diff --git a/schemas/aep/_base.schema.json b/schemas/aep/_base.schema.json
new file mode 100644
index 0000000..dd6c0e6
--- /dev/null
+++ b/schemas/aep/_base.schema.json
@@ -0,0 +1,56 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "urn:wasmagent:aep:base",
+  "title": "AEPEnvelopeBase",
+  "description": "Shared base properties for all AEP evidence envelope schemas. Not registered in the schema index — intended to be composed via $ref / allOf.",
+  "type": "object",
+  "required": [
+    "schema_version",
+    "created_at_ms"
+  ],
+  "properties": {
+    "schema_version": {
+      "type": "string",
+      "description": "Semver-like schema identifier, e.g. aep/v0.1"
+    },
+    "trace_id": {
+      "type": "string",
+      "description": "Optional correlation ID linking related evidence records"
+    },
+    "created_at_ms": {
+      "type": "number",
+      "description": "UTC epoch milliseconds when the record was created (envelope timestamp)"
+    },
+    "metadata": {
+      "type": "object",
+      "description": "Optional key-value metadata about the evidence record",
+      "additionalProperties": {
+        "type": ["string", "number", "boolean", "null", "array", "object"]
+      }
+    },
+    "signature": {
+      "type": "object",
+      "description": "Optional tamper-evident signature over the record content",
+      "required": [
+        "alg",
+        "key_id",
+        "sig"
+      ],
+      "properties": {
+        "alg": {
+          "type": "string",
+          "description": "Signature algorithm, e.g. ed25519"
+        },
+        "key_id": {
+          "type": "string",
+          "description": "Identifier of the signing key"
+        },
+        "sig": {
+          "type": "string",
+          "description": "Base64-encoded signature bytes"
+        }
+      },
+      "additionalProperties": false
+    }
+  }
+}
diff --git a/schemas/aep/evidence-envelope.schema.json b/schemas/aep/evidence-envelope.schema.json
index 227648e..e6e3440 100644
--- a/schemas/aep/evidence-envelope.schema.json
+++ b/schemas/aep/evidence-envelope.schema.json
@@ -2,48 +2,33 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://wasmagent.dev/schemas/aep/evidence-envelope.schema.json",
   "title": "EvidenceEnvelope",
-  "description": "Shared base fields for all AEP evidence records \u2014 schema_version, trace_id, created_at_ms, and optional signature. Every evidence type $refs this schema for its envelope fields.",
-  "type": "object",
-  "required": [
-    "schema_version",
-    "created_at_ms"
-  ],
-  "properties": {
-    "schema_version": {
-      "type": "string",
-      "description": "Semver-like schema identifier, e.g. aep/v0.1"
+  "description": "Shared envelope base for all AEP evidence records. Every evidence type wraps its payload in this envelope, which carries schema_version, trace_id, created_at_ms, evidence_type, payload, metadata, and an optional signature.",
+  "allOf": [
+    {
+      "$ref": "_base.schema.json"
     },
-    "trace_id": {
-      "type": "string",
-      "description": "Optional correlation ID linking related evidence records"
-    },
-    "created_at_ms": {
-      "type": "number",
-      "description": "UTC epoch milliseconds when the record was created"
-    },
-    "signature": {
+    {
       "type": "object",
-      "description": "Optional tamper-evident signature over the record content",
       "required": [
-        "alg",
-        "key_id",
-        "sig"
+        "evidence_type",
+        "payload"
       ],
       "properties": {
-        "alg": {
+        "evidence_type": {
           "type": "string",
-          "description": "Signature algorithm, e.g. ed25519"
+          "description": "Discriminator for the kind of evidence enclosed, e.g. 'execution', 'artifact', 'checkpoint', 'replay', 'memory'"
         },
-        "key_id": {
-          "type": "string",
-          "description": "Identifier of the signing key"
-        },
-        "sig": {
-          "type": "string",
-          "description": "Base64-encoded signature bytes"
+        "payload": {
+          "description": "The evidence content — structure depends on evidence_type",
+          "type": "object",
+          "properties": {
+            "contents": {
+              "description": "The primary evidence payload (opaque per evidence_type)"
+            }
+          },
+          "additionalProperties": true
         }
-      },
-      "additionalProperties": false
+      }
     }
-  }
+  ]
 }
diff --git a/tests/fixtures/valid/evidence-envelope/example.json b/tests/fixtures/valid/evidence-envelope/example.json
index e8a3ef0..34a896a 100644
--- a/tests/fixtures/valid/evidence-envelope/example.json
+++ b/tests/fixtures/valid/evidence-envelope/example.json
@@ -2,6 +2,14 @@
   "schema_version": "aep/v0.1",
   "trace_id": "trace-abc123",
   "created_at_ms": 1737600000000,
+  "evidence_type": "artifact",
+  "payload": {
+    "contents": "artifact-evidence-data"
+  },
+  "metadata": {
+    "source": "wasmagent-js",
+    "correlation_tag": "run-42"
+  },
   "signature": {
     "alg": "ed25519",
     "key_id": "key-001",