Anthropic's server-side refusal-fallback feature
(`server-side-fallback-2026-06-01`) can emit an output-only `{ "type":
"fallback", ... }` block inside an assistant response. That block is
valid on the response path but rejected on the request path, so when a
client replays the assistant turn the next request can 400 with an
invalid `fallback` input tag.
This strips output-only request blocks in the shared request-body
readers before forwarding. `read_request_json_with_bytes` re-encodes raw
bytes only when stripping occurs, so byte-faithful passthrough paths
cannot leak the invalid block while clean requests keep their original
bytes. If stripping empties an assistant content array, the helper
backfills a benign text block so the request remains schema-valid.
## Type of Change
- [x] Bug fix
## Changes Made
- Add `strip_output_only_request_blocks` in `headroom/proxy/helpers.py`.
- Strip fallback blocks in both `_read_request_json` and
`read_request_json_with_bytes`.
- Add regression tests for stripping, empty-turn backfill, raw-byte
re-encoding, and byte-identical clean requests.
- Update `CHANGELOG.md`.
## Testing
Focused verification on the current PR head after merging `main`:
```text
python -m pytest tests/test_output_only_request_blocks.py -q
4 passed
uvx ruff==0.15.17 check headroom/proxy/helpers.py tests/test_output_only_request_blocks.py
All checks passed!
uvx ruff==0.15.17 format --check headroom/proxy/helpers.py tests/test_output_only_request_blocks.py
2 files already formatted
```
## Notes
No live Anthropic request was run; the regression is covered at the
shared request-reader layer used by the proxy forwarding paths.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: JerrettDavis <mxjerrett@gmail.com>
Co-authored-by: Tejas Chopra <chopratejas@gmail.com>
