Claude Code builds Anthropic-Beta per request instead of sending a fixed list.
Captured from an isolated 2.1.220 profile pointed at api.anthropic.com through
a local proxy, over two rounds covering 11 model IDs and the [1m] variants:
constant claude-code, interleaved-thinking, redact-thinking,
thinking-token-count, context-management, prompt-caching-scope
tools advanced-tool-use-2025-11-20 only when tools are declared
model mid-conversation-system-2026-04-07 only on models that accept a
role=system turn
[1m] context-1m-2025-08-07, directly after claude-code-20250219 rather
than at the end
trailing effort-2025-11-24, then server-side-fallback-2026-06-01
claude-sonnet-5 emits mid-conversation-system-2026-04-07, so it accepts a
role=system turn and must not sit in the legacy reminder whitelist.
count_tokens does not reuse the inference fingerprint. Running /context in an
interactive session issues 37 identical calls, which made the endpoint
observable for the first time: four betas only, and 21 headers rather than 22
because X-Stainless-Timeout is absent. The profile is selected from the request
path so no call site has to thread another flag.
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