Builder agent
The Builder agent describes the exact code changes a task needs, file by file. It runs in six built-in workflows before the code pass writes the real diff.
The Builder agent implements. Given a plan, it describes the exact code changes to make: per file, with the key code or pseudocode, and why. It also notes anything that needs follow-up.
Where it runs
| Workflow | Step |
|---|---|
| Plan, Build, Review & Test | Build |
| Security-Sensitive Change | Implement |
| UI Implementation | Implement |
| Incident Response | Mitigation |
| Performance Tuning | Apply |
| Quality-Gated Change | Build |
Inputs
- The session task.
- A read-only snapshot of relevant files from the project's repository, or nothing when the project starts from scratch.
- The earlier steps. In most workflows that is a plan from the Architect or the UI Designer. In Security-Sensitive Change it also includes the Security Auditor's risk analysis. In Quality-Gated Change the Builder runs first and works from the task alone.
Outputs
The Builder's step returns text: the changes per file and the reasoning behind them. It is one input to what happens next.
After all of a workflow's steps finish, the session runs a code pass. That pass reads the task, the code and the team's work (the Builder's description included) and returns the complete new content of every file it changes. The platform writes those files into a throwaway clone and takes a real git diff. That diff is what you review, what the project's tests run against, and what lands as a pull request after approval.
Example run
Illustrative example, not a recorded session. In Incident Response, the Incident Responder has triaged a spike of 500 errors on checkout, and the Architect has traced it to a null currency code from one payment provider. The Builder's Mitigation step proposes a guard in the payment adapter that falls back to the order's currency, a log line when the fallback fires, and a follow-up note to fix the provider mapping properly. The code pass turns that into a two-file diff, and the Tester step has already listed the cases that should prove it.
Limits
- It does not run commands, compile code or execute tests. Tests run later, in a sandbox, against the diff.
- What it describes and what the code pass writes can differ. Review the diff, not the description.
- The code pass reads a fixed amount of the team's output. In long workflows, late steps can be cut from what it sees.
- A change too large to review ends the session with that reason. It is never silently cut short.
- Your workspace spend cap applies to the whole session. There is no separate cap per agent. See cost controls.
Frequently asked questions
Does the Builder agent edit my repository directly?
No. It describes the change. A separate code pass writes complete files into a throwaway copy of the repository and produces a diff, which only lands after the release gate and approval.
Which model does the Builder use?
The session's model. On the Auto tier its step goes to the Economy tier first, and the code pass that writes the files goes to the Standard tier first.