diff --git a/scripts/collaboration/protocol.ts b/scripts/collaboration/protocol.ts
new file mode 100644
index 0000000..cb72553
--- /dev/null
+++ b/scripts/collaboration/protocol.ts
@@ -0,0 +1,230 @@
+#!/usr/bin/env bun
+/**
+ * Cross-Harness Collaboration Protocol
+ *
+ * Enables two sessions (Claude+Claude, Claude+Codex, Codex+Codex) to
+ * co-work on the same repo via shared artifacts and event messages.
+ *
+ * Communication is file-based and poll-based — no shared process or socket.
+ * Sessions write events to a shared JSONL file and poll for new messages.
+ *
+ * Usage:
+ *   bun run scripts/collaboration/protocol.ts init [session-name]
+ *   bun run scripts/collaboration/protocol.ts send [to] [type] [message]
+ *   bun run scripts/collaboration/protocol.ts poll [session-name]
+ *   bun run scripts/collaboration/protocol.ts status
+ *   bun run scripts/collaboration/protocol.ts claim [file-path]
+ *   bun run scripts/collaboration/protocol.ts release [file-path]
+ */
+
+import { mkdirSync, readFileSync, writeFileSync, appendFileSync, existsSync } from "fs";
+import { join } from "path";
+
+const COLLAB_DIR = join(process.cwd(), ".productionos", "collaboration");
+const MESSAGES_FILE = join(COLLAB_DIR, "messages.jsonl");
+const STATUS_FILE = join(COLLAB_DIR, "status.json");
+const PLAN_FILE = join(COLLAB_DIR, "plan.md");
+const OWNERSHIP_FILE = join(COLLAB_DIR, "ownership.json");
+
+interface CollabMessage {
+  timestamp: string;
+  from: string;
+  to: string | "*";
+  type: "task-assigned" | "task-complete" | "finding" | "question" | "status-update" | "file-claim" | "file-release";
+  payload: Record<string, unknown>;
+}
+
+interface SessionStatus {
+  sessions: Record<string, {
+    name: string;
+    started: string;
+    lastActive: string;
+    status: "active" | "paused" | "done";
+    worktree?: string;
+    currentTask?: string;
+  }>;
+}
+
+interface OwnershipMap {
+  files: Record<string, {
+    owner: string;
+    claimed: string;
+    mode: "exclusive" | "readonly";
+  }>;
+}
+
+function ensureDir(): void {
+  mkdirSync(COLLAB_DIR, { recursive: true });
+  if (!existsSync(MESSAGES_FILE)) writeFileSync(MESSAGES_FILE, "");
+  if (!existsSync(STATUS_FILE)) writeFileSync(STATUS_FILE, JSON.stringify({ sessions: {} }, null, 2));
+  if (!existsSync(OWNERSHIP_FILE)) writeFileSync(OWNERSHIP_FILE, JSON.stringify({ files: {} }, null, 2));
+}
+
+function readStatus(): SessionStatus {
+  try {
+    return JSON.parse(readFileSync(STATUS_FILE, "utf-8"));
+  } catch {
+    return { sessions: {} };
+  }
+}
+
+function writeStatus(status: SessionStatus): void {
+  writeFileSync(STATUS_FILE, JSON.stringify(status, null, 2));
+}
+
+function readOwnership(): OwnershipMap {
+  try {
+    return JSON.parse(readFileSync(OWNERSHIP_FILE, "utf-8"));
+  } catch {
+    return { files: {} };
+  }
+}
+
+function writeOwnership(ownership: OwnershipMap): void {
+  writeFileSync(OWNERSHIP_FILE, JSON.stringify(ownership, null, 2));
+}
+
+function sendMessage(msg: CollabMessage): void {
+  appendFileSync(MESSAGES_FILE, JSON.stringify(msg) + "\n");
+}
+
+function readMessages(since?: string): CollabMessage[] {
+  if (!existsSync(MESSAGES_FILE)) return [];
+  const lines = readFileSync(MESSAGES_FILE, "utf-8").trim().split("\n").filter(Boolean);
+  const messages = lines.map((line) => {
+    try { return JSON.parse(line) as CollabMessage; }
+    catch { return null; }
+  }).filter(Boolean) as CollabMessage[];
+  if (since) {
+    return messages.filter((m) => m.timestamp > since);
+  }
+  return messages;
+}
+
+// === Commands ===
+
+const [, , command, ...args] = process.argv;
+
+switch (command) {
+  case "init": {
+    const sessionName = args[0] || `session-${Date.now()}`;
+    ensureDir();
+    const status = readStatus();
+    status.sessions[sessionName] = {
+      name: sessionName,
+      started: new Date().toISOString(),
+      lastActive: new Date().toISOString(),
+      status: "active",
+    };
+    writeStatus(status);
+    if (!existsSync(PLAN_FILE)) {
+      writeFileSync(PLAN_FILE, `# Collaboration Plan\n\nCreated: ${new Date().toISOString()}\nSessions: ${sessionName}\n\n## Tasks\n\n- [ ] (assign tasks here)\n`);
+    }
+    console.log(JSON.stringify({ ok: true, session: sessionName, dir: COLLAB_DIR }));
+    break;
+  }
+
+  case "send": {
+    const [to, type, ...messageParts] = args;
+    if (!to || !type) {
+      console.error("Usage: send [to] [type] [message]");
+      process.exit(1);
+    }
+    ensureDir();
+    const msg: CollabMessage = {
+      timestamp: new Date().toISOString(),
+      from: process.env.COLLAB_SESSION || "unknown",
+      to,
+      type: type as CollabMessage["type"],
+      payload: { message: messageParts.join(" ") },
+    };
+    sendMessage(msg);
+    console.log(JSON.stringify({ ok: true, sent: msg }));
+    break;
+  }
+
+  case "poll": {
+    const sessionName = args[0];
+    const since = args[1]; // ISO timestamp
+    const messages = readMessages(since);
+    const relevant = sessionName
+      ? messages.filter((m) => m.to === sessionName || m.to === "*")
+      : messages;
+    console.log(JSON.stringify({ messages: relevant, count: relevant.length }));
+    break;
+  }
+
+  case "status": {
+    ensureDir();
+    const status = readStatus();
+    const ownership = readOwnership();
+    console.log(JSON.stringify({ sessions: status.sessions, files: ownership.files }, null, 2));
+    break;
+  }
+
+  case "claim": {
+    const filePath = args[0];
+    const sessionName = args[1] || process.env.COLLAB_SESSION || "unknown";
+    if (!filePath) {
+      console.error("Usage: claim [file-path] [session-name]");
+      process.exit(1);
+    }
+    ensureDir();
+    const ownership = readOwnership();
+    if (ownership.files[filePath] && ownership.files[filePath].owner !== sessionName) {
+      console.log(JSON.stringify({ ok: false, error: `File claimed by ${ownership.files[filePath].owner}` }));
+      process.exit(1);
+    }
+    ownership.files[filePath] = {
+      owner: sessionName,
+      claimed: new Date().toISOString(),
+      mode: "exclusive",
+    };
+    writeOwnership(ownership);
+    sendMessage({
+      timestamp: new Date().toISOString(),
+      from: sessionName,
+      to: "*",
+      type: "file-claim",
+      payload: { file: filePath },
+    });
+    console.log(JSON.stringify({ ok: true, claimed: filePath }));
+    break;
+  }
+
+  case "release": {
+    const filePath = args[0];
+    if (!filePath) {
+      console.error("Usage: release [file-path]");
+      process.exit(1);
+    }
+    ensureDir();
+    const ownership = readOwnership();
+    const sessionName = args[1] || process.env.COLLAB_SESSION || "unknown";
+    delete ownership.files[filePath];
+    writeOwnership(ownership);
+    sendMessage({
+      timestamp: new Date().toISOString(),
+      from: sessionName,
+      to: "*",
+      type: "file-release",
+      payload: { file: filePath },
+    });
+    console.log(JSON.stringify({ ok: true, released: filePath }));
+    break;
+  }
+
+  default:
+    console.log(`Cross-Harness Collaboration Protocol
+
+Commands:
+  init [name]              Register this session
+  send [to] [type] [msg]   Send message to another session
+  poll [name] [since]      Read messages for this session
+  status                   Show active sessions and file ownership
+  claim [file] [name]      Claim exclusive access to a file
+  release [file]           Release file ownership
+
+Message types: task-assigned, task-complete, finding, question, status-update, file-claim, file-release
+`);
+}