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Provider `context_length_exceeded` 400s permanently wedged agent sessions: the turn errored, the oversized history persisted in the in-memory session, and the usage baseline stayed frozen at the last successful sub-threshold reading (failed requests report no usage), so the preflight handoff gate never fired again — every later prompt failed identically until an agent restart. The byte-truncation fallback never intervened because it is a request-body limiter (`estimated_bytes`), not a context-window defence; at context-window scale it is a measured no-op. This adds the reactive recovery path: - **Typed classification.** `AgentError::LlmContextExceeded` is classified at both non-success provider terminals — the shared `post()` (Anthropic, OpenAI, Databricks) and `openrouter_post()` — on `status == 400` plus a context-window body match, so ordinary 400s stay terminal. - **Forced handoff.** A context-400 forces a summarize-handoff that bypasses `should_handoff()` and `BUZZ_AGENT_MAX_HANDOFFS`, bounded by its own per-turn budget (`MAX_CONTEXT_RECOVERIES_PER_RUN = 3`). - **Shrink ladder.** The summarize prompt budget halves from the observed rejected history size — not from `max_context_tokens`, the number the provider just contradicted — rung to rung, with a 4096-byte floor. A summarize call rejected for the same reason takes the next rung instead of re-sticking. At the floor (overflow dominated by unshrinkable frame: system prompt, tool schemas, live prompt) recovery is refused and the provider error surfaces clearly instead of self-healing. - **Baseline reset.** The stale usage baseline is cleared when a request fails, so the preflight gate cannot stay frozen sub-threshold on retries. Named behavior changes: 1. **Anthropic and OpenRouter errors now carry the `(model)` stamp.** Provider arms return their `Result` into the central error mapper instead of early-returning past it, making the code match its documented single-convergence contract at that mapper. 2. **`max_rounds` now counts completions the loop acts on.** A request rejected with a context-400 that is then successfully recovered refunds its round before the retry, paired 1:1 with a consumed recovery rung, so the round cap is neither weakened nor able to drop a recovered turn unanswered. Related: #4805 — the complementary proactive fix (per-session handoff-cap kill switch that let sessions grow to the provider wall). #4805 prevents reaching the wall; this PR recovers at it. --------- Co-authored-by: npub17jjz49l9jjmhhk7cac63j8yt9z555n9cw8vk7v5jz4vzw4ppld5qgj57cc <f4a42a97e594b77bdbd8ee35191c8b28a94a4cb871d96f32921558275421fb68@buzz.block.builderlab.xyz> Co-authored-by: Duncan <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>