Classification: layer=E (verification) + C (meta); mechanism=scaffold F.

Intent: Engineer linter/checker output as agent input — stable, structured, citing the violated requirement and the fix surface — because error text is the highest-leverage prompt in the loop.

Also Known As: actionable diagnostics; SARIF output; fix-it hints; structured errors; machine-readable lint; ESLint `messageId`.

Applicability / Recognition: Apply when — a custom `@typescript-eslint` rule (or a thrown domain error) emits free text; an error states what failed but not which REQ or where to fix; tool output is unstable across runs. *Detector seed:* a custom eslint/check message or a domain error without a `spec://` REQ URI and a fix-surface hint → recognition fires (tool output is the agent's percept, R3-011). Note: `tsc`'s own diagnostics (TS2322 etc.) are already coded — wrap them with REQ context, do not replace them.

Motivation: A weak agent sees `Type 'string' is not assignable to type 'never'` from a `satisfies never` exhaustiveness check — true but opaque. With the custom rule emitting `violates REQ spec://errors/r6: error union not exhaustively handled; fix surface: add a case for variant 'Timeout' before assertNever`, the agent acts directly. The strong author's "what to do when this fails" is materialized in the message.

Structure & Participants: *Diagnostic* (an eslint report with a stable `messageId`) · *REQ citation* (`spec://` URI) · *Fix-surface hint* (where/what) · *Stable format* (SARIF / fixed grammar).

Collaborations: Carries failures from Classes C/D/E and the §8 bans; feeds the agent loop's next prompt; in raids, structured diagnostics let the orchestrator triage misfires.

Goals / Non-Goals: *Goals:* every custom check is agent-actionable. *Non-Goals:* NOT rewriting `tsc`'s own diagnostics (already good — wrap them); does NOT replace the contract that defines correctness.

Consequences: (+) iterations-to-green drop, more for weaker models; (+) diagnostics double as a navigable requirement map. (−) message authoring cost; (−) verbosity vs token budget — keep a compact grammar.

Alternatives: free-text errors (wasted conditioning); silent failure (worst). Neither acceptable for an agent loop.

Risks & Assumptions: assumes a stable REQ namespace exists (it does — specmap, guide §9). *Sunset:* none material.

Evidence & Transfer-strength: R3-011 (tool output is highest-leverage prompt, theory), R2C-004 (agent conditions on tool text, benchmark). Class: benchmark + theory. Tag: **[E-mid]**.