Add SSE keep-alive and prepare invocations 1.0.0

[Shiva S (Shivakishore14)]

Description

Adds configurable SSE keep-alive comments to idle POST /invocations event streams. When SSE_KEEPALIVE_INTERVAL is configured, the host emits : keep-alive comments without reordering source events, while preserving streaming context, error propagation, and cancellation cleanup.

Keep-alive injection is limited to text/event-stream; other StreamingResponse media types such as NDJSON remain unchanged.

The issue was reproduced in Azure hosted agents: silent streams consistently reset near 269-270 seconds. A fixed image successfully completed the same 270-second idle stream and delivered the final event and [DONE] sentinel.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes.

Testing:

• uv run --group dev pytest -q tests/test_sse_keep_alive.py tests/test_invoke.py — 17 passed
• Azure hosted-agent idle stream at 270 seconds — 7/7 yacflow assertions passed

[azure-pipelines]

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[Shiva S (Shivakishore14)]

Long-duration hosted validation completed successfully:

• Idle interval: 1,800 seconds (30 minutes)
• Result: 7/7 yacflow assertions passed
• Total duration: 31m 52.5s
• Image: cr5bu5pbkxrzvny.azurecr.io/sample-agent-python:invocations-sse-keepalive-30m-ce15c6d521
• Digest: sha256:19afb8f4f40b0b2f4004b1a89bd9a3c075f8ab53b98a8403ae3698b7deba5f57

The stream delivered started, finished, and [DONE] after the full silent interval.

[Copilot]

Pull request overview

Adds configurable keep-alive comments to idle SSE invocation streams while preserving event ordering and cleanup.

Changes:

• Adds SSE keep-alive stream wrapping.
• Limits injection to text/event-stream.
• Adds integration and lifecycle tests plus release notes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
_sse.py	Implements keep-alive injection.
_invocation.py	Applies keep-alives to SSE responses.
test_sse_keep_alive.py	Tests timing, errors, cleanup, and media filtering.
CHANGELOG.md	Documents the fix.

[Shiva S (Shivakishore14)]

Fixed in 8d81498. The producer is now request-driven: one persistent task owns and advances the source (preserving contextvars), but it only calls anext(source) when the consumer requests the next chunk. This restores backpressure without reintroducing cross-task context issues. Added test_with_keep_alive_preserves_source_backpressure to verify a paused consumer does not advance the source.