fix(core): create new McpClient on restart to apply updated config

[h30s]

Summary

Fix McpClient instance reuse during server restart/reload that silently ignores updated configurations. Always create a new McpClient with the current config instead of reusing the stale instance.

Details

In maybeDiscoverMcpServer(), the existing McpClient was disconnected but then reused via existing ?? new McpClient(...). Since McpClient stores its serverConfig as private readonly, any updated configuration (changed command, args, env, timeout) was silently ignored. The fix disconnects and removes the old client, then always constructs a new McpClient with the updated config.

Related Issues

Fixes #19792

How to Validate

1. Run the new test directly:

   npm test -w @google/gemini-cli-core -- src/tools/mcp-client-manager.test.ts

2. Verify the new test should create a new McpClient with updated config on restart passes.
3. Verify all 20 existing tests in the file continue to pass.
4. Run full CI checks:

   npm run typecheck
   npm run lint
   npm run test

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• 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
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello @h30s, 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 bug where configuration changes for McpClient were not being applied after a server restart or reload due to the reuse of stale client instances. The fix ensures that the system always creates a fresh McpClient with the latest configuration, thereby guaranteeing that all updates are correctly recognized and utilized. This improves the reliability and responsiveness of the application to configuration changes.

Highlights

• Configuration Reload Fix: Resolved an issue where McpClient instances were being reused during server restarts or reloads, causing updated configurations to be silently ignored. The system now ensures a new McpClient is created with the current configuration.
• Client Management Logic: Modified the McpClientManager to explicitly disconnect and remove old McpClient instances from its internal map before constructing a new one, guaranteeing that all configuration changes are applied.
• New Test Case: Added a dedicated test case to verify that McpClient correctly re-initializes with updated configurations upon a simulated restart.

Changelog

• packages/core/src/tools/mcp-client-manager.test.ts
  • Added a new test case to validate that a new McpClient is created with updated configuration on restart.
• packages/core/src/tools/mcp-client-manager.ts
  • Modified maybeDiscoverMcpServer to explicitly delete the existing McpClient from the clients map before creating a new one.
  • Changed the client instantiation logic to always create a new McpClient instance with the current configuration, instead of potentially reusing an old one.
  • Ensured that the newly created client is always set in the clients map and that the mcp-client-update event is emitted.

Activity

• No human activity has occurred on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a bug where updated configurations for MCP servers were ignored on restart or reload. The fix ensures that a new McpClient instance is always created with the latest configuration, instead of reusing a stale one. The accompanying test effectively validates this change. I've added one suggestion to improve the robustness of the client disconnection logic to prevent potential issues with stale clients if the disconnection process fails.

[h30s]

@scidomino Please take a look whenever you have time. The PR is ready from my side 😊