[rudycelekli]

The graphicle program's MCP server answers a method a connection does not serve with JSON-RPC error -32602 ("Invalid params") instead of -32601 ("Method not found").

What happens

On a workspace connection (which offers no resources):

→ {"jsonrpc":"2.0","id":1,"method":"resources/list","params":{}}
← {"jsonrpc":"2.0","id":1,"error":{"code":-32602,"message":"Unsupported MCP method"}}

The same happens for prompts/list, completion/complete, logging/setLevel or a misspelt method.

Why it matters

JSON-RPC 2.0 (which MCP builds on) reserves -32601 for a method the server does not have. Clients rely on that code to treat an optional method as absent, and to tell that case apart from a call to a method the server has that was refused (-32602). The graph engine's own MCP server already answers -32601 (serve.rs).

Change

mcp_in answers -32601 when the method is not served. The message, "Unsupported MCP method", is unchanged. Every other error (an unknown tool, bad arguments, an access refusal) keeps -32602.

Test

workspace_scope::a_workspace_connection_serves_the_graph_tools_of_the_project_repo_names already checks the message resources/list gets on a workspace connection. It now also checks that the code is -32601, and that a tools/call naming an unknown tool still gets -32602.

Verification

In crates/graphicle, with Rust 1.95.0:

• cargo fmt --check: clean
• cargo clippy --locked --all-targets -- -D warnings: clean on Rust 1.95.0 (macOS). Not run locally with --features local-inference; CI covers that.
• cargo test --locked --all-targets: 690 passed, 0 failed.

I left CHANGELOG.md to you.

🤖 Generated with Claude Code

[VikramLex]

Thank you, @rudycelekli. Merged for 0.1.2. -32601 for a method the connection does not serve, and -32602 kept for a refused call, lets clients tell the two apart. Thanks for the extra assertions. The CHANGELOG credits you.