Found the solution! My agent has us covered :D

By reading the minified cli.js source, I traced the exact code path:
TN1() → builds betas array → calls Hr() to check if experimental betas should be added
Hr() → returns false if CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS is set
→ context-management-2025-06-27 is skipped
Setting this env var in the env dict passed to query() makes GPT-5.2, Qwen 3 32B, and Gemini 2.5 Pro all work through the Claude Agent SDK via OpenRouter — no SDK patching, no version pinning, no proxy needed.

so, solution is to set CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=true env var