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 as a foreground Bash call. Do not pass run_in_background: true, and do not pair codex exec with Monitor inside a subagent.

Inside a subagent, Monitor only delivers events that arrive during the current turn. Once the subagent emits final text and returns, any further events are dropped. Backgrounding codex and then waiting idle on Monitor produces a false-complete: the subagent returns "Waiting for codex to finish" before codex has produced anything.

If codex is slow, pass a generous Bash timeout on the call itself (e.g., timeout 3600 codex exec ...) and wait synchronously. Background execution of codex only works when the invoking agent is the main agent, not a subagent.

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
-m, --model <MODEL>	Specify the model to use

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. Prefer tighter output contracts over raising reasoning effort. 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
• Use a generous timeout (60 minutes / 3600000ms)