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Covered in AI EngineeringModels & BenchmarksAgents & Protocols, where the background and the sources for this subject live.
Claude cache diagnostics compares the current request with the response `id` of the previous diagnosed turn and reports the first divergence across model, system, tools, or messages. Since 23 September 2026 the `diagnostics` field is always present, so reliable debugging means reading that field together with `usage.cache_read_input_tokens`.
4 min read
Claude Opus 5.5 lists 20 percent below Claude Opus 5 on input and output, at $4 and $20 per million tokens. Its cache reads fall 60 percent, to $0.20, because the multiplier changed from 0.1x to 0.05x. Thinking cannot be disabled, and 25 percent more output tokens erases the output cut.
4 min read
Google ADK v2.10.0 adds `persistent`, `bounded` and `ephemeral` skill lifecycles behind `ADK_ENABLE_SKILL_LIFECYCLE=1`, but the source code is clear that only `bounded` skills count against `max_active_skills`. `ephemeral` skills expire at turn end, except under `run_live`, where they stay active for the full stream.
4 min read
Anthropic's mid-conversation-tool-changes-2026-07-01 beta lets a system-role message add or remove already-declared tools partway through a conversation without invalidating the cached prefix above it. LangChain wired this into ChatAnthropic on 23 September 2026, and its wrapper silently drops any other content type from that message with only a logged warning.
5 min read
openai-python v3.15.0 adds a prewarm field to the Responses API's prompt cache options. Setting it true forces generate to false, writing the cache with no output, at a TTL fixed to 30 minutes. Claude has offered the same effect since launch via a max_tokens=0 request, with a 5 minute default or a paid 1 hour TTL.
4 min read
Claude Code and the Agent SDK record a session's system prompt on its first request and keep reusing that copy after resume, so a changed append or custom prompt has no effect until the session compacts or you start a new one. The snapshot field controls this, and it defaults to off in bare mode.
5 min read
MCP revision 2026-07-28 requires ttlMs and cacheScope on every list result and tells servers to return tools in a deterministic order. Both are filed as minor changes. They are the most direct cost change in the revision, because tool definitions sit in the cacheable prefix of every agent step.
3 min read
Prompt caching stores a prefix of your prompt so later requests reuse it instead of reprocessing it. Reads are far cheaper than base input tokens, but writing the cache costs a premium and entries expire. It pays whenever a large stable prefix is reused several times inside the TTL, and loses on one-shot traffic.
6 min read