fix(core): respect NO_PROXY for network-based MCP servers

[cocosheng-g]

Summary

Fixes issue where local MCP servers (localhost/127.0.0.1) are unreachable when a global proxy is configured, even if NO_PROXY is set. Introduced a surgical, proxy-aware fetcher specifically for MCP network transports that respects NO_PROXY.

Details

• Replaced globalThis.fetch with a localized undici.fetch using EnvHttpProxyAgent in mcp-client.ts.
• EnvHttpProxyAgent correctly handles NO_PROXY and no_proxy environment variables.
• The change is scoped strictly to StreamableHTTPClientTransport and SSEClientTransport initialization, ensuring zero risk to core Gemini API traffic.
• Added type-safe response conversion to maintain compatibility with the MCP SDK.

Related Issues

Fixes #3340

How to Validate

1. Start a local HTTP MCP server.
2. Set HTTP_PROXY to an invalid address (e.g., http://127.0.0.1:8888).
3. Set NO_PROXY=127.0.0.1.
4. Run gemini chat and attempt to use a tool from the local MCP server.
5. Verify the connection succeeds bypassing the proxy.

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

[gemini-code-assist]

Summary of Changes

Hello, 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 connectivity issues for local MCP servers in environments with global proxy configurations. By integrating a specialized fetcher that respects the NO_PROXY environment variable, the changes ensure that traffic to local endpoints bypasses unnecessary proxy routing while maintaining existing error handling logic for SSE streams.

Highlights

• Proxy Handling: Introduced a proxy-aware fetcher using undici's EnvHttpProxyAgent to ensure network-based MCP servers correctly respect NO_PROXY environment variables.
• Connection Reliability: Resolved issues where local MCP servers (localhost/127.0.0.1) were unreachable when a global proxy was configured.
• Compatibility: Implemented response conversion to maintain compatibility between undici fetch results and the existing MCP SDK requirements.

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[gemini-code-assist]

Code Review

This pull request introduces proxy-aware fetching in the MCP client by utilizing undici's EnvHttpProxyAgent to respect the NO_PROXY environment variable. This change ensures that requests to local MCP servers bypass global proxies when configured. The implementation includes logic to wrap the fetch operation, conditionally using the proxy agent and converting undici responses to standard Response objects for compatibility. I have no feedback to provide.

[github-actions]

✅ 80 tests passed successfully on gemini-3-flash-preview.

🧠 Model Steering Guidance

This PR modifies files that affect the model's behavior (prompts, tools, or instructions).

• ⚠️ Consider adding Evals: No behavioral evaluations (evals/*.eval.ts) were added or updated in this PR. Consider adding a test case to verify the new behavior and prevent regressions.
• 🚀 Maintainer Reminder: Please ensure that these changes do not regress results on benchmark evals before merging.

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This is an automated guidance message triggered by steering logic signatures.

[adamfweidman]

Approved with a comment