Bug: /executeCommand SSE stream omits blank-line event delimiters

[Pluviobyte]

What happened?

The A2A server streaming /executeCommand endpoint writes Server-Sent Events with a single trailing newline:

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

A spec-compliant SSE/EventSource client requires a blank line (\n\n) to dispatch an event. Without that blank line, consecutive data: lines are folded into one record and the final event is never properly terminated, so clients cannot parse individual streaming updates.

What did you expect to happen?

Each JSON-RPC response emitted by the streaming endpoint should be a separate SSE event, delimited by a blank line:

data: {...}\n\n

Root cause

The handler emits \n instead of \n\n. The existing streaming test did not catch this because it used a Mocha-style done callback, which Vitest 3 does not honor, so the test returned before its asynchronous assertions ran.

Proposed fix

Append the missing blank line to each event and convert the streaming test to async/await so it genuinely validates emitted events.

Related PR: #27549

[gemini-cli]

The A2A server's SSE implementation is missing the required blank line delimiter, causing client-side hangs. Fixing this requires a server-side change and a non-trivial refactor of the test suite from legacy done callbacks to async/await for Vitest 3 compatibility.

The streaming endpoint is missing required delimiters in the Server-Sent Events, which prevents standard clients from correctly parsing the data. Please provide your CLI version.

Effort Analysis:
In packages/a2a-server/src/http/app.ts, the handleExecuteCommand function (line 173) emits SSE data with only a single \n, violating the EventSource specification which requires \n\n to dispatch an event. This leads to hanging IPC connections where the client never terminates the record. Furthermore, the corresponding test in packages/a2a-server/src/http/app.test.ts (line 1137) incorrectly uses a legacy done() callback which Vitest 3 does not reliably honor, masking the bug. Resolving this involves refactoring the asynchronous flow of the streaming tests to use async/await patterns and ensuring proper event termination, fitting the 'Asynchronous Flow & Integration' category for effort/medium due to the complexity of debugging IPC and framework-level test flakiness.

[Pluviobyte]

Additional info for triage:

• CLI version / code version: reproduced against current main source at commit 0139140 (@google/gemini-cli package version 0.45.0-nightly.20260521.g854f811be in this checkout).
• This is a source-level SSE framing bug in the A2A server endpoint rather than an installed-binary-only report. The affected code path is the streaming /executeCommand handler in packages/a2a-server/src/http/app.ts.
• Reproduction covered by PR fix(a2a-server): delimit SSE events with a blank line in /executeCommand #27549: before the fix, the handler writes data: <json>\n for each event. A spec-compliant SSE parser only dispatches an event after a blank line (\n\n), so two emitted responses are parsed as one malformed/incomplete record. After the fix each event is delimited with \n\n and parses as an individual event.
• Validation in PR fix(a2a-server): delimit SSE events with a blank line in /executeCommand #27549: converted the streaming test from the ineffective Mocha-style done callback to async/await; the test fails before the production change with expected 1 to be 2 and passes after.

Related PR: #27549