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fetch: map Pydantic ValidationError to INVALID_PARAMS; add robots.txt timeout; tests - #4599

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eltypical wants to merge 3 commits into
modelcontextprotocol:mainfrom
eltypical:fix-validate-args-and-timeout
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fetch: map Pydantic ValidationError to INVALID_PARAMS; add robots.txt timeout; tests#4599
eltypical wants to merge 3 commits into
modelcontextprotocol:mainfrom
eltypical:fix-validate-args-and-timeout

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@eltypical

@eltypical eltypical commented Jul 31, 2026

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Problem

  • Tool invocation path (call_tool) only caught ValueError when parsing arguments into the Fetch pydantic model. On pydantic>=2, invalid/missing inputs raise ValidationError which does not inherit from ValueError, bubbling up as unhandled exceptions, violating MCP error contract and causing integrity failures in multi-tenant tool pipelines.
  • robots.txt fetch in check_may_autonomously_fetch_url had no explicit timeout, risking event-loop starvation / availability loss when upstream hosts hang.

Fix

  • Add parse_fetch_args() helper wrapping Fetch(**arguments) and catching ValidationError, re-raising McpError(ErrorData(code=INVALID_PARAMS, ...)).
  • Use parse_fetch_args in call_tool; preserve explicit URL-blank guard.
  • Add timeout=15 to robots.txt GET call.

Tests

  • New TestArgumentParsing::test_parse_fetch_args_maps_validation_error asserts INVALID_PARAMS mapping for missing required fields.
  • New TestCheckMayAutonomouslyFetchUrl::test_robots_fetch_has_timeout verifies timeout kwarg passed to httpx.AsyncClient.get("/robots.txt", ..., timeout=15).

Security/Resilience Impact

  • CIA: Integrity improved by ensuring invalid input never contaminates execution path; Availability improved with bounded robots.txt fetch.

Signed-off-by: Kiell Tampubolon yeheskieltampubolon@gmail.com

eltypical added 2 commits July 31, 2026 10:31
@eltypical
fetch/server: catch Pydantic ValidationError on input parsing and fac…
ab62677 
…tor parse helper; add robots.txt timeout

call_tool previously caught ValueError only, but pydantic>=2 raises ValidationError on model construction. That propagated as an unhandled exception instead of standardized INVALID_PARAMS.

- Add parse_fetch_args() catching ValidationError and raising McpError(INVALID_PARAMS)
- Use helper in call_tool
- Add timeout=15 to robots.txt fetch to avoid hanging on slow sites

Signed-off-by: Kiell Tampubolon <yeheskieltampubolon@gmail.com>
@eltypical
tests(fetch): add regression tests for ValidationError mapping and ro…
eee7f22 
…bots.txt timeout

- Simulate pydantic ValidationError on Fetch construction and assert INVALID_PARAMS via McpError
- Ensure robots.txt fetch passes timeout to AsyncClient.get

Signed-off-by: Kiell Tampubolon <yeheskieltampubolon@gmail.com>
Copilot AI review requested due to automatic review settings July 31, 2026 02:32
Copilot AI reviewed Jul 31, 2026

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Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

@eltypical
fetch: fix robots.txt can_fetch argument order (user_agent, url)
5264af6 
Protego.can_fetch expects (user_agent, url). Passing (url, user_agent) could allow or deny erroneously under certain robots.txt, causing CI test failures in the fetch server package.

Also retains explicit 15s timeout for robots.txt preflight and centralized Pydantic ValidationError→MCP INVALID_PARAMS mapping.

Signed-off-by: Kiell Tampubolon <113831023+eltypical@users.noreply.github.com>
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@eltypical