Root cause of 'Invalid signature in thinking block' (400 at
messages.1.content.0, Claude models): build_messages_request replayed
EVERY stored Reasoning item as a thinking block with no origin check —
history synthesized by other backends (encrypted_content: None → the
mandatory signature field serialized as ""), Responses-API tco_* blobs
(signature bytes, no text), and blocks signed by a DIFFERENT model after
a mid-session /model switch. Anthropic validates every replayed
signature (model-bound), so such histories 400 deterministically. The
platform split was circumstantial: Windows sessions started on the
default model and switched to Claude; macOS sessions were Claude-native
from turn 1. Adversarially verified — no platform-divergent byte path
exists in capture, storage, or replay.
New prune_replayed_thinking pass (Pi/Claude Code replay policy): keep
exactly the final assistant message's thinking when its tool loop is
still open (request ends on the tool results — an open loop can never
span a model switch) and the block is genuinely signed; strip every
other thinking block (the API ignores valid prior-turn thinking and
rejects invalid). Assistant messages emptied by the strip (thinking-only
aborted turns) are removed — empty content arrays are rejected too.
Tests: three unit tests pin strip-outside-loop (unsigned, tco_*, stale
signed), keep-in-active-loop (verbatim text+signature at content.0), and
emptied-message removal; the legacy-upgrade integration test now proves
both wire fidelity in the active loop AND stripping once the loop
closes.
Verified: sampling-types + sampler + chat-state + shell 6072 tests
green, clippy clean.
ZacharyZhang-NY
6ba24db019