- **fix(providers):** when `OPENCODE_SYNTHESIZE_CLI_HEADERS=true`, a non-CLI client User-Agent (e.g. `curl/8.5.0`, SDKs) on opencode-go/opencode-zen/opencode-free requests is now REPLACED with the synthesized `opencode-cli/1.0.0` instead of being honored — opencode.ai's free tier (`/zen/v1`) returns `FreeUsageLimitError` 429 for generic client UAs egressing from datacenter IPs, which made the #5997 CLI-identity synthesis ineffective for non-CLI clients. Client UAs already matching `opencode-cli/…` are preserved (the real CLI's versioned identity stays intact); all other client-supplied `x-opencode-*` headers keep client-wins. Regression guard: `tests/unit/opencode-cli-headers-synthesis-5997.test.ts` (7, incl. non-CLI UA replaced + CLI UA preserved). (#5997 follow-up)