[hermes-agent / Gemini] bound_model_input_without_hygiene can slice on assistant tool_call causing HTTP 400

## Observed symptom [Bug]: bound_model_input_without_hygiene can slice on an assistant tool_call, causing Gemini HTTP 400 INVALID_ARGUMENT ### Bug Description When session hygiene compression does not commit in-line (e.g. `hygiene_max_turn_hold_seconds` turn-hold budget expires while the summary is still streaming), `bound_model_input_without_hygiene(history, hs.hard_msg_limit)` bounds the model payload in memory to prevent sending an unbounded transcript. If the computed `tail_start` lands on an `assistant` message (or advances past `tool` results into an `assistant` message with `tool_calls`), the kept transcript starts with `[system, assistant(tool_calls), ...]`. Google Gemini's native API (`generateContent`) enforces strict turn ordering where a model `functionCall` cannot be the opening turn and must immediately follow either a `user` turn or a `functionResponse` turn. Google rejects the request with HTTP 400: ``` Gem ## Literal error text ``` bound_model_input_without_hygiene ``` ## Environment GitHub issue NousResearch/hermes-agent #121880; state=open ## Trigger / context ### Bug Description When session hygiene compression does not commit in-line (e.g. `hygiene_max_turn_hold_seconds` turn-hold budget expires while the summary is still streaming), `bound_model_input_without_hygiene(history, hs.hard_msg_limit)` bounds the model payload in memory to prevent sending an unbounded transcript. If the computed `tail_start` lands on an `assistant` message (or advances past `tool` results into an `assistant` message with `tool_calls`), the kept transcript starts with `[system, assistant(tool_calls), ...]`. Google Gemini's native API (`generateContent`) enforces strict turn ordering where a model `functionCall` cannot be the opening turn and must immediately follow either a `user` turn or a `functionResponse` turn. Google rejects the request with HTTP 400: ``` Gemini HTTP 400 (INVALID_ARGUMENT): Please ensure that function call turn comes immediately after a user turn or after a function response turn. ``` This is treated as a fatal non-retryable error, breaking the turn and forcing the user to `/new`. ### Steps to Reproduce 1. Run a long-running session on `gemini-flash-latest` (or any Gemini model via the native adapter) accumulating >400 messages with multiple tool execution rounds (e.g. `terminal`). 2. Allow session hygiene auto-compression to fire and exceed `hygiene_max_turn_hold_seconds` (10s default), causing compression to defer with the notice: `ℹ️ Context compression deferred — summary still streaming. Continuing without compression t ## Source evidence - https://github.com/NousResearch/hermes-agent/issues/121880 (2026-09-28T17:36:32Z) — GitHub issue #121880 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: C8159f63e38 ## 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.