fix done marker for error frame

[akshaydeo]

Summary

Fixes a bug where a data: [DONE] marker was emitted after an error frame in a streaming response, causing clients keyed on that marker to interpret a failed or truncated stream as a clean, successful completion. This was the root ambiguity behind truncated upstream streams appearing successful (issue #5546).

Changes

• Introduced a sawErrorChunk boolean in handleStreamingResponse that is set when a chunk carrying a BifrostError is written to the wire. If this flag is set, the [DONE] marker is suppressed at the end of the stream, since the error frame already serves as a terminal signal.
• The flag is checked after marshalling to avoid suppressing the marker for chunks that never actually reach the client.
• runCompleter errors are intentionally not factored into this decision — those reflect post-processing or plugin failures, not an incomplete stream, so [DONE] remains accurate in that case.
• Added streamdonemarker_test.go with two regression tests: one asserting [DONE] is absent after an error frame, and one guarding against over-suppression by confirming clean streams still receive the marker.

Type of change

• Bug fix
• Feature
• Refactor
• Documentation
• Chore/CI

Affected areas

• Core (Go)
• Transports (HTTP)
• Providers/Integrations
• Plugins
• UI (React)
• Docs

How to test

go test ./transports/bifrost-http/handlers/...

TestStreamingResponseSkipsDoneAfterErrorChunk verifies that a stream ending with an error frame does not emit [DONE].
TestStreamingResponseSendsDoneOnCleanStream verifies that a clean stream still receives [DONE].

Breaking changes

• Yes
• No

Related issues

Closes #5546

Security considerations

None.

Checklist

• I read docs/contributing/README.md and followed the guidelines
• I added/updated tests where appropriate
• I updated documentation where needed
• I verified builds succeed (Go and UI)
• I verified the CI pipeline passes locally if applicable

[coderabbitai]

📝 Walkthrough

Walkthrough

Streaming responses now track Bifrost error chunks and omit the terminal [DONE] marker after an error. New in-process SSE tests verify both error-stream suppression and clean-stream completion behavior.

Changes

SSE termination handling

Layer / File(s)	Summary
Track streamed error chunks transports/bifrost-http/handlers/inference.go	handleStreamingResponse records error-containing chunks and requires a clean stream before emitting [DONE].
Validate terminal marker behavior transports/bifrost-http/handlers/streamdonemarker_test.go	In-process SSE tests verify error content is delivered without [DONE], while clean content streams include [DONE].

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• maximhq/bifrost#5475: Both changes modify SSE handling around streamed Bifrost errors.
• maximhq/bifrost#5567: Both changes modify termination behavior for truncated upstream streams.

Suggested reviewers: tejasghatte, bearts, roroghost17

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code matches #5546 by stopping [DONE] after a serialized error frame while preserving clean-stream completion.
Out of Scope Changes check	✅ Passed	The changes stay focused on the SSE done-marker bug fix and related regression tests.
Title check	✅ Passed	The title is concise and clearly describes the main fix to suppress the DONE marker after error frames.
Description check	✅ Passed	The PR description follows the template well, covering summary, changes, testing, impact, related issue, security, and checklist.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 07-27-adds_stream_done_marker_for_inference_interaface

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[akshaydeo]

• fix done marker for error frame #5568 👈 (View in Graphite)
• adds EOF handler support to all providers #5567
• adds ssetruncation interface for handling EOF cases #5566
• dev

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[akshaydeo]

@coderabbitai full review

[coderabbitai]

✅ Action performed

Full review finished.

[akshaydeo]

Merge activity

• Jul 27, 9:25 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 27, 9:28 AM UTC: Graphite couldn't merge this pull request because a downstack PR adds EOF handler support to all providers #5567 failed to merge.
• Jul 27, 9:46 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 27, 9:47 AM UTC: @akshaydeo merged this pull request with Graphite.

The base branch was changed.