fix(fetch): handle malformed input without crashing#3515
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olaservo merged 1 commit intomodelcontextprotocol:mainfrom Mar 15, 2026
Merged
fix(fetch): handle malformed input without crashing#3515olaservo merged 1 commit intomodelcontextprotocol:mainfrom
olaservo merged 1 commit intomodelcontextprotocol:mainfrom
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mcp-server-fetch crashes on any malformed JSON-RPC input because server.run() is called with raise_exceptions=True. A single invalid byte on stdin terminates the server process via unhandled ExceptionGroup. Change to raise_exceptions=False to match the behavior of other reference servers (e.g., mcp-server-time, mcp-server-sqlite) which handle parse errors gracefully and continue serving. Fuzz testing showed fetch crashed on 61/65 test cases while other servers using raise_exceptions=False survived all 65. Fixes modelcontextprotocol#3359
olaservo
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Clean, well-motivated fix. The raise_exceptions=True setting was a debugging aid that causes the server to crash on any malformed JSON-RPC input — the linked issue (#3359) demonstrates 61 out of 65 fuzz test inputs crashing the fetch server.
Changing to raise_exceptions=False (the SDK default) means exceptions are returned as error responses to the client instead of terminating the server process. This is consistent with mcp-server-time, which already uses the default and survives all malformed inputs.
Verified:
- Fetch server explicitly sets
raise_exceptions=Trueat line 288 - Time server omits it (uses SDK default
False) - Git server has the same
Truesetting (good candidate for a follow-up PR) - SDK documents this parameter with clear semantics
LGTM — thanks for the contribution @anshul-garg27!
Reviewed with the assistance of Claude Code (claude-opus-4-6). Claims were independently verified against the source code and linked issues.
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…ocol#3515) fix(fetch): handle malformed input without crashing Changes `raise_exceptions=True` to `raise_exceptions=False` in the fetch server's `Server.run()` call, preventing the server from crashing on malformed JSON-RPC input. This aligns with the SDK's intended default behavior and is consistent with other reference servers. Fixes modelcontextprotocol#3359
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Summary
Change
raise_exceptions=Truetoraise_exceptions=Falseinmcp-server-fetch/server.pyto prevent the server from crashing on malformed JSON-RPC input.Fixes #3359
Problem
mcp-server-fetchterminates on any malformed JSON-RPC message becauseserver.run()is called withraise_exceptions=True(line 288). A single invalid byte on stdin kills the server process via unhandledExceptionGroup.Fuzz testing from the issue showed:
raise_exceptions=True)raise_exceptions=False)Fix
One-line change:
raise_exceptions=True→raise_exceptions=False, consistent with all other reference servers exceptgit(which also has this issue).Test plan
echo "NOT VALID JSON" | mcp-server-fetchshould no longer crash