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Add quiz example app, fix dev server empty string args#3700

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jlowin merged 2 commits intomainfrom
example-quiz
Mar 30, 2026
Merged

Add quiz example app, fix dev server empty string args#3700
jlowin merged 2 commits intomainfrom
example-quiz

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@jlowin jlowin commented Mar 30, 2026

A quiz/trivia app that demonstrates multi-turn state building with FastMCPApp. The LLM generates questions and passes them to take_quiz, the user answers via buttons, each answer calls a backend tool to grade it, and the final score gets sent back to the conversation via SendMessage.

Also fixes the dev server sending empty strings for unfilled optional form fields, which caused validation errors against non-string types.

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@marvin-context-protocol marvin-context-protocol Bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. cli Related to FastMCP CLI commands (run, dev, install) or CLI functionality. labels Mar 30, 2026
@jlowin jlowin merged commit f01d0c5 into main Mar 30, 2026
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@jlowin jlowin deleted the example-quiz branch March 30, 2026 01:17
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Test Failure Analysis

Summary: Two CI jobs failed after this PR was merged: the lowest-direct dependency resolution job and the Python 3.13 unit tests.

Root Cause:

  1. Tests with lowest-direct dependencies — This PR added prefab-ui>=0.18.0 to the apps optional dependency. When CI ran, uv reported only prefab-ui<=0.17.0 is available, making the dependency unsatisfiable. Since prefab-ui 0.18.0 is now available on PyPI, this was likely a transient propagation delay (the package was published around the same time as this merge).

  2. Tests: Python 3.13 on ubuntu-latesttest_rate_limiting_with_different_operations (in tests/server/middleware/test_rate_limiting.py:381) failed with "DID NOT RAISE ToolError". The test uses max_requests_per_second=9.0, burst_capacity=5 and expects exactly 5 MCP operations to exhaust the token bucket before a 3rd call_tool. The heavy_computation tool has a time.sleep(0.01), and at 9 tokens/second refill, 10ms is enough to restore ~0.09 tokens — creating a razor-thin margin. On Python 3.13, different async scheduling means the timing diverges just enough that the burst isn't exhausted.

Suggested Solution:

  1. prefab-ui failure: Re-running CI should resolve this — 0.18.0 is now indexed. No code change needed.

  2. Rate limiting test: Use a near-zero refill rate to eliminate timing sensitivity, matching the approach in test_rate_limiting_blocks_rapid_requests:

    # Change this:
    RateLimitingMiddleware(max_requests_per_second=9.0, burst_capacity=5)
    # To this (near-zero refill so no tokens regenerate during the test):
    RateLimitingMiddleware(max_requests_per_second=0.001, burst_capacity=5)

    This ensures the burst is what limits requests, not a race between token refill and operation speed.

Detailed Analysis

Failure 1 — Dependency resolution:

× No solution found when resolving dependencies:
╰─▶ Because only prefab-ui<=0.17.0 is available and fastmcp[apps] depends
    on prefab-ui>=0.18.0, we can conclude that fastmcp[apps]'s requirements
    are unsatisfiable.

pyproject.toml (added in this PR): apps = ["prefab-ui>=0.18.0"]

Failure 2 — Rate limiting test (Python 3.13 only):

>           with pytest.raises(ToolError, match="Rate limit exceeded"):
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E           Failed: DID NOT RAISE <class 'fastmcp.exceptions.ToolError'>
tests/server/middleware/test_rate_limiting.py:393

The debug log shows the MCP operation sequence: list_tools → call_tool quick_action → list_tools → call_tool heavy_computation. With burst_capacity=5, this exhausts exactly 4–5 tokens (depending on init). The refill rate of 9/sec means time.sleep(0.01) inside heavy_computation adds back ~0.09 tokens — a margin that apparently makes the difference on Python 3.13's async scheduler vs 3.10.

The companion test test_custom_client_identification explicitly accounts for token count with the comment: burst_capacity=4 # init + list_tools + call + list_tools = 4, so 2nd call fails. The failing test lacks this precision.

Related Files
  • pyproject.toml — added apps = ["prefab-ui>=0.18.0"] (new dependency causing lowest-direct failure)
  • tests/server/middleware/test_rate_limiting.py:381test_rate_limiting_with_different_operations (timing-sensitive, fails on Python 3.13)
  • src/fastmcp/server/middleware/rate_limiting.py:22TokenBucketRateLimiter.consume() (refills tokens using time.time() elapsed)

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]>
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)
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