**1 issue found** across 2 files Confidence score: **3/5** - There is a meaningful regression risk: `packages/core/lib/v3/api.ts` appears to include Stagehand REST API client/server breaking changes without the required integration coverage under `packages/server/test` (severity 7/10, confidence 9/10). - This is likely user-impacting if behavior diverges between client and server, so the PR is not a low-risk merge until integration tests validate the contract change. - Pay close attention to `packages/core/lib/v3/api.ts` and `packages/server/test` - API contract changes need integration tests to catch breakage across client/server boundaries.

Prompt for AI agents (unresolved issues)

```text Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately. P1: Custom agent: **Any breaking changes to Stagehand REST API client / server implementation must be covered by an integration test under packages/server/test** Rule d1dcea1f-c2ef-4728-9dd1-a92623cd374f is violated: this REST API client behavior change (optional `modelApiKey` / conditional `x-model-api-key`) is not covered by a server integration test for session start without a model key. ```

Architecture diagram

```mermaid sequenceDiagram participant User as Client Application participant SDK as StagehandAPIClient participant API as Stagehand API Server User->>SDK: init(params) Note over SDK: CHANGED: No longer throws if
modelApiKey is missing SDK->>SDK: prepareModelConfig(model) alt apiKey provided in params or instance SDK->>SDK: Set apiKey in model configuration else apiKey omitted SDK->>SDK: NEW: Return model configuration
without apiKey field end SDK->>SDK: Construct Headers opt modelApiKey is present Note over SDK: Set x-model-api-key header end SDK->>API: POST /session/start (fetch) Note over SDK,API: Headers: x-api-key, x-bb-session-id, ... alt modelApiKey provided Note over SDK,API: CHANGED: Includes x-model-api-key else modelApiKey omitted Note over SDK,API: NEW: Excludes x-model-api-key end API-->>SDK: SessionStartResult (200 OK) SDK-->>User: Initialized Instance ```

_{Reply with feedback, questions, or to request a fix. Tag `@cubic-dev-ai` to re-run a review, or [fix all with cubic](https://www.cubic.dev/action/fix/pr/browserbase/stagehand/1953/ai_pr_review_1775258954037_441eda56-787f-4c6f-8630-e683d3722300?entrySource=github_ui_to_cubic_ui).}