Fix MCP header encoding for non-ASCII values

[jstar0]

Summary

Fixes #25668.

MCP HTTP transports now encode configured header values that cannot be passed to Fetch as ByteString values. This prevents discovery from failing when a configured header contains Unicode text such as mąka.

Changes

• Normalize MCP transport header values before they are passed into Fetch-backed transports.
• Preserve existing values that are already valid ByteString values.
• Encode only non-ByteString values as UTF-8 bytes represented in Latin-1, which keeps Fetch Headers validation from throwing while preserving the intended bytes.
• Add a regression test for an httpUrl MCP server configured with a non-ASCII header value.

Verification

npm exec prettier -- --check packages/core/src/tools/mcp-client.ts packages/core/src/tools/mcp-client.test.ts
npm exec --workspace @google/gemini-cli-core -- vitest run src/tools/mcp-client.test.ts
npm run typecheck --workspace @google/gemini-cli-core
npm exec eslint -- packages/core/src/tools/mcp-client.ts packages/core/src/tools/mcp-client.test.ts

[github-actions]

📊 PR Size: size/M

• Lines changed: 50
• Additions: +46
• Deletions: -4
• Files changed: 2

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[github-actions]

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[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 an issue where MCP HTTP transports failed to handle non-ASCII characters in configured headers. By implementing a normalization step that encodes these values into a format compatible with Fetch headers, the change ensures that discovery processes remain robust even when using Unicode text in header configurations.

Highlights

• Header Encoding: Introduced a normalization mechanism for MCP transport headers to ensure non-ASCII values are correctly encoded for Fetch-backed transports.
• ByteString Validation: Added a utility to identify if header values are valid ByteStrings, preserving them as-is while encoding others as UTF-8 bytes represented in Latin-1.
• Regression Testing: Added a new test case to verify that MCP servers configured with non-ASCII header values do not cause discovery failures.

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[gemini-code-assist]

Code Review

This pull request introduces header value encoding for HTTP and SSE transports in the MCP client to prevent errors when headers contain non-ByteString characters (such as UTF-8 characters). It adds helper functions encodeHeaderValuesForFetch and isByteString to convert non-ByteString values into Latin-1, and applies this encoding in createTransportRequestInit and createSSETransportWithAuth. A corresponding unit test has been added to verify this behavior. No review comments were provided, so there is no additional feedback to address.