majiayu000 Codex Exec
Autonomous task execution via the codex CLI. Runs non-interactively. Progress streams to stderr; final result on stdout.
codex exec "task description" < /dev/null
For large context, pipe it via stdin. The prompt stays as the argument, context is passed as <stdin> automatically:
cat context.txt | codex exec "question about the context"
Sandbox
All codex Bash calls require dangerouslyDisableSandbox: true (network access to OpenAI API). Without it, codex crashes with an Operation not permitted panic from the system-configuration crate before the model runs.
Stdin Gotcha
Codex reads from stdin whenever stdin is non-TTY (per codex exec --help: "If stdin is piped and a prompt is also provided, stdin is appended as a <stdin> block"). In subagent and subprocess contexts the harness leaves stdin connected to a pipe that never EOFs, so a bare codex exec "..." hangs forever, printing only Reading additional input from stdin....
Always redirect stdin on non-piped invocations:
codex exec "task description" < /dev/null
The piped form (cat context.txt | codex exec "...") is safe — cat closes the pipe after the file, sending EOF.
Synchronous Execution
Run codex via the Bash tool (timeout: 3600000, do not set run_in_background). On long runs the harness may force the call into the background anyway; the Bash result then contains a background ID instead of codex output. Poll the background ID with BashOutput until codex completes, then treat the final output as the synchronous result.
If you are a subagent, do not pair codex exec with Monitor. Wait for the Bash call to return before emitting final text. Monitor only delivers events during your current turn; events after you emit final text are dropped. Backgrounding codex and idling on Monitor produces a false-complete: you return "Waiting for codex to finish" before codex has produced anything. The same false-empty trap applies if the harness force-backgrounds the call and you return the background ID as if codex finished — see /peer-review subagent-wrapping for the recovery pattern.
Permission Levels
| Level | Flag | When to Use |
|---|---|---|
| Read-only (default) | (none) | Analysis, code reading, generating reports |
| Workspace write | --sandbox workspace-write |
Editing files within the project |
| Full access | --sandbox danger-full-access |
Installing packages, running tests, system operations |
| Full auto | --full-auto |
Combined with a sandbox level for unattended execution |
For fix or implementation tasks, default to --sandbox workspace-write --full-auto so Codex can edit files without confirmation prompts. Use read-only for analysis or research tasks.
Options
| Option | Description |
|---|---|
--full-auto |
Allow file edits without confirmation prompts |
--sandbox <level> |
Permission level: danger-full-access, workspace-write |
--json |
JSON Lines output (progress + final message) |
-o <path> |
Write final message to a file |
--output-schema <path> |
Enforce JSON Schema on the output |
--ephemeral |
No persisted session files |
--skip-git-repo-check |
Bypass git repository requirement |
Prompt Shaping
Codex uses XML tags in its own context scaffolding, so the model parses them natively. Structure prompts with XML tags for clearer responses:
<task>: The concrete job and relevant context.<structured_output_contract>: Required output shape, ordering, and format.<compact_output_contract>: Same purpose but for concise prose responses.<grounding_rules>: When claims must be evidence-based.<dig_deeper_nudge>: Push past surface-level findings to check for second-order failures.<verification_loop>: When correctness matters — ask Codex to verify before finalizing.
Keep prompts compact, with tight output contracts. One clear task per exec call.
Parallel Execution
Codex supports parallel sub-agents via spawn_agent / wait_agent. The model will not fan out unless the prompt explicitly requests it. See references/parallel-execution.md for patterns and limitations.
Interpreting Results
- Exec output is a starting point, not a guaranteed solution
- Cross-reference suggestions with project documentation and conventions
- Test incrementally rather than applying all changes at once
- For file-editing tasks, always review the diff before committing