Codex needs more than a list of features. It needs to understand the current project, the result you want and the boundaries it must preserve. The project plan should help it make implementation decisions without inventing customer requirements.
State whether this is a new product or an existing repository. Name the relevant source files, runtime, data store and deployment environment when they are known. For an existing product, explicitly list the behavior that must keep working.
Define the first response you need
Ask Codex to read the supplied brief, explain the intended product, identify missing decisions and outline a bounded first milestone. Request an estimate with assumptions and remaining work. Confirm that it has read the actual source rather than merely opened a link.
Separate planning from implementation
A QERA brief is input to the coding task. It does not grant repository permissions, establish service accounts or prove that the finished product exists. Review the plan and authorize the appropriate implementation work. Keep deployment and irreversible actions under the controls your project requires.