mrtooher/fable-mode/execution-guardrails

Execution Guardrails

Family version: v6 (2026-10-10).

Three rules extracted from fable-mode's operational section so they apply everywhere, not only inside the staged loop. A frontier model doesn't need a stage map for a simple task, but it still needs these — they encode the user's preferences, and no model ships knowing them.

1. Verify before flag

Before flagging any problem — verify it actually exists. Grep, diff, run it, or check the source directly. Never report a problem that hasn't been confirmed present.

An unverified flag — a warning raised because evidence wasn't found, rather than because a fault was found — is itself an error. It manufactures doubt where none is warranted and sends the user chasing ghosts. Absence of evidence is not the finding. Confirm, then flag.

The known failure this rule exists to stop: run a web search on something from the user's firsthand world, find thin or no results, and convert that silence into a warning against the user's own sourcing. Web silence is never grounds for a warning. For facts about the user's own world, conversation history outranks the web (see the source-of-truth skill).

A capability flag follows the same standard. "This may be beyond me" must name what was attempted and where it failed — not a vague appeal to difficulty.

2. Warning threshold

Across any run, minor concerns accumulate that aren't worth halting on individually. Keep a running count. At the threshold — default three, tunable if the user sets a different number — stop and surface all of them to the user at once before continuing.

Rationale: three small things pointing the same direction usually mean one real problem worth a decision. Below threshold, keep working; a drip of trivial caveats is noise. At threshold, batch them — one interruption with full context beats three fragmentary ones.

A concern that independently meets the verify-before-flag bar and is material on its own does not wait for the threshold. The threshold governs minor concerns only.

3. Find-and-replace safety

When editing files with sed (or any substring replace), always anchor on word boundaries to avoid corrupting compound words — a bare edge replace will mangle Ledger into garbage. Use \bword\b, not bare word.

After any sed pass on a file, grep for glued or malformed compound words before presenting the result. A replace that silently corrupts neighboring tokens is the most common self-inflicted error in file edits. The check is cheap; the silent corruption is not.

Preferred order of tools: targeted string-replace on a unique anchor > word-boundary sed > bare sed (never). If the string to replace isn't unique in the file, widen the anchor until it is — do not replace-all and hope.

Relationship to fable-mode

fable-mode's staged loop is optional and gated on task size. These guardrails are not optional and not gated. When fable-mode runs, its step 4 (self-critique) and all file edits inherit these rules. When fable-mode doesn't run, these rules apply anyway. The per-model runners — fable-opus and fable-sonnet — inline these rules verbatim into every subagent briefing: cloud subagents could load this skill (2026-10-10), other surfaces are unchecked, and the inline copy is shorter than a skill load.