## Description
Closes#902
Mid-turn user interjections (steering) silently dropped through the
Bedrock streaming path. The _stream_response_bedrock code path
reconstructs Anthropic SSE events from parsed StreamEvent objects
instead of passing raw bytes through, so SSE-level ping keepalives are
never forwarded to Claude Code. Claude Code relies on ping events to arm
its mid-turn steering / interruptible state; without them, queued
interjections are discarded instead of sent.
Root cause (confirmed):
- Standard direct-Anthropic path does a raw yield-chunk passthrough —
pings flow unchanged.
- Bedrock path (_stream_response_bedrock.generate()) reconstructs events
from litellm/anyllm
stream_message() output, which only yields semantic events
(message_start, content_block_*,
message_delta, message_stop, error). No pings, ever.
Fix: emit a synthetic 'event: ping / data: {}' at stream start (before
the first message_start)
so downstream clients see the same ping-then-content cadence as a real
Anthropic stream.
Note: periodic pings for very long responses (>~25s) may be needed if
steering disarms on a timer.
This commit arms it at turn start; follow-up if reporters confirm
steering still drops on long turns.
The causal link (ping → steering) is the reporter's hypothesis from
hands-on debugging.
The observable defect (zero pings in stream) is confirmed and fixed.
## Type of Change
- [x] Bug fix (non-breaking change that fixes an issue)
## Changes Made
- headroom/proxy/handlers/streaming.py: yield ping event before the
event loop in _stream_response_bedrock.generate()
- tests/test_proxy/test_bedrock_sse_ping.py: 3 new tests asserting ping
appears before message_start
## Testing
- [x] Unit tests pass (pytest)
- [x] Linting passes (ruff check .)
- [x] New tests added for new functionality
### Test Output
```
tests/test_proxy/test_bedrock_sse_ping.py::test_bedrock_stream_emits_ping_before_message_start PASSED
tests/test_proxy/test_bedrock_sse_ping.py::test_bedrock_stream_ping_has_empty_data PASSED
tests/test_proxy/test_bedrock_sse_ping.py::test_bedrock_stream_contains_message_stop PASSED
tests/test_backend_streaming_cache_metrics.py (4 tests) PASSED
7 passed in 4.41s
```
## Real Behavior Proof
- Environment: macOS, Python 3.11, headroom unit tests
- Exact command / steps: pytest
tests/test_proxy/test_bedrock_sse_ping.py -v
- Observed result: 3 new tests pass; ping appears before message_start
in Bedrock stream
- Not tested: end-to-end against live Bedrock + Claude Code (no Bedrock
credentials available)
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
## Checklist
- [x] Code follows project style guidelines
- [x] Code is commented where non-obvious
- [x] No new warnings
- [x] Tests added and passing
## Additional Notes
The Rust proxy files mentioned in the issue (sse/framing.rs,
sse/anthropic.rs) are NOT part of this fix.
Those drops are in a telemetry-only tee task that never affects the
client byte path — the Rust proxy
does a raw bytes passthrough for all responses. The defect is
Python-only, confined to the Bedrock path.
---------
Co-authored-by: JerrettDavis <mxjerrett@gmail.com>
