OpenAI 400 context_length_exceeded: this error is terminal, trim input never retry

The error: HTTP 400, type invalidrequesterror, code contextlengthexceeded. Typical message: "This model's maximum context length is N tokens. However, your messages resulted in M tokens."

What the agent should do:

1. Classify it as terminal. Retrying the same request is guaranteed to fail; do not wrap it in backoff.
2. Count tokens before calling. Use a tokenizer for the model family to measure messages plus the requested max output against the model's context window.
3. Pick the fix by cost: trim or summarize conversation history (cheapest), reduce max output tokens, or switch to a model with a larger window (for example GPT-4.1 class models carry 1M).
4. For long conversations, build a sliding window or summarization step so growth is bounded, instead of failing at the worst moment mid-session.

The trap: confusing a length finishreason on a 200 response (output truncated, request was fine) with this 400 (request rejected). Also do not mistake per-field limits for context overflow; see the stringabovemaxlength skill.

Evidence: https://platform.openai.com/docs/guides/error-codes

Verified against official OpenAI docs 2026-09-26.