Unstable prompt cache hits for muse-spark-1.3-contributor in CLI usage (routing suspected, repro via /chat/completions and /responses) #1007

Background (how this was found)

Normal CLI usage with meta/muse-spark-1.3-contributor (GOAT plan, cmd v1.79.2, macOS): prompt-cache hits are abnormal/unstable in long sessions (high then dropping to 0, flapping). Only after noticing this did I repro directly against the Provider API with /chat/completions and /responses using a fixed prefix — the API results below are the controlled confirmation, not the original symptom.

Repro (Provider API, controlled)

Fixed ~30k-char prefix, per-round only Round N: reply exactly RN, 1.5s gap, store:false on /responses:

# /responses per round:
{"model":"meta/muse-spark-1.3-contributor","instructions":"<FIXED ~30k chars>","input":"Round N: reply exactly RN","store":false,"max_output_tokens":50}
# /chat/completions per round:
{"model":"meta/muse-spark-1.3-contributor","messages":[{"role":"system","content":"<SAME FIXED TEXT>"},{"role":"user","content":"Round N: reply exactly RN"}],"temperature":0,"max_tokens":100,"stream":false}
# read cached tokens + system_fingerprint per round

Results (meta/muse-spark-1.3-contributor, input=6926)

Reads like a routing problem, not a fixed 0%: same-prefix turns scatter (fingerprint changes every round on chat; responses only sticks with a manual key). responses envelope also has no system_fingerprint to diagnose routing. And the CLI exposes no prompt_cache_key, so there is no in-product workaround.

Ask

1. Sticky/session-aware routing (or implicit stable cache key) so plain same-prefix CLI turns hit stably without manual prompt_cache_key.
2. Expose prompt_cache_key (or equivalent) in CLI/BYOK options for pinned sessions.
3. Please also check other models for similar cache/endpoint inconsistencies (spot check hit one model returning 400 unsupported_model on /responses while serving fine on /chat/completions, and another flapping hit-then-miss); worth an audit across models/endpoints.

Env: cmd v1.79.2, macOS, GOAT plan, 2026-10-09.