Better prompt caching for GPT-6: higher hit rates, diagnostics, and agent-friendly controls

Why it matters

• OpenAI published Better prompt caching for GPT-6 (primary: openai.com/index/better-prompt-caching-for-gpt-6 RSS pubDate 2026-09-22T21:00:00Z).
• For persistent multi-turn agents, shared instructions, tool definitions, and context can be cached across requests—cutting latency and offering up to ~90% discount on cached input tokens.
• GPT-6 family raises default hit rates; eligible shared prefixes reused within a ~30-minute window get cache discounts.
• New Prompt Caching Dashboard and diagnostics explain misses (e.g. tools_changed).
• Optional controls: explicit cache breakpoints
• change reasoning effort without breaking cache
• keep tool defs/schemas/order stable (prefer allowed_tools / tool_choice=none over removing defs)
• append developer messages
• optional prewarming.
• Customer quotes on the page are not independently verified here.
• Editorial: infra/observability for agent economics