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1448718fca
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>
102 lines
3.4 KiB
Python
102 lines
3.4 KiB
Python
"""Output-only content blocks must be stripped from request messages.
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Anthropic's server-side refusal-fallback feature emits an output-only
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``{"type": "fallback", ...}`` block inside an assistant response. It is valid on
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the response path but rejected on the request path, so replaying that assistant
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turn 400s the whole request. The shared body readers must drop it before
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forwarding. See ``strip_output_only_request_blocks`` in ``headroom.proxy.helpers``.
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"""
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import asyncio
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import json
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from headroom.proxy.helpers import (
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read_request_json_with_bytes,
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strip_output_only_request_blocks,
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)
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_FALLBACK = {
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"type": "fallback",
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"from": {"model": "claude-fable-5"},
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"to": {"model": "claude-opus-4-8"},
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}
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class _FakeHeaders:
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def __init__(self, d=None):
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self._d = {k.lower(): v for k, v in (d or {}).items()}
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def get(self, k, default=None):
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return self._d.get(k.lower(), default)
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class _FakeRequest:
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def __init__(self, raw, headers=None):
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self._raw = raw
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self.headers = _FakeHeaders(headers)
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async def body(self):
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return self._raw
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def _has_fallback(messages):
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for msg in messages:
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content = msg.get("content")
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if isinstance(content, list):
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for block in content:
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if isinstance(block, dict) and block.get("type") == "fallback":
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return True
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return False
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def test_strip_removes_fallback_and_backfills_emptied_turn():
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messages = [
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{"role": "user", "content": "hi"},
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# assistant turn that is ONLY a fallback signal (the crash case)
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{"role": "assistant", "content": [dict(_FALLBACK)]},
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# fallback prefix + real content
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{"role": "assistant", "content": [dict(_FALLBACK), {"type": "text", "text": "A."}]},
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]
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assert strip_output_only_request_blocks(messages) is True
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assert not _has_fallback(messages)
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# emptied turn is backfilled with a single benign text block
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assert messages[1]["content"] == [{"type": "text", "text": "(model fallback)"}]
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# mixed turn keeps only the real content
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assert [b["type"] for b in messages[2]["content"]] == ["text"]
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# idempotent
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assert strip_output_only_request_blocks(messages) is False
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def test_strip_is_noop_on_clean_or_invalid_input():
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assert strip_output_only_request_blocks(None) is False
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assert strip_output_only_request_blocks([{"role": "user", "content": "hi"}]) is False
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assert (
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strip_output_only_request_blocks(
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[{"role": "user", "content": [{"type": "text", "text": "x"}]}]
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)
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is False
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)
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def test_reader_strips_and_reencodes_raw_bytes():
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body = {
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"model": "claude-fable-5",
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"messages": [
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{"role": "user", "content": "hi"},
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{"role": "assistant", "content": [dict(_FALLBACK)]},
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],
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}
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raw = json.dumps(body).encode("utf-8")
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result, out_raw = asyncio.run(read_request_json_with_bytes(_FakeRequest(raw)))
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assert not _has_fallback(result["messages"])
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# raw bytes re-encoded so byte-faithful passthrough cannot leak the pre-strip body
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assert not _has_fallback(json.loads(out_raw)["messages"])
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assert json.loads(out_raw) == result
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def test_reader_leaves_clean_requests_byte_identical():
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raw = json.dumps({"model": "x", "messages": [{"role": "user", "content": "hi"}]}).encode(
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"utf-8"
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)
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_, out_raw = asyncio.run(read_request_json_with_bytes(_FakeRequest(raw)))
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assert out_raw == raw
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