diff --git a/README.md b/README.md
index 6f211947..39b2e0bb 100644
--- a/README.md
+++ b/README.md
@@ -49,12 +49,26 @@ Hi have a look at program.md and let's kick off a new experiment! let's do the s
 
 The `program.md` file is essentially a super lightweight "skill".
 
+## Multi-GPU (running multiple agents)
+
+If you have multiple GPUs, you can run agents in parallel using `swarm.py`. It creates isolated git worktrees so each agent has its own `train.py` and branch:
+
+```bash
+# Set up 4 agents on 4 GPUs, optionally run baselines
+uv run swarm.py --tag mar10 --gpus 0,1,2,3 --baseline
+
+# Then open Claude Code / Codex in each worktree directory
+# Cleanup when done
+uv run swarm.py --cleanup
+```
+
 ## Project structure
 
 ```
 prepare.py      — constants, data prep + runtime utilities (do not modify)
 train.py        — model, optimizer, training loop (agent modifies this)
 program.md      — agent instructions
+swarm.py        — multi-GPU launcher (creates worktrees for parallel agents)
 pyproject.toml  — dependencies
 ```
 
diff --git a/swarm.py b/swarm.py
new file mode 100644
index 00000000..b3372d80
--- /dev/null
+++ b/swarm.py
@@ -0,0 +1,129 @@
+"""
+Multi-GPU launcher for autoresearch experiments.
+
+Creates isolated git worktrees so multiple agents (Claude Code, Codex, etc.)
+can run experiments in parallel, each on its own GPU and branch.
+
+Usage:
+    uv run swarm.py --tag mar10 --gpus 0,1,2,3
+    uv run swarm.py --tag mar10 --gpus 0,1 --baseline   # also run baseline
+    uv run swarm.py --cleanup                            # remove all worktrees
+"""
+
+import argparse
+import os
+import subprocess
+import sys
+
+BASE_DIR = os.path.dirname(os.path.abspath(__file__))
+WORKTREE_DIR = os.path.join(BASE_DIR, "worktrees")
+
+
+def run(cmd, **kwargs):
+    return subprocess.run(cmd, cwd=BASE_DIR, capture_output=True, text=True, **kwargs)
+
+
+def setup_worktrees(tag, gpu_ids):
+    os.makedirs(WORKTREE_DIR, exist_ok=True)
+    agents = []
+    for i, gpu in enumerate(gpu_ids):
+        branch = f"autoresearch/{tag}/agent-{i}"
+        agent_dir = os.path.join(WORKTREE_DIR, f"agent-{i}")
+        if os.path.isdir(agent_dir):
+            print(f"  agent-{i}: reusing existing worktree")
+        else:
+            result = run(["git", "worktree", "add", "-b", branch, agent_dir, "HEAD"])
+            if result.returncode != 0:
+                print(f"  agent-{i}: failed — {result.stderr.strip()}")
+                continue
+            print(f"  agent-{i}: created on branch {branch}")
+        agents.append({"id": i, "gpu": gpu, "dir": agent_dir, "branch": branch})
+    return agents
+
+
+def run_baselines(agents):
+    """Run baseline train.py on all agents in parallel, one per GPU."""
+    procs = []
+    for a in agents:
+        log_path = os.path.join(a["dir"], "run.log")
+        env = {**os.environ, "CUDA_VISIBLE_DEVICES": str(a["gpu"])}
+        log_f = open(log_path, "w")
+        proc = subprocess.Popen(
+            [sys.executable, os.path.join(a["dir"], "train.py")],
+            cwd=a["dir"], env=env, stdout=log_f, stderr=subprocess.STDOUT,
+        )
+        procs.append((a, proc, log_f))
+        print(f"  agent-{a['id']} (GPU {a['gpu']}): running...")
+
+    print(f"\nWaiting for {len(procs)} baseline runs (~5 min)...\n")
+    for a, proc, log_f in procs:
+        proc.wait()
+        log_f.close()
+        log_path = os.path.join(a["dir"], "run.log")
+        bpb = "N/A"
+        try:
+            with open(log_path) as f:
+                for line in f:
+                    if line.startswith("val_bpb:"):
+                        bpb = line.split(":")[1].strip()
+        except FileNotFoundError:
+            pass
+        status = "OK" if proc.returncode == 0 else "FAILED"
+        print(f"  agent-{a['id']} (GPU {a['gpu']}): {status}, val_bpb={bpb}")
+
+
+def cleanup():
+    result = run(["git", "worktree", "list", "--porcelain"])
+    for line in result.stdout.splitlines():
+        if line.startswith("worktree ") and "/worktrees/agent-" in line:
+            path = line.split("worktree ", 1)[1]
+            run(["git", "worktree", "remove", "--force", path])
+            print(f"  removed {path}")
+    if os.path.isdir(WORKTREE_DIR):
+        try:
+            os.rmdir(WORKTREE_DIR)
+        except OSError:
+            pass
+    print("Cleanup done.")
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Multi-GPU launcher for autoresearch")
+    parser.add_argument("--tag", help="Experiment tag (e.g. mar10)")
+    parser.add_argument("--gpus", help="Comma-separated GPU IDs (e.g. 0,1,2,3)")
+    parser.add_argument("--baseline", action="store_true", help="Run baseline on all agents")
+    parser.add_argument("--cleanup", action="store_true", help="Remove all worktrees")
+    args = parser.parse_args()
+
+    if args.cleanup:
+        cleanup()
+        return
+
+    if not args.tag or not args.gpus:
+        parser.error("--tag and --gpus are required (or use --cleanup)")
+
+    gpu_ids = [int(g.strip()) for g in args.gpus.split(",")]
+    n = len(gpu_ids)
+
+    print(f"Setting up {n} agent worktrees (tag={args.tag}):\n")
+    agents = setup_worktrees(args.tag, gpu_ids)
+
+    if not agents:
+        print("No agents created.")
+        return
+
+    if args.baseline:
+        print(f"\nRunning baselines:\n")
+        run_baselines(agents)
+
+    print(f"\n{'='*60}")
+    print(f"Ready! Open your agent in each worktree:\n")
+    for a in agents:
+        print(f"  GPU {a['gpu']}: cd {a['dir']}")
+    print(f"\nPrompt each agent:")
+    print(f'  "Set CUDA_VISIBLE_DEVICES={agents[0]["gpu"]}. Read program.md and start."')
+    print(f"\nCleanup later: uv run swarm.py --cleanup")
+
+
+if __name__ == "__main__":
+    main()