docs: note prefix caching enabled on frank-default-small (opt-in for hybrid; ~4x TTFT on repeated prefix)

docs/frank-redo-2026-08-17.md

@@ -189,3 +189,14 @@ cache for rollback.
 `MAX_NUM_BATCHED_TOKENS` lowered from 16384 to 8192 (user request).
 vLLM has no separate ubatch knob; the scheduler issues one batch per
 step up to this token cap.
+
+### 4. Prefix caching enabled
+
+Added `--enable-prefix-caching`. The model is hybrid (GatedDeltaNet +
+full attention); vLLM keeps prefix caching **opt-in** for hybrid models
+(`default_prefix_caching = is_prefix_caching_supported and not
+is_hybrid`), so it was off by default. Enabling it reuses the KV of
+repeated prompt prefixes. Measured on a 2023-token prefix (2026-08-17
+evening): 4.6s cold -> 1.1s cached (~4x TTFT), output unchanged. This is
+the main TTFT lever for pi, which re-sends the same system prompt + tool
+schema every turn. Toggled via `ENABLE_PREFIX_CACHING` in the env file.