RFC-ATF-13: Agent Trust Fabric -- Agent Capability Emergence Engine (ACEE). Closing the Multi-Agent Capability Gap with Capability Cluster Detection, Necessity Proof Protocol, and PQC-Signed Capability Witness Records

RFC-ATF-13 specifies the Agent Capability Emergence Engine (ACEE) -- the thirteenth RFC in the ATF Open Standard series published by OMNIX QUANTUM LTD. RFC-ATF-13 closes the Multi-Agent Capability Gap (MAC-Gap): the structural blind spot where two tool calls -- each individually ALLOWED under TCGL -- combine to constitute a capability that is CATASTROPHIC in the composed multi-agent context. Prior ATF layers evaluate tool calls in isolation; ACEE evaluates the composition surface. Five core components: CCG (Capability Cluster Graph): A directed graph where nodes are individual tool calls and weighted edges represent semantic composition proximity. CCG operates pre-execution across all agents in the governance session, identifying clusters whose combined path score exceeds severity thresholds. ACEE-INV-001: CCG scan is mandatory before any multi-agent tool call sequence. NPP (Necessity Proof Protocol): For any cluster with severity CONCERNING or above, NPP requires a structured necessity argument demonstrating that the capability composition is required by the declared mandate. pathₛcore in 0. 0, 1. 0 is a deterministic pure function of CCG edge weights. CWR (Capability Witness Record): A PQC-signed (ML-DSA-65) governance artifact issued per cluster, containing: clusterᵢd, member tool calls, pathₛcore, severity classification, necessity proof, and PoGR anchor. Offline-verifiable. ACEE-INV-003: CWR is pre-execution and immutable. CCM (Cross-Session Capability Memory): Append-only registry of capability cluster patterns across sessions. ACEE-INV-008: CCM entries are append-only. Enables TEG computation. TEG (Temporal Emergence Gradient): Measures the rate of capability emergence across sessions, detecting capability drift before it becomes CATASTROPHIC. Five severity classes: BENIGN (pathₛcore=0) -- SUSPICIOUS (0. 0-0. 3) -- CONCERNING (0. 3-0. 6) -- CRITICAL (0. 6-0. 9, TCGL DEFERREDHAR) -- CATASTROPHIC (>=0. 9, SYSTEM HALT). ACEE-INV-005: CRITICAL and CATASTROPHIC clusters MUST route through the TCGL gate before any tool in the cluster executes. 12 Emergence Classification Table (ECT) entries define the normative mapping from (toolcategoryₚair, compositionₜype) to severity class. ECT is immutable once published in the RFC. 12 new invariants are introduced: ACEE-INV-001 through ACEE-INV-012. Combined with the 272 invariants of RFC-ATF-1 through RFC-ATF-12, the ATF stack reaches 284 formally specified invariants across 36 protocol families. An implementation complying with RFC-ATF-1 through RFC-ATF-13 is designated ATF-ACEE-Compliant -- the highest compliance tier in the ATF stack as of RFC-ATF-13. Persistence schema: 3 new tables -- atfcwrᵣecords, atfcapabilityclusters, atfccmₚatterns. All append-only. Regulatory alignment: EU AI Act Art. 9 (multi-agent risk documentation), Art. 10 (training data governance for capability detection), Art. 13 (transparency of composed capabilities), Art. 72 (market surveillance via CWR public verification) ; NIST AI RMF GV-1. 1 (multi-agent governance scope) ; ISO/IEC 42001 Clause 8. 4 (AI system design and composition risk). Related ADR: ADR-218. Test suite: 93/93 PASS. Intellectual origin of CCG graph-theoretic formulation: OMNIX QUANTUM LTD R&D team, June 2026.