This implements [SEP-1303](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1303).
Input validation errors (missing required fields, wrong types) during
tool calls
are now returned as `CallToolResult` with `IsError=true` instead of
JSON-RPC
`-32602` errors. This allows LLMs to see the validation error and
self-correct,
per the MCP spec.
The existing pattern for handler-returned errors (lines 347-350 in
server.go)
already converts regular errors to tool results with `IsError=true`.
This change
applies the same pattern to argument schema validation and unmarshal
errors,
resolving the `TODO(#450)` in the code.
Thanks to @iwyrkore for filing the issue and proposing the approach, and
@findleyr for the cross-SDK analysis in the discussion.
Fixes#450
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
Expose the jsonrpc.Error type to allow access to underlying JSON-RPC
error codes. Also, expose common JSON-RPC error codes in the jsonrpc
package, and MCP error codes in the mcp package.
Also, implement support for the MCP URL elicitation required error
(-32042), enabling servers to request out-of-band user authorization and
clients to automatically handle these requests with retry logic.
Server-side:
- Add CodeURLElicitationRequired constant
- Add URLElicitationRequiredError() constructor.
Client-side:
- Add urlElicitationMiddleware to intercept URLElicitationRequired
errors and retry calls.
- Update callElicitationCompleteHandler to signal waiting operations.
- Fix capability advertisement to support both form and URL modes.
Fixes#452Fixes#623
