[ni (nniiovoo)]

Motivation & Context

AGUIHttpService only parsed SSE lines starting with data: . The space after data: is optional in SSE, so a server that writes data:{...} gave an empty response with no error. Repro is in #10248.

Description & Review Guide

• What are the major changes? post_run now matches data: and removes one leading space if there is one, as the SSE spec says.
• What is the impact of these changes? Events from servers that omit the space are no longer dropped. Streams that use data: are parsed the same as before.
• What do you want reviewers to focus on? I kept it to the prefix check. Multi-line data: events are still not joined, same as before.

Tests: I added a test that streams data:{...} lines through httpx.MockTransport (it fails on main).

Related Issue

Fixes #10248

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: issue analysis, implementation, tests, and this description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

No active review issues were identified for the change supporting SSE events without a space after data:.

No open issues 🎉

What changed in this PR

Fixes #10248 by allowing the Python AG-UI client to parse server-sent events without a space after data:, while preserving support for the existing spaced format.

Changes:

• Matches data: and removes one leading space when present.
• Adds a regression test using httpx.MockTransport for streams without the space.
• Leaves multi-line event handling unchanged.

File	Description
python/​packages/​ag-ui/​tests/​ag_ui/​test_http_service.py	Adds regression coverage for data:{...} event streams.
python/​packages/​ag-ui/​agent_framework_ag_ui/​_http_service.py	Accepts both SSE data-prefix formats.

_{Review effort: Balanced}