From 8a0190ca3bc4253529be0075c0ceab2702b8d804 Mon Sep 17 00:00:00 2001
From: HyeongHo Jun <88872409+kamja44@users.noreply.github.com>
Date: Sat, 3 Jan 2026 01:06:56 +0900
Subject: [PATCH] fix(core): handle unhandled promise rejection in
 mcp-client-manager (#14701)

---
 .../core/src/tools/mcp-client-manager.test.ts | 36 ++++++++++++++++++
 packages/core/src/tools/mcp-client-manager.ts | 38 +++++++++++--------
 2 files changed, 59 insertions(+), 15 deletions(-)

diff --git a/packages/core/src/tools/mcp-client-manager.test.ts b/packages/core/src/tools/mcp-client-manager.test.ts
index 0ff201e8e6..d2fdc5d119 100644
--- a/packages/core/src/tools/mcp-client-manager.test.ts
+++ b/packages/core/src/tools/mcp-client-manager.test.ts
@@ -238,4 +238,40 @@ describe('McpClientManager', () => {
       );
     });
   });
+
+  describe('Promise rejection handling', () => {
+    it('should handle errors thrown during client initialization', async () => {
+      vi.mocked(McpClient).mockImplementation(() => {
+        throw new Error('Client initialization failed');
+      });
+
+      mockConfig.getMcpServers.mockReturnValue({
+        'test-server': {},
+      });
+
+      const manager = new McpClientManager({} as ToolRegistry, mockConfig);
+
+      await expect(manager.startConfiguredMcpServers()).resolves.not.toThrow();
+    });
+
+    it('should handle errors thrown in the async IIFE before try block', async () => {
+      let disconnectCallCount = 0;
+      mockedMcpClient.disconnect.mockImplementation(async () => {
+        disconnectCallCount++;
+        if (disconnectCallCount === 1) {
+          throw new Error('Disconnect failed unexpectedly');
+        }
+      });
+      mockedMcpClient.getServerConfig.mockReturnValue({});
+
+      mockConfig.getMcpServers.mockReturnValue({
+        'test-server': {},
+      });
+
+      const manager = new McpClientManager({} as ToolRegistry, mockConfig);
+      await manager.startConfiguredMcpServers();
+
+      await expect(manager.restartServer('test-server')).resolves.not.toThrow();
+    });
+  });
 });
diff --git a/packages/core/src/tools/mcp-client-manager.ts b/packages/core/src/tools/mcp-client-manager.ts
index fbc3b3e423..7a1443e096 100644
--- a/packages/core/src/tools/mcp-client-manager.ts
+++ b/packages/core/src/tools/mcp-client-manager.ts
@@ -163,8 +163,7 @@ export class McpClientManager {
       return;
     }
 
-    const currentDiscoveryPromise = new Promise<void>((resolve, _reject) => {
-      // eslint-disable-next-line @typescript-eslint/no-floating-promises
+    const currentDiscoveryPromise = new Promise<void>((resolve, reject) => {
       (async () => {
         try {
           if (existing) {
@@ -212,6 +211,13 @@ export class McpClientManager {
               );
             }
           }
+        } catch (error) {
+          const errorMessage = getErrorMessage(error);
+          coreEvents.emitFeedback(
+            'error',
+            `Error initializing MCP server '${name}': ${errorMessage}`,
+            error,
+          );
         } finally {
           // This is required to update the content generator configuration with the
           // new tool configuration.
@@ -221,28 +227,30 @@ export class McpClientManager {
           }
           resolve();
         }
-      })();
+      })().catch(reject);
     });
 
     if (this.discoveryPromise) {
-      this.discoveryPromise = this.discoveryPromise.then(
-        () => currentDiscoveryPromise,
-      );
+      // Ensure the next discovery starts regardless of the previous one's success/failure
+      this.discoveryPromise = this.discoveryPromise
+        .catch(() => {})
+        .then(() => currentDiscoveryPromise);
     } else {
       this.discoveryState = MCPDiscoveryState.IN_PROGRESS;
       this.discoveryPromise = currentDiscoveryPromise;
     }
     this.eventEmitter?.emit('mcp-client-update', this.clients);
     const currentPromise = this.discoveryPromise;
-    // eslint-disable-next-line @typescript-eslint/no-floating-promises
-    currentPromise.then((_) => {
-      // If we are the last recorded discoveryPromise, then we are done, reset
-      // the world.
-      if (currentPromise === this.discoveryPromise) {
-        this.discoveryPromise = undefined;
-        this.discoveryState = MCPDiscoveryState.COMPLETED;
-      }
-    });
+    void currentPromise
+      .finally(() => {
+        // If we are the last recorded discoveryPromise, then we are done, reset
+        // the world.
+        if (currentPromise === this.discoveryPromise) {
+          this.discoveryPromise = undefined;
+          this.discoveryState = MCPDiscoveryState.COMPLETED;
+        }
+      })
+      .catch(() => {}); // Prevents unhandled rejection from the .finally branch
     return currentPromise;
   }