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A known method whose params do not deserialize is answered with -32601 Method not found

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Seit 17.10.2026.

  • #1375 von @talhayme — offen

Bewertung

Schwierigkeit
4/5
Geschätzter Aufwand
3-5 Tage
Anfängerfreundlichkeit
35/100
Issue-Typ
Bug
Klarheit
Größtenteils klar
Aktivitätsstatus
Aktiv
Tech-Stack
rust
Bereich
backend-api-design

Rechercherichtung

Start by reading the ClientRequest untagged enum and the default on_custom_request handler described in the issue; compare how typed request deserialization failures reach that handler with how unknown methods are handled. Check the existing request error tests and add cases for the malformed tools/call, prompts/get, and resources/read params. Done means known methods with invalid params return -32602, while an unknown method still returns -32601.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

bug P1 ready for work T-handler T-model

Describe the bug

When a request names a standard method but its params do not fit that method's type, the server answers -32601 with the method name as the message. The client is told the server has no tools/call, when the problem is in its own arguments.

To Reproduce (main at 808124b, also 3.5.1; stdio, after a 2025-11-25 handshake, server built with #[tool_router] / #[prompt_router])

request response
tools/call, no params -32601 tools/call
tools/call, params: {} -32601 tools/call
tools/call, params: {"name": 7, "arguments": {}} -32601 tools/call
tools/call, params: {"name": "ok", "arguments": "x"} -32601 tools/call
prompts/get, params: {"name": 7} -32601 prompts/get
resources/read, params: {"uri": 7} -32601 resources/read
fixture/unknown (control) -32601 fixture/unknown

On 2026-07-28 the rows that carry params behave the same; the row without params is correctly answered with -32602 for the missing _meta.

Why

ClientRequest is #[serde(untagged)]. When the typed variant for the method fails, the message falls through to CustomRequest, and the default on_custom_request returns METHOD_NOT_FOUND. Same shape as #1345, one level down.

Expected behavior

-32602 Invalid params when the method is one the SDK defines and its params do not deserialize. The tools page lists "Malformed requests (requests that fail to satisfy CallToolRequest schema)" under protocol errors, and the Go, Python and TypeScript 2.x SDKs answer these requests with -32602.

Related, smaller

A request whose params is not an object ("params": []) is answered with {"jsonrpc":"2.0","error":{"code":-32600,"message":"Invalid request"}}. The response has no id member, although the request's id was readable, so the sender cannot match it. I can split this into its own issue if you prefer.

I can send a PR for the first part.

AI disclosure: investigated and drafted with Claude Code; I reviewed it.

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Rust
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Gemergte PRs (30 T.)
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