Codex Implement

Codex Implement

Delegate bounded implementation tasks to OpenAI Codex CLI. Claude acts as Architect (decomposes, selects context, verifies); Codex acts as Editor (implements each bounded task).

When to Use

• Ad-hoc implementation: "have Codex build this"
• Cross-model implementation for blind-spot coverage
• When you want Claude to orchestrate and verify, not write the code itself

Arguments

• Positional: Spec file path OR quoted text description (optional)
• --consult — Run /codex-consult on the implementation plan before executing
• --no-commit — Skip commits after each task (leave changes unstaged)
• --dry-run — Show Implementation Brief + task decomposition without executing
• --model MODEL — Override Codex model for this invocation

Workflow

Copy this checklist and track progress:

Implementation Progress:
- [ ] Phase 1: Pre-flight checks
- [ ] Phase 2: Gather input
- [ ] Phase 3: Analyze and decompose
- [ ] Phase 4: Optional consultation (if --consult)
- [ ] Phase 5: Execute tasks
- [ ] Phase 6: Summary

Phase 1: Pre-flight

Guard Checks

# Already inside Codex?
if [ -n "$CODEX_SANDBOX" ]; then
  echo "Already running inside Codex. Skipping."
  exit 0
fi

# Codex CLI installed?
codex --version

# Authenticated?
codex login status

If Codex CLI is not available or not authenticated, report the error and STOP:

ERROR: Codex CLI required
==========================
Install: npm install -g @openai/codex
Auth:    codex login

Load Configuration

Read .claude/settings.local.json:

CODEX_MODEL=$(jq -r '.codexImplement.model // .codexReview.codeModel // empty' .claude/settings.local.json 2>/dev/null)
CODEX_EFFORT=$(jq -r '.codexImplement.effort // .codexReview.effort // empty' .claude/settings.local.json 2>/dev/null)
TIMEOUT_MINS=$(jq -r '.codexImplement.timeoutMinutes // .codexReview.taskTimeoutMinutes // 60' .claude/settings.local.json 2>/dev/null || echo "60")
MAX_TASKS=$(jq -r '.codexImplement.maxTasks // 5' .claude/settings.local.json 2>/dev/null || echo "5")
ENABLED=$(jq -r '.codexImplement.enabled // true' .claude/settings.local.json 2>/dev/null || echo "true")

If --model flag provided, override CODEX_MODEL. If enabled is false, report and STOP.

Parse Flags

Extract from argument string:

• --consult → CONSULT=true
• --no-commit → NO_COMMIT=true
• --dry-run → DRY_RUN=true
• --model VALUE → override CODEX_MODEL
• Remaining text → INPUT (file path or description)

Phase 2: Gather Input

Determine input mode from INPUT:

Mode A: Spec File

If INPUT is a file path that exists:

1. Read the file
2. Extract: problem statement, requirements, acceptance criteria, scope boundaries
3. Present summary to user via AskUserQuestion: "This is what I'll have Codex implement. Correct?"
4. If user modifies, incorporate changes

Mode B: Inline Text

If INPUT is quoted text (not a file path):

1. Claude explores the codebase to understand the affected area:
   • Glob for relevant files
   • Read key files to understand patterns
   • Identify what exists vs. what needs to be built
2. Generate an Implementation Brief (internal, not written to disk):
   • What to build (from user's description)
   • Affected files (from codebase exploration)
   • Success criteria (Claude generates testable criteria)
   • Constraints (from AGENTS.md and codebase conventions)
3. Present brief to user via AskUserQuestion for confirmation

Mode C: Interactive

If no INPUT provided:

1. Ask one question at a time via AskUserQuestion (conversational, discover-style)
2. Follow threads: if an answer raises something interesting, explore it
3. Smart-skip: don't ask what's already clear from context
4. Typical questions:
   • "What would you like Codex to build?"
   • "Where in the codebase should this live?" (after exploring to suggest options)
   • "Any constraints or patterns it should follow?"
   • "How should we verify it works?"
5. Escape hatch: if user says "just do it", ask one final critical question then proceed
6. Move on when sufficient clarity achieved (typically 3-5 questions)
7. Build Implementation Brief from answers (same structure as Mode B)

Implementation Brief Structure

All modes produce this internal structure:

IMPLEMENTATION BRIEF
====================
Description: {what to build}
Affected files: {list of files Codex should read/modify}
Success criteria:
  1. {testable criterion}
  2. {testable criterion}
Constraints:
  - {from AGENTS.md}
  - {from codebase conventions}

Phase 3: Analyze and Decompose

See DECOMPOSITION.md for the full decision tree.

Claude reads the Implementation Brief and decides single vs. multi-task:

Single task if ALL of:

• Touches ≤ 3 files
• Single concern (one logical change)
• Estimated ≤ ~80 lines of change
• No cross-cutting concerns

Multiple tasks if ANY of:

• Touches > 3 files
• Multiple concerns
• Cross-cutting layers (API + frontend + database)
• Estimated > 80 lines

Decomposition Rules

• Each task: single bounded concern
• Order by dependency (foundational first)
• Cap at MAX_TASKS (default 5)
• If exceeds cap: warn and suggest full /feature-spec → /feature-plan workflow, but allow override if user confirms

User Confirmation

Present the task list to the user:

IMPLEMENTATION PLAN
===================
Tasks: {N}

1. {task description} — touches {files}
2. {task description} — touches {files}
...

Estimated Codex time: ~{N * timeout/4} minutes (worst case: {N * timeout} minutes)

Ask: "Ready to execute?" with options:

• "Execute (Recommended)"
• "Modify tasks"
• "Cancel"

Dry Run Exit

If --dry-run: display the Implementation Brief and task list, then STOP.

Phase 4: Optional Consultation

If --consult flag is set:

1. Write Implementation Brief to temp file
2. Invoke /codex-consult on the temp file
3. Present Codex's findings to user
4. Ask: "Incorporate suggestions?" → refine brief if yes
5. Clean up temp file
6. Proceed to Phase 5

Phase 5: Execute Tasks

Safety Guard Pattern

Apply this pattern before and after every Codex invocation to prevent accidental commits from persisting:

# Before invocation — fail fast on dirty worktree, record HEAD
if [ -n "$(git status --porcelain)" ]; then
  echo "WARNING: Worktree has uncommitted changes. Commit or stash before running Codex."
fi
HEAD_BEFORE=$(git rev-parse HEAD)

# ... invoke Codex ...

# After invocation — revert any commits Codex made
HEAD_AFTER=$(git rev-parse HEAD)
if [ "$HEAD_BEFORE" != "$HEAD_AFTER" ]; then
  echo "WARNING: Codex made commits. Reverting."
  git reset --soft "$HEAD_BEFORE"
  git restore --staged .
fi

For each task (sequentially):

Step 1: Capture Safety State

Apply the safety guard pattern (pre-invocation half) defined above.

Step 2: Build Task Prompt

See PROMPT_TEMPLATE.md for the full template.

Write prompt to temp file. Include:

• AGENTS.md context
• ONLY files relevant to this specific task (not all affected files)
• Success criteria for THIS task only
• Constraints: follow patterns, don't commit, stay in scope

Step 3: Invoke Codex

# Build model flag
MODEL_FLAG=""
if [ -n "$CODEX_MODEL" ]; then
  MODEL_FLAG="--model $CODEX_MODEL"
fi

# Build effort flag
EFFORT_ARGS=()
if [ -n "$CODEX_EFFORT" ]; then
  EFFORT_ARGS=(-c "model_reasoning_effort=\"$CODEX_EFFORT\"")
fi

PROMPT_FILE="/tmp/codex-impl-task-${TASK_NUM}.md"
OUTPUT_FILE="/tmp/codex-impl-task-${TASK_NUM}-output.txt"

cat $PROMPT_FILE | codex exec \
  --sandbox danger-full-access \
  -c 'approval_policy="never"' \
  -c 'features.search=true' \
  $MODEL_FLAG \
  "${EFFORT_ARGS[@]}" \
  -o $OUTPUT_FILE \
  -
EXIT_CODE=$?

Use Bash tool timeout parameter: TIMEOUT_MINS * 60 * 1000 milliseconds.

MANDATORY rules:

• NEVER use run_in_background
• NEVER retry with modified syntax
• NEVER use 2>&1 (corrupts output parsing)
• Use Bash tool timeout, not shell timeout

Step 4: Safety Check

Apply the safety guard pattern (post-invocation half) defined above.

Step 5: Parse Result

if [ $EXIT_CODE -eq 124 ]; then
  STATUS="FAILED"; ISSUE="Timeout after ${TIMEOUT_MINS} minutes"
elif [ $EXIT_CODE -ne 0 ]; then
  STATUS="FAILED"; ISSUE="Codex exited with code $EXIT_CODE"
elif [ ! -f "$OUTPUT_FILE" ]; then
  STATUS="FAILED"; ISSUE="No output file produced"
else
  STATUS=$(grep "^Status:" "$OUTPUT_FILE" | head -1 | awk '{print $2}')
  if [ -z "$STATUS" ]; then
    STATUS=$(grep "Status:" "$OUTPUT_FILE" | head -1 | awk '{print $2}')
  fi
  if [ -z "$STATUS" ]; then
    STATUS="FAILED"; ISSUE="Could not parse task result"
  fi
fi

Step 6: Verify

See VERIFICATION.md for the full verification strategy.

4-tier verification:

1. Scope guard: git diff --name-only — did Codex only touch expected files?
2. Automated checks: Run tests/lint/typecheck if configured
3. Claude review: Read git diff, compare against success criteria
4. Cross-model review (only with --consult): Run /codex-review on the changes

Step 7: Commit or Rollback

On success (unless --no-commit):

• Stage changed files: git add {specific files}
• Commit: impl: {task description} [codex-implement]

On failure:

• Show what went wrong (Codex output + verification results)
• Ask user via AskUserQuestion:
  • "Retry this task"
  • "Skip and continue"
  • "Abort remaining tasks"
  • "I'll implement this one manually"
• After 2 consecutive failures on same task: recommend manual implementation

Step 8: Clean Up

rm -f $PROMPT_FILE $OUTPUT_FILE

Phase 6: Summary

IMPLEMENTATION COMPLETE
=======================
Tasks: {completed}/{total}
Files created: {count}
Files modified: {count}
Commits: {count}

Changes:
  {commit message 1}
  {commit message 2}
  ...

Next steps:
  - Run full test suite: {test command}
  - Review changes: git diff HEAD~{N}

If any tasks failed or were skipped, include them in the summary with recommendations.

Error Handling

Configuration

Add to .claude/settings.local.json:

{
  "codexImplement": {
    "enabled": true,
    "model": "gpt-5.3-codex",
    "effort": "xhigh",
    "timeoutMinutes": 60,
    "maxTasks": 5
  }
}

Fallback chain: codexImplement → codexReview → defaults.

REMINDER: Codex Invocation Rules

These rules are mandatory for every codex exec invocation:

• NEVER use run_in_background — always execute synchronously
• NEVER retry with modified syntax — report exit code and skip
• NEVER use 2>&1 — corrupts output parsing (Codex streams progress to stderr)
• Use Bash tool timeout parameter — not shell timeout command
• Always capture HEAD before and after — revert accidental commits