RunbookFX: Type- and Effect-Safe LLM Synthesis for Executable Incident Diagnosis and Mitigation

Large language models are increasingly deployed as autonomous agents for cloud incident response, yet their direct use admits hallucinated diagnoses, unauthorized actions, irreversible changes, and unauditable decision trails. We present RunbookFX, a typed functional domain-specific language that elevates incident response from natural-language suggestions to executable programs whose safety is established statically. The key insight is that incident-response safety decomposes into three interacting dimensions: risk severity, exercised capabilities, and rollback resource availability. RunbookFX formalizes this decomposition as a product effect algebra Risk × K × ℕ whose four cross-component interaction axioms yield domain-specific safety theorems unexpressible in flat effect frameworks; a strong handler parametricity result then transfers these guarantees from a replay handler to any bisimilar live handler, bridging offline verification and production deployment. An LLM proposes candidate programs that a CEGIS-style verifier filters by static type checking and dynamic contract replay. A ∼2,200-line Coq development discharges the product effect algebra, its composition-preservation property, and four core safety theorems: Effect WF Preservation, Progress, single-step No Unauthorized Action, and Rollback Linearity. Of the 27 supporting obligations in the substitution and multi-step layers, 18 now close with Qed—including all Canonical Forms, all effect-operation Inversion lemmas, Value Typing, de Bruijn weakening, and the typing-respecting reduction cases for observe, act, rollback, and the affirmative guard; the remaining nine trace back to the de Bruijn substitution lemma, whose proof skeleton follows Pierce et al. [2019]. Evaluated on RCAEval for root cause analysis and ITBench for end-to-end mitigation, RunbookFX achieves 64% Top-1 RCA accuracy against 53% for the best LLM baseline and 38% mitigation success at 3.3× the official ITBench agent, with zero safety violations and 100% rollback coverage by construction.