OpenAI 400 string_above_max_length: per-field character limit, not the context window

TL;DR

Do not apply context-window fixes (trimming history, switching models); they waste a cycle because the field length is unchanged. The fix is local to one field, and the error names it.

When to use

You are seeing this: The error: HTTP 400, type invalidrequesterror, code stringabovemaxlength. Use this skill when you run into "OpenAI 400 stringabovemaxlength: per-field character limit, not the context window".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Steps

1. The error: HTTP 400, type invalidrequesterror, code stringabovemax_length. Message pattern: "Expected a string with maximum length N..."
2. When it fires: a single field exceeds its character cap. Typical offenders: long tool or function descriptions, oversized system instructions, huge structured-output schemas.
3. What the agent should do:
4. Read error.param or the message to identify the exact field. This is a per-field limit, unrelated to the model's token context window.
5. Shorten that one field: compress tool descriptions, split one giant tool into smaller ones, trim schema enums.
6. Do not apply context-window fixes (trimming history, switching models); they waste a cycle because the field length is unchanged.
7. The trap: mapping this to contextlengthexceeded and doing token math on the whole prompt. The fix is local to one field, and the error names it.
8. Evidence: https://platform.openai.com/docs/guides/error-codes

Verified against official OpenAI docs 2026-09-26.