SEP-1303: Input validation errors should return Tool Execution Errors, not Protocol Errors

Summary

SDK v1.29.0 still throws McpError(ErrorCode.InvalidParams, ...) when tool input validation fails, which conflicts with SEP-1303 (merged into spec 2025-11-25).

Per the 2025-11-25 changelog:

Clarify that input validation errors should be returned as Tool Execution Errors rather than Protocol Errors to enable model self-correction (SEP-1303).

Current SDK behavior prevents the agent model from seeing the error message and self-correcting.

Current Behavior

packages/sdk/src/server/mcp.ts (compiled: dist/esm/server/mcp.js:174-180):

const parseResult = await safeParseAsync(schemaToParse, args);
if (!parseResult.success) {
    const error = 'error' in parseResult ? parseResult.error : 'Unknown error';
    const errorMessage = getParseErrorMessage(error);
    throw new McpError(
      ErrorCode.InvalidParams,
      `Input validation error: Invalid arguments for tool ${toolName}: ${errorMessage}`
    );
}

Result to the client: JSON-RPC error -32602 InvalidParams (Protocol Error).

Expected Behavior (SEP-1303)

Return a successful CallToolResult with isError: true and the validation message in content:

{
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Input validation error for tool X: <Zod message>"
      }
    ],
    "isError": true
  }
}

This lets the model see the error and retry with corrected arguments — exactly the motivation behind SEP-1303.

Scope

Only input validation should switch. Keep Protocol Errors for:

• Unknown tool (-32602) — already correct, and SDK 2.0.0-alpha.1 reinforced this for unknown tools
• Disabled tool — correct
• Structural JSON-RPC errors — correct

Impact

Any server built on this SDK is non-compliant with spec 2025-11-25 for this specific clause. MCPWebStore (Trusteed) hit this during a 2025-11-25 alignment audit — no workaround exists short of patching the SDK or wrapping every registerTool call with permissive input schemas and manual Zod validation inside the handler.

Suggested Fix

Transform the throw into a structured CallToolResult:

if (!parseResult.success) {
    const errorMessage = getParseErrorMessage(
      'error' in parseResult ? parseResult.error : 'Unknown error'
    );
    return {
        content: [{
            type: 'text',
            text: `Input validation error for tool ${toolName}: ${errorMessage}`
        }],
        isError: true,
    };
}

Same for the symmetric output validation check at line ~201 — though SEP-1303 language targets input validation primarily; output validation may warrant a separate decision.

References

• Spec 2025-11-25 changelog (minor change #5): https://modelcontextprotocol.io/specification/2025-11-25/changelog
• SEP-1303 discussion: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1303
• SDK source (v1.29.0): src/server/mcp.ts (tool handler path in CallToolRequestSchema)

Happy to open a PR if maintainers confirm the direction.

Start in packages/sdk/src/server/mcp.ts at the CallToolRequestSchema tool handler path and inspect the input and symmetric output validation checks. Compare the current McpError handling with the expected CallToolResult shape and the scope for unknown-tool and structural errors. Done means input validation reaches the client as content with isError: true while the listed protocol errors remain unchanged.