fix(anthropic): accept done stream terminator

[bhuvan002]

What

Teach the native Anthropic streaming backend to recognize the OpenAI-style
[DONE] SSE sentinel and stop the stream when it appears.

Add a regression test that verifies events after the sentinel are not emitted.
Existing coverage still verifies normal Anthropic streams that end without
[DONE].

Why

OpenRouter's Anthropic-compatible endpoint can append data: [DONE] after
otherwise valid Anthropic SSE events. Switchyard previously tried to parse that
sentinel as JSON, turning a successful response into a mid-stream parse error.

Reproduction

With Switchyard running on port 4001 and openrouter-opus configured as an
Anthropic-format route to anthropic/claude-opus-4.8 on OpenRouter:

curl --no-buffer http://127.0.0.1:4001/v1/messages \
  -H 'content-type: application/json' \
  -H 'anthropic-version: 2023-06-01' \
  --data '{
    "model": "openrouter-opus",
    "max_tokens": 16,
    "stream": true,
    "messages": [
      {"role": "user", "content": "Reply with exactly: OK"}
    ]
  }'

Before this fix, the response contains the normal Anthropic events through
message_stop, then ends with:

event: error
data: {"type": "error", "error": {"message": "SwitchyardProcessorError('processor failed: SwitchyardUpstreamError: upstream failed: Anthropic stream emitted invalid JSON frame: expected value at line 1 column 2')", "type": "internal_error"}}

OpenRouter has appended data: [DONE]; Switchyard tries to decode it as JSON
and emits the error above. After this fix, [DONE] terminates the stream
cleanly and no error event is emitted.

How tested

• uv run ruff check . clean
• uv run mypy switchyard clean
• uv run pytest tests/ -v -m 'not integration' — 1949 passed, 12 skipped
• cargo fmt --all --check clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo test --workspace green
• User-confirmed OpenRouter streaming smoke against /api/v1/messages

Checklist

• One class per file; filename = snake_case of the primary class. (N/A: no classes added.)
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use. (N/A: no public symbols added.)
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed. (N/A: compatibility fix only.)
• Commits signed off (Signed-off-by: Bhuvan Agrawal <11240550+bhuvan002@users.noreply.github.com>) per the DCO.

Notes for reviewers

The full live E2E suite reached 17 passing tests, then hit its five-error limit
because the shared fixture on current main still invokes the removed
switchyard passthrough command. That harness issue is unrelated to this
two-file parser change; 19 live tests were therefore not run.

Summary by CodeRabbit

• Bug Fixes

  • Fixed Anthropic streaming responses to stop processing immediately at the [DONE] marker.
  • Prevented events sent after stream completion from appearing in responses.
  • Improved handling when a connection closes without a final separator.

• Tests

  • Added coverage verifying that events after [DONE] are ignored.

[coderabbitai]

Walkthrough

Anthropic native SSE streaming now treats [DONE] as a terminal marker, including buffered final-frame handling. An integration test verifies frames after [DONE] are not emitted.

Changes

Anthropic SSE termination

Layer / File(s)	Summary
SSE termination and validation crates/switchyard-components/src/backends/anthropic.rs, crates/switchyard-components/tests/adversarial_native_backends.rs	anthropic_sse_stream stops on ParsedSseFrame::Done, applies the sentinel to buffered frames, and verifies that later events are excluded.

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

Poem

I’m a rabbit hopping through the stream,
[DONE] ends the SSE dream.
No later carrots pass the gate,
The test confirms they’re stopped in wait.
Thump, thump—clean termination’s great!

🚥 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 clearly matches the main change: Anthropic streaming now accepts the [DONE] terminator.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

🧹 Nitpick comments (1)

crates/switchyard-components/tests/adversarial_native_backends.rs (1)

1043-1077: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Cover the unterminated [DONE] frame.

This test exercises only the normal SSE-frame path because [DONE] is followed by \n\n. Add a case where the stream ends with data: [DONE] without a final separator, so the buffered-frame handling at anthropic.rs Line 539 is regression-tested.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/switchyard-components/tests/adversarial_native_backends.rs` around
lines 1043 - 1077, The test `anthropic_streaming_stops_at_done_marker` currently
covers only a `[DONE]` frame terminated by a blank-line separator. Add a
separate streaming test case whose SSE payload ends exactly with `data: [DONE]`
without a trailing newline or separator, then collect the response and assert it
emits only the preceding `message_start` event while exercising the
buffered-frame handling in the Anthropic backend.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@crates/switchyard-components/tests/adversarial_native_backends.rs`:
- Around line 1043-1077: The test `anthropic_streaming_stops_at_done_marker`
currently covers only a `[DONE]` frame terminated by a blank-line separator. Add
a separate streaming test case whose SSE payload ends exactly with `data:
[DONE]` without a trailing newline or separator, then collect the response and
assert it emits only the preceding `message_start` event while exercising the
buffered-frame handling in the Anthropic backend.

---

ℹ️ Review info

⚙️ Run configuration

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Run ID: cd36e2e8-2a0b-40b0-a899-473640bd05e7

📥 Commits

Reviewing files that changed from the base of the PR and between 3269dce and 45a8acf.

📒 Files selected for processing (2)

• crates/switchyard-components/src/backends/anthropic.rs
• crates/switchyard-components/tests/adversarial_native_backends.rs