Client.callTool() throws -32602 after tools/list_changed replaces an in-flight call's output schema

Start in src/client/index.ts and inspect Client.callTool(), especially the validator lookup relative to the awaited request. Reproduce the race by refreshing tools while a delayed call is pending, then verify that the original result is checked against the schema active when the call began. Done means the old-generation result succeeds without the -32602 validation error.

Description

Client.callTool() retrieves the cached output validator after awaiting the tools/call response.

If listTools() runs while that request is pending, cacheToolMetadata() clears and replaces the validator cache. The completed call can therefore be validated against a newer schema than the one active when the call started.

Reproduction

1. List a tool whose output schema requires { generation: "old" }.
2. Start a delayed callTool() for that tool.
3. While it is pending, refresh the tool list with a schema requiring
   { generation: "new" }.
4. Complete the original call with { generation: "old" }.

Actual result:
McpError -32602: Structured content does not match the tool's output schema

Expected result:* The call is validated using the schema generation active when the call began.

Proposed fix

Capture the validator before yielding to the request:

const validator = this.getToolOutputValidator(params.name);
const result = await this.request(
  { method: "tools/call", params },
  resultSchema,
  options,
);

This is consistent with the existing required-task check, which is also performed before the request.

diff --git a/src/client/index.ts b/src/client/index.ts
--- a/src/client/index.ts
+++ b/src/client/index.ts
@@
         if (this.isToolTaskRequired(params.name)) {
             throw new McpError(
                 ErrorCode.InvalidRequest,
                 `Tool "${params.name}" requires task-based execution. Use client.experimental.tasks.callToolStream() instead.`
             );
         }

-        const result = await this.request({ method: 'tools/call', params }, resultSchema, options);
-
-        // Check if the tool has an outputSchema
+        // A concurrent listTools() can replace the cache while this request is
+        // in flight. Capture the validator used when the call is dispatched.
         const validator = this.getToolOutputValidator(params.name);
+        const result = await this.request({ method: 'tools/call', params }, resultSchema, options);

         if (validator) {
             // If tool has outputSchema, it MUST return structuredContent (unless it's an error)
             if (!result.structuredContent && !result.isError) {

Affected versions

Reproduced with @modelcontextprotocol/sdk 1.30.0.