The best backend for Codex and Windsurf is one both agents can operate through the same repository instructions and scoped development contract. Their interfaces differ, but project creation, authorization, safe writes, tests, deployment verification, and cleanup should not depend on which coding agent is active.
Use AGENTS.md for shared behavior. Add client-specific configuration only for tool connection and permissions.
Codex and Windsurf meet the backend differently
Codex can work through local and hosted agent workflows, long-running goals, skills, and tools. OpenAI's Codex use cases include repository analysis, implementation, testing, automation, and durable objectives.
Windsurf's Cascade works inside the editor with Code and Chat modes, terminal execution, MCP, rules, checkpoints, and planning. Its Cascade documentation describes those capabilities and publishes an agent-readable docs index.
The useful difference is supervision. An IDE session often has a person close by. A queued or long-running task may reach a provisioning or permission decision after that person has left. The backend contract must remain safe in both cases.
Use one shared repository contract
Put these rules in AGENTS.md or another shared project instruction file:
Use development resources only.
Read current first-party service docs before calling external tools.
Keep management and application credentials in server-only code.
Reuse one idempotency key for retries of the same logical write.
Ask before claiming, paying, upgrading, or deleting external state.
Verify the public application route before reporting deployment success.
Record every resource created and its cleanup path.
The backend operability guide explains the API behavior those rules expect.
Configure tools by least authority
For Codex, expose only the skills, MCP servers, commands, and network access needed by the task. For Windsurf, configure only the required MCP server and terminal commands, then keep automatic execution within a reviewed allowlist.
Neither agent should receive a production administrator credential for a development task. A local approval prompt cannot narrow what a leaked credential can do later.
Pick the backend by project state
No backend and no provider account
Cohesivity is a direct path when the agent needs supported backend resources before the human creates accounts. Either agent can bootstrap an ephemeral tenant, fetch live offering contracts, and provision through one project context. A later claim remains a human decision.
Existing Supabase or Firebase project
Use the platform already chosen. Supabase and Firebase both publish official MCP servers for coding agents. Scope them to a development project and follow their production-access warnings. The backend platform guide compares those operating models.
Direct cloud or internal platform
Use the team's existing CLI, infrastructure code, and restricted role. Give the agent a task-specific environment or plan-only access before any apply step. Do not bypass established deployment review because the agent can run the command.
Compare outcomes instead of agent personalities
Run one fixed task and score:
| Measure | What to record |
|---|---|
| Correctness | Tests and acceptance criteria passed |
| Human interruption | Every login, approval, and missing input |
| External effects | Resources, writes, deploys, and messages |
| Retry behavior | Duplicate calls and idempotency results |
| Security | Credential placement and permission scope |
| Recovery | State after interruption and resume |
| Cleanup | Resources removed or handed to a human |
Do not infer that one agent is better because its response sounds more confident. Inspect the repository and backend state.
Make the task portable
The task should name the desired outcome, constraints, tests, and approval boundaries without encoding one client's UI. This also makes the result usable in Claude Code, Cursor, or another agent later.
The prompt below produces that shared task and comparison rubric before either agent changes the project.