diff --git a/package.json b/package.json
index 5e07a3c..55fcc91 100644
--- a/package.json
+++ b/package.json
@@ -12,15 +12,40 @@
   },
   "categories": ["Other"],
   "main": "./out/extension.cjs",
-  "activationEvents": [],
+  "activationEvents": ["onStartupFinished"],
   "contributes": {
     "commands": [
       {
         "command": "cowriting.showClineSdkInfo",
         "title": "Cowriting: Show Cline SDK Info",
         "category": "Cowriting"
-      }
-    ]
+      },
+      {
+        "command": "cowriting.createThread",
+        "title": "Cowriting: Add Coauthoring Thread on Selection",
+        "category": "Cowriting"
+      },
+      { "command": "cowriting.reply", "title": "Reply", "category": "Cowriting" },
+      { "command": "cowriting.resolveThread", "title": "Resolve Thread", "category": "Cowriting" },
+      { "command": "cowriting.reopenThread", "title": "Reopen Thread", "category": "Cowriting" }
+    ],
+    "menus": {
+      "comments/commentThread/context": [
+        { "command": "cowriting.reply", "group": "inline", "when": "commentController == cowriting.threads" }
+      ],
+      "comments/commentThread/title": [
+        {
+          "command": "cowriting.resolveThread",
+          "group": "inline",
+          "when": "commentController == cowriting.threads && commentThread =~ /^open$/"
+        },
+        {
+          "command": "cowriting.reopenThread",
+          "group": "inline",
+          "when": "commentController == cowriting.threads && commentThread =~ /^resolved$/"
+        }
+      ]
+    }
   },
   "scripts": {
     "build": "node esbuild.mjs",
diff --git a/src/extension.ts b/src/extension.ts
index 1f10d70..8671a27 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -1,40 +1,56 @@
 import * as vscode from "vscode";
 import { fetchSdkSummary } from "./cline";
+import { CoauthorStore } from "./store";
+import { ThreadController } from "./threadController";
 
 const CHANNEL_NAME = "Cowriting (Cline SDK)";
 
-export function activate(context: vscode.ExtensionContext): void {
+export interface CowritingApi {
+  threadController: ThreadController;
+}
+
+export function activate(context: vscode.ExtensionContext): CowritingApi | undefined {
+  // --- POC command (Feature #2), unchanged ---
   const output = vscode.window.createOutputChannel(CHANNEL_NAME);
   context.subscriptions.push(output);
-
-  const command = vscode.commands.registerCommand(
-    "cowriting.showClineSdkInfo",
-    async () => {
+  context.subscriptions.push(
+    vscode.commands.registerCommand("cowriting.showClineSdkInfo", async () => {
       try {
         const summary = await fetchSdkSummary();
         output.clear();
         output.appendLine(`@cline/sdk version: ${summary.version}`);
         output.appendLine(`Builtin tools (${summary.tools.length}):`);
-        for (const tool of summary.tools) {
-          output.appendLine(`  • ${tool.id} — ${tool.description}`);
-        }
+        for (const tool of summary.tools) output.appendLine(`  • ${tool.id} — ${tool.description}`);
         output.show(true);
         await vscode.window.showInformationMessage(
-          `Cline SDK ${summary.version} loaded — ${summary.tools.length} builtin tools. See the "${CHANNEL_NAME}" output channel.`
+          `Cline SDK ${summary.version} loaded — ${summary.tools.length} builtin tools. See the "${CHANNEL_NAME}" output channel.`,
         );
       } catch (err) {
         const message = err instanceof Error ? err.message : String(err);
         output.appendLine(`Failed to drive @cline/sdk: ${message}`);
         output.show(true);
-        await vscode.window.showErrorMessage(
-          `Cowriting: failed to load @cline/sdk — ${message}`
-        );
+        await vscode.window.showErrorMessage(`Cowriting: failed to load @cline/sdk — ${message}`);
       }
-    }
+    }),
   );
-  context.subscriptions.push(command);
+
+  // --- F2: region-anchored threads (Feature #4) ---
+  const root = vscode.workspace.workspaceFolders?.[0]?.uri.fsPath;
+  if (!root) return undefined; // no workspace → nothing to anchor against
+  const store = new CoauthorStore(root);
+  const threadController = new ThreadController(store, root);
+  context.subscriptions.push(threadController);
+
+  // Render threads for already-open editors, and on future opens.
+  const renderIfOpen = (doc: vscode.TextDocument) => {
+    if (doc.uri.scheme === "file" && doc.uri.fsPath.startsWith(root)) threadController.renderAll(doc);
+  };
+  vscode.workspace.textDocuments.forEach(renderIfOpen);
+  context.subscriptions.push(vscode.workspace.onDidOpenTextDocument(renderIfOpen));
+
+  return { threadController };
 }
 
 export function deactivate(): void {
-  // Nothing to clean up beyond the disposables registered on the context.
+  // Disposables registered on the context handle cleanup.
 }
diff --git a/src/threadController.ts b/src/threadController.ts
new file mode 100644
index 0000000..0631440
--- /dev/null
+++ b/src/threadController.ts
@@ -0,0 +1,291 @@
+/**
+ * ThreadController — the thin editor-facing layer (spec §6.4). Wires
+ * CoauthorStore + Anchorer + threadModel to vscode.comments: create-on-
+ * selection, reply, resolve, render, live-range tracking, reload, and
+ * FileSystemWatcher-driven external re-anchoring (SLICE-4). Threads are NEVER
+ * silently moved — an unresolvable anchor renders as an orphaned thread (INV-1).
+ */
+import * as vscode from "vscode";
+import { CoauthorStore } from "./store";
+import { emptyArtifact, type Artifact, type Provenance } from "./model";
+import { buildFingerprint, resolve, shift, type OffsetRange } from "./anchorer";
+import { addThread, appendMessage, setStatus } from "./threadModel";
+
+/** Test-facing snapshot of what is currently rendered for a document. */
+export interface RenderedThread {
+  id: string;
+  status: "open" | "resolved";
+  orphaned: boolean;
+  range: { startLine: number; endLine: number };
+}
+
+interface DocState {
+  docPath: string;
+  uri: vscode.Uri;
+  artifact: Artifact;
+  /** thread id -> vscode thread. */
+  vsThreads: Map<string, vscode.CommentThread>;
+  /** thread id -> live offset range (within-session optimization, INV-3). */
+  live: Map<string, OffsetRange>;
+  /** thread id -> orphaned flag for the current render. */
+  orphaned: Map<string, boolean>;
+}
+
+export class ThreadController implements vscode.Disposable {
+  private readonly controller: vscode.CommentController;
+  private readonly disposables: vscode.Disposable[] = [];
+  private readonly docs = new Map<string, DocState>(); // keyed by docPath
+  /** sidecar paths we just wrote, to ignore our own watcher events. */
+  private readonly selfWrites = new Set<string>();
+
+  constructor(private readonly store: CoauthorStore, private readonly rootDir: string) {
+    this.controller = vscode.comments.createCommentController("cowriting.threads", "Coauthoring Threads");
+    this.controller.commentingRangeProvider = {
+      provideCommentingRanges: (document) => {
+        if (!this.isInRoot(document.uri)) return [];
+        return [new vscode.Range(0, 0, Math.max(0, document.lineCount - 1), 0)];
+      },
+    };
+    this.disposables.push(this.controller);
+
+    this.disposables.push(
+      vscode.commands.registerCommand("cowriting.createThread", () => this.createThreadOnSelection()),
+      vscode.commands.registerCommand("cowriting.reply", (r: vscode.CommentReply) => this.reply(r)),
+      vscode.commands.registerCommand("cowriting.resolveThread", (t: vscode.CommentThread) =>
+        this.setThreadStatus(t, "resolved"),
+      ),
+      vscode.commands.registerCommand("cowriting.reopenThread", (t: vscode.CommentThread) =>
+        this.setThreadStatus(t, "open"),
+      ),
+      vscode.workspace.onDidChangeTextDocument((e) => this.onDidChange(e)),
+      vscode.workspace.onDidSaveTextDocument((d) => this.onDidSave(d)),
+    );
+
+    // Re-anchor on external change to any sidecar (a git pull, a manual edit).
+    const watcher = vscode.workspace.createFileSystemWatcher("**/.threads/**/*.json");
+    const onSidecar = (uri: vscode.Uri) => this.onExternalSidecarChange(uri);
+    watcher.onDidChange(onSidecar);
+    watcher.onDidCreate(onSidecar);
+    this.disposables.push(watcher);
+  }
+
+  private isInRoot(uri: vscode.Uri): boolean {
+    return uri.scheme === "file" && uri.fsPath.startsWith(this.rootDir);
+  }
+
+  private docPathOf(uri: vscode.Uri): string {
+    return vscode.workspace.asRelativePath(uri, false);
+  }
+
+  private currentAuthor(): Provenance {
+    const id = vscode.workspace.getConfiguration("git").get<string>("user.name") || process.env.USER || "human";
+    return { kind: "human", id };
+  }
+
+  private persist(state: DocState): void {
+    this.selfWrites.add(this.store.sidecarPath(state.docPath));
+    this.store.save(state.docPath, state.artifact);
+  }
+
+  private ensureState(document: vscode.TextDocument): DocState {
+    const docPath = this.docPathOf(document.uri);
+    let state = this.docs.get(docPath);
+    if (!state) {
+      state = {
+        docPath,
+        uri: document.uri,
+        artifact: this.store.load(docPath) ?? emptyArtifact(docPath),
+        vsThreads: new Map(),
+        live: new Map(),
+        orphaned: new Map(),
+      };
+      this.docs.set(docPath, state);
+    }
+    return state;
+  }
+
+  // ---- PUC-1: create on selection -------------------------------------------------
+
+  async createThreadOnSelection(firstBody = "New thread"): Promise<string | undefined> {
+    const editor = vscode.window.activeTextEditor;
+    if (!editor || editor.selection.isEmpty || !this.isInRoot(editor.document.uri)) return undefined;
+    const document = editor.document;
+    const state = this.ensureState(document);
+    const offsets: OffsetRange = {
+      start: document.offsetAt(editor.selection.start),
+      end: document.offsetAt(editor.selection.end),
+    };
+    const fp = buildFingerprint(document.getText(), offsets);
+    const { threadId } = addThread(state.artifact, fp, { author: this.currentAuthor(), body: firstBody });
+    this.persist(state);
+    this.renderThread(document, state, threadId, offsets, false);
+    return threadId;
+  }
+
+  // ---- PUC-2: reply / resolve -----------------------------------------------------
+
+  reply(r: vscode.CommentReply): void {
+    const threadId = this.threadIdOf(r.thread);
+    const state = this.stateOfThread(r.thread);
+    if (!threadId || !state) return;
+    appendMessage(state.artifact, threadId, { author: this.currentAuthor(), body: r.text });
+    this.persist(state);
+    this.refreshComments(r.thread, state, threadId);
+  }
+
+  private setThreadStatus(vsThread: vscode.CommentThread, status: "open" | "resolved"): void {
+    const threadId = this.threadIdOf(vsThread);
+    const state = this.stateOfThread(vsThread);
+    if (!threadId || !state) return;
+    setStatus(state.artifact, threadId, status);
+    this.persist(state);
+    vsThread.state = status === "resolved" ? vscode.CommentThreadState.Resolved : vscode.CommentThreadState.Unresolved;
+    vsThread.contextValue = status;
+  }
+
+  // ---- PUC-3/4: render, reload, re-anchor -----------------------------------------
+
+  /** Load + (re)render every thread for a document at its resolved anchor (or orphaned). */
+  renderAll(document: vscode.TextDocument): void {
+    const docPath = this.docPathOf(document.uri);
+    const state = this.ensureState(document);
+    // fresh artifact from disk (reload / external change)
+    state.artifact = this.store.load(docPath) ?? emptyArtifact(docPath);
+    for (const vsThread of state.vsThreads.values()) vsThread.dispose();
+    state.vsThreads.clear();
+    state.live.clear();
+    state.orphaned.clear();
+    const text = document.getText();
+    for (const thread of state.artifact.threads) {
+      const fp = state.artifact.anchors[thread.anchorId]?.fingerprint;
+      const resolved = fp ? resolve(text, fp) : "orphaned";
+      if (resolved === "orphaned") {
+        const line = fp ? Math.min(fp.lineHint, Math.max(0, document.lineCount - 1)) : 0;
+        const off = document.offsetAt(new vscode.Position(line, 0));
+        this.renderThread(document, state, thread.id, { start: off, end: off }, true);
+      } else {
+        this.renderThread(document, state, thread.id, resolved, false);
+      }
+    }
+  }
+
+  private onExternalSidecarChange(uri: vscode.Uri): void {
+    const p = uri.fsPath;
+    if (this.selfWrites.has(p)) {
+      this.selfWrites.delete(p);
+      return; // our own write
+    }
+    for (const state of this.docs.values()) {
+      if (this.store.sidecarPath(state.docPath) === p) {
+        const doc = vscode.workspace.textDocuments.find((d) => this.docPathOf(d.uri) === state.docPath);
+        if (doc) this.renderAll(doc);
+      }
+    }
+  }
+
+  private onDidChange(e: vscode.TextDocumentChangeEvent): void {
+    const state = this.docs.get(this.docPathOf(e.document.uri));
+    if (!state || state.live.size === 0) return;
+    for (const change of e.contentChanges) {
+      const edit = { start: change.rangeOffset, end: change.rangeOffset + change.rangeLength, newLength: change.text.length };
+      for (const [id, range] of state.live) {
+        const next = shift(range, edit);
+        state.live.set(id, next);
+        const vsThread = state.vsThreads.get(id);
+        if (vsThread && !state.orphaned.get(id)) {
+          vsThread.range = new vscode.Range(e.document.positionAt(next.start), e.document.positionAt(next.end));
+        }
+      }
+    }
+  }
+
+  private onDidSave(document: vscode.TextDocument): void {
+    const state = this.docs.get(this.docPathOf(document.uri));
+    if (!state || state.live.size === 0) return;
+    const text = document.getText();
+    let changed = false;
+    for (const thread of state.artifact.threads) {
+      if (state.orphaned.get(thread.id)) continue;
+      const range = state.live.get(thread.id);
+      if (!range) continue;
+      state.artifact.anchors[thread.anchorId] = { fingerprint: buildFingerprint(text, range) };
+      changed = true;
+    }
+    if (changed) this.persist(state);
+  }
+
+  // ---- rendering helpers ----------------------------------------------------------
+
+  private renderThread(
+    document: vscode.TextDocument,
+    state: DocState,
+    threadId: string,
+    offsets: OffsetRange,
+    orphaned: boolean,
+  ): void {
+    const thread = state.artifact.threads.find((t) => t.id === threadId)!;
+    const range = new vscode.Range(document.positionAt(offsets.start), document.positionAt(offsets.end));
+    const vsThread = this.controller.createCommentThread(
+      document.uri,
+      range,
+      thread.messages.map((m) => this.toComment(m.body, m.author.id)),
+    );
+    vsThread.label = orphaned ? "⚠ Orphaned thread (anchor not found)" : undefined;
+    vsThread.contextValue = orphaned ? "orphaned" : thread.status;
+    vsThread.state = thread.status === "resolved" ? vscode.CommentThreadState.Resolved : vscode.CommentThreadState.Unresolved;
+    vsThread.collapsibleState = vscode.CommentThreadCollapsibleState.Collapsed;
+    state.vsThreads.set(threadId, vsThread);
+    state.live.set(threadId, offsets);
+    state.orphaned.set(threadId, orphaned);
+  }
+
+  private refreshComments(vsThread: vscode.CommentThread, state: DocState, threadId: string): void {
+    const thread = state.artifact.threads.find((t) => t.id === threadId)!;
+    vsThread.comments = thread.messages.map((m) => this.toComment(m.body, m.author.id));
+  }
+
+  private toComment(body: string, authorName: string): vscode.Comment {
+    return {
+      body: new vscode.MarkdownString(body),
+      mode: vscode.CommentMode.Preview,
+      author: { name: authorName },
+    };
+  }
+
+  private threadIdOf(vsThread: vscode.CommentThread): string | undefined {
+    for (const state of this.docs.values()) {
+      for (const [id, t] of state.vsThreads) if (t === vsThread) return id;
+    }
+    return undefined;
+  }
+
+  private stateOfThread(vsThread: vscode.CommentThread): DocState | undefined {
+    for (const state of this.docs.values()) {
+      for (const t of state.vsThreads.values()) if (t === vsThread) return state;
+    }
+    return undefined;
+  }
+
+  // ---- test-facing surface --------------------------------------------------------
+
+  getRendered(docPath: string): RenderedThread[] {
+    const state = this.docs.get(docPath);
+    if (!state) return [];
+    const out: RenderedThread[] = [];
+    for (const [id, vsThread] of state.vsThreads) {
+      const t = state.artifact.threads.find((x) => x.id === id)!;
+      const r = vsThread.range;
+      out.push({
+        id,
+        status: t.status,
+        orphaned: !!state.orphaned.get(id),
+        range: { startLine: r ? r.start.line : 0, endLine: r ? r.end.line : 0 },
+      });
+    }
+    return out;
+  }
+
+  dispose(): void {
+    for (const d of this.disposables) d.dispose();
+  }
+}
diff --git a/src/threadModel.ts b/src/threadModel.ts
new file mode 100644
index 0000000..cded93a
--- /dev/null
+++ b/src/threadModel.ts
@@ -0,0 +1,48 @@
+/**
+ * vscode-free mutations on the Artifact (spec §6.5). Keeping thread logic here
+ * (not in the vscode controller) keeps it unit-testable; ThreadController is a
+ * thin adapter that calls these and persists via CoauthorStore.
+ */
+import {
+  newId,
+  type Artifact,
+  type Fingerprint,
+  type Provenance,
+  type ThreadStatus,
+} from "./model";
+
+export interface NewMessage {
+  author: Provenance;
+  body: string;
+}
+
+function message(m: NewMessage) {
+  return { id: newId("m"), author: m.author, body: m.body, createdAt: new Date().toISOString() };
+}
+
+/** Lazily-created anchor + thread for a new region discussion (PUC-1). */
+export function addThread(
+  artifact: Artifact,
+  fingerprint: Fingerprint,
+  first: NewMessage,
+): { threadId: string; anchorId: string } {
+  const anchorId = newId("a");
+  const threadId = newId("t");
+  artifact.anchors[anchorId] = { fingerprint };
+  artifact.threads.push({ id: threadId, anchorId, status: "open", messages: [message(first)] });
+  return { threadId, anchorId };
+}
+
+/** Append a reply (PUC-2). Throws if the thread is unknown. */
+export function appendMessage(artifact: Artifact, threadId: string, m: NewMessage): void {
+  const t = artifact.threads.find((x) => x.id === threadId);
+  if (!t) throw new Error(`unknown thread id: ${threadId}`);
+  t.messages.push(message(m));
+}
+
+/** Resolve/reopen a thread (PUC-2). Throws if the thread is unknown. */
+export function setStatus(artifact: Artifact, threadId: string, status: ThreadStatus): void {
+  const t = artifact.threads.find((x) => x.id === threadId);
+  if (!t) throw new Error(`unknown thread id: ${threadId}`);
+  t.status = status;
+}
diff --git a/test/threadModel.test.ts b/test/threadModel.test.ts
new file mode 100644
index 0000000..ab6dd4d
--- /dev/null
+++ b/test/threadModel.test.ts
@@ -0,0 +1,49 @@
+import { describe, it, expect } from "vitest";
+import { emptyArtifact, type Provenance } from "../src/model";
+import { addThread, appendMessage, setStatus } from "../src/threadModel";
+
+const BEN: Provenance = { kind: "human", id: "ben" };
+const fp = { text: "anchored", before: "a", after: "b", lineHint: 3 };
+
+describe("addThread", () => {
+  it("adds an anchor + an open thread with the first message and returns their ids", () => {
+    const a = emptyArtifact("docs/x.md");
+    const { threadId, anchorId } = addThread(a, fp, { author: BEN, body: "first" });
+    expect(Object.keys(a.anchors)).toContain(anchorId);
+    expect(a.anchors[anchorId].fingerprint).toEqual(fp);
+    const t = a.threads.find((x) => x.id === threadId)!;
+    expect(t.anchorId).toBe(anchorId);
+    expect(t.status).toBe("open");
+    expect(t.messages).toHaveLength(1);
+    expect(t.messages[0].body).toBe("first");
+    expect(t.messages[0].author).toEqual(BEN);
+    expect(t.messages[0].id).toMatch(/^m_/);
+    expect(t.messages[0].createdAt).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+  });
+});
+
+describe("appendMessage", () => {
+  it("appends a message to the named thread", () => {
+    const a = emptyArtifact("docs/x.md");
+    const { threadId } = addThread(a, fp, { author: BEN, body: "first" });
+    appendMessage(a, threadId, { author: BEN, body: "reply" });
+    const t = a.threads.find((x) => x.id === threadId)!;
+    expect(t.messages.map((m) => m.body)).toEqual(["first", "reply"]);
+  });
+
+  it("throws for an unknown thread id", () => {
+    const a = emptyArtifact("docs/x.md");
+    expect(() => appendMessage(a, "nope", { author: BEN, body: "x" })).toThrow(/nope/);
+  });
+});
+
+describe("setStatus", () => {
+  it("flips a thread between open and resolved", () => {
+    const a = emptyArtifact("docs/x.md");
+    const { threadId } = addThread(a, fp, { author: BEN, body: "first" });
+    setStatus(a, threadId, "resolved");
+    expect(a.threads[0].status).toBe("resolved");
+    setStatus(a, threadId, "open");
+    expect(a.threads[0].status).toBe("open");
+  });
+});