diff --git a/src/model.ts b/src/model.ts
new file mode 100644
index 0000000..c1113de
--- /dev/null
+++ b/src/model.ts
@@ -0,0 +1,124 @@
+/**
+ * Versioned coauthoring artifact schema (spec §6.3) — the shared git-native
+ * envelope under every coauthoring capability. `anchors` and the `Provenance`
+ * author field are SHARED primitives (INV-4): F3 (attributions) and F4
+ * (proposals) add arrays that reference the same shapes without a format break.
+ *
+ * vscode-free and pure, so it is unit-testable in Node (mirrors src/cline.ts).
+ */
+import { randomUUID } from "node:crypto";
+
+export const SCHEMA_VERSION = 1 as const;
+
+/** Anchored text + bounded context + a line tie-breaker (spec §6.3). */
+export interface Fingerprint {
+  text: string;
+  /** <= ~3 lines / 120 chars leading context. */
+  before: string;
+  /** <= ~3 lines / 120 chars trailing context. */
+  after: string;
+  /** 0-based line of the anchor start; a tie-breaker, NOT truth (INV-3). */
+  lineHint: number;
+}
+
+export interface Anchor {
+  fingerprint: Fingerprint;
+}
+
+/** The reusable author field (spec §6.3). `agent` only present when kind=agent. */
+export type Provenance =
+  | { kind: "human"; id: string }
+  | { kind: "agent"; id: string; agent: { sdk: string; model: string; sessionId: string } };
+
+export interface Message {
+  id: string;
+  author: Provenance;
+  body: string;
+  /** ISO-8601. */
+  createdAt: string;
+}
+
+export type ThreadStatus = "open" | "resolved";
+
+export interface Thread {
+  id: string;
+  anchorId: string;
+  status: ThreadStatus;
+  messages: Message[];
+}
+
+export interface Artifact {
+  schemaVersion: number;
+  /** repo-relative path; the sidecar key. */
+  document: { path: string };
+  /** SHARED primitive (INV-4). */
+  anchors: Record<string, Anchor>;
+  threads: Thread[];
+  /** F3 extension point — reuses anchors[]; not implemented in F2. */
+  attributions: unknown[];
+  /** F4 extension point — reuses anchors[] + provenance; not in F2. */
+  proposals: unknown[];
+}
+
+export function emptyArtifact(docPath: string): Artifact {
+  return {
+    schemaVersion: SCHEMA_VERSION,
+    document: { path: docPath },
+    anchors: {},
+    threads: [],
+    attributions: [],
+    proposals: [],
+  };
+}
+
+/** Stable, generated id (spec §6.3). */
+export function newId(prefix: string): string {
+  return `${prefix}_${randomUUID()}`;
+}
+
+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 };
+}
+
+/**
+ * Pretty-printed JSON with STABLE key ordering (anchors sorted by id), so
+ * sidecar diffs stay minimal and merge-friendly (spec §6.3, INV-2). Always
+ * ends with a trailing newline.
+ */
+export function serializeArtifact(a: Artifact): string {
+  const canonical = {
+    schemaVersion: a.schemaVersion,
+    document: { path: a.document.path },
+    anchors: Object.fromEntries(
+      Object.keys(a.anchors)
+        .sort()
+        .map((k) => [
+          k,
+          {
+            fingerprint: {
+              text: a.anchors[k].fingerprint.text,
+              before: a.anchors[k].fingerprint.before,
+              after: a.anchors[k].fingerprint.after,
+              lineHint: a.anchors[k].fingerprint.lineHint,
+            },
+          },
+        ]),
+    ),
+    threads: a.threads.map((t) => ({
+      id: t.id,
+      anchorId: t.anchorId,
+      status: t.status,
+      messages: t.messages.map((m) => ({
+        id: m.id,
+        author: serializeProvenance(m.author),
+        body: m.body,
+        createdAt: m.createdAt,
+      })),
+    })),
+    attributions: a.attributions,
+    proposals: a.proposals,
+  };
+  return JSON.stringify(canonical, null, 2) + "\n";
+}
diff --git a/src/store.ts b/src/store.ts
new file mode 100644
index 0000000..f96acd3
--- /dev/null
+++ b/src/store.ts
@@ -0,0 +1,31 @@
+/**
+ * CoauthorStore — load/save the per-document `.threads/` sidecar (spec §6.4).
+ * Git-native, serverless, plain pretty JSON (INV-2). vscode-free (Node fs only),
+ * so it is unit-testable; the FileSystemWatcher that triggers re-anchoring on
+ * external change is wired in the vscode layer (ThreadController, SLICE-4).
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { serializeArtifact, type Artifact } from "./model";
+
+export class CoauthorStore {
+  /** @param rootDir absolute workspace-folder root that owns the `.threads/` tree. */
+  constructor(private readonly rootDir: string) {}
+
+  /** `.threads/<repo-relative-docPath>.json` (spec §6.3). */
+  sidecarPath(docPath: string): string {
+    return path.join(this.rootDir, ".threads", `${docPath}.json`);
+  }
+
+  load(docPath: string): Artifact | null {
+    const p = this.sidecarPath(docPath);
+    if (!fs.existsSync(p)) return null;
+    return JSON.parse(fs.readFileSync(p, "utf8")) as Artifact;
+  }
+
+  save(docPath: string, artifact: Artifact): void {
+    const p = this.sidecarPath(docPath);
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    fs.writeFileSync(p, serializeArtifact(artifact), "utf8");
+  }
+}
diff --git a/test/model.test.ts b/test/model.test.ts
new file mode 100644
index 0000000..53d5805
--- /dev/null
+++ b/test/model.test.ts
@@ -0,0 +1,58 @@
+import { describe, it, expect } from "vitest";
+import {
+  SCHEMA_VERSION,
+  emptyArtifact,
+  serializeArtifact,
+  type Artifact,
+} from "../src/model";
+
+function sampleArtifact(): Artifact {
+  return {
+    schemaVersion: SCHEMA_VERSION,
+    document: { path: "docs/spec.md" },
+    anchors: {
+      a2: { fingerprint: { text: "beta", before: "x", after: "y", lineHint: 5 } },
+      a1: { fingerprint: { text: "alpha", before: "", after: "", lineHint: 1 } },
+    },
+    threads: [
+      {
+        id: "t1",
+        anchorId: "a1",
+        status: "open",
+        messages: [
+          { id: "m1", author: { kind: "human", id: "ben" }, body: "hi", createdAt: "2026-06-10T00:00:00.000Z" },
+        ],
+      },
+    ],
+    attributions: [],
+    proposals: [],
+  };
+}
+
+describe("serializeArtifact", () => {
+  it("round-trips through JSON.parse", () => {
+    const a = sampleArtifact();
+    const parsed = JSON.parse(serializeArtifact(a)) as Artifact;
+    expect(parsed).toEqual(a);
+  });
+
+  it("emits stable, sorted anchor keys and a trailing newline regardless of insertion order", () => {
+    const a = sampleArtifact();
+    const out = serializeArtifact(a);
+    expect(out.endsWith("\n")).toBe(true);
+    // a1 must serialize before a2 even though a2 was inserted first
+    expect(out.indexOf('"a1"')).toBeLessThan(out.indexOf('"a2"'));
+    // byte-identical on re-serialize
+    expect(serializeArtifact(JSON.parse(out))).toBe(out);
+  });
+
+  it("emptyArtifact has the shared F3/F4 extension points present and empty", () => {
+    const e = emptyArtifact("docs/x.md");
+    expect(e.schemaVersion).toBe(SCHEMA_VERSION);
+    expect(e.document.path).toBe("docs/x.md");
+    expect(e.anchors).toEqual({});
+    expect(e.threads).toEqual([]);
+    expect(e.attributions).toEqual([]);
+    expect(e.proposals).toEqual([]);
+  });
+});
diff --git a/test/store.test.ts b/test/store.test.ts
new file mode 100644
index 0000000..20eb736
--- /dev/null
+++ b/test/store.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { CoauthorStore } from "../src/store";
+import { emptyArtifact, newId, serializeArtifact, type Artifact } from "../src/model";
+
+let root: string;
+
+beforeEach(() => {
+  root = fs.mkdtempSync(path.join(os.tmpdir(), "coauthor-store-"));
+});
+afterEach(() => {
+  fs.rmSync(root, { recursive: true, force: true });
+});
+
+function withThread(): Artifact {
+  const a = emptyArtifact("docs/spec.md");
+  a.anchors["a1"] = { fingerprint: { text: "hello", before: "", after: "", lineHint: 0 } };
+  a.threads.push({
+    id: "t1",
+    anchorId: "a1",
+    status: "open",
+    messages: [{ id: newId("m"), author: { kind: "human", id: "ben" }, body: "note", createdAt: "2026-06-10T00:00:00.000Z" }],
+  });
+  return a;
+}
+
+describe("CoauthorStore", () => {
+  it("returns null for a document with no sidecar", () => {
+    const store = new CoauthorStore(root);
+    expect(store.load("docs/spec.md")).toBeNull();
+  });
+
+  it("computes the sidecar path as .threads/<docPath>.json", () => {
+    const store = new CoauthorStore(root);
+    expect(store.sidecarPath("docs/spec.md")).toBe(path.join(root, ".threads", "docs", "spec.md.json"));
+  });
+
+  it("save then load round-trips the artifact", () => {
+    const store = new CoauthorStore(root);
+    const a = withThread();
+    store.save("docs/spec.md", a);
+    expect(store.load("docs/spec.md")).toEqual(a);
+  });
+
+  it("writes byte-stable, pretty JSON (re-save is identical)", () => {
+    const store = new CoauthorStore(root);
+    const a = withThread();
+    store.save("docs/spec.md", a);
+    const first = fs.readFileSync(store.sidecarPath("docs/spec.md"), "utf8");
+    store.save("docs/spec.md", store.load("docs/spec.md")!);
+    const second = fs.readFileSync(store.sidecarPath("docs/spec.md"), "utf8");
+    expect(second).toBe(first);
+    expect(first).toBe(serializeArtifact(a));
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index ed8bf77..1a98d14 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -4,5 +4,6 @@ export default defineConfig({
   test: {
     environment: "node",
     include: ["test/**/*.test.ts"],
+    exclude: ["test/e2e/**", "node_modules/**"],
   },
 });