Claude Code -> claude-opus-4-8 turns intermittently died with 'tool call could not be parsed (retry also failed)'. OmniRoute's claude identity cloak rebuilt the anthropic-beta header from scratch and unconditionally forced interleaved-thinking-2025-05-14 (+ advanced-tool-use / effort for heavy agents), even when the client never negotiated them. The forced interleaved-thinking conflicts with tool_choice-forced turns, producing malformed opus tool_use streams (and sibling 400 'Thinking may not be enabled when tool_choice forces tool use').
selectBetaFlags now takes the client's inbound anthropic-beta: when present, thinking/effort betas are only emitted if the client requested them. Opaque clients (no header — the OAuth cloak path) keep the full set unchanged, so existing behavior and the #2454 model-tier gating are preserved.
Co-authored-by: Forcerecon <Forcerecon@users.noreply.github.com>
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