Enforce with a hook

​

The question

Why not just tell the agent in the system prompt to use the cheaper model?

​

The answer

A prompt-based instruction is a suggestion. A hook enforces it in code. A system-prompt instruction is a suggestion the model can drift from under a long, noisy context — and there’s no way to verify it was followed. A PreToolUse hook runs synchronously in the execution path itself, before the tool call fires, so it can inspect the call and block, modify, or reroute it — compliance isn’t optional.

• 90% — fewer tokens when a large file read is delegated instead of handled by the frontier model itself
• 350 lines — a read request above this size is what automatically triggers the reroute
• 10–30s — added latency per delegated call

• Targeted read (offset/limit) → frontier model

Claude already knows what it needs — reading it directly is correct and cheap.

• Untargeted, >350 lines → cheap worker

Volume is the cost, not judgment — the hook blocks this automatically.

• Edit / debug / architecture → frontier model

Excluded by design, not detected per-call — the worker missed a bug the frontier caught in seconds.

​

Source

Spotify Engineering — Portal / shunt, Sept 2026 — Gemini 2.5 Flash worker model · self-reported, four-scenario benchmark.

​

Continue the walkthrough