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.