diff --git a/src/proposalModel.ts b/src/proposalModel.ts
new file mode 100644
index 0000000..2f699cd
--- /dev/null
+++ b/src/proposalModel.ts
@@ -0,0 +1,47 @@
+/**
+ * proposalModel — pure helpers over the F4 `proposals[]` section (spec §6.3,
+ * INV-13: pending-only). Mirrors threadModel.ts: vscode-free mutations against
+ * an Artifact, used inside CoauthorStore.update() by ProposalController.
+ * The diff body is PRESENTATION ONLY — the truth is fingerprint.text →
+ * replacement (INV-11); a whole-range -/+ rendering, not an LCS diff (§6.7).
+ */
+import { newId, type Artifact, type Fingerprint, type Proposal, type Provenance } from "./model";
+
+export function addProposal(
+  artifact: Artifact,
+  fp: Fingerprint,
+  replacement: string,
+  author: Provenance,
+  opts?: { turnId?: string; instruction?: string },
+): { proposalId: string; anchorId: string } {
+  const anchorId = newId("a");
+  const proposalId = newId("pr");
+  artifact.anchors[anchorId] = { fingerprint: fp };
+  artifact.proposals.push({
+    id: proposalId,
+    anchorId,
+    replacement,
+    author,
+    createdAt: new Date().toISOString(),
+    ...(opts?.turnId !== undefined ? { turnId: opts.turnId } : {}),
+    ...(opts?.instruction !== undefined ? { instruction: opts.instruction } : {}),
+  });
+  return { proposalId, anchorId };
+}
+
+/** Remove a pending proposal (accept and reject both end here — INV-13). */
+export function removeProposal(artifact: Artifact, proposalId: string): boolean {
+  const before = artifact.proposals.length;
+  artifact.proposals = artifact.proposals.filter((p) => p.id !== proposalId);
+  return artifact.proposals.length < before;
+}
+
+/** Markdown comment body: instruction header + fenced whole-range diff. */
+export function proposalBody(targetText: string, p: Proposal): string {
+  const header = p.instruction ? `**Claude proposes** — _${p.instruction}_` : "**Claude proposes**";
+  const diffLines = [
+    ...targetText.split("\n").map((l) => `- ${l}`),
+    ...p.replacement.split("\n").map((l) => `+ ${l}`),
+  ].join("\n");
+  return `${header}\n\n\`\`\`diff\n${diffLines}\n\`\`\`\n\n✓ Accept applies this replacement · ✗ Reject discards it`;
+}
diff --git a/test/proposalModel.test.ts b/test/proposalModel.test.ts
index 43eedbc..9fce315 100644
--- a/test/proposalModel.test.ts
+++ b/test/proposalModel.test.ts
@@ -9,6 +9,7 @@ import {
   type Proposal,
 } from "../src/model";
 import { CoauthorStore } from "../src/store";
+import { addProposal, proposalBody, removeProposal } from "../src/proposalModel";
 
 const agent = {
   kind: "agent" as const,
@@ -73,3 +74,34 @@ describe("CoauthorStore prune with proposals (spec §6.3)", () => {
     }
   });
 });
+
+describe("proposalModel helpers (spec §6.4)", () => {
+  const fp = { text: "old line one\nold line two", before: "", after: "", lineHint: 3 };
+
+  it("addProposal creates the anchor + pending record; removeProposal deletes by id", () => {
+    const a = emptyArtifact("docs/x.md");
+    const { proposalId, anchorId } = addProposal(a, fp, "new line", agent, {
+      turnId: "turn-9", instruction: "shorten",
+    });
+    expect(a.anchors[anchorId].fingerprint).toEqual(fp);
+    expect(a.proposals).toHaveLength(1);
+    expect(a.proposals[0].id).toBe(proposalId);
+    expect(a.proposals[0].replacement).toBe("new line");
+    expect(a.proposals[0].turnId).toBe("turn-9");
+    expect(a.proposals[0].instruction).toBe("shorten");
+    expect(removeProposal(a, proposalId)).toBe(true);
+    expect(a.proposals).toHaveLength(0);
+    expect(removeProposal(a, proposalId)).toBe(false);
+  });
+
+  it("proposalBody renders instruction + a fenced unified diff of old → new", () => {
+    const a = emptyArtifact("docs/x.md");
+    addProposal(a, fp, "new only line", agent, { instruction: "merge the lines" });
+    const body = proposalBody(fp.text, a.proposals[0]);
+    expect(body).toContain("**Claude proposes** — _merge the lines_");
+    expect(body).toContain("```diff");
+    expect(body).toContain("- old line one");
+    expect(body).toContain("- old line two");
+    expect(body).toContain("+ new only line");
+  });
+});