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Anthropic tells you to remove token-efficient-tools-2025-02-19 because it has no effect. The page that defined it now 307s to a migration guide index that mentions neither the feature nor beta headers. Measured October 2026: 0 occurrences across a 760-URL documentation index, and the whole surviving description is one sentence on one of four model migration guides.
The PDF support page states its page ceiling with a condition you cannot evaluate from that page. Three other statements of the same limit key it off the model instead. Measured on October 9, 2026, plus the arithmetic that makes 600 unreachable on a dense document.
Anthropic's Files API guide says expires_at appears on every file response. Its own migration table says the field is not returned under the legacy beta header. Dated mirrors put both sentences on the page since September 1, 2026, and neither API reference page can carry the warning.
A thinking block stays valid against the bytes you sent. The prompt cache keys on the prompt the model renders. The rows where those two disagree cost money with no error.
Two stop reasons mean your response was cut off, and the continuation helper on Anthropic's own stop reasons page only handles one of them. model_context_window_exceeded falls out of the loop and is returned as complete. Adding it to the condition is worse: the continuation re-sends a full window as input, which is documented as a 400 on every model.
An Anthropic Priority Tier commitment is drawn down alongside your ordinary rate limit, not on top of it. Reading the service tiers page next to the pricing page and the models list turns up a 20x burndown spread, a single cache read rate that only holds because of the exclusion list, and three of four current models excluded.
disable_parallel_tool_use means at most one tool call under tool_choice auto, and exactly one under any or tool. Claude Opus 5.5, Fable 5.1 and Mythos 5.1 reject any and tool with a 400, so the exactly-one contract is not expressible on them at all.
A Claude refusal is an HTTP 200 with a fallback subsystem behind it. The handoff is marked by a content block, and after the first fallback the routing goes sticky and that marker disappears while the fallback model keeps answering.
The Files API left beta, but the old header did not become inert. It now selects which response shape you get, and the one field it hides is the one that tells you a file has stopped working.
Structured outputs and strict tool use turn your JSON schema into a separate cached artifact. It has its own 24 hour lifetime, invalidation rules that invert what you would guess, and a retention boundary that zero data retention does not cover.
A task budget is not a per-task cost ceiling. It resets whenever you send a user message carrying no tool results, which is exactly what most harnesses do when Claude finishes gracefully.
Context rot is real and Anthropic now names it in its own docs. The useful question in 2026 is narrower: your Claude model ships with one of two self regulation mechanisms, or with neither, and they do not overlap. Sonnet and Haiku get context awareness free and automatic. Opus needs a beta header. Opus 4.6 gets nothing.
