diff --git a/src/model.ts b/src/model.ts
index c1113de..968578b 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -47,6 +47,20 @@ export interface Thread {
   messages: Message[];
 }
 
+/** F3 (spec §6.3): an anchored, author-attributed span — state, not history. */
+export interface AttributionRecord {
+  id: string;
+  /** shared anchors map — same Fingerprint primitive (INV-4). */
+  anchorId: string;
+  author: Provenance;
+  /** ISO-8601. */
+  createdAt: string;
+  /** ISO-8601; bumped when the span's extent/fingerprint changes. */
+  updatedAt: string;
+  /** groups all spans applied by one live turn. */
+  turnId?: string;
+}
+
 export interface Artifact {
   schemaVersion: number;
   /** repo-relative path; the sidecar key. */
@@ -54,8 +68,8 @@ export interface Artifact {
   /** SHARED primitive (INV-4). */
   anchors: Record<string, Anchor>;
   threads: Thread[];
-  /** F3 extension point — reuses anchors[]; not implemented in F2. */
-  attributions: unknown[];
+  /** F3 fills this (spec §6.3, INV-4): typed attribution records anchored via anchors[]. */
+  attributions: AttributionRecord[];
   /** F4 extension point — reuses anchors[] + provenance; not in F2. */
   proposals: unknown[];
 }
@@ -117,7 +131,14 @@ export function serializeArtifact(a: Artifact): string {
         createdAt: m.createdAt,
       })),
     })),
-    attributions: a.attributions,
+    attributions: a.attributions.map((at) => ({
+      id: at.id,
+      anchorId: at.anchorId,
+      author: serializeProvenance(at.author),
+      createdAt: at.createdAt,
+      updatedAt: at.updatedAt,
+      ...(at.turnId !== undefined ? { turnId: at.turnId } : {}),
+    })),
     proposals: a.proposals,
   };
   return JSON.stringify(canonical, null, 2) + "\n";
diff --git a/test/model.test.ts b/test/model.test.ts
index 53d5805..c0bea25 100644
--- a/test/model.test.ts
+++ b/test/model.test.ts
@@ -56,3 +56,35 @@ describe("serializeArtifact", () => {
     expect(e.proposals).toEqual([]);
   });
 });
+
+describe("attributions[] (F3 SLICE-1)", () => {
+  it("round-trips an attribution record through serialize → parse", () => {
+    const a = emptyArtifact("docs/x.md");
+    a.anchors["a_1"] = { fingerprint: { text: "alpha", before: "", after: " beta", lineHint: 0 } };
+    a.attributions.push({
+      id: "at_1",
+      anchorId: "a_1",
+      author: { kind: "agent", id: "claude", agent: { sdk: "@cline/sdk", model: "sonnet", sessionId: "run-1" } },
+      createdAt: "2026-06-10T00:00:00.000Z",
+      updatedAt: "2026-06-10T00:00:00.000Z",
+      turnId: "turn-1",
+    });
+    const parsed = JSON.parse(serializeArtifact(a)) as Artifact;
+    expect(parsed.attributions).toEqual(a.attributions);
+  });
+
+  it("omits turnId when undefined and serializes stably", () => {
+    const a = emptyArtifact("docs/x.md");
+    a.attributions.push({
+      id: "at_2",
+      anchorId: "a_2",
+      author: { kind: "human", id: "ben" },
+      createdAt: "2026-06-10T00:00:00.000Z",
+      updatedAt: "2026-06-10T00:00:00.000Z",
+    });
+    const s1 = serializeArtifact(a);
+    expect(s1.includes("turnId")).toBe(false);
+    const s2 = serializeArtifact(JSON.parse(s1) as Artifact);
+    expect(s2).toBe(s1);
+  });
+});