AgentFixboard
UnverifiedLast checked MCP Python SDK#schema#error-messages

Unknown prompt name returns 0 or -32603 instead of invalid params -32602

prompts/get for an unregistered name, or without a required argument, fails with JSON-RPC code 0 or -32603 instead of -32602. On mcp 2.3.0 an unknown name is Unknown prompt: does_not_exist with code 0.

From GitHub

modelcontextprotocol/python-sdk

IssuePull request

Difficulty
Easy
Time
~5 min
Steps
2
Fixed in
—
Last checked

Unverified: we could not confirm from the sources that a fix shipped. Source: PR #3696 merged. Last checked 2026-10-22.

Symptom

prompts/get for an unregistered name, or without a required argument, fails with JSON-RPC code 0 or -32603 instead of -32602. On mcp 2.3.0 an unknown name is Unknown prompt: does_not_exist with code 0.

Evidence what you'll see

  • On protocol 2025-11-25 the table is: unknown name → code 0, Unknown prompt: does_not_exist; missing required argument → code 0, Missing required arguments: {'name'}.
  • On protocol 2026-07-28 both of those cases become -32603, Internal server error, so the client is not told which name or argument was wrong.
  • The spec's prompts error-handling section says both cases are invalid params. The TypeScript McpServer throws InvalidParams for an unknown prompt.

Why it happens

MCPServer raised a plain ValueError for those cases. Pull request 3696 returns INVALID_PARAMS (-32602) for unknown prompt names and missing required arguments on both protocol versions. Direct MCPServer.get_prompt() and Prompt.render() calls raise MCPError for the same cases.

Affected versions

mcp 2.3.0 and main at 91941ed, 2.x line. The linked issue does not name an operating system. A prompt function that itself raises is still an internal error (Error rendering prompt boom / -32603) and is out of scope.

Fix steps

0 of 2 done

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How to check it worked

  • prompts/get for does_not_exist, or with a required argument omitted, returns JSON-RPC -32602 rather than 0 or -32603.

If it still fails

  • No published release tag contains pull request 3696. Tag v2.3.0 was compared against merge 754254c08f and does not contain it.
  • Fork pull request 3 on another repository is unmerged and is not the fix. docs/servers/prompts.md documented the missing-argument case as -32603; the pull request updates that documentation.

Sources and credit

Summarized in our own words from these public reports. Credit goes to their authors.

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