fix(core): handle unhandled promise rejection in mcp-client-manager #14701

kamja44 · 2025-12-08T10:14:31Z

Add outer try-catch block to catch initialization errors
Connect async IIFE rejection to Promise reject handler
Add tests for error scenarios during client initialization
Prevents unhandled promise rejection when McpClient constructor throws
Ensures one server failure doesn't stop other servers from starting

Summary

Fixes a structural issue in McpClientManager.addOrUpdateServer() that causes unhandled promise rejections during MCP server initialization. The Promise constructor received an unused _reject parameter, and the async IIFE lacked proper error handling, causing crashes when errors occurred in the McpClient constructor or disconnect method.

Details

Root cause: The current implementation creates a Promise with _reject parameter but never uses it. The async IIFE has no .catch() handler, so errors thrown before the try block become unhandled rejections.

Changes made:

Changed _reject to reject and properly connected it
Added outer try-catch to catch initialization errors (constructor, disconnect)
Added .catch(reject) to async IIFE to handle any uncaught errors
Added proper error logging via coreEvents.emitFeedback

Files modified:

packages/core/src/tools/mcp-client-manager.ts (lines 162-220)
packages/core/src/tools/mcp-client-manager.test.ts (added 2 test cases)

Note: This is NOT a duplicate of #8365. That issue was about MCP timeout errors in gemini.mjs and is already closed. This PR fixes a different structural problem in mcp-client-manager.ts that could cause unhandled rejections during initialization.

Related Issues

Fixes #14700

How to Validate

Run unit tests:

npm run test mcp-client-manager.test.ts

## Pre-Merge Checklist

<!-- Check all that apply before requesting review or merging. -->

- [ ] Updated relevant documentation and README (if needed)
- [O] Added/updated tests (if needed)
- [ ] Noted breaking changes (if any)
- [ ] Validated on required platforms/methods:
  - [ ] MacOS
    - [ ] npm run
    - [ ] npx
    - [ ] Docker
    - [ ] Podman
    - [ ] Seatbelt
  - [ ] Windows
    - [ ] npm run
    - [ ] npx
    - [ ] Docker
  - [ ] Linux
    - [O] npm run
    - [ ] npx
    - [ ] Docker

- Add outer try-catch block to catch initialization errors - Connect async IIFE rejection to Promise reject handler - Add tests for error scenarios during client initialization - Prevents unhandled promise rejection when McpClient constructor throws - Ensures one server failure doesn't stop other servers from starting Fixes google-gemini#14700

gemini-code-assist · 2025-12-08T10:14:45Z

Summary of Changes

Hello @kamja44, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical structural flaw in the McpClientManager that led to unhandled promise rejections during MCP server initialization. By enhancing error handling within the Promise constructor and the asynchronous IIFE, the changes ensure that the application remains stable even when McpClient operations, such as construction or disconnection, encounter errors. This prevents crashes and allows other servers to initialize successfully, improving overall system resilience.

Highlights

Error Handling Improvement: Implemented an outer try-catch block to gracefully handle initialization errors within the McpClientManager.
Promise Rejection Connection: The _reject parameter in the Promise constructor was renamed to reject and properly connected to catch asynchronous errors.
Asynchronous IIFE Error Catching: Added a .catch(reject) handler to the async Immediately Invoked Function Expression (IIFE) to prevent unhandled promise rejections.
Robust Server Initialization: Ensures that a failure in one server's initialization or disconnection process does not halt the startup of other servers.
Enhanced Error Logging: Integrated coreEvents.emitFeedback for proper error logging when MCP server initialization fails.
New Test Cases: Added dedicated test cases to validate the new error handling mechanisms during client initialization and within the async IIFE.

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kamja44 · 2025-12-08T10:16:15Z

I also left this explanation in the ISSUE, but I am attaching it here in the PR comments as well to clarify why this is not a duplicate.

Why this is NOT a duplicate of #8365

#8365 (Closed):

Issue: MCP timeout error during user confirmation

Location: gemini.mjs timeout handler

Cause: MCP error -32001: Request timed out

Status: Already fixed and closed

This PR (#14700):

Issue: Structural problem in promise error handling during MCP initialization

Location: packages/core/src/tools/mcp-client-manager.ts (addOrUpdateServer method)

Cause: Unused _reject parameter + missing .catch() in the async IIFE

Status: Preventive fix to avoid potential crashes

Key differences:

Different code files and methods

Different root causes (timeout vs improper promise error handling)

Different scenarios (user confirmation timeout vs client initialization failures)

This PR is a preventive robustness improvement, not a fix for a previously reported crash

This PR addresses a structural issue that may lead to similar symptoms but has an entirely different root cause.

gemini-code-assist

Code Review

This pull request effectively addresses an unhandled promise rejection within McpClientManager by introducing proper error handling inside the promise executor. The new tests also correctly validate the fix for server initialization failures. However, my review has identified a critical side effect of the change: it introduces a new path for promise rejection that leads to another unhandled promise rejection later in the same function. My comment provides a detailed explanation of this issue and a recommended solution to make the fix complete.

packages/core/src/tools/mcp-client-manager.ts

Address review feedback: Change .then() to .finally() on line 238 to ensure cleanup logic runs regardless of promise resolution, preventing unhandled rejection when currentDiscoveryPromise rejects. - Replace .then((_) => ...) with void finally(() => ...) - Remove eslint-disable comment (no longer needed) - Ensures discoveryState cleanup happens on both success and failure

Adib234 · 2025-12-30T21:38:31Z

hi, sorry that no one has reviewed your PR! I can take a look at this if you are still working on it!

kamja44 · 2025-12-31T00:19:05Z

hi, sorry that no one has reviewed your PR! I can take a look at this if you are still working on it!

Hi, thanks so much for taking a look — I really appreciate it!
Yes, I’m still working on this and would love your feedback whenever you have time

Adib234 · 2026-01-02T14:55:21Z

packages/core/src/tools/mcp-client-manager.ts

    const currentPromise = this.discoveryPromise;
-    // eslint-disable-next-line @typescript-eslint/no-floating-promises
-    currentPromise.then((_) => {
+    void currentPromise.finally(() => {


Can we attach a catch to this promise?

void currentPromise .finally(() => { if (currentPromise === this.discoveryPromise) { this.discoveryPromise = undefined; this.discoveryState = MCPDiscoveryState.COMPLETED; } }) .catch(() => {}); // Prevents unhandled rejection from the .finally branch

Added the .catch() to handle any errors from the finally block. Thanks!

Adib234 · 2026-01-02T15:01:32Z

packages/core/src/tools/mcp-client-manager.ts

We have another unhandled promise on line 234

gemini-cli/packages/core/src/tools/mcp-client-manager.ts

Line 234 in d344ceb

this.discoveryPromise = this.discoveryPromise.then(

If the promise gets rejected there's nothing to handle this, so can we add the catch as well?

if (this.discoveryPromise) { // Ensure the next discovery starts regardless of the previous one's success/failure this.discoveryPromise = this.discoveryPromise .catch(() => {}) .then(() => currentDiscoveryPromise); }

Without the .catch(), the next discovery wouldn't run if the previous one failed. Fixed by adding .catch() before .then(). Thanks for catching this important issue!

… chain - Add catch to finally chain to prevent unhandled rejections - Add catch before then to ensure discovery continues on previous failure

Adib234

Thank you for this! LGTM 🚀

…oogle-gemini#14701)

…on in mcp-client-manager (google-gemini#14701)

kamja44 requested a review from a team as a code owner December 8, 2025 10:14

gemini-code-assist bot reviewed Dec 8, 2025

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iqbalbhatti49 approved these changes Dec 8, 2025

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kamja44 added 5 commits December 10, 2025 11:38

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

3f06e0e

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

adf9dd0

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

73da3ca

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

d5b0bcf

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

f6b9139

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

d344ceb

Adib234 reviewed Jan 2, 2026

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kamja44 and others added 2 commits January 3, 2026 00:10

fix(core): add missing catch handlers in mcp-client-manager discovery…

1f6178b

… chain - Add catch to finally chain to prevent unhandled rejections - Add catch before then to ensure discovery continues on previous failure

Merge branch 'main' into fix/mcp-client-manager-unhandled-rejection

bb55c44

Adib234 approved these changes Jan 2, 2026

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Adib234 added this pull request to the merge queue Jan 2, 2026

Merged via the queue into google-gemini:main with commit 8a0190c Jan 2, 2026
20 checks passed

y0usaf pushed a commit to y0usaf/gemini-cli that referenced this pull request Jan 19, 2026

fix(core): handle unhandled promise rejection in mcp-client-manager (g…

6999b1f

…oogle-gemini#14701)

thacio added a commit to thacio/auditaria that referenced this pull request Jan 24, 2026

Merge Commit '8a0190ca3': fix(core): handle unhandled promise rejecti…

076fa7e

…on in mcp-client-manager (google-gemini#14701)

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kamja44 commented Dec 8, 2025

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