fix: reply -32700 on stdio parse errors instead of closing

[DaleSeo]

Fixes #825

Motivation and Context

The RMCP stdio transport used to close the stream silently when it encountered a malformed line. This made it hard for clients to tell the difference between a parse failure and a normal end-of-file initiated by the peer. Just one bad byte could bring down the entire session.

Now, the stdio transport responds with a JSON-RPC -32700 Parse error when it receives a malformed line, instead of closing the stream silently. It also removes a leading UTF-8 BOM, so messages from Windows tools that have a BOM prefix can be parsed correctly.

How Has This Been Tested?

Added new tests

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[alexhancock]

is there really no way to build something we can serialize to get this value instead of hardcoding it?

[DaleSeo]

Great question, @alexhancock! This was a workaround because JsonRpcError requires its id field and RequestId has no null variant.

I looked into the JSON RPC spec and the MCP spec. I noticed that the MCP spec separated the Response schema into ResultResponse and ErrorResponse in the 2025-11-25 version to match the JSON RPC spec.

2025-06-18	2025-11-25

As seen above, the id field is required for ResultResponse while is optional for ErrorResponse. I checked other SDKs, and they all follow this, so I updated our JsonRpcError::id struct to be optional.

rust-sdk/crates/rmcp/src/model.rs

Lines 462 to 470 in 690df59

	pub struct JsonRpcError {
	pub jsonrpc: JsonRpcVersion2_0,
	// MCP 2025-11-25 §Error Responses: `id` is optional and omitted when the
	// server cannot read the request id (e.g. parse error / invalid request).
	// https://modelcontextprotocol.io/specification/2025-11-25/basic#error-responses
	#[serde(default, skip_serializing_if = "Option::is_none")]
	pub id: Option<RequestId>,
	pub error: ErrorData,
	}