From 684f8fdee15e8800770570b9a33ce02ed2a18786 Mon Sep 17 00:00:00 2001
From: Vibe Nuage Agent <vibe@mistral.ai>
Date: Mon, 2 Nov 2026 13:48:41 +0000
Subject: [PATCH] feat(conformance): TC-L4-01..05 Layer 4 audit trail
 validators (RFC-0005)

Co-authored-by: emmanuel-a-otchere <emmanuel-a-otchere@users.noreply.github.com>
---
 src/contracts/layer4-integrity.ts    | 138 +++++++++++++++++++++
 src/fixtures/l4-conformant.json      | 171 +++++++++++++++++++++++++
 src/fixtures/l4-dangling-event.json  | 171 +++++++++++++++++++++++++
 src/fixtures/l4-legacy.json          | 157 +++++++++++++++++++++++
 src/fixtures/l4-non-append-only.json | 178 +++++++++++++++++++++++++++
 src/fixtures/l4-rerun-unlinked.json  | 171 +++++++++++++++++++++++++
 src/runner.ts                        |   8 +-
 tests/layer4.test.ts                 |  66 ++++++++++
 8 files changed, 1059 insertions(+), 1 deletion(-)
 create mode 100644 src/contracts/layer4-integrity.ts
 create mode 100644 src/fixtures/l4-conformant.json
 create mode 100644 src/fixtures/l4-dangling-event.json
 create mode 100644 src/fixtures/l4-legacy.json
 create mode 100644 src/fixtures/l4-non-append-only.json
 create mode 100644 src/fixtures/l4-rerun-unlinked.json
 create mode 100644 tests/layer4.test.ts

diff --git a/src/contracts/layer4-integrity.ts b/src/contracts/layer4-integrity.ts
new file mode 100644
index 0000000..1086489
--- /dev/null
+++ b/src/contracts/layer4-integrity.ts
@@ -0,0 +1,138 @@
+/**
+ * Layer 4 (Audit Trail) integrity validators. Implements TC-L4-01..05 per
+ * RFC-0005: the audit trail is lifecycle-complete and append-only, not
+ * merely present.
+ *
+ * See: https://github.com/Source-Engineering/source-engineering-specifications/blob/main/compatibility-test-suite.md
+ */
+
+import type { Cots, AuditEntry } from "../types.js";
+import { type Result, combine, fail, warn } from "../result.js";
+
+const URN_RE = /^[a-z][a-z0-9+\-.]*:/i;
+
+/** TC-L4-01: every decision's audit trail starts with "created" and every entry is complete. */
+export const validateAuditTrailComplete = (cots: Cots): Result => {
+  const errors: Result[] = [];
+
+  for (let i = 0; i < cots.decisions.length; i++) {
+    const history = cots.decisions[i].audit_history;
+    if (history.length === 0) {
+      errors.push(fail("TC-L4-01", `decisions[${i}].audit_history is empty (must start with "created")`));
+      continue;
+    }
+    if (history[0].action !== "created") {
+      errors.push(fail("TC-L4-01", `decisions[${i}].audit_history[0] is not "created" (got "${history[0].action}")`));
+    }
+    for (let j = 0; j < history.length; j++) {
+      const e: AuditEntry = history[j];
+      if (!e.id || !URN_RE.test(e.id)) {
+        errors.push(fail("TC-L4-01", `decisions[${i}].audit_history[${j}].id is missing or not a valid URI ('${e.id}')`));
+      }
+      if (!e.action) {
+        errors.push(fail("TC-L4-01", `decisions[${i}].audit_history[${j}].action is empty`));
+      }
+      if (!e.performed_by) {
+        errors.push(fail("TC-L4-01", `decisions[${i}].audit_history[${j}].performed_by is empty`));
+      }
+      if (Number.isNaN(Date.parse(e.performed_at))) {
+        errors.push(fail("TC-L4-01", `decisions[${i}].audit_history[${j}].performed_at is not a valid ISO-8601 timestamp ('${e.performed_at}')`));
+      }
+    }
+  }
+
+  return combine(...errors);
+};
+
+/** TC-L4-02: append-only verifiable: unique entry ids and monotonic time order. */
+export const validateAuditTrailAppendOnly = (cots: Cots): Result => {
+  const errors: Result[] = [];
+
+  for (let i = 0; i < cots.decisions.length; i++) {
+    const history = cots.decisions[i].audit_history;
+    const seen = new Set<string>();
+    for (let j = 0; j < history.length; j++) {
+      const e = history[j];
+      if (seen.has(e.id)) {
+        errors.push(fail("TC-L4-02", `decisions[${i}].audit_history contains duplicate entry id '${e.id}' (append-only violated)`));
+      }
+      seen.add(e.id);
+      if (j > 0) {
+        const prev = Date.parse(history[j - 1].performed_at);
+        const curr = Date.parse(e.performed_at);
+        if (!Number.isNaN(prev) && !Number.isNaN(curr) && curr < prev) {
+          errors.push(fail("TC-L4-02", `decisions[${i}].audit_history[${j}] is earlier than its predecessor (append-only violated: ${e.performed_at} < ${history[j - 1].performed_at})`));
+        }
+      }
+    }
+  }
+
+  return combine(...errors);
+};
+
+/** TC-L4-03: every non-"created" entry names a decision that exists in the COTS. */
+export const validateLifecycleEventsResolve = (cots: Cots): Result => {
+  const errors: Result[] = [];
+  const decisionIds = new Set(cots.decisions.map((d) => d.id));
+
+  for (let i = 0; i < cots.decisions.length; i++) {
+    for (let j = 0; j < cots.decisions[i].audit_history.length; j++) {
+      const e = cots.decisions[i].audit_history[j];
+      if (e.action === "created") continue;
+      const m = e.notes.match(/decision_id=(\S+)/);
+      if (!m) {
+        errors.push(fail("TC-L4-03", `decisions[${i}].audit_history[${j}] (${e.action}) does not name a decision_id; the lifecycle event dangles`));
+      } else if (!decisionIds.has(m[1])) {
+        errors.push(fail("TC-L4-03", `decisions[${i}].audit_history[${j}] references unknown decision '${m[1]}'`));
+      }
+    }
+  }
+
+  return combine(...errors);
+};
+
+/** TC-L4-04: rerun entries link to the original decision; superseded entries name the superseding decision. */
+export const validateLifecycleLinkage = (cots: Cots): Result => {
+  const errors: Result[] = [];
+  const decisionIds = new Set(cots.decisions.map((d) => d.id));
+
+  for (let i = 0; i < cots.decisions.length; i++) {
+    for (let j = 0; j < cots.decisions[i].audit_history.length; j++) {
+      const e = cots.decisions[i].audit_history[j];
+      if (e.action === "rerun") {
+        const m = e.notes.match(/rerun_of=(\S+)/);
+        if (!m) {
+          errors.push(fail("TC-L4-04", `decisions[${i}].audit_history[${j}] is a rerun without rerun_of linkage`));
+        } else if (!decisionIds.has(m[1])) {
+          errors.push(fail("TC-L4-04", `decisions[${i}].audit_history[${j}] rerun_of '${m[1]}' does not resolve to a decision in the COTS`));
+        }
+      }
+      if (e.action === "superseded") {
+        const m = e.notes.match(/superseded_by=(\S+)/);
+        if (!m) {
+          errors.push(fail("TC-L4-04", `decisions[${i}].audit_history[${j}] is superseded without superseded_by linkage`));
+        } else if (!decisionIds.has(m[1])) {
+          errors.push(fail("TC-L4-04", `decisions[${i}].audit_history[${j}] superseded_by '${m[1]}' does not resolve to a decision in the COTS`));
+        }
+      }
+    }
+  }
+
+  return combine(...errors);
+};
+
+/** TC-L4-05: a pre-RFC-0005 COTS with a created-only history passes with a legacy warning. */
+export const validateLegacyAuditTrail = (cots: Cots): Result => {
+  const warnings: Result[] = [];
+
+  for (let i = 0; i < cots.decisions.length; i++) {
+    const history = cots.decisions[i].audit_history;
+    if (history.length === 1 && history[0].action === "created") {
+      warnings.push(
+        warn("TC-L4-05", `decisions[${i}] has a created-only audit history (pre-RFC-0005 producer; no lifecycle events recorded)`)
+      );
+    }
+  }
+
+  return combine(...warnings);
+};
diff --git a/src/fixtures/l4-conformant.json b/src/fixtures/l4-conformant.json
new file mode 100644
index 0000000..bc02869
--- /dev/null
+++ b/src/fixtures/l4-conformant.json
@@ -0,0 +1,171 @@
+{
+  "cots_version": "0.1.0",
+  "cots_id": "urn:source-engineering:cots:00000000-0000-0000-0000-000000000001",
+  "produced_at": "2026-10-11T12:00:00Z",
+  "produced_by": {
+    "system_name": "test-fixture",
+    "system_version": "0.1.0"
+  },
+  "inputs": {
+    "source_artifacts": [
+      {
+        "id": "urn:source-engineering:source:contract-2026-001",
+        "origin": {
+          "system_name": "Acme Contracts",
+          "system_version": "1.0.0",
+          "configuration_hash": "0000000000000000000000000000000000000000000000000000000000000001"
+        },
+        "version": {
+          "content_hash": "1111111111111111111111111111111111111111111111111111111111111111",
+          "issued_at": "2026-10-10T08:00:00Z"
+        },
+        "authority": {
+          "asserted_by": "acme-contracts",
+          "trust_score": 0.95
+        },
+        "temporal_validity": {
+          "valid_from": "2026-10-10T08:00:00Z",
+          "valid_until": null
+        },
+        "content_reference": {
+          "uri": "https://example.com/contracts/2026-001.pdf",
+          "media_type": "application/pdf"
+        }
+      }
+    ]
+  },
+  "context": {
+    "model": {
+      "model_name": "claude-sonnet-4-5",
+      "model_version": "2026-09-15"
+    },
+    "parameters": {
+      "temperature": 0.3,
+      "top_p": 0.95,
+      "prompt_cache_id": null,
+      "tool_budget": 8,
+      "retrieval_depth": 5
+    },
+    "constraints": [
+      {
+        "id": "urn:source-engineering:constraint:jurisdiction-001",
+        "type": "jurisdiction",
+        "scope": "all_decisions",
+        "value": "US-NY"
+      }
+    ]
+  },
+  "decisions": [
+    {
+      "id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+      "decision_text": "The contract is a standard commercial services agreement with mutual indemnification, a 30-day termination notice, and a New York governing-law clause.",
+      "basis_id": "urn:source-engineering:basis:00000000-0000-0000-0000-000000000001",
+      "evidence_chain_ref": "urn:source-engineering:evidence-chain:00000000-0000-0000-0000-000000000001",
+      "validation_status": {
+        "status": "pending",
+        "last_changed_at": "2026-10-11T12:00:00Z",
+        "last_changed_by": "test-fixture"
+      },
+      "responsible_authority": {
+        "authority_id": "claude-sonnet-4-5",
+        "authority_type": "automated"
+      },
+      "audit_history": [
+        {
+          "id": "urn:source-engineering:audit:00000000-0000-0000-0000-000000000001",
+          "action": "created",
+          "performed_by": "test-fixture",
+          "performed_at": "2026-10-11T12:00:00Z",
+          "notes": "Initial creation."
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-002",
+          "action": "appealed",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T09:00:00Z",
+          "notes": "User appealed. decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-003",
+          "action": "rerun",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T10:00:00Z",
+          "notes": "Re-run on model v2. rerun_of=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001 decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        }
+      ],
+      "created_at": "2026-10-11T12:00:00Z"
+    }
+  ],
+  "evidence": {
+    "evidence_chain": [
+      {
+        "decision_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "supporting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "extract_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-7"
+              ],
+              "output": "Section 7.1: Each party shall indemnify and hold harmless...",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.92,
+              "basis": "Direct extraction from the section 7.1 text."
+            },
+            "polarity": "supports"
+          }
+        ],
+        "contradicting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "search_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-12"
+              ],
+              "output": "Section 12.4: This agreement shall be governed by the laws of the State of Delaware.",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.78,
+              "basis": "Contradictory governing-law clause in section 12.4 (the model rejected it because section 14.1 is the prevailing clause)."
+            },
+            "polarity": "contradicts"
+          }
+        ],
+        "claims": [
+          {
+            "id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "decision_record_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+            "claim_text": "The contract contains a mutual indemnification clause in section 7.1.",
+            "claim_type": "factual",
+            "supporting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001"
+            ],
+            "contradicting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002"
+            ]
+          }
+        ]
+      }
+    ]
+  },
+  "validation": {
+    "feedback_surfaces": [
+      {
+        "id": "urn:source-engineering:surface:00000000-0000-0000-0000-000000000001",
+        "type": "appeal",
+        "target": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "endpoint": "https://example.com/appeal/00000000-0000-0000-0000-000000000001",
+        "description": "File an appeal against this decision."
+      }
+    ]
+  }
+}
\ No newline at end of file
diff --git a/src/fixtures/l4-dangling-event.json b/src/fixtures/l4-dangling-event.json
new file mode 100644
index 0000000..329af9f
--- /dev/null
+++ b/src/fixtures/l4-dangling-event.json
@@ -0,0 +1,171 @@
+{
+  "cots_version": "0.1.0",
+  "cots_id": "urn:source-engineering:cots:00000000-0000-0000-0000-000000000001",
+  "produced_at": "2026-10-11T12:00:00Z",
+  "produced_by": {
+    "system_name": "test-fixture",
+    "system_version": "0.1.0"
+  },
+  "inputs": {
+    "source_artifacts": [
+      {
+        "id": "urn:source-engineering:source:contract-2026-001",
+        "origin": {
+          "system_name": "Acme Contracts",
+          "system_version": "1.0.0",
+          "configuration_hash": "0000000000000000000000000000000000000000000000000000000000000001"
+        },
+        "version": {
+          "content_hash": "1111111111111111111111111111111111111111111111111111111111111111",
+          "issued_at": "2026-10-10T08:00:00Z"
+        },
+        "authority": {
+          "asserted_by": "acme-contracts",
+          "trust_score": 0.95
+        },
+        "temporal_validity": {
+          "valid_from": "2026-10-10T08:00:00Z",
+          "valid_until": null
+        },
+        "content_reference": {
+          "uri": "https://example.com/contracts/2026-001.pdf",
+          "media_type": "application/pdf"
+        }
+      }
+    ]
+  },
+  "context": {
+    "model": {
+      "model_name": "claude-sonnet-4-5",
+      "model_version": "2026-09-15"
+    },
+    "parameters": {
+      "temperature": 0.3,
+      "top_p": 0.95,
+      "prompt_cache_id": null,
+      "tool_budget": 8,
+      "retrieval_depth": 5
+    },
+    "constraints": [
+      {
+        "id": "urn:source-engineering:constraint:jurisdiction-001",
+        "type": "jurisdiction",
+        "scope": "all_decisions",
+        "value": "US-NY"
+      }
+    ]
+  },
+  "decisions": [
+    {
+      "id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+      "decision_text": "The contract is a standard commercial services agreement with mutual indemnification, a 30-day termination notice, and a New York governing-law clause.",
+      "basis_id": "urn:source-engineering:basis:00000000-0000-0000-0000-000000000001",
+      "evidence_chain_ref": "urn:source-engineering:evidence-chain:00000000-0000-0000-0000-000000000001",
+      "validation_status": {
+        "status": "pending",
+        "last_changed_at": "2026-10-11T12:00:00Z",
+        "last_changed_by": "test-fixture"
+      },
+      "responsible_authority": {
+        "authority_id": "claude-sonnet-4-5",
+        "authority_type": "automated"
+      },
+      "audit_history": [
+        {
+          "id": "urn:source-engineering:audit:00000000-0000-0000-0000-000000000001",
+          "action": "created",
+          "performed_by": "test-fixture",
+          "performed_at": "2026-10-11T12:00:00Z",
+          "notes": "Initial creation."
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-002",
+          "action": "appealed",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T09:00:00Z",
+          "notes": "User appealed. decision_id=urn:source-engineering:decision:nope"
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-003",
+          "action": "rerun",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T10:00:00Z",
+          "notes": "Re-run on model v2. rerun_of=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001 decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        }
+      ],
+      "created_at": "2026-10-11T12:00:00Z"
+    }
+  ],
+  "evidence": {
+    "evidence_chain": [
+      {
+        "decision_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "supporting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "extract_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-7"
+              ],
+              "output": "Section 7.1: Each party shall indemnify and hold harmless...",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.92,
+              "basis": "Direct extraction from the section 7.1 text."
+            },
+            "polarity": "supports"
+          }
+        ],
+        "contradicting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "search_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-12"
+              ],
+              "output": "Section 12.4: This agreement shall be governed by the laws of the State of Delaware.",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.78,
+              "basis": "Contradictory governing-law clause in section 12.4 (the model rejected it because section 14.1 is the prevailing clause)."
+            },
+            "polarity": "contradicts"
+          }
+        ],
+        "claims": [
+          {
+            "id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "decision_record_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+            "claim_text": "The contract contains a mutual indemnification clause in section 7.1.",
+            "claim_type": "factual",
+            "supporting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001"
+            ],
+            "contradicting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002"
+            ]
+          }
+        ]
+      }
+    ]
+  },
+  "validation": {
+    "feedback_surfaces": [
+      {
+        "id": "urn:source-engineering:surface:00000000-0000-0000-0000-000000000001",
+        "type": "appeal",
+        "target": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "endpoint": "https://example.com/appeal/00000000-0000-0000-0000-000000000001",
+        "description": "File an appeal against this decision."
+      }
+    ]
+  }
+}
\ No newline at end of file
diff --git a/src/fixtures/l4-legacy.json b/src/fixtures/l4-legacy.json
new file mode 100644
index 0000000..6c1e28a
--- /dev/null
+++ b/src/fixtures/l4-legacy.json
@@ -0,0 +1,157 @@
+{
+  "cots_version": "0.1.0",
+  "cots_id": "urn:source-engineering:cots:00000000-0000-0000-0000-000000000001",
+  "produced_at": "2026-10-11T12:00:00Z",
+  "produced_by": {
+    "system_name": "test-fixture",
+    "system_version": "0.1.0"
+  },
+  "inputs": {
+    "source_artifacts": [
+      {
+        "id": "urn:source-engineering:source:contract-2026-001",
+        "origin": {
+          "system_name": "Acme Contracts",
+          "system_version": "1.0.0",
+          "configuration_hash": "0000000000000000000000000000000000000000000000000000000000000001"
+        },
+        "version": {
+          "content_hash": "1111111111111111111111111111111111111111111111111111111111111111",
+          "issued_at": "2026-10-10T08:00:00Z"
+        },
+        "authority": {
+          "asserted_by": "acme-contracts",
+          "trust_score": 0.95
+        },
+        "temporal_validity": {
+          "valid_from": "2026-10-10T08:00:00Z",
+          "valid_until": null
+        },
+        "content_reference": {
+          "uri": "https://example.com/contracts/2026-001.pdf",
+          "media_type": "application/pdf"
+        }
+      }
+    ]
+  },
+  "context": {
+    "model": {
+      "model_name": "claude-sonnet-4-5",
+      "model_version": "2026-09-15"
+    },
+    "parameters": {
+      "temperature": 0.3,
+      "top_p": 0.95,
+      "prompt_cache_id": null,
+      "tool_budget": 8,
+      "retrieval_depth": 5
+    },
+    "constraints": [
+      {
+        "id": "urn:source-engineering:constraint:jurisdiction-001",
+        "type": "jurisdiction",
+        "scope": "all_decisions",
+        "value": "US-NY"
+      }
+    ]
+  },
+  "decisions": [
+    {
+      "id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+      "decision_text": "The contract is a standard commercial services agreement with mutual indemnification, a 30-day termination notice, and a New York governing-law clause.",
+      "basis_id": "urn:source-engineering:basis:00000000-0000-0000-0000-000000000001",
+      "evidence_chain_ref": "urn:source-engineering:evidence-chain:00000000-0000-0000-0000-000000000001",
+      "validation_status": {
+        "status": "pending",
+        "last_changed_at": "2026-10-11T12:00:00Z",
+        "last_changed_by": "test-fixture"
+      },
+      "responsible_authority": {
+        "authority_id": "claude-sonnet-4-5",
+        "authority_type": "automated"
+      },
+      "audit_history": [
+        {
+          "id": "urn:source-engineering:audit:00000000-0000-0000-0000-000000000001",
+          "action": "created",
+          "performed_by": "test-fixture",
+          "performed_at": "2026-10-11T12:00:00Z",
+          "notes": "Initial creation."
+        }
+      ],
+      "created_at": "2026-10-11T12:00:00Z"
+    }
+  ],
+  "evidence": {
+    "evidence_chain": [
+      {
+        "decision_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "supporting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "extract_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-7"
+              ],
+              "output": "Section 7.1: Each party shall indemnify and hold harmless...",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.92,
+              "basis": "Direct extraction from the section 7.1 text."
+            },
+            "polarity": "supports"
+          }
+        ],
+        "contradicting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "search_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-12"
+              ],
+              "output": "Section 12.4: This agreement shall be governed by the laws of the State of Delaware.",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.78,
+              "basis": "Contradictory governing-law clause in section 12.4 (the model rejected it because section 14.1 is the prevailing clause)."
+            },
+            "polarity": "contradicts"
+          }
+        ],
+        "claims": [
+          {
+            "id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "decision_record_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+            "claim_text": "The contract contains a mutual indemnification clause in section 7.1.",
+            "claim_type": "factual",
+            "supporting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001"
+            ],
+            "contradicting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002"
+            ]
+          }
+        ]
+      }
+    ]
+  },
+  "validation": {
+    "feedback_surfaces": [
+      {
+        "id": "urn:source-engineering:surface:00000000-0000-0000-0000-000000000001",
+        "type": "appeal",
+        "target": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "endpoint": "https://example.com/appeal/00000000-0000-0000-0000-000000000001",
+        "description": "File an appeal against this decision."
+      }
+    ]
+  }
+}
\ No newline at end of file
diff --git a/src/fixtures/l4-non-append-only.json b/src/fixtures/l4-non-append-only.json
new file mode 100644
index 0000000..0d2dbd8
--- /dev/null
+++ b/src/fixtures/l4-non-append-only.json
@@ -0,0 +1,178 @@
+{
+  "cots_version": "0.1.0",
+  "cots_id": "urn:source-engineering:cots:00000000-0000-0000-0000-000000000001",
+  "produced_at": "2026-10-11T12:00:00Z",
+  "produced_by": {
+    "system_name": "test-fixture",
+    "system_version": "0.1.0"
+  },
+  "inputs": {
+    "source_artifacts": [
+      {
+        "id": "urn:source-engineering:source:contract-2026-001",
+        "origin": {
+          "system_name": "Acme Contracts",
+          "system_version": "1.0.0",
+          "configuration_hash": "0000000000000000000000000000000000000000000000000000000000000001"
+        },
+        "version": {
+          "content_hash": "1111111111111111111111111111111111111111111111111111111111111111",
+          "issued_at": "2026-10-10T08:00:00Z"
+        },
+        "authority": {
+          "asserted_by": "acme-contracts",
+          "trust_score": 0.95
+        },
+        "temporal_validity": {
+          "valid_from": "2026-10-10T08:00:00Z",
+          "valid_until": null
+        },
+        "content_reference": {
+          "uri": "https://example.com/contracts/2026-001.pdf",
+          "media_type": "application/pdf"
+        }
+      }
+    ]
+  },
+  "context": {
+    "model": {
+      "model_name": "claude-sonnet-4-5",
+      "model_version": "2026-09-15"
+    },
+    "parameters": {
+      "temperature": 0.3,
+      "top_p": 0.95,
+      "prompt_cache_id": null,
+      "tool_budget": 8,
+      "retrieval_depth": 5
+    },
+    "constraints": [
+      {
+        "id": "urn:source-engineering:constraint:jurisdiction-001",
+        "type": "jurisdiction",
+        "scope": "all_decisions",
+        "value": "US-NY"
+      }
+    ]
+  },
+  "decisions": [
+    {
+      "id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+      "decision_text": "The contract is a standard commercial services agreement with mutual indemnification, a 30-day termination notice, and a New York governing-law clause.",
+      "basis_id": "urn:source-engineering:basis:00000000-0000-0000-0000-000000000001",
+      "evidence_chain_ref": "urn:source-engineering:evidence-chain:00000000-0000-0000-0000-000000000001",
+      "validation_status": {
+        "status": "pending",
+        "last_changed_at": "2026-10-11T12:00:00Z",
+        "last_changed_by": "test-fixture"
+      },
+      "responsible_authority": {
+        "authority_id": "claude-sonnet-4-5",
+        "authority_type": "automated"
+      },
+      "audit_history": [
+        {
+          "id": "urn:source-engineering:audit:00000000-0000-0000-0000-000000000001",
+          "action": "created",
+          "performed_by": "test-fixture",
+          "performed_at": "2026-10-11T12:00:00Z",
+          "notes": "Initial creation."
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-002",
+          "action": "appealed",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T09:00:00Z",
+          "notes": "User appealed. decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-003",
+          "action": "rerun",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T10:00:00Z",
+          "notes": "Re-run on model v2. rerun_of=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001 decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-002",
+          "action": "appealed",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T09:00:00Z",
+          "notes": "User appealed. decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        }
+      ],
+      "created_at": "2026-10-11T12:00:00Z"
+    }
+  ],
+  "evidence": {
+    "evidence_chain": [
+      {
+        "decision_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "supporting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "extract_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-7"
+              ],
+              "output": "Section 7.1: Each party shall indemnify and hold harmless...",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.92,
+              "basis": "Direct extraction from the section 7.1 text."
+            },
+            "polarity": "supports"
+          }
+        ],
+        "contradicting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "search_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-12"
+              ],
+              "output": "Section 12.4: This agreement shall be governed by the laws of the State of Delaware.",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.78,
+              "basis": "Contradictory governing-law clause in section 12.4 (the model rejected it because section 14.1 is the prevailing clause)."
+            },
+            "polarity": "contradicts"
+          }
+        ],
+        "claims": [
+          {
+            "id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "decision_record_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+            "claim_text": "The contract contains a mutual indemnification clause in section 7.1.",
+            "claim_type": "factual",
+            "supporting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001"
+            ],
+            "contradicting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002"
+            ]
+          }
+        ]
+      }
+    ]
+  },
+  "validation": {
+    "feedback_surfaces": [
+      {
+        "id": "urn:source-engineering:surface:00000000-0000-0000-0000-000000000001",
+        "type": "appeal",
+        "target": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "endpoint": "https://example.com/appeal/00000000-0000-0000-0000-000000000001",
+        "description": "File an appeal against this decision."
+      }
+    ]
+  }
+}
\ No newline at end of file
diff --git a/src/fixtures/l4-rerun-unlinked.json b/src/fixtures/l4-rerun-unlinked.json
new file mode 100644
index 0000000..8728177
--- /dev/null
+++ b/src/fixtures/l4-rerun-unlinked.json
@@ -0,0 +1,171 @@
+{
+  "cots_version": "0.1.0",
+  "cots_id": "urn:source-engineering:cots:00000000-0000-0000-0000-000000000001",
+  "produced_at": "2026-10-11T12:00:00Z",
+  "produced_by": {
+    "system_name": "test-fixture",
+    "system_version": "0.1.0"
+  },
+  "inputs": {
+    "source_artifacts": [
+      {
+        "id": "urn:source-engineering:source:contract-2026-001",
+        "origin": {
+          "system_name": "Acme Contracts",
+          "system_version": "1.0.0",
+          "configuration_hash": "0000000000000000000000000000000000000000000000000000000000000001"
+        },
+        "version": {
+          "content_hash": "1111111111111111111111111111111111111111111111111111111111111111",
+          "issued_at": "2026-10-10T08:00:00Z"
+        },
+        "authority": {
+          "asserted_by": "acme-contracts",
+          "trust_score": 0.95
+        },
+        "temporal_validity": {
+          "valid_from": "2026-10-10T08:00:00Z",
+          "valid_until": null
+        },
+        "content_reference": {
+          "uri": "https://example.com/contracts/2026-001.pdf",
+          "media_type": "application/pdf"
+        }
+      }
+    ]
+  },
+  "context": {
+    "model": {
+      "model_name": "claude-sonnet-4-5",
+      "model_version": "2026-09-15"
+    },
+    "parameters": {
+      "temperature": 0.3,
+      "top_p": 0.95,
+      "prompt_cache_id": null,
+      "tool_budget": 8,
+      "retrieval_depth": 5
+    },
+    "constraints": [
+      {
+        "id": "urn:source-engineering:constraint:jurisdiction-001",
+        "type": "jurisdiction",
+        "scope": "all_decisions",
+        "value": "US-NY"
+      }
+    ]
+  },
+  "decisions": [
+    {
+      "id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+      "decision_text": "The contract is a standard commercial services agreement with mutual indemnification, a 30-day termination notice, and a New York governing-law clause.",
+      "basis_id": "urn:source-engineering:basis:00000000-0000-0000-0000-000000000001",
+      "evidence_chain_ref": "urn:source-engineering:evidence-chain:00000000-0000-0000-0000-000000000001",
+      "validation_status": {
+        "status": "pending",
+        "last_changed_at": "2026-10-11T12:00:00Z",
+        "last_changed_by": "test-fixture"
+      },
+      "responsible_authority": {
+        "authority_id": "claude-sonnet-4-5",
+        "authority_type": "automated"
+      },
+      "audit_history": [
+        {
+          "id": "urn:source-engineering:audit:00000000-0000-0000-0000-000000000001",
+          "action": "created",
+          "performed_by": "test-fixture",
+          "performed_at": "2026-10-11T12:00:00Z",
+          "notes": "Initial creation."
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-002",
+          "action": "appealed",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T09:00:00Z",
+          "notes": "User appealed. decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        },
+        {
+          "id": "urn:source-engineering:audit:l4-003",
+          "action": "rerun",
+          "performed_by": "reviewer-1",
+          "performed_at": "2026-10-12T10:00:00Z",
+          "notes": "Re-run. decision_id=urn:source-engineering:decision:00000000-0000-0000-0000-000000000001"
+        }
+      ],
+      "created_at": "2026-10-11T12:00:00Z"
+    }
+  ],
+  "evidence": {
+    "evidence_chain": [
+      {
+        "decision_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "supporting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "extract_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-7"
+              ],
+              "output": "Section 7.1: Each party shall indemnify and hold harmless...",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.92,
+              "basis": "Direct extraction from the section 7.1 text."
+            },
+            "polarity": "supports"
+          }
+        ],
+        "contradicting_evidence": [
+          {
+            "id": "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002",
+            "claim_id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "source_id": "urn:source-engineering:source:contract-2026-001",
+            "transformation": {
+              "operation": "search_clause",
+              "inputs": [
+                "https://example.com/contracts/2026-001.pdf#section-12"
+              ],
+              "output": "Section 12.4: This agreement shall be governed by the laws of the State of Delaware.",
+              "recorded_at": "2026-10-11T12:00:00Z"
+            },
+            "uncertainty": {
+              "calibrated_score": 0.78,
+              "basis": "Contradictory governing-law clause in section 12.4 (the model rejected it because section 14.1 is the prevailing clause)."
+            },
+            "polarity": "contradicts"
+          }
+        ],
+        "claims": [
+          {
+            "id": "urn:source-engineering:claim:00000000-0000-0000-0000-000000000001",
+            "decision_record_id": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+            "claim_text": "The contract contains a mutual indemnification clause in section 7.1.",
+            "claim_type": "factual",
+            "supporting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000001"
+            ],
+            "contradicting_evidence": [
+              "urn:source-engineering:evidence:00000000-0000-0000-0000-000000000002"
+            ]
+          }
+        ]
+      }
+    ]
+  },
+  "validation": {
+    "feedback_surfaces": [
+      {
+        "id": "urn:source-engineering:surface:00000000-0000-0000-0000-000000000001",
+        "type": "appeal",
+        "target": "urn:source-engineering:decision:00000000-0000-0000-0000-000000000001",
+        "endpoint": "https://example.com/appeal/00000000-0000-0000-0000-000000000001",
+        "description": "File an appeal against this decision."
+      }
+    ]
+  }
+}
\ No newline at end of file
diff --git a/src/runner.ts b/src/runner.ts
index 6666216..fabb1e9 100644
--- a/src/runner.ts
+++ b/src/runner.ts
@@ -20,6 +20,7 @@ import { validateFiveParts, validateFeedbackSurfaces } from "./contracts/five-pa
 import { validateLayer2Records } from "./contracts/layer2-integrity.js";
 import { validateUrnGrammar, validateChainResolution, validateTemporalOrdering, validateRecursiveChains } from "./contracts/semantic-integrity.js";
 import { validateGuardrailDecisions } from "./contracts/layer3-integrity.js";
+import { validateAuditTrailComplete, validateAuditTrailAppendOnly, validateLifecycleEventsResolve, validateLifecycleLinkage, validateLegacyAuditTrail } from "./contracts/layer4-integrity.js";
 
 export const run = (cots: Cots): Result => {
   return combine(
@@ -41,7 +42,12 @@ export const run = (cots: Cots): Result => {
     validateChainResolution(cots),
     validateTemporalOrdering(cots),
     validateRecursiveChains(cots),
-    validateGuardrailDecisions(cots)
+    validateGuardrailDecisions(cots),
+    validateAuditTrailComplete(cots),
+    validateAuditTrailAppendOnly(cots),
+    validateLifecycleEventsResolve(cots),
+    validateLifecycleLinkage(cots),
+    validateLegacyAuditTrail(cots)
   );
 };
 
diff --git a/tests/layer4.test.ts b/tests/layer4.test.ts
new file mode 100644
index 0000000..d4a4bb9
--- /dev/null
+++ b/tests/layer4.test.ts
@@ -0,0 +1,66 @@
+/**
+ * TC-L4 test cases (RFC-0005, Layer 4 audit trail floor).
+ *
+ * See: https://github.com/Source-Engineering/source-engineering-specifications/blob/main/compatibility-test-suite.md
+ */
+
+import { describe, it, expect } from "vitest";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+import { run } from "../src/runner.js";
+import type { Cots } from "../src/types.js";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const FIXTURES = join(__dirname, "..", "src", "fixtures");
+
+const load = (name: string): Cots => {
+  const p = resolve(FIXTURES, name);
+  return JSON.parse(readFileSync(p, "utf-8"));
+};
+
+describe("TC-L4: Layer 4 audit trail (RFC-0005)", () => {
+  it("TC-L4-01: passes for a complete audit trail", () => {
+    const result = run(load("l4-conformant.json"));
+    const errors = result.errors.filter((e) => e.test_id === "TC-L4-01");
+    expect(errors).toHaveLength(0);
+  });
+
+  it("TC-L4-02: passes for unique ids in monotonic order; fails on duplicates", () => {
+    const ok = run(load("l4-conformant.json"));
+    expect(ok.errors.filter((e) => e.test_id === "TC-L4-02")).toHaveLength(0);
+    const bad = run(load("l4-non-append-only.json"));
+    const errors = bad.errors.filter((e) => e.test_id === "TC-L4-02");
+    expect(errors.length).toBeGreaterThan(0);
+    expect(errors[0].message).toContain("append-only");
+  });
+
+  it("TC-L4-03: fails when a lifecycle event names an unknown decision", () => {
+    const result = run(load("l4-dangling-event.json"));
+    const errors = result.errors.filter((e) => e.test_id === "TC-L4-03");
+    expect(errors.length).toBeGreaterThan(0);
+    expect(errors[0].message).toContain("urn:source-engineering:decision:nope");
+  });
+
+  it("TC-L4-04: fails when a rerun lacks rerun_of linkage to a known decision", () => {
+    const result = run(load("l4-rerun-unlinked.json"));
+    const errors = result.errors.filter((e) => e.test_id === "TC-L4-04");
+    expect(errors.length).toBeGreaterThan(0);
+    expect(errors[0].message).toContain("rerun_of");
+  });
+
+  it("TC-L4-05: a created-only history passes with a legacy warning", () => {
+    const result = run(load("l4-legacy.json"));
+    expect(result.status).toBe("pass");
+    const warnings = result.warnings.filter((w) => w.test_id === "TC-L4-05");
+    expect(warnings.length).toBeGreaterThan(0);
+    expect(warnings[0].message).toContain("pre-RFC-0005");
+  });
+
+  it("the conformant fixture has no TC-L4 errors end-to-end", () => {
+    const result = run(load("l4-conformant.json"));
+    const errors = result.errors.filter((e) => e.test_id.startsWith("TC-L4"));
+    expect(errors).toHaveLength(0);
+  });
+});