The anthropic-transport-stream was overwriting thinkingSignature on each
signature_delta event instead of appending. Since Anthropic sends the
thinking block signature across multiple streaming chunks, only the last
chunk survived. The truncated signature was persisted to session JSONL,
causing all subsequent replay attempts to fail with HTTP 400:
thinking or redacted_thinking blocks in the latest assistant message
cannot be modified
This permanently bricked sessions with no user recovery path.
Fix: accumulate signature_delta values by concatenating instead of
overwriting, matching the correct implementation in the LLM provider
layer (src/llm/providers/anthropic.ts:629-634).
Includes real-scenario proof against live Anthropic API validating that
correct signatures replay successfully while truncated signatures are
rejected.
Fixes#87574
Refs #80625, #85781, #87475
8dc9cfe734