[hermes-agent] Anthropic /v1/messages 400 'thinking blocks cannot be modified': heartbeat continuation duplicates thinking blocks into one assistant message

## Observed symptom Anthropic /v1/messages 400 'thinking blocks cannot be modified': heartbeat continuation duplicates thinking blocks into one assistant message ## Summary Running several company-profile agents (dispatched by Paperclip via `hermes -p <profile> chat`) against native Anthropic (`claude-sonnet-5` / `claude-opus-5-5`, extended thinking + interleaved-thinking beta), a recurring subset of heartbeat runs fail immediately with: ``` Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'messages.1.content.1: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.'}} ``` This is **not** a proxy/compression artifact — I run these agents through a Headroom proxy and confirmed via its logs that the outbound ## Literal error text ``` thinking blocks cannot be modified ``` ## Environment GitHub NousResearch/hermes-agent #123923 ## Trigger / context ## Summary Running several company-profile agents (dispatched by Paperclip via `hermes -p <profile> chat`) against native Anthropic (`claude-sonnet-5` / `claude-opus-5-5`, extended thinking + interleaved-thinking beta), a recurring subset of heartbeat runs fail immediately with: ``` Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'messages.1.content.1: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.'}} ``` This is **not** a proxy/compression artifact — I run these agents through a Headroom proxy and confirmed via its logs that the outbound request was forwarded byte-identical to what Hermes sent (Headroom detected the signed thinking block and dropped its own compression edits, `dropped_mutation_reasons=structural_diff_vs_original`). Th ## Source evidence - https://github.com/NousResearch/hermes-agent/issues/123923 (2026-09-26T12:50:59Z) — #123923 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:52:00.922591Z - scout_id: C14e6fb4a76 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.