diff --git a/src/model.ts b/src/model.ts
index 968578b..4ac79f1 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -61,6 +61,22 @@ export interface AttributionRecord {
   turnId?: string;
 }
 
+/** F4 (spec §6.3, INV-13): a PENDING machine edit — state, not history. */
+export interface Proposal {
+  id: string;
+  /** shared anchors map; fingerprint.text IS the exact target text (INV-11). */
+  anchorId: string;
+  /** the full proposed text for the anchored range. */
+  replacement: string;
+  author: Provenance;
+  /** ISO-8601. */
+  createdAt: string;
+  /** groups N proposals born of one live turn. */
+  turnId?: string;
+  /** what the human asked for (review context). */
+  instruction?: string;
+}
+
 export interface Artifact {
   schemaVersion: number;
   /** repo-relative path; the sidecar key. */
@@ -70,8 +86,8 @@ export interface Artifact {
   threads: Thread[];
   /** 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[];
+  /** F4 (spec §6.3, INV-13): pending proposals on shared anchors[] + Provenance. */
+  proposals: Proposal[];
 }
 
 export function emptyArtifact(docPath: string): Artifact {
@@ -139,7 +155,15 @@ export function serializeArtifact(a: Artifact): string {
       updatedAt: at.updatedAt,
       ...(at.turnId !== undefined ? { turnId: at.turnId } : {}),
     })),
-    proposals: a.proposals,
+    proposals: a.proposals.map((p) => ({
+      id: p.id,
+      anchorId: p.anchorId,
+      replacement: p.replacement,
+      author: serializeProvenance(p.author),
+      createdAt: p.createdAt,
+      ...(p.turnId !== undefined ? { turnId: p.turnId } : {}),
+      ...(p.instruction !== undefined ? { instruction: p.instruction } : {}),
+    })),
   };
   return JSON.stringify(canonical, null, 2) + "\n";
 }
diff --git a/test/proposalModel.test.ts b/test/proposalModel.test.ts
new file mode 100644
index 0000000..ad8d3a5
--- /dev/null
+++ b/test/proposalModel.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import {
+  emptyArtifact,
+  serializeArtifact,
+  type Artifact,
+  type Proposal,
+} from "../src/model";
+
+const agent = {
+  kind: "agent" as const,
+  id: "claude",
+  agent: { sdk: "@cline/sdk", model: "sonnet", sessionId: "s1" },
+};
+
+describe("Proposal model (spec §6.3, INV-13)", () => {
+  it("serializes proposals with stable field order and survives a round-trip", () => {
+    const a = emptyArtifact("docs/x.md");
+    a.anchors["a_1"] = {
+      fingerprint: { text: "old text", before: "", after: "", lineHint: 0 },
+    };
+    const p: Proposal = {
+      id: "pr_1",
+      anchorId: "a_1",
+      replacement: "new text",
+      author: agent,
+      createdAt: "2026-06-10T00:00:00.000Z",
+      turnId: "turn-1",
+      instruction: "tighten",
+    };
+    a.proposals.push(p);
+    const reloaded = JSON.parse(serializeArtifact(a)) as Artifact;
+    expect(reloaded.proposals).toHaveLength(1);
+    expect(reloaded.proposals[0]).toEqual(p);
+    // optional fields are OMITTED (not null) when absent
+    const bare: Proposal = {
+      id: "pr_2", anchorId: "a_1", replacement: "r", author: agent,
+      createdAt: "2026-06-10T00:00:00.000Z",
+    };
+    a.proposals.push(bare);
+    const again = JSON.parse(serializeArtifact(a)) as Artifact;
+    expect("turnId" in again.proposals[1]).toBe(false);
+    expect("instruction" in again.proposals[1]).toBe(false);
+    // serialization is byte-stable (INV-2)
+    expect(serializeArtifact(again)).toBe(serializeArtifact(a));
+  });
+});