diff --git a/CLAUDE.md b/CLAUDE.md
index 6d53f15..287dc09 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -31,6 +31,7 @@ Repo: https://gitea.apointless.space/bsncubed/aes67-ESP32-P4
 - Flash over the network (normal way since step 2b; keep USB for recovery):
   `curl -f --data-binary @build/aes67_p4.bin -H 'Content-Type: application/octet-stream' http://p4-aes67/api/ota`
   The board reboots into the new image on trial; check `GET /api/ota` shows the new version with `pending_verify: false`.
+- Another board, over USB: `tools/flash_board.py` (checks chip revision and flash size, optional erase, flashes, finds the IP in the boot log, sets hostname / stream name / multicast so boards don't clash). Needs a build and esptool (IDF env or `pip install esptool`).
 - Serial: opening /dev/ttyACM0 can reset the board. Don't open it while an OTA image is on trial (a reset then counts as a failed boot and rolls back).
 - Rollback test build (self-test always fails), in its own build dir:
   `idf.py -B build-selftest-fail -DSDKCONFIG=build-selftest-fail/sdkconfig -DSDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.selftest_fail" build`
diff --git a/tools/flash_board.py b/tools/flash_board.py
new file mode 100755
index 0000000..c9450be
--- /dev/null
+++ b/tools/flash_board.py
@@ -0,0 +1,222 @@
+#!/usr/bin/env python3
+"""Flash this firmware onto another ESP32-P4 board over USB, then give it its own identity.
+
+Steps: pick the port, check the chip (ESP32-P4, revision the image supports, flash size),
+optionally erase, write bootloader + partition table + app, wait for the board's IP in its boot
+log, then set hostname / stream name / multicast address over the web API so it doesn't clash
+with other boards on the network.
+
+Needs esptool (in the ESP-IDF Python env, or `pip install esptool`) and a build (`idf.py build`).
+Every question can also be answered with an option, e.g.:
+  tools/flash_board.py --port /dev/ttyACM0 --erase --hostname p4-aes67-2 --name "P4 AES67 2" \\
+                       --mcast 239.69.1.11
+Boards already running this firmware and on the network update over the web (OTA) instead.
+"""
+import argparse
+import json
+import os
+import re
+import struct
+import subprocess
+import sys
+import time
+import urllib.request
+
+REPO = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+MIN_FLASH_MB = 32   # the bootloader header says 32 MB (sdkconfig); smaller chips aren't supported
+
+
+def die(msg):
+    print(f"\nERROR: {msg}")
+    sys.exit(1)
+
+
+def ask(prompt, default=None):
+    s = input(f"{prompt}{f' [{default}]' if default is not None else ''}: ").strip()
+    return s or (default if default is not None else "")
+
+
+def yes(prompt, default):
+    s = input(f"{prompt} [{'Y/n' if default else 'y/N'}]: ").strip().lower()
+    return default if not s else s.startswith("y")
+
+
+def esptool(port, *args, capture=True):
+    cmd = [sys.executable, "-m", "esptool", "--chip", "esp32p4", "--port", port, *args]
+    if not capture:
+        if subprocess.call(cmd) != 0:
+            die("esptool failed (see above)")
+        return ""
+    r = subprocess.run(cmd, capture_output=True, text=True)
+    if r.returncode != 0:
+        die(f"esptool {' '.join(args)} failed:\n{r.stdout}{r.stderr}")
+    return r.stdout
+
+
+def pick_port(given):
+    if given:
+        return given
+    try:
+        from serial.tools import list_ports
+    except ImportError:
+        die("pyserial missing: run inside the ESP-IDF environment or `pip install esptool`")
+    ports = [p for p in list_ports.comports() if "ACM" in p.device or "USB" in p.device]
+    if not ports:
+        die("no USB serial port found: connect the board's USB-C port (not the PoE/Ethernet side)")
+    for i, p in enumerate(ports, 1):
+        print(f"  {i}) {p.device}  {p.description}")
+    if len(ports) == 1:
+        return ports[0].device if yes(f"Use {ports[0].device}?", True) else die("no port chosen")
+    n = ask("Port number", "1")
+    return ports[int(n) - 1].device
+
+
+def load_images(build):
+    """(offset, path) pairs and flash options from build/flash_args."""
+    fa = os.path.join(build, "flash_args")
+    if not os.path.exists(fa):
+        die(f"{fa} not found: build first (idf.py build)")
+    lines = open(fa).read().split("\n")
+    opts = lines[0].split()
+    images = []
+    for line in lines[1:]:
+        if line.strip():
+            off, path = line.split()
+            images.append((off, os.path.join(build, path)))
+    return opts, images
+
+
+def image_rev_range(app):
+    """Chip revision range from the app image header (esp_image_header_t), as (min, max) * 100."""
+    h = open(app, "rb").read(24)
+    if h[0] != 0xE9:
+        die(f"{app} is not an ESP app image")
+    min_full, max_full = struct.unpack_from("<HH", h, 15)
+    return min_full, max_full
+
+
+def app_version(app):
+    # esp_app_desc_t after the image header (24 B) and first segment header (8 B): version at +16.
+    d = open(app, "rb").read(24 + 8 + 256)[32:]
+    return d[16:48].split(b"\0")[0].decode(errors="replace")
+
+
+def check_chip(port, app):
+    out = esptool(port, "flash_id")
+    m = re.search(r"ESP32-P4 \(revision v(\d+)\.(\d+)\)", out)
+    if not m:
+        die(f"not an ESP32-P4?\n{out}")
+    rev = int(m.group(1)) * 100 + int(m.group(2))
+    lo, hi = image_rev_range(app)
+    print(f"  chip: ESP32-P4 revision v{rev // 100}.{rev % 100}")
+    if not lo <= rev <= hi:
+        die(f"this build supports chip revisions v{lo // 100}.{lo % 100} to v{hi // 100}.{hi % 100}; "
+            f"the board is v{rev // 100}.{rev % 100}. Build with the matching "
+            "CONFIG_ESP32P4_SELECTS_REV_LESS_V3 setting (see CLAUDE.md).")
+    m = re.search(r"Detected flash size: (\d+)MB", out)
+    if not m:
+        die(f"could not read the flash size:\n{out}")
+    mb = int(m.group(1))
+    print(f"  flash: {mb} MB")
+    if mb < MIN_FLASH_MB:
+        die(f"{mb} MB flash; this build needs {MIN_FLASH_MB} MB")
+
+
+def wait_for_ip(port, timeout=90):
+    """Read the boot log until the board reports its IP (the port open may reset it: fine here)."""
+    import serial
+    print(f"  waiting for the board's IP (boot log on {port}, up to {timeout} s; Ethernet plugged in?)")
+    end = time.time() + timeout
+    with serial.Serial(port, 115200, timeout=1) as s:
+        buf = b""
+        while time.time() < end:
+            buf += s.read(4096)
+            m = re.search(rb"got IP (\d+\.\d+\.\d+\.\d+)", buf)
+            if m:
+                return m.group(1).decode()
+            buf = buf[-4096:]
+    return None
+
+
+def api(ip, method, path, body=None):
+    req = urllib.request.Request(f"http://{ip}{path}", method=method,
+                                 data=json.dumps(body).encode() if body is not None else None,
+                                 headers={"Content-Type": "application/json"})
+    with urllib.request.urlopen(req, timeout=10) as r:
+        t = r.read()
+        return json.loads(t) if t else {}
+
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("--port")
+    ap.add_argument("--build", default=os.path.join(REPO, "build"), help="build directory (default: build/)")
+    ap.add_argument("--erase", action=argparse.BooleanOptionalAction,
+                    help="erase the whole flash first (clears any stored config)")
+    ap.add_argument("--hostname")
+    ap.add_argument("--name", help="stream / Spotify device name")
+    ap.add_argument("--mcast", help="AES67 multicast address")
+    ap.add_argument("--no-config", action="store_true", help="flash only, keep the default identity")
+    a = ap.parse_args()
+
+    opts, images = load_images(a.build)
+    app = next(p for off, p in images if p.endswith("aes67_p4.bin"))
+    print(f"Firmware: {app_version(app)}  ({app})\n")
+
+    print("1) Port")
+    port = pick_port(a.port)
+
+    print("\n2) Checking the board")
+    check_chip(port, app)
+
+    print("\n3) Flashing")
+    erase = a.erase if a.erase is not None else yes(
+        "  Erase the whole flash first? (yes for a new board or to clear an old config)", True)
+    if erase:
+        esptool(port, "erase_flash", capture=False)
+    args = ["write_flash", *opts]
+    for off, path in images:
+        args += [off, path]
+    esptool(port, *args, capture=False)
+
+    if a.no_config:
+        print("\nDone (default identity: hostname p4-aes67, multicast 239.69.1.10).")
+        return
+
+    print("\n4) Finding the board on the network")
+    ip = wait_for_ip(port)
+    if not ip:
+        die("no IP seen in the boot log. Check the Ethernet cable/DHCP; the board is flashed, set the "
+            "identity later in the web UI.")
+    print(f"  board is at {ip}")
+    for _ in range(30):     # web server comes up right after the IP
+        try:
+            cfg = api(ip, "GET", "/api/config")
+            break
+        except OSError:
+            time.sleep(1)
+    else:
+        die(f"web UI at http://{ip}/ does not answer")
+
+    print("\n5) Identity (must differ from every other board on this network)")
+    host = a.hostname or ask("  Hostname", "p4-aes67-2")
+    name = a.name or ask("  Stream / Spotify device name", "P4 AES67 2")
+    mcast = a.mcast or ask("  AES67 multicast address", "239.69.1.11")
+    cfg["net"]["hostname"] = host
+    cfg["aes67"]["name"] = name
+    cfg["aes67"]["mcast"] = mcast
+    cfg["source"]["spotify_name"] = name
+    try:
+        api(ip, "POST", "/api/config", cfg)
+    except urllib.error.HTTPError as e:
+        die(f"config rejected: {e.read().decode()}")
+    print(f"\nDone. Web UI: http://{ip}/  or  http://{host}.local/")
+    print("Still to set in the web UI if needed: Spotify client ID/secret, source mode, PTP role.")
+
+
+if __name__ == "__main__":
+    try:
+        main()
+    except KeyboardInterrupt:
+        print("\naborted")
+        sys.exit(1)