Calling a tool that does not exist returned success. A client has no way to tell a typo from a working call, and a model that hallucinates a tool name is told it was right.

How to fix it

1. Return -32601 for an unknown method or tool`Method not found`. A model that receives an error learns the tool does not exist; one that receives success learns the opposite.

What this check asserts

The server answered a request whose Origin header named a site it has no reason to trust. Through DNS rebinding, any web page the user opens can point a hostname at this server's address and send it requests from their browser, with whatever access that network position gives.

How to fix it

1. Check Origin on every request, and refuse with 403Compare it against an allowlist of the origins your clients actually use; a request with no Origin is a non-browser client and is unaffected. The Streamable HTTP transport requires it.
2. Bind a local server to 127.0.0.1, not 0.0.0.0That keeps the rest of the network out. It does not keep a browser on the same machine out, which is why the Origin check is needed as well.

A public endpoint only warns: the rule still applies, but DNS rebinding is an attack on what a browser can reach that the attacker cannot, which a public server is not.

What this check asserts

A parameter with a type and no description tells the model nothing about what to put in it. The call is still well-formed, so the protocol is satisfied and nothing fails a conformance check — the model simply guesses, and the failure surfaces later as a wrong answer rather than as an error. A required parameter is the acute case: the model has to supply it.

How to fix it

1. Describe every required parameter firstOne sentence saying what the value is and where the caller gets it. `id` is not a description; "the account id from list_accounts" is, and it tells the model which other tool to call first.
2. Constrain what you canAn `enum` removes a whole class of guesses. A `pattern`, a `format`, a numeric bound or a `default` each narrow the space the model is choosing from, and all of them are cheaper than the prose that would otherwise be needed.
3. Add an example where the shape is not obvious`examples` on a property costs a few tokens and removes the most common category of malformed call: the right type in the wrong shape.

What this check asserts

A request missing a required parameter was answered as though it were valid. The parameter list is part of the contract; not enforcing it means the contract is advisory.

How to fix it

1. Return -32602 for bad params`Invalid params` is the specified answer. Include which parameter and why in the error message — that message reaches the model.

What this check asserts

The tool accepted a call with a required argument missing. The schema said the argument was required and the server did not enforce it, which means the schema is describing an intention rather than a contract. A model that gets a plausible answer to an incomplete call has no way to learn it made a mistake.

How to fix it

1. Validate arguments against your own schemaBefore the tool body runs, check the arguments against the `inputSchema` you published. Most SDKs will do this for you if you let them.
2. Fail with -32602Return the JSON-RPC `Invalid params` error rather than a success with a guess in it. An error the model can read is a correction; a plausible wrong answer is not.

What this check asserts