diff --git a/.changeset/fix-call-tool-schema-generation.md b/.changeset/fix-call-tool-schema-generation.md
new file mode 100644
index 0000000000..bc951b650a
--- /dev/null
+++ b/.changeset/fix-call-tool-schema-generation.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Validate each `callTool()` result against the output schema cached when the call begins, so a concurrent tool-list refresh cannot apply a newer schema to an in-flight result.
diff --git a/src/client/index.ts b/src/client/index.ts
index ae0e698ec3..818505ddb8 100644
--- a/src/client/index.ts
+++ b/src/client/index.ts
@@ -724,10 +724,11 @@ export class Client<
             );
         }
 
+        // Capture the validator before dispatch so a concurrent listTools() refresh cannot change
+        // the schema generation used for this in-flight call.
+        const validator = this.getToolOutputValidator(params.name);
         const result = await this.request({ method: 'tools/call', params }, resultSchema, options);
 
-        // Check if the tool has an outputSchema
-        const validator = this.getToolOutputValidator(params.name);
         if (validator) {
             // If tool has outputSchema, it MUST return structuredContent (unless it's an error)
             if (!result.structuredContent && !result.isError) {
diff --git a/test/client/index.test.ts b/test/client/index.test.ts
index f5c6a348d1..660615987c 100644
--- a/test/client/index.test.ts
+++ b/test/client/index.test.ts
@@ -1779,6 +1779,77 @@ test('should handle partial listChanged capability support', async () => {
 });
 
 describe('outputSchema validation', () => {
+    test('should validate an in-flight call against the output schema active when it started', async () => {
+        const client = new Client(
+            {
+                name: 'test-client',
+                version: '1.0.0'
+            },
+            {
+                capabilities: {}
+            }
+        );
+
+        let catalogGeneration = 'old';
+        let callCount = 0;
+        let releaseFirstCall!: () => void;
+        const firstCallMayReturn = new Promise<void>(resolve => {
+            releaseFirstCall = resolve;
+        });
+
+        vi.spyOn(client, 'request').mockImplementation(async request => {
+            if (request.method === 'tools/list') {
+                return {
+                    tools: [
+                        {
+                            name: 'versioned-tool',
+                            inputSchema: {
+                                type: 'object',
+                                properties: {}
+                            },
+                            outputSchema: {
+                                type: 'object',
+                                properties: {
+                                    generation: { const: catalogGeneration }
+                                },
+                                required: ['generation'],
+                                additionalProperties: false
+                            }
+                        }
+                    ]
+                };
+            }
+
+            if (request.method === 'tools/call') {
+                callCount += 1;
+                const callGeneration = catalogGeneration;
+                if (callCount === 1) {
+                    await firstCallMayReturn;
+                }
+                return {
+                    content: [],
+                    structuredContent: { generation: callGeneration }
+                };
+            }
+
+            throw new Error(`Unexpected request: ${request.method}`);
+        });
+
+        await client.listTools();
+        const firstCall = client.callTool({ name: 'versioned-tool' });
+
+        catalogGeneration = 'new';
+        await client.listTools();
+        releaseFirstCall();
+
+        await expect(firstCall).resolves.toMatchObject({
+            structuredContent: { generation: 'old' }
+        });
+        await expect(client.callTool({ name: 'versioned-tool' })).resolves.toMatchObject({
+            structuredContent: { generation: 'new' }
+        });
+    });
+
     /***
      * Test: Validate structuredContent Against outputSchema
      */