the stream-json result event carries modelUsage.<model>.contextWindow
which is the actual per-inference active window the model enforces.
for claude-sonnet-4-6 this is 200k even though the full prompt cache
can hold millions of tokens via accumulated cache reads.
with the nix-configured sonnet = 1000000 the proactive compact watermark
sat at 750k and was never reached. agents grew context until prompt_too_long
at ~170k — reactive compact, no checkpoint turn.
changes:
- bus gains api_context_window field seeded from modelUsage.*.contextWindow
in each turn's result event. authoritative; falls back to env var, then 200k.
- new effective_context_window(bus) helper used by both watermark functions
- compact_watermark (75%) and auto_reset_watermark (50%) call effective_context_window
- context_tokens() docstring clarified: all three token fields (input +
cache_read + cache_creation) count against the per-inference contextWindow
limit. the large cache_read values seen in the result event are cumulative
across all inferences in a turn, not per-inference.
- /api/state context_window_tokens now reflects the calibrated window
closes#129
b0f6bd8ece