diff --git a/src/globalSidecarStore.ts b/src/globalSidecarStore.ts
new file mode 100644
index 0000000..e7c9f4b
--- /dev/null
+++ b/src/globalSidecarStore.ts
@@ -0,0 +1,89 @@
+/**
+ * GlobalSidecarStore — the out-of-workspace/untitled authoring sidecar (F8 spec
+ * §6.2/§6.4), mirroring BaselineStore (src/baselineStore.ts): vscode-free (Node
+ * fs/crypto only), one `Artifact` JSON per document in VS Code's per-extension
+ * GLOBAL storage, NEVER the repo (INV-19/24). The key passed in is the DOCUMENT
+ * KEY (the URI string for the docs this store serves):
+ *   - `file:` URI  → disk at `<dir>/sidecars/<sha256(uri)>.json`.
+ *   - `untitled:`  → an in-memory Map only (no durable identity — the F6
+ *     degrade); lost on reload, and never read by mergeArtifacts (INV-25).
+ * `consumeSelfWrite` is a no-op (these artifacts are outside the `.threads/`
+ * watcher, so there is no self-write storm to suppress).
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { createHash } from "node:crypto";
+import {
+  SCHEMA_VERSION,
+  emptyArtifact,
+  isNewerMajor,
+  serializeArtifact,
+  type Artifact,
+} from "./model";
+import type { SidecarStore } from "./sidecarStore";
+
+export class GlobalSidecarStore implements SidecarStore {
+  /** untitled keys live here only — no durable identity to persist against. */
+  private readonly memory = new Map<string, Artifact>();
+
+  /** @param storageDir absolute VS Code GLOBAL-storage dir (context.globalStorageUri.fsPath). */
+  constructor(private readonly storageDir: string) {}
+
+  private isUntitled(key: string): boolean {
+    return key.startsWith("untitled:");
+  }
+
+  /** Filesystem-safe storage key for a persistable (file:) doc: sha256 of its URI. */
+  private storageKey(key: string): string {
+    return createHash("sha256").update(key).digest("hex");
+  }
+
+  /** Disk path for a `file:` key, or undefined for an in-memory untitled key. */
+  sidecarPath(key: string): string | undefined {
+    if (this.isUntitled(key)) return undefined;
+    return path.join(this.storageDir, "sidecars", `${this.storageKey(key)}.json`);
+  }
+
+  load(key: string): Artifact | null {
+    if (this.isUntitled(key)) return this.memory.get(key) ?? null;
+    const p = this.sidecarPath(key)!;
+    if (!fs.existsSync(p)) return null;
+    return JSON.parse(fs.readFileSync(p, "utf8")) as Artifact;
+  }
+
+  save(key: string, artifact: Artifact): void {
+    if (this.isUntitled(key)) {
+      this.memory.set(key, artifact);
+      return;
+    }
+    const p = this.sidecarPath(key)!;
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    fs.writeFileSync(p, serializeArtifact(artifact), "utf8");
+  }
+
+  /** See CoauthorStore.update — same INV-16 throw + anchor prune, no self-write mark. */
+  update(key: string, mutate: (artifact: Artifact) => void): Artifact {
+    const artifact = this.load(key) ?? emptyArtifact(key);
+    if (isNewerMajor(artifact)) {
+      throw new Error(
+        `refusing to write ${key}: sidecar schemaVersion ${artifact.schemaVersion} > supported ${SCHEMA_VERSION} (INV-16)`,
+      );
+    }
+    mutate(artifact);
+    const referenced = new Set<string>([
+      ...artifact.threads.map((t) => t.anchorId),
+      ...artifact.attributions.map((a) => a.anchorId),
+      ...artifact.proposals.map((p) => p.anchorId),
+    ]);
+    for (const id of Object.keys(artifact.anchors)) {
+      if (!referenced.has(id)) delete artifact.anchors[id];
+    }
+    this.save(key, artifact);
+    return artifact;
+  }
+
+  /** No-op: global artifacts are outside the `.threads/` watcher. */
+  consumeSelfWrite(_fsPath: string): boolean {
+    return false;
+  }
+}
diff --git a/src/sidecarStore.ts b/src/sidecarStore.ts
new file mode 100644
index 0000000..6116573
--- /dev/null
+++ b/src/sidecarStore.ts
@@ -0,0 +1,35 @@
+/**
+ * SidecarStore — the storage surface the three authoring controllers depend on
+ * (F8 spec §6.2/§6.4). One per-document `Artifact` keyed by a string `key` (the
+ * document key — repo-relative path in-workspace, URI string otherwise; the
+ * router's keyOf is the single source). Two implementations:
+ *   - CoauthorStore (src/store.ts) — the committable repo `.threads/` sidecar,
+ *     keyed by repo-relative path (INV-2, byte-for-byte unchanged; conforms
+ *     structurally — not modified for F8).
+ *   - GlobalSidecarStore (src/globalSidecarStore.ts) — out-of-workspace `file:`
+ *     and `untitled:` docs, in VS Code GLOBAL storage keyed by sha256(uri)
+ *     (INV-19/24/25; untitled in-memory only).
+ * SidecarRouter (src/sidecarRouter.ts) implements this AND adds keyOf/sidecarPath;
+ * the controllers receive the router.
+ */
+import type { Artifact } from "./model";
+
+export interface SidecarStore {
+  /** Load the artifact for `key`, or null if none persisted. */
+  load(key: string): Artifact | null;
+  /** Overwrite the artifact for `key` (newest wins, no history). */
+  save(key: string, artifact: Artifact): void;
+  /**
+   * Read-modify-write: load (or empty), apply `mutate`, prune anchors referenced
+   * by no thread/attribution/proposal, persist, return the result. MUST be
+   * synchronous (the F3 co-ownership contract). Throws on a newer-major sidecar
+   * (INV-16 backstop).
+   */
+  update(key: string, mutate: (artifact: Artifact) => void): Artifact;
+  /**
+   * Decrement-and-report whether `fsPath` is a sidecar this store just wrote
+   * (repo sidecars only; the global store is outside the `.threads/` watcher so
+   * its impl is a no-op returning false).
+   */
+  consumeSelfWrite(fsPath: string): boolean;
+}
diff --git a/test/globalSidecarStore.test.ts b/test/globalSidecarStore.test.ts
new file mode 100644
index 0000000..e3a0a4f
--- /dev/null
+++ b/test/globalSidecarStore.test.ts
@@ -0,0 +1,103 @@
+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 { createHash } from "node:crypto";
+import { GlobalSidecarStore } from "../src/globalSidecarStore";
+import { emptyArtifact, SCHEMA_VERSION, type Artifact } from "../src/model";
+
+let dir: string;
+beforeEach(() => {
+  dir = fs.mkdtempSync(path.join(os.tmpdir(), "global-sidecar-"));
+});
+afterEach(() => {
+  fs.rmSync(dir, { recursive: true, force: true });
+});
+
+const FILE_URI = "file:///abs/notes/chapter-1.md";
+const hash = (uri: string) => createHash("sha256").update(uri).digest("hex");
+
+function withThread(uri: string): Artifact {
+  const a = emptyArtifact(uri);
+  a.anchors["an_1"] = { fingerprint: { text: "hi", before: "", after: "", lineHint: 0 } };
+  a.threads.push({ id: "t_1", anchorId: "an_1", status: "open", messages: [] });
+  return a;
+}
+
+describe("GlobalSidecarStore — file: URIs persist to <dir>/sidecars/<sha256(uri)>.json", () => {
+  it("returns null for a key with no sidecar", () => {
+    expect(new GlobalSidecarStore(dir).load(FILE_URI)).toBeNull();
+  });
+
+  it("save then load round-trips the artifact at the hashed path", () => {
+    const store = new GlobalSidecarStore(dir);
+    const a = withThread(FILE_URI);
+    store.save(FILE_URI, a);
+    const onDisk = path.join(dir, "sidecars", `${hash(FILE_URI)}.json`);
+    expect(fs.existsSync(onDisk)).toBe(true);
+    expect(store.load(FILE_URI)).toEqual(a);
+  });
+
+  it("sidecarPath returns the hashed disk path for a file: URI", () => {
+    const store = new GlobalSidecarStore(dir);
+    expect(store.sidecarPath(FILE_URI)).toBe(path.join(dir, "sidecars", `${hash(FILE_URI)}.json`));
+  });
+
+  it("overwrites in place (newest wins, no history)", () => {
+    const store = new GlobalSidecarStore(dir);
+    store.save(FILE_URI, withThread(FILE_URI));
+    const empty = emptyArtifact(FILE_URI);
+    store.save(FILE_URI, empty);
+    expect(store.load(FILE_URI)).toEqual(empty);
+  });
+
+  it("consumeSelfWrite is a no-op returning false (outside the .threads watcher)", () => {
+    const store = new GlobalSidecarStore(dir);
+    store.save(FILE_URI, withThread(FILE_URI));
+    expect(store.consumeSelfWrite(store.sidecarPath(FILE_URI)!)).toBe(false);
+  });
+
+  it("update applies the mutation, prunes unreferenced anchors, and persists", () => {
+    const store = new GlobalSidecarStore(dir);
+    const result = store.update(FILE_URI, (a) => {
+      a.anchors["orphan"] = { fingerprint: { text: "x", before: "", after: "", lineHint: 0 } };
+      a.anchors["kept"] = { fingerprint: { text: "y", before: "", after: "", lineHint: 0 } };
+      a.threads.push({ id: "t", anchorId: "kept", status: "open", messages: [] });
+    });
+    expect(Object.keys(result.anchors)).toEqual(["kept"]); // "orphan" pruned
+    expect(store.load(FILE_URI)!.threads.length).toBe(1);
+  });
+
+  it("update throws on a newer-major sidecar (INV-16 backstop)", () => {
+    const store = new GlobalSidecarStore(dir);
+    const future = { ...emptyArtifact(FILE_URI), schemaVersion: SCHEMA_VERSION + 1 };
+    store.save(FILE_URI, future as Artifact);
+    expect(() => store.update(FILE_URI, () => {})).toThrow(/INV-16/);
+  });
+});
+
+describe("GlobalSidecarStore — untitled: keys are in-memory only (no disk, lost on reload)", () => {
+  const UNTITLED = "untitled:Untitled-1";
+
+  it("save then load round-trips within the same instance (session)", () => {
+    const store = new GlobalSidecarStore(dir);
+    const a = emptyArtifact(UNTITLED);
+    a.threads.push({ id: "t", anchorId: "an", status: "open", messages: [] });
+    a.anchors["an"] = { fingerprint: { text: "z", before: "", after: "", lineHint: 0 } };
+    store.save(UNTITLED, a);
+    expect(store.load(UNTITLED)).toEqual(a);
+  });
+
+  it("writes NO disk file for an untitled key", () => {
+    const store = new GlobalSidecarStore(dir);
+    store.save(UNTITLED, emptyArtifact(UNTITLED));
+    expect(store.sidecarPath(UNTITLED)).toBeUndefined();
+    expect(fs.existsSync(path.join(dir, "sidecars"))).toBe(false);
+  });
+
+  it("a fresh instance (simulated reload) has no untitled state", () => {
+    const store = new GlobalSidecarStore(dir);
+    store.save(UNTITLED, emptyArtifact(UNTITLED));
+    expect(new GlobalSidecarStore(dir).load(UNTITLED)).toBeNull();
+  });
+});