fix: suppress output schema for ToolResult subclass annotations#3548
fix: suppress output schema for ToolResult subclass annotations#3548
Conversation
jlowin
added
the
bug
marvin-context-protocol
Bot
added
the
server
![chatgpt-codex-connector[bot]](https://avatars.githubusercontent.com/in/1144995?s=60&v=4){kind=small}
There was a problem hiding this comment.
💡 Codex Review
Here are some automated review suggestions for this pull request.
Reviewed commit: b91d5950cd
ℹ️ About Codex in GitHub
Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you
- Open a pull request for review
- Mark a draft as ready
- Comment "@codex review".
If Codex has suggestions, it will comment; otherwise it will react with 👍.
Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".
| # ToolResult subclasses should suppress schema generation just | ||
| # like ToolResult itself — replace_type only does exact matching. | ||
| if _contains_tool_result_type(output_type): | ||
| output_type = _UnserializableType |
There was a problem hiding this comment.
When this branch forces ParsedFunction.output_schema to None for a ToolResult subclass, Tool.from_tool(..., transform_fn=...) now takes the output_schema is None fallback in src/fastmcp/tools/tool_transform.py:493-506 and inherits the parent tool's schema unless the annotation is exactly ToolResult. In the common case where the parent has an output schema and the custom transform returns a ToolResult subclass with different structured_content, the transformed tool will advertise the parent schema even though run() returns the subclass payload, which can still fail MCP output validation at runtime.
Useful? React with 👍 / 👎.
Test Failure AnalysisSummary: The Tests with lowest-direct dependencies job fails in — this failure is unrelated to this PR's changes and is a pre-existing issue already being fixed in PR #3549. Root Cause: The test at Suggested Solution: This is being fixed in PR #3549 ( Detailed AnalysisFailing test: # tests/utilities/test_components.py:232
def test_warning_for_missing_key_prefix(self):
with warnings.catch_warnings(record=True) as w:
warnings.simplefilter("always")
class NoPrefix(FastMCPComponent):
pass
assert len(w) == 1 # fails: 5 warnings emitted with pydantic 2.11.7Log excerpt: Fix from PR #3549: key_prefix_warnings = [
x for x in w if "does not define KEY_PREFIX" in str(x.message)
]
assert len(key_prefix_warnings) == 1The same failure is reproducible across multiple unrelated PRs (e.g., #3549 was opened specifically to fix it), confirming this is not a regression from this PR. Related Files
🤖 Analysis by marvin — the FastMCP triage bot |
There was a problem hiding this comment.
💡 Codex Review
Here are some automated review suggestions for this pull request.
Reviewed commit: 2eae136da7
ℹ️ About Codex in GitHub
Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you
- Open a pull request for review
- Mark a draft as ready
- Comment "@codex review".
If Codex has suggestions, it will comment; otherwise it will react with 👍.
Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".
| return_annotation = inspect.signature( | ||
| transform_fn | ||
| ).return_annotation | ||
| if return_annotation is ToolResult: | ||
| if issubclass_safe(return_annotation, ToolResult): |
There was a problem hiding this comment.
Fresh evidence: with from __future__ import annotations (which this repo already exercises in tests/tools/test_tool_future_annotations.py), inspect.signature(transform_fn).return_annotation here is the string 'MyToolResult', so issubclass_safe(...) returns false even though ParsedFunction.from_function() has already resolved the return type and set output_schema=None. In that case Tool.from_tool(..., transform_fn=...) falls back to the parent tool's schema, and a custom transform that returns a ToolResult subclass can still advertise the wrong output schema and fail MCP output validation at runtime.
Useful? React with 👍 / 👎.
Co-authored-by: Claude Opus 4.6 <[email protected]> Co-authored-by: Jeremiah Lowin <[email protected]> Co-authored-by: Marvin Context Protocol <41898282+Marvin Context [email protected]> Co-authored-by: voidborne-d <[email protected]> Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com> Co-authored-by: Claude <[email protected]> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com> Co-authored-by: d 🔹 <[email protected]> Co-authored-by: Jeremiah Lowin <[email protected]> Co-authored-by: nightcityblade <[email protected]> Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]> Co-authored-by: Claude Sonnet 4.6 <[email protected]> Co-authored-by: Bill Easton <[email protected]> Co-authored-by: Sumanshu Nankana <[email protected]> Co-authored-by: Eric Robinson <[email protected]> Co-authored-by: Martim Santos <[email protected]> Co-authored-by: d 🔹 <[email protected]> Co-authored-by: Matthieu B <[email protected]> Co-authored-by: Sascha Buehrle <[email protected]> Co-authored-by: Hakancan <[email protected]> Co-authored-by: nightcityblade <[email protected]> Co-authored-by: Matt Hallowell <[email protected]> Co-authored-by: nate nowack <[email protected]> Co-authored-by: Bill Easton <[email protected]> Co-authored-by: Marcus Shu <[email protected]> Co-authored-by: Rushabh Doshi <[email protected]> Co-authored-by: AIKAWA Shigechika <[email protected]> Co-authored-by: Jeremy Simon <[email protected]> Co-authored-by: Miguel Miranda Dias <[email protected]> Co-authored-by: Anthony James Padavano <[email protected]> Co-authored-by: Mostafa Kamal <[email protected]> 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)
1 participant
replace_typeuses exact type identity to recognizeToolResultand suppress automatic output schema generation — but subclasses slip through. A tool annotated-> MyToolResult(ToolResult)gets a spurious schema generated from the container's internal Pydantic fields (content,structured_content, etc.), which then fails MCP SDK validation at runtime because the actual structured content doesn't match the container's shape.The fix adds
_contains_tool_result_type(), mirroring the existing_contains_prefab_type()pattern — a subclass-aware check that recurses through unions andAnnotated, applied beforereplace_typeruns.Closes #3528