| name | fable-understand |
| description | Source-grounded codebase understanding for behavior, architecture, boot flow, data flow, integration wiring, or "how does this work" questions. Use when the user asks Codex to explain a system, trace a path, map a subsystem, or answer behavior questions with citations and unknowns. |
Fable Understand
Answer from implementation evidence, not memory or stale docs.
For ECF-style governed runs, use ../../references/ecf-run-contract.md. For large or high-risk understanding tasks, use real Codex subagents when the runtime exposes a subagent tool and the user has not opted out; treat cross-module architecture, boot flow, data flow, integration wiring, security/privacy/money/data/API, or many-file mapping questions as large by default. Otherwise run single-agent multi-lens and say why no subagents were used when workflow trace is requested.
For large or high-risk Fable tasks, recommend running the parent task on gpt-5.6-sol with Ultra (model_reasoning_effort = "ultra") when available. Ultra may delegate proactively, but still explicitly request parallel delegation for disjoint Fable lenses when the runtime supports subagents; otherwise use single-agent multi-lens and report the reason.
Workflow
- Restate the question and scope.
- Restate authority boundaries and declare the ECF run mode when the user asks for ECF, subagents, or a receipt.
- Read repo instructions and the most direct source files.
- Trace from entrypoint to effects:
- route/command/UI entry
- service/module boundaries
- data reads/writes
- external calls
- errors, retries, and fallbacks
- tests and docs that confirm or contradict behavior
- Inspect callers and importers before answering behavior questions.
- Use a small diagram or ordered flow when it improves clarity.
- Include unknowns, assumptions, and stale-doc risks.
Evidence Safety
Never print raw secrets, tokens, private keys, wallet keys, credential files, or .env values. Redact secret-like values and cite only the file/path/key name needed to explain the issue.
Output
Prefer this shape:
- direct answer
- source-backed flow
- important edge cases
- unknowns or verification gaps
- useful next probe, only if needed
Every non-obvious claim should have a file, line, command, artifact, or runtime citation.
When requested, include a compact Workflow Trace with mode, ECF contract status, lenses covered, spawned agents or no-subagent reason, verification method, and coverage gaps.
