PrefectHQ/fastmcp v4.1.0 on GitHub

FastMCP 4.1.0 is a maintenance release at heart: a broad sweep of fixes across OpenAPI, proxies, auth, tasks, and the CLI, plus full Python 3.15 support. It doesn't change how most servers behave. A few security fixes do tighten inputs that FastMCP previously accepted, though, and our versioning policy keeps breaking changes out of patch releases, so this ships as a point-one release.

Check these before upgrading:

MultiAuth now qualifies client IDs by source, so existing MultiAuth sessions and tasks start new ownership scopes. Finish pending tasks before upgrading.
Regex tool search uses Pydantic's regex engine, which doesn't support lookarounds or backreferences.
Skill files must resolve inside their configured directories.
OpenAPI path parameters reject . and .. segments.
HTTP sessions now expire after the MCP SDK's 30-minute idle default instead of never.
CodeMode requires Monty 1.1, which renames max_duration_secs to max_feed_duration_secs.

Thanks to our 48 contributors, including 21 first-time contributors!

What's Changed

Breaking Changes ⚠️

fix(http): preserve the SDK's default session idle timeout by @asts-top in #5229
Restore full Python 3.15 support by @Kludex in #5558

Enhancements ✨

enhancement: Configure token introspection audiences by @jlowin and @sajdakabir in #5461
fix: Make proxy HTTP header forwarding configurable by @jlowin in #5462
enhancement: Configure upload retention in memory by @jlowin and @warlocksmurf in #5463
Clarify maintainer review decisions by @jlowin in #5466
feat: persist encrypted CLI OAuth tokens by @andreivince in #5405
feat(auth): add TokenCache support to GoogleTokenVerifier and GoogleProvider by @jerive in #5366
Add gitleaks secret scanning to prek by @strawgate in #5355
Keep review design decisions with maintainers by @jlowin in #5468

Security 🔒

fix!: Qualify MultiAuth client identities by @jlowin in #5455
fix!: Validate OpenAPI path parameter segments by @jlowin in #5457
fix!: Resolve skill files within their configured directories by @jlowin and @GEONWOOHAN in #5459
fix!: Use Pydantic’s regex engine for tool search by @jlowin in #5467

Fixes 🐞

fix: Share JWKS refreshes across key lookups by @jlowin in #5456
fix: Reuse shared OpenAPI composition members by @jlowin in #5460
Resolve app tool names against the transformed catalog by @jlowin in #5464
fix: keep generator iteration inside dependency lifetime by @mikamikasuki in #5438
fix(tools): consume synchronous generators within dispatched invocation by @HootRock in #5376
fix(tools): reject results returned after timeout deadlines by @HootRock in #5379
Fix #5368: Tool.from_tool: a transform_fn that declares ctx: Context fails on eve by @nandanadileep and @chrikrah in #5371
fix(auth): preserve AuthKit issuer identity by @mikamikasuki and @paddynatte in #5452
fix(auth): advertise resource_metadata only when a route serves it by @alex-feel in #5363
fix: decode escaped JSON Pointer tokens during hydration by @HuaTNA in #5407
fix: read MCP config files as UTF-8 in the install commands by @ch-z-hc in #5359
Add regression coverage for root-context schema caching by @hulkbig in #5397
Keep the original cause when ErrorHandlingMiddleware re-raises an error unchanged by @asasemahmed in #5344
Resolve ASGIServer.http_client relative URLs against the server origin by @asasemahmed in #5341
Raise startup failures from run_server_async instead of hanging by @asasemahmed in #5340
Restrict Windows CLI state access without requiring SeSecurityPrivilege by @asasemahmed in #5312
Advertise the same list_changed capabilities over stdio as other transports by @asasemahmed in #5311
Validate responses to dict and nested-list elicitation choices by @asasemahmed in #5343
Inline enums shared by several fields in elicitation schemas by @asasemahmed in #5342
Honor the SDK ElicitResult returned by an elicitation handler by @asasemahmed in #5339
Look up call_tool output schemas across all tools/list pages by @asasemahmed in #5338
Deduplicate versions in PromptsAsTools and ResourcesAsTools list tools by @asasemahmed in #5331
Map File text formats to canonical text MIME types by @asasemahmed in #5334
Keep generated OpenAPI component names unique when a suffixed name is taken by @asasemahmed in #5337
Keep is_error when ResponseLimitingMiddleware truncates a result by @asasemahmed in #5309
Read +json OpenAPI resource responses as JSON text by @asasemahmed in #5310
Preserve provider challenge defaults through MultiAuth by @taylorwilsdon in #5454
Write generate-cli output as UTF-8 by @asasemahmed and @ch-z-hc in #5335
Strip internal visibility meta from prompt results and static resources by @asasemahmed in #5330
Register browser OAuth clients as public clients by @goran-revops in #5231
fix(client): preserve unique JSON arrays by @lzyruc in #5270
Keep filesystem reloads serialized during cancellation by @sherxlg-gif in #5184
Treat upstream HTTP errors from OpenAPI tools as HTTP status errors by @asasemahmed in #5308
chore(ty): pin the checked platform to the one CI uses (#5302) by @sclfcz in #5303
fix(tools): take partial tool names and docstrings from the wrapped function by @BlueX888 and @Ares16x16 in #5269
fix(tools): keep structured content for object schemas without explicit type by @BlueX888 in #5271
fix: return current tool schemas from cached BM25 searches by @baba9811 in #5153
fix(azure): send prompt=select_account to Entra instead of the consent URL by @mayankkumar-cmd in #5162
fix(auth): preserve upstream refresh error semantics by @77cjj in #5011
Clean up run_server_async when startup exits early by @syf2211 and @asasemahmed in #5469
Pass resource security policy through FileSystemProvider discovery by @asasemahmed in #5498
Forward annotations on proxied resources and resource templates by @asasemahmed in #5499
Forward the resources/read result meta through proxies by @asasemahmed in #5500
Fix proxy dropping log messages whose data is not a msg/extra dict by @asasemahmed in #5501
Forward stop_sequences in the proxy's sampling relay by @asasemahmed in #5502
Treat non-Bearer Authorization schemes as missing authentication by @asasemahmed in #5503
Reject positional-only parameters in prompts and resource templates by @asasemahmed in #5505
Carry domain and prefers_border onto the Prefab renderer resource by @asasemahmed in #5611
Keep GenerativeUI CSP on the renderer resource only by @asasemahmed in #5612
Keep OpenAPI path segments that contain a colon by @asasemahmed in #5613
Print install mcp-json and inspect JSON without Rich wrapping by @asasemahmed in #5615
Coerce optional parameters in fastmcp call by their non-null type by @asasemahmed in #5616
Return the existing component when an ignored duplicate is added by @asasemahmed in #5617
Keep a config client's servers connected when a new() clone connects by @asasemahmed in #5618
Strip discriminator under properties named default, const, enum or examples by @asasemahmed in #5619
Keep a finished task's status when it is cancelled by @asasemahmed in #5621
Forward --skip-source to the uv run subprocess in fastmcp run by @asasemahmed in #5623
Send numeric and boolean OpenAPI raw bodies as text by @asasemahmed in #5624
Map Anthropic stop_sequence to the MCP stopSequence stop reason by @asasemahmed in #5625
Escape the server spec and name in the generate-cli header docstring by @asasemahmed in #5626
Fix context docs: request_context.meta is a dict, not an attribute object by @asasemahmed in #5631
Validate strict tool input in JSON mode by @asasemahmed in #5504
Raise when a required OpenAPI path parameter is missing by @asasemahmed in #5614
Omit writeOnly properties from OpenAPI tool output schemas by @asasemahmed in #5336
fix(tasks): keep stored arguments alive with the other per-task keys by @sclfcz in #5144
fix(auth): omit scopes from generic authorization code exchange by @liang0417 and @kiranthakkar in #5183
Reject missing and unexpected arguments at task submission by @asasemahmed in #5622
fix(auth): preserve verifier-defined challenge scopes through wrappers by @mikamikasuki in #5478
fix(server): VersionFilter get_tool/get_resource/get_prompt respect include_unversioned=False by @Ashfaqbs and @Purin1410 in #4949
Stop asserting that the GitHub remote server lacks structured content by @jlowin in #5634
Skip skills whose SKILL.md is not valid UTF-8 during directory discovery by @asasemahmed in #5497
Fix tool body validation errors at the MCP boundary by @qingshungLI and @alastair in #5485
Detect client extensions without requiring clientInfo by @alex-feel in #5196
Propagate child cancellation from gather by @dennisimoo and @lokesh0186 in #5161
fix(resources): separate URI fragments from query values by @Harsh23Kashyap in #5132
fix: preserve input requests in resource and prompt tools by @HootRock and @andreivince in #5476
fix(docstrings): don't leak Returns/Raises into description for parameterless functions by @Ashfaqbs and @pvliesdonk in #5067
fix(client): hydrate formatted scalars and positional array items by @FanouZeng-TT, @MarcHuang168, @lzyruc, and @onk3sh in #5203

Docs 📚

docs: credit contributors in v4.0.11 changelog entries by @jlowin in #5449
Clarify CodeMode execution context by @jlowin in #5458
docs: explain Inspector entrypoint initialization by @andreivince and @nick-youngblut in #5404
Docs: show post-call inspection of tool text results by @Amazinghorseli and @roli-lpci in #5385
Document FastMCP Client use cases by @andreivince in #5482
docs: show tests for MCP mounted in FastAPI by @andreivince and @DeoLeung in #5471
Document OpenTelemetry log export by @andreivince in #5483
docs: explain OpenAPI client timeouts by @andreivince and @dimitribarbot in #5470
Clarify Python projects and Cursor workspaces in install docs by @andreivince and @nick-youngblut in #5475
Remove comments from fastmcp.json docs examples by @asasemahmed in #5627
Fix mcp-json Advanced Configuration example output by @asasemahmed in #5628
Remove FASTMCP_SERVER_DEPENDENCIES from the settings reference by @asasemahmed in #5629
Fix versioning docs: get_tool takes a VersionSpec and returns None for a missing version by @asasemahmed in #5630
Fix authorization docs: AccessToken field types and built-in check count by @asasemahmed in #5632

Examples & Contrib 💡

Don't evaluate properties when registering MCPMixin methods by @asasemahmed in #5620

Dependencies 📦

chore(deps): bump the uv group across 2 directories with 2 updates by @dependabot[bot] in #5393

New Contributors

@mikamikasuki made their first contribution in #5438
@HootRock made their first contribution in #5376
@alex-feel made their first contribution in #5363
@HuaTNA made their first contribution in #5407
@andreivince made their first contribution in #5404
@ch-z-hc made their first contribution in #5359
@Amazinghorseli made their first contribution in #5385
@hulkbig made their first contribution in #5397
@jerive made their first contribution in #5366
@goran-revops made their first contribution in #5231
@lzyruc made their first contribution in #5270
@sherxlg-gif made their first contribution in #5184
@baba9811 made their first contribution in #5153
@mayankkumar-cmd made their first contribution in #5162
@77cjj made their first contribution in #5011
@liang0417 made their first contribution in #5183
@Ashfaqbs made their first contribution in #4949
@qingshungLI made their first contribution in #5485
@dennisimoo made their first contribution in #5161
@FanouZeng-TT made their first contribution in #5203
@GEONWOOHAN made their first contribution in #5459

Full Changelog: v4.0.11...v4.1.0

PrefectHQ/fastmcp v4.1.0 v4.1.0: Fourth and One on GitHub