fix(a2a-server): delimit SSE events with a blank line in /executeCommand

[Pluviobyte]

Summary

The streaming /executeCommand endpoint in the A2A server emits Server-Sent Events that are not separated by a blank line, so a spec-compliant SSE/EventSource client cannot parse the individual events. This is a one-line framing fix plus a test that actually exercises the path.

Details

In packages/a2a-server/src/http/app.ts each event was written as:

res.write(`data: ${JSON.stringify(jsonRpcResponse)}\n`);

Per the SSE spec, an event is only dispatched when a blank line (\n\n) is encountered. With a single \n, consecutive data: lines are concatenated into one record and the final event is never terminated, so a real EventSource consumer ends up with one malformed event instead of N. The rest of the codebase already uses \n\n for SSE (see packages/core/src/code_assist/server.test.ts). The fix appends the missing newline.

While fixing this I found the existing streaming test never actually asserted anything: it used the Mocha-style done callback, which Vitest 3 does not honor, so the test body returned before the res.on('end') assertions ran and it passed regardless of output. I converted it to async/await (matching the other SSE tests in the file) so it genuinely validates the stream — it fails before this fix (expected 1 to be 2) and passes after.

Related Issues

Fixes #27587

How to Validate

cd packages/a2a-server
npm run typecheck
npx vitest run src/http/app.test.ts   # 20 passed

To see the bug without the production fix, revert only the app.ts change and re-run the streaming test — it fails with expected 1 to be 2, confirming the new test guards the regression.

Pre-Merge Checklist

• Updated relevant documentation and README (not needed — internal behavior fix)
• Added/updated tests (converted the ineffective streaming test into a real regression test)
• Noted breaking changes (none — output becomes spec-compliant SSE)
• Validated on required platforms/methods:
  • Linux
    • npm run (vitest + tsc --noEmit for @google/gemini-cli-a2a-server)

Note: this is a platform-independent string-framing change covered by automated unit tests, so it was validated via the package test suite on Linux rather than the full per-OS runtime matrix.

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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 a protocol non-compliance issue in the A2A server's streaming /executeCommand endpoint. By correctly delimiting SSE events with a blank line, it ensures that standard EventSource clients can properly parse individual events. Additionally, the existing streaming test was refactored to be truly asynchronous, providing a reliable regression test for this fix.

Highlights

• SSE Protocol Compliance: Updated the /executeCommand endpoint to append a double newline (\n\n) to SSE events, ensuring compliance with the Server-Sent Events specification for proper event dispatching.
• Test Suite Refactoring: Converted the streaming test from a callback-based implementation to an async/await pattern to ensure assertions are correctly executed and validated.

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[gemini-code-assist]

Code Review

This pull request updates the SSE streaming response in packages/a2a-server/src/http/app.ts to terminate events with a double newline (\n\n) to comply with the Server-Sent Events specification. It also refactors the /executeCommand streaming E2E test in app.test.ts to use async/await and parse the full response text directly, simplifying the test logic. There are no review comments, so I have no feedback to provide.