diff --git a/src/model.ts b/src/model.ts
index 4ac79f1..b59d3b3 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -25,10 +25,25 @@ export interface Anchor {
   fingerprint: Fingerprint;
 }
 
-/** The reusable author field (spec §6.3). `agent` only present when kind=agent. */
+/**
+ * The reusable author field (contract §3.3). `agent` only present when
+ * kind=agent. F5 (spec §6.3, fork c): `email` is the cross-rung join key —
+ * git's own identity model (forges map email → account); `onBehalfOf` records
+ * the operator a machine acted for (rfc-app#46 §6.5 made data).
+ */
 export type Provenance =
-  | { kind: "human"; id: string }
-  | { kind: "agent"; id: string; agent: { sdk: string; model: string; sessionId: string } };
+  | { kind: "human"; id: string; email?: string }
+  | {
+      kind: "agent";
+      id: string;
+      email?: string;
+      agent: {
+        sdk: string;
+        model: string;
+        sessionId: string;
+        onBehalfOf?: { id: string; email?: string };
+      };
+    };
 
 export interface Message {
   id: string;
@@ -107,9 +122,27 @@ export function newId(prefix: string): string {
 }
 
 function serializeProvenance(p: Provenance): Record<string, unknown> {
-  return p.kind === "agent"
-    ? { kind: p.kind, id: p.id, agent: { sdk: p.agent.sdk, model: p.agent.model, sessionId: p.agent.sessionId } }
-    : { kind: p.kind, id: p.id };
+  const known: Record<string, unknown> = {
+    kind: p.kind,
+    id: p.id,
+    ...(p.email !== undefined ? { email: p.email } : {}),
+  };
+  if (p.kind === "agent") {
+    known.agent = {
+      sdk: p.agent.sdk,
+      model: p.agent.model,
+      sessionId: p.agent.sessionId,
+      ...(p.agent.onBehalfOf !== undefined
+        ? {
+            onBehalfOf: {
+              id: p.agent.onBehalfOf.id,
+              ...(p.agent.onBehalfOf.email !== undefined ? { email: p.agent.onBehalfOf.email } : {}),
+            },
+          }
+        : {}),
+    };
+  }
+  return known;
 }
 
 /**
diff --git a/test/model.test.ts b/test/model.test.ts
index c0bea25..a536af9 100644
--- a/test/model.test.ts
+++ b/test/model.test.ts
@@ -57,6 +57,52 @@ describe("serializeArtifact", () => {
   });
 });
 
+describe("cross-rung identity (F5 SLICE-2)", () => {
+  it("round-trips email on human provenance, omitted when absent", () => {
+    const a = emptyArtifact("docs/x.md");
+    a.anchors["a_1"] = { fingerprint: { text: "alpha", before: "", after: "", lineHint: 0 } };
+    a.threads.push({
+      id: "t_1",
+      anchorId: "a_1",
+      status: "open",
+      messages: [
+        { id: "m_1", author: { kind: "human", id: "ben", email: "ben@wiggleverse.org" }, body: "hi", createdAt: "2026-06-10T00:00:00.000Z" },
+        { id: "m_2", author: { kind: "human", id: "anon" }, body: "yo", createdAt: "2026-06-10T00:00:01.000Z" },
+      ],
+    });
+    const out = serializeArtifact(a);
+    const parsed = JSON.parse(out) as Artifact;
+    expect(parsed.threads[0].messages[0].author).toEqual({ kind: "human", id: "ben", email: "ben@wiggleverse.org" });
+    expect("email" in parsed.threads[0].messages[1].author).toBe(false);
+    expect(serializeArtifact(parsed)).toBe(out);
+  });
+
+  it("round-trips agent.onBehalfOf and omits it when absent", () => {
+    const a = emptyArtifact("docs/x.md");
+    a.attributions.push({
+      id: "at_1",
+      anchorId: "a_1",
+      author: {
+        kind: "agent",
+        id: "claude",
+        agent: { sdk: "@cline/sdk", model: "sonnet", sessionId: "run-1", onBehalfOf: { id: "benstull", email: "ben@wiggleverse.org" } },
+      },
+      createdAt: "2026-06-10T00:00:00.000Z",
+      updatedAt: "2026-06-10T00:00:00.000Z",
+    });
+    const out = serializeArtifact(a);
+    const parsed = JSON.parse(out) as Artifact;
+    const author = parsed.attributions[0].author;
+    expect(author.kind).toBe("agent");
+    if (author.kind === "agent") {
+      expect(author.agent.onBehalfOf).toEqual({ id: "benstull", email: "ben@wiggleverse.org" });
+    }
+    expect(out.includes("onBehalfOf")).toBe(true);
+    const a2 = emptyArtifact("docs/x.md");
+    expect(serializeArtifact(a2).includes("onBehalfOf")).toBe(false);
+  });
+});
+
 describe("attributions[] (F3 SLICE-1)", () => {
   it("round-trips an attribution record through serialize → parse", () => {
     const a = emptyArtifact("docs/x.md");