D1303 — A malformed or unknown top-level tools/call is -32602; a sibling's refusal stays a result

Decision. A tools/call whose arguments is not an object, or that names a tool the server does not have, answers JSON-RPC -32602 (invalid params) naming the tool, with no Go type in the text. The Atlas API takes limit only as a positive integer on every route that has one and answers 400 otherwise. The routed kb "x": tool not found: <name>, where a sibling KB has the tool and the named one does not (routedmount.go), stays an isError result.

Why. The MCP spec makes an unknown tool invalid params, and a Go unmarshal message tells a caller nothing. The routed refusal is a different fact: the tool exists on the connection and one KB declines it, as a denial does, so a caller that branches on isError for refusals keeps working.

Alternatives rejected. Keep the unknown tool as an isError result: contradicts the spec and hides the typo among tool failures. Make the routed refusal -32602 too: the name is valid on the connection, so the call is not malformed. Clamp a non-positive Atlas limit to the default: limit=-1 then answers 200 for a request that makes no sense.

Consequences. A client or script that greps an unknown-tool answer for tool not found must also look for a JSON-RPC error (test/e2e scenario 20 and the fake server in cmd/cartographer/toolprefix_client_test.go do). The unknown call is still audited as unknown_tool, and a pre-D288 prefixed name keeps its hint after unknown tool <name>.