fix(translation): reject incomplete anthropic streams

[bhuvan002]

What

Explicitly throws and SSE error in case the upstream Anthropic stream ends prematurely without a message_stop event.

Why

The motivation — what problem does this solve, or which ticket does it close?

Closes #

How tested

• uv run ruff check . clean
• uv run mypy switchyard clean
• uv run pytest tests/ green
• Manual smoke (describe what was run)

Checklist

• One class per file; filename = snake_case of the primary class.
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use.
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed.
• Commits signed off (Signed-off-by: Your Name <email>) per the DCO.

Notes for reviewers

Anything reviewers should pay extra attention to — risky paths, follow-up tickets, intentional trade-offs.

Summary by CodeRabbit

• Bug Fixes
  • Improved Anthropic stream handling to detect incomplete responses.
  • Added an error when a stream ends without the expected completion event.
  • Prevented incomplete streams from being treated as successfully completed.

[coderabbitai]

Walkthrough

Anthropic stream decoding now tracks message_stop events in normal and trailing frames. It reports a response-translation error when the stream ends without one. Tests verify the error and prevent synthetic stop events.

Changes

Anthropic stream completion validation

Layer / File(s)	Summary
Track and validate message_stop completion crates/switchyard-translation/src/helpers.rs	The decoder tracks message_stop events across all frames and reports an error for incomplete streams. Tests verify the error and confirm that no synthetic stop event is generated.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Poem

A rabbit watched the stream frames flow,
“Stop” must come before we go.
No stop? An error hops in sight,
No made-up event takes flight.
The decoder now ends right.

🚥 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 describes the main change: rejecting incomplete Anthropic streams without a message_stop event.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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-translation/src/helpers.rs (1)

584-608: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Cover both terminal-frame paths.

This test covers only EOF after a normal message_start frame. It does not cover a valid message_stop in the normal path or the trailing-frame path at Lines 233-241. Add success cases for both paths. Add a concise comment stating that incomplete Anthropic streams must fail without synthesizing message_stop.

🤖 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-translation/src/helpers.rs` around lines 584 - 608, Extend
the Anthropic stream tests around encode_stream to cover successful completion
with a valid message_stop in both the normal terminal-frame path and the
trailing-frame path near the existing terminal handling. Add a concise comment
explaining that incomplete streams must fail without synthesizing message_stop,
while preserving the existing EOF failure assertion.

Source: Coding guidelines

🤖 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-translation/src/helpers.rs`:
- Around line 584-608: Extend the Anthropic stream tests around encode_stream to
cover successful completion with a valid message_stop in both the normal
terminal-frame path and the trailing-frame path near the existing terminal
handling. Add a concise comment explaining that incomplete streams must fail
without synthesizing message_stop, while preserving the existing EOF failure
assertion.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 83567ab8-0e78-43bc-a42d-6b34f6f7d41e

📥 Commits

Reviewing files that changed from the base of the PR and between 8d47c2f and 74042e7.

📒 Files selected for processing (1)

• crates/switchyard-translation/src/helpers.rs