From a0350ac3d8f1e147d81ad4d6e267112276a94a1d Mon Sep 17 00:00:00 2001
From: harshasiddartha <siddartha2367@gmail.com>
Date: Thu, 6 Aug 2026 14:14:59 +0530
Subject: [PATCH] fix(client): keep tool metadata intact when a listTools()
 refresh fails

cacheToolMetadata() cleared the output-validator and task-support caches
before compiling the replacement catalog. If a later output schema failed
to compile, listTools() rejected but the client was left with empty or
partially replaced metadata, so subsequent callTool() invocations skipped
output validation and lost task-support information.

Build the replacement Map/Set locally and publish them only once every
tool has been processed, so a failed refresh leaves the metadata from the
last successful listTools() in effect.
---
 src/client/index.ts       | 18 ++++++---
 test/client/index.test.ts | 85 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+), 6 deletions(-)

diff --git a/src/client/index.ts b/src/client/index.ts
index ae0e698ec3..6621fca8be 100644
--- a/src/client/index.ts
+++ b/src/client/index.ts
@@ -785,26 +785,32 @@ export class Client<
      * Called after listTools() to pre-compile validators for better performance.
      */
     private cacheToolMetadata(tools: Tool[]): void {
-        this._cachedToolOutputValidators.clear();
-        this._cachedKnownTaskTools.clear();
-        this._cachedRequiredTaskTools.clear();
+        // Build the replacement collections separately, so that a schema which fails to
+        // compile leaves the metadata of the last successful listTools() untouched.
+        const toolOutputValidators = new Map<string, JsonSchemaValidator<unknown>>();
+        const knownTaskTools = new Set<string>();
+        const requiredTaskTools = new Set<string>();
 
         for (const tool of tools) {
             // If the tool has an outputSchema, create and cache the validator
             if (tool.outputSchema) {
                 const toolValidator = this._jsonSchemaValidator.getValidator(tool.outputSchema as JsonSchemaType);
-                this._cachedToolOutputValidators.set(tool.name, toolValidator);
+                toolOutputValidators.set(tool.name, toolValidator);
             }
 
             // If the tool supports task-based execution, cache that information
             const taskSupport = tool.execution?.taskSupport;
             if (taskSupport === 'required' || taskSupport === 'optional') {
-                this._cachedKnownTaskTools.add(tool.name);
+                knownTaskTools.add(tool.name);
             }
             if (taskSupport === 'required') {
-                this._cachedRequiredTaskTools.add(tool.name);
+                requiredTaskTools.add(tool.name);
             }
         }
+
+        this._cachedToolOutputValidators = toolOutputValidators;
+        this._cachedKnownTaskTools = knownTaskTools;
+        this._cachedRequiredTaskTools = requiredTaskTools;
     }
 
     /**
diff --git a/test/client/index.test.ts b/test/client/index.test.ts
index f5c6a348d1..fcc2c3b5a4 100644
--- a/test/client/index.test.ts
+++ b/test/client/index.test.ts
@@ -2333,6 +2333,91 @@ describe('outputSchema validation', () => {
             /Structured content does not match the tool's output schema/
         );
     });
+
+    /***
+     * Test: Keep Previously Cached Metadata when a Later listTools() Fails to Compile
+     */
+    test('should keep previously cached tool metadata when a later listTools() fails to compile', async () => {
+        const server = new Server(
+            {
+                name: 'test-server',
+                version: '1.0.0'
+            },
+            {
+                capabilities: {
+                    tools: {}
+                }
+            }
+        );
+
+        server.setRequestHandler(InitializeRequestSchema, async request => ({
+            protocolVersion: request.params.protocolVersion,
+            capabilities: {},
+            serverInfo: {
+                name: 'test-server',
+                version: '1.0.0'
+            }
+        }));
+
+        const validTools: Tool[] = [
+            {
+                name: 'validated-tool',
+                inputSchema: { type: 'object', properties: {} },
+                outputSchema: {
+                    type: 'object',
+                    properties: { result: { type: 'string' } },
+                    required: ['result'],
+                    additionalProperties: false
+                }
+            },
+            {
+                name: 'task-only-tool',
+                inputSchema: { type: 'object', properties: {} },
+                execution: { taskSupport: 'required' }
+            }
+        ];
+
+        // Accepted by ListToolsResultSchema (the root is a valid object schema), but
+        // rejected by the JSON Schema validator because of the unknown nested type.
+        const uncompilableTools: Tool[] = [
+            {
+                name: 'broken-tool',
+                inputSchema: { type: 'object', properties: {} },
+                outputSchema: {
+                    type: 'object',
+                    properties: { value: { type: 'not-a-json-schema-type' } }
+                }
+            }
+        ];
+
+        const catalogs = [validTools, uncompilableTools];
+        server.setRequestHandler(ListToolsRequestSchema, async () => ({ tools: catalogs.shift() ?? validTools }));
+
+        server.setRequestHandler(CallToolRequestSchema, async () => ({
+            structuredContent: { unexpected: 'value' }
+        }));
+
+        const client = new Client({
+            name: 'test-client',
+            version: '1.0.0'
+        });
+
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+        await Promise.all([client.connect(clientTransport), server.connect(serverTransport)]);
+
+        // Cache the metadata of the first, valid catalog.
+        await client.listTools();
+
+        // The refresh fails while compiling the replacement output schema.
+        await expect(client.listTools()).rejects.toThrow();
+
+        // The metadata of the last successful catalog must still be in effect.
+        await expect(client.callTool({ name: 'validated-tool' })).rejects.toThrow(
+            /Structured content does not match the tool's output schema/
+        );
+        await expect(client.callTool({ name: 'task-only-tool' })).rejects.toThrow(/requires task-based execution/);
+    });
 });
 
 describe('Task-based execution', () => {