Return Invalid Params for unknown prompts and missing prompt arguments

[latent-9]

Summary

prompts/get returns Internal Error (-32603) when the prompt name is unknown or a required argument is missing. Both are client input errors, so the JSON-RPC error code should be Invalid Params (-32602), which is what the sibling handlers already return for the same conditions.

Current behavior

Requesting an unknown prompt:

{"jsonrpc":"2.0","id":1,"error":{"code":-32603,"message":"Internal error","data":"Prompt not found ghost"}}

completion/complete (Server#complete), asked to complete an argument for that same unknown prompt, already returns -32602. The two handlers return different codes for the one condition.

Why Invalid Params

The not-found and missing-input paths of the neighboring methods already map to -32602:

• tools/call unknown tool, in Server#call_tool
• resources/read unknown resource, via ResourceNotFoundError, standardized to -32602 in SEP-2164
• completion/complete unknown prompt, in Server#complete

prompts/get was the only not-found path left on -32603.

Change

Add an explicit error_code: JsonRpcHandler::ErrorCode::INVALID_PARAMS at the two prompts/get raise sites:

• unknown prompt, in Server#get_prompt
• missing required argument, in Prompt.validate_arguments!

This follows the ResourceNotFoundError pattern already in the tree: the wire code becomes -32602 and the descriptive message is surfaced, while error_type stays :prompt_not_found / :missing_required_arguments so the existing instrumentation labels are unchanged.

After:

{"jsonrpc":"2.0","id":1,"error":{"code":-32602,"message":"Prompt not found ghost","data":"Prompt not found ghost"}}

Tests

Extended the two existing prompts/get error tests to assert the -32602 code; they fail on the current code and pass with the fix. bundle exec rake is green (1701 runs, 0 failures), RuboCop reports no offenses, and the conformance baseline is unchanged.

[koic]

Thanks!