prompt decorators (#3540) fix: filter warnings by message in KEY_PREFIX test (#3549) fix: suppress output schema for ToolResult subclass annotations (#3548) fix: increase sleep duration in proxy cache tests (#3567) fix: store absolute token expiry to prevent stale expires_in on reload (#3572) fix: preserve tool properties named 'title' during schema compression (#3582) Fix loopback redirect URI port matching per RFC 8252 §7.3 (#3589) Fix app tool routing: visibility check and middleware propagation (#3591) Fix query parameter serialization to respect OpenAPI explode/style settings (#3595) Fix dev apps form: union types, textarea support, JSON parsing (#3597) fix(google): replace deprecated /oauth2/v1/tokeninfo with /oauth2/v3/userinfo (#3603) fix: resolve EntraOBOToken dependency injection through MultiAuth (#3609) fix(docs): correct misleading stateless_http header (#3622) fix: filesystem provider import machinery (#3626) Closes #3625 (issues 2, 3, 6) fix: recover StdioTransport after subprocess exits (#3630) fix(server): preserve mounted tool task metadata (#3632) fix: scope deprecation warning filter to FastMCPDeprecationWarning (#3649) fix imports, add PrefabAppConfig (#3650) fix: resolve CurrentFastMCP/ctx.fastmcp to child server in mounted background tasks (#3651) Fix blocking docs issues: chart imports, Select API, Rx consistency (#3652) closed by default (#3657) Fix prompt caching middleware missing wrap/unwrap round-trip (#3666) fix: serialize object query params per OpenAPI style/explode rules (#3662) Fixes #2857 fix: HTTP request headers not accessible in background task workers (#3631) fix: restore HTTP headers in worker execution path for background tasks (#3681) fix: strip discriminator after dereferencing schemas (#3682) fix: remove stale ty:ignore directives for ty 0.0.26 (#3684) Fix docs gaps in app provider pages (#3690) fix: dev apps log panel UX improvements (#3698) fix dev server empty string args (#3700)

marvin-context-protocol bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. server Related to FastMCP server implementation or server-side functionality. labels Mar 26, 2026

jlowin requested changes Mar 27, 2026

View reviewed changes

pandego added 2 commits March 27, 2026 13:52

fix(server): preserve mounted tool task metadata

9c566bd

fix(server): move task execution metadata to base tool

71efe67

pandego force-pushed the fix/3569-mounted-tasksupport branch from 6d55530 to 71efe67 Compare March 27, 2026 12:54

cleanup: remove stale import, tighten execution metadata guard

288e804

jlowin approved these changes Mar 27, 2026

View reviewed changes

jlowin merged commit 59a126a into PrefectHQ:main Mar 27, 2026
14 of 15 checks passed

marvin-context-protocol bot mentioned this pull request Apr 13, 2026

raise_on_error not forwarded during task graceful degradation #3908

Open

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

fix(server): preserve mounted tool task metadata#3632

fix(server): preserve mounted tool task metadata#3632
jlowin merged 3 commits intoPrefectHQ:mainfrom
pandego:fix/3569-mounted-tasksupport

pandego commented Mar 26, 2026

Uh oh!

jlowin left a comment

Uh oh!

pandego commented Mar 27, 2026

Uh oh!

marvin-context-protocol bot commented Mar 27, 2026 •

edited

Loading

Uh oh!

jlowin left a comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Conversation

pandego commented Mar 26, 2026

Uh oh!

jlowin left a comment

Choose a reason for hiding this comment

Uh oh!

pandego commented Mar 27, 2026

Uh oh!

marvin-context-protocol bot commented Mar 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Test Failure Analysis

Uh oh!

jlowin left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

marvin-context-protocol bot commented Mar 27, 2026 •

edited

Loading