max_output_tokens is a hard wall the endpoint enforces mid-sentence. The
```state block is emitted after the narration, so a long turn hits the wall
partway through the block and the deltas are lost — silently, since nothing
reads finish_reason.
builder.length_hint() derives a word limit from the cap ((cap - 50 headroom)
* 0.75 words/token * 0.90 buffer) and injects it just above EMIT_REMINDER,
which keeps the last slot it needs. Reserved in build_context like the
reminder is.
Phrased as a ceiling, not a budget. Measured against gemma-4-26b at cap 800,
n=5 per arm: no hint 174 words, "keep this turn under about N words" 246,
"hard limit ... a typical turn is much shorter" 170. A budget reads as a
target to fill — every budget run was longer than every unhinted one, pushing
turns toward the wall the hint exists to avoid. Ceiling phrasing still works
at tight caps: at 250, unhinted hit finish_reason=length 2/6, hinted 0/6.
tests/test_length_hint.py, 11 tests; each mechanism verified by sabotage.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UeQVy5bEjLhfgWNc27Efet
