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fix: scope deprecation warning filter to FastMCPDeprecationWarning#3649

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jlowin merged 1 commit intomainfrom
fix/3646-deprecation-warning-side-effects
Mar 27, 2026
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fix: scope deprecation warning filter to FastMCPDeprecationWarning#3649
jlowin merged 1 commit intomainfrom
fix/3646-deprecation-warning-side-effects

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@jlowin jlowin commented Mar 27, 2026

FastMCP was calling warnings.simplefilter("default", DeprecationWarning) at import time, which inserted a process-wide filter that made every library's deprecation warnings visible — not just ours. When FastMCP ran alongside Django or other frameworks, their internal deprecation warnings (Django 6.0 migrations, functools changes, etc.) would surface unexpectedly.

The root cause: this global filter was added to ensure FastMCP's own deprecation warnings were visible in library contexts (Python suppresses DeprecationWarning outside __main__ by default). But each deprecation warning site already had a per-site if settings.deprecation_warnings: guard, so the global filter was belt-and-suspenders — and the belt was catching everyone else's pants too.

The fix introduces FastMCPDeprecationWarning(DeprecationWarning) and uses it everywhere FastMCP issues a deprecation warning. The simplefilter call now targets only this subclass, so FastMCP's warnings remain visible while other libraries' warnings follow standard Python behavior.

# Before: surfaced ALL DeprecationWarnings process-wide
warnings.simplefilter("default", DeprecationWarning)

# After: only FastMCP's warnings
warnings.simplefilter("default", FastMCPDeprecationWarning)

Since FastMCPDeprecationWarning is a subclass of DeprecationWarning, existing user code that filters on DeprecationWarning continues to work.

Closes #3646

@jlowin jlowin added the bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. label Mar 27, 2026
@jlowin jlowin merged commit 77d8b3a into main Mar 27, 2026
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@jlowin jlowin deleted the fix/3646-deprecation-warning-side-effects branch March 27, 2026 13:54
jlowin added a commit that referenced this pull request Mar 30, 2026
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]>
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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]>
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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]>
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Co-authored-by: Bill Easton <[email protected]>
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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)
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deprecation warning has unwanted sideeffects

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