AsyncPipeline.stream POC

[anakin87]

Related Issues

• tentative implementation for https://github.com/deepset-ai/haystack-private/issues/328
• Related: Support both callback and generator-based streaming in all Chat Generators #8742, enhancement: make streaming more convenient #9347

Proposed Changes:

• expose an AsyncPipeline.stream method that returns a PipelineStreamHandle
  • async iterator enabling async for chunk in handle
  • exposes a result field to get the final result

Choices and limitations:

• at pipeline level: this unlocks most features without changing much code
• async-only: the use case is async; trying to make it work with sync could create more problems than benefits
• to be able to return the final result, I am not using a generator but an async iterator (PipelineStreamHandle). This currently makes the integration with Hayhooks not exactly ergonomic, but we can work to improve it

Does not contain breaking changes but it's thought for Haystack 3.

How did you test it?

• New tests, CI
• Realistic use cases (demo + Hayhooks) available at https://github.com/anakin87/haystack-streaming-poc

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ready	Preview, Comment	May 14, 2026 2:04pm

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/core/pipeline
async_pipeline.py					760
Project Total

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