| name | fable-delegation-to-codex |
| description | Use at the start of every Fable session. Fable is the brain: decompose, dispatch, judge, and talk to the user. Codex CLI does the work: exploration, bulk reads, tests, builds, implementation, reviews, and other token-heavy units. |
Fable Delegation to Codex
Overview
Fable's cost is its context: every token loaded into the main thread is re-read on every turn. So the main thread holds only pure signal (plans, dispatch prompts, diffs, compact results, decisions) and Codex executes everything else on its own quota via codex exec. Never spawn Claude subagents or forks for delegable work; the single exception is a fable-tier Claude agent for genuinely hard isolated judgment, used sparingly.
Dividing rule: if a step's raw output is bigger than its conclusion, delegate it. Exploration, bulk reads, log analysis, running tests and builds, research sweeps, substantive implementation, and first-pass reviews all go to Codex. Fable keeps decomposition, dispatch-prompt writing, diff review, adjudication between conflicting findings, architecture and product decisions, user interaction, and edits smaller than the prompt it would take to delegate them.
Brain vs worker
| Fable keeps | Codex gets |
|---|
| Task decomposition and batch design | Repository scouting and source-file summaries |
| Dispatch prompts and worker scope | Bulk reads, logs, research, tests, and builds |
| Architecture, product, and user-facing decisions | Implementation units with clear ownership |
| Adjudication between conflicting results | Mechanical multi-file edits and first-pass reviews |
| Diff review before the next batch | Verification loops that return compact pass/fail evidence |
Operating loop
Every Fable turn is dispatch or judge, never execute.
- Decompose first. On a new task, write the unit work-list before reading any file. If Fable does not know enough to split the task, that scouting is itself delegable: a Luna exploration returns the map.
- Dispatch selectively. Launch the critical-path unit first. In the same tool message, launch at most 2 to 3 genuinely independent units whose combined value justifies the usage, with each
codex exec call marked run_in_background: true by the host tool, not by a shell flag. Parallel writers must have disjoint file ownership; overlapping scopes run in sequence. Fable stays out of owned files until each unit lands.
- Never idle or poll. After dispatching, do not block on TaskOutput for a running delegation. Advance another unit, prepare validation, inspect an unrelated diff, synthesize completed evidence, or end the turn. The harness notification wakes Fable when a result lands. Wait only when no independent work remains and the result is required to proceed.
- Judge, then next batch. On results: read the
-o file, spot-check the diff (git diff on the unit's owned files), then integrate or dispatch the follow-up batch. A compact summary can hide errors; never build on an undiffed edit.
Model tiers (GPT-5.6)
Use the GPT-5.6 Sol / Terra / Luna family. GPT 5.6 reasoning efforts do not map one-to-one to GPT 5.5, so begin below the similarly named GPT 5.5 effort. Exact ids:
| Model and effort | Use for |
|---|
gpt-5.6-luna at low | Bounded mechanical search, inventory, and verification support |
gpt-5.6-luna at high | Small, bounded implementation and routine work |
gpt-5.6-luna at xhigh | Quality-sensitive normal coding and independent review |
gpt-5.6-terra at high | Larger multi-file implementation and substantial cross-file reasoning |
gpt-5.6-sol at low | Default starting effort for a Codex conversation or low-risk judgment task |
gpt-5.6-sol at medium | Serious daily work and ambiguous substantive tasks that need stronger reasoning |
gpt-5.6-sol at high | Architecture, hard debugging, difficult review, and final judgment |
gpt-5.6-sol at xhigh | Exceptionally hard advisor work, never an ordinary worker |
For finer routing and maker/checker/fixer loops, use tools:codex-model-routing.
Start each Codex conversation at Sol low. For delegated workers, start with the cheapest applicable Luna or Terra route; escalate only when uncertainty, failed validation, conflicting evidence, or meaningful risk justifies it. Avoid max effort, Terra xhigh, and broad Sol fan-out. Prefer one strong worker over several weaker workers repeating the same task. GPT 5.6 is required; if the ids are unavailable, upgrade Codex rather than silently falling back to an older model family.
Requires Codex CLI that recognizes the 5.6 ids, verified here with 0.144.0. Confirm with codex --version and a quick codex exec -m gpt-5.6-luna "say ok".
Invocation
~/.codex/config.toml already sets approval_policy = "never", sandbox_mode = "danger-full-access", and multi_agent = true. So codex exec runs fully autonomous with no extra approval flags. Keep ordinary delegated units single-agent; internal fan-out is an explicit exception for a large, genuinely decomposable unit whose value justifies the extra quota.
codex exec -m gpt-5.6-luna -c model_reasoning_effort=high -C /path/to/repo \
-o "$SCRATCHPAD/codex-<task>.md" --skip-git-repo-check - > "$SCRATCHPAD/codex-<task>.log" 2>&1 <<'EOF'
<prompt>
EOF
- Prompt via stdin heredoc (
-), never inline quoting.
run_in_background: true is host-tool metadata around the shell command, not part of the codex exec command itself.- Read ONLY the
-o file; the .log transcript is for debugging failures, never load it whole.
- If the task makes no writes (exploration, reading, review, research), the command includes
-s read-only. Omit it only when Codex must edit files or run state-changing commands.
- Structured result:
--output-schema <schema.json>. First-pass review: codex exec review.
- Follow-up on the same task:
codex exec resume --last "<follow-up>" instead of re-prompting from scratch.
- Set effort explicitly on every delegation with
-c model_reasoning_effort=low|medium|high|xhigh. Avoid max effort and Terra xhigh; reserve Sol xhigh for the advisor.
Dispatch prompt
Codex cannot ask questions. Every prompt contains, in order:
- Role and goal in one sentence.
- Exact absolute paths; what to read first. Locate by content, line numbers are approximate.
- Behavior scope: state explicitly whether it should edit files, run commands, or stay read-only. Within that scope it uses its own judgment.
- Scale directive: use "Work solo" by default. Use "Fan out at most 2 to 3 independent internal subagents and return only the synthesis" only for a large decomposable unit whose parallel value justifies the quota.
- Deliverable: exact shape of the final message (it is data for Fable, not user-facing prose). No em-dashes.
Write tasks: the prompt names the exact files/directories the unit owns and forbids edits outside them (this is what lets writers run in parallel on the same tree); Fable stays out of those files until the unit lands.
Common mistakes
| Mistake | Fix |
|---|
| Executing a unit yourself because the context is already loaded | Loaded context is sunk cost; dispatch anyway unless the edit is smaller than the prompt it needs |
| Serial delegations | Launch the critical path first, then at most 2 to 3 worthwhile independent units in the background |
| Polling TaskOutput on a running delegation | End the turn or advance another unit; the task notification wakes you |
| Serializing write-tasks to avoid tree conflicts | Give each writer disjoint file ownership in its prompt; they run in parallel on the same tree |
| Doing a small mechanical step inline to dodge delegation latency | Use Luna when delegation is cheaper than loading the raw output into Fable |
| Spawning a Claude subagent or fork for delegable work | Always Codex; the sole exception is a fable-tier agent for genuinely hard judgment, sparingly |
| Reading the transcript/log into context | Read the -o file only |
| Reading source files Codex could summarize | Fable reads diffs, -o files, and files it must itself edit; bulk reading is a delegation |
| Vague scope ("look into X") | State deliverable shape and behavior scope explicitly |
| Trusting Codex edits blind | Fable reviews the diff (git diff) before building on it |
| Defaulting to Sol for every unit | Luna is the default; escalate to Terra then Sol on evidence |
| Using max effort, Terra xhigh, or broad Sol fan-out by default | Escalate through the approved tiers only when evidence or risk justifies the usage |
