fix(claude): accept data-only Responses SSE (#700)

[snowyukitty]

Closes #700.

Summary

• accept Responses-compatible data-only SSE records in the Claude Messages bridge by falling back
  to the parsed JSON payload's type when the optional SSE event: field is absent;
• preserve explicit event-name precedence and strict terminal-frame validation;
• add streaming, non-streaming, mixed-framing, precedence, ignored-frame, and truncation
  regressions.

Problem

The Claude Messages bridge currently requires every upstream Responses SSE record to carry both an
event: line and a data: line before it parses the JSON:

if (!eventName || !dataLine) continue;

The provider in #700 emits valid data-only records whose Responses event type is in the JSON
payload:

data: {"type":"response.output_text.delta",...}

data: {"type":"response.completed",...}

data: [DONE]

The HTML SSE standard permits data-only messages; event: is not required and the default event
type is message. Because the bridge drops these records before parsing them, it never observes
response.completed. Both Claude streaming and non-streaming requests consequently end as:

upstream stream ended before a terminal frame (truncated response)

The two Claude response modes share this converter, so one parser gate causes both failures.

Fix

Parse any record with a non-empty data: payload, then resolve its Responses event name as:

1. the explicit SSE event: value, when present; otherwise
2. the parsed JSON payload's string type.

The scope remains narrow:

• explicit event: values still take precedence over a conflicting payload type;
• untyped data-only JSON objects remain ignored;
• non-JSON sentinels such as [DONE] remain ignored;
• response.completed, response.incomplete, or response.failed is still required before EOF;
• cancellation, keepalive, terminal, error-taxonomy, image, and tool-call paths are unchanged.

This accepts the gateway's framing without weakening the existing fail-closed truncation contract.

Tests

New regressions cover:

• a fully data-only Responses stream through Anthropic SSE output;
• data-only frames through non-streaming aggregation;
• explicit and data-only records interleaved in one stream;
• explicit event-name precedence over a conflicting payload type;
• ignoring untyped data-only records;
• [DONE] without a Responses terminal frame still producing a truncation error rather than
  message_stop.

Activation was verified before the source change: the streaming and non-streaming data-only tests
failed against the old gate, while the explicit-precedence control passed.

Final verification on commit fcd3298f:

• bun test tests/claude-outbound.test.ts — 30 passed, 0 failed
• bun test tests/claude-messages-endpoint.test.ts — 26 passed, 0 failed
• bun run typecheck — passed
• bun run privacy:scan — passed
• git diff --check — passed
• full isolated suite with Bun 1.3.14:
  bun scripts/test.ts --max-concurrency 4 ./tests/ —
  5,977 passed, 1 Windows-only skip, 0 failed across 429 files

The bounded concurrency is a local WSL scheduling accommodation, not a test exclusion. At Bun's
default concurrency, two pre-existing deadline-sensitive tests intermittently starved under load;
the same tests passed 10/10 and 5/5 in isolated repetitions, and the complete lower-concurrency run
was clean.

Notes

• No authentication, credential, logging, provider-routing, or workflow path changes.
• No live provider credential was used; the tests encode the redacted wire shape from [Provider compatibility]: Claude Messages bridge rejects data-only Responses SSE as truncated #700.
• This is the TypeScript dev contribution. Any dev2-go port remains the maintainer's transition
  responsibility under the repository policy.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed. (No user-facing configuration or API
  documentation changes are required.)
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

Summary by CodeRabbit

• Bug Fixes
  • Improved streaming response handling for gateways that omit SSE event names.
  • Preserved text updates and message aggregation across mixed and data-only streams.
  • Added safer handling for untyped or incomplete terminal messages to prevent incorrect stream completion.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 1c42fd5e-1f2b-45b8-ab75-c482cd08570d

📥 Commits

Reviewing files that changed from the base of the PR and between c0ad57a and fcd3298.

📒 Files selected for processing (2)

• src/claude/outbound.ts
• tests/claude-outbound.test.ts

---

📝 Walkthrough

Walkthrough

The Claude outbound SSE converter now accepts frames without event: by inferring event names from JSON type values. Tests cover mixed framing, aggregation, precedence, ignored untyped frames, and truncated data-only streams.

Changes

Responses SSE compatibility

Layer / File(s)	Summary
Data-only SSE frame parsing src/claude/outbound.ts	Frames now require only a data payload, validate parsed JSON objects, infer missing event names from data.type, and continue enforcing resolvable events and terminal-state handling.
Data-only stream validation tests/claude-outbound.test.ts	Tests cover data-only event sequences, mixed explicit and inferred events, non-streaming aggregation, explicit-event precedence, untyped-frame ignoring, and fail-closed [DONE] handling.

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

Possibly related PRs

• lidge-jun/opencodex#302: Updates fail-closed handling for response.completed and response.incomplete in the same outbound translation path.
• lidge-jun/opencodex#316: Adjusts SSE event-name and terminal framing behavior for optional event: fields.

Suggested reviewers: wibias, ingwannu, lidge-jun

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: accepting data-only Responses SSE in Claude.
Linked Issues check	✅ Passed	The code and tests match #700 by inferring event types from data-only payloads, preserving explicit event precedence, and keeping truncation checks.
Out of Scope Changes check	✅ Passed	The changes stay focused on SSE parsing and regression tests, with no evident unrelated or extraneous modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[snowyukitty]

CI note: the Windows job reached the workflow's 12-minute cap after the root test suite, GUI tests, privacy scan, release-helper syntax check, and GUI lint had all passed. It was cancelled during GUI build; Ubuntu, macOS, all npm-global jobs, React Doctor, target enforcement, and CodeRabbit passed. The contributor account cannot rerun an upstream Actions job, so a maintainer rerun should be sufficient. I have not changed the reviewed commit just to retrigger CI.

[lidge-jun]

Heads-up on a policy change that affects the port note in this PR's description: the dev2-go line is retired, so no Go counterpart is expected for this change and nothing further is owed after it lands on dev.

dev (Bun-native TypeScript) is the single integration line again. Keeping two integration lines in sync cost more than the Go port returned, so the carry/port obligation and the needs-go-port label are both gone. The dev2-go history is archived at lidge-jun/opencodex-go-archive with its final tip tagged archive/dev2-go here.

Nothing is required from you — this PR is unaffected other than that the port line in the description no longer applies. Sorry for the mixed signals while the policy was in flux.