[claude-code-router] [Bug] Managed compact handoff strips max_tokens → anthropic_messages upstream 400 'body.max_tokens Field required', whole fallback chain fails

## Observed symptom [Bug] Managed compact handoff strips max_tokens → anthropic_messages upstream 400 'body.max_tokens Field required', whole fallback chain fails Managed compact handoff requests sent over `anthropic_messages` fail with a deterministic `400` because `sanitizeCompactHandoffRequest()` strips **all** output-token-limit keys (including `max_tokens`) from the body, and Anthropic-Messages-compatible upstreams that validate the body with pydantic require `max_tokens` as a mandatory field. The stripped body is reused by every provider in the fallback chain, so **all attempts fail** and the client only sees `All target providers failed. [upstream_response|400] Upstream request failed.` Host deployment, gateway `[local]`, 2026-09-30 09:36–09:42 UTC. Client: `Profile: Claude Code` (Claude Desktop Code tab, model discovery `model-discovery.claude-app`). ## Literal error text ``` {"error":{"message":"1 validation error:\n {'type': 'missing', 'loc': 'body.max_tokens', 'msg': 'Field required', 'input': '<dict of 7 items>'}","type":"Bad Request","param":"body.max_tokens","code":400}} ``` ## Environment GitHub musistudio/claude-code-router #1843 ## Trigger / context Public GitHub report in musistudio/claude-code-router (FRESH_7D). ## Source evidence - https://github.com/musistudio/claude-code-router/issues/1843 (2026-09-30T12:55:09Z) — evidence_class: PRIMARY_ISSUE_REPORT — #1843 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:50:53.329738Z - scout_id: C7a6e42df8c ## 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. Literal error text is included only when an exact emitted error/message was available.