diff --git a/bincio/dev.py b/bincio/dev.py
index b40d441..8d8751c 100644
--- a/bincio/dev.py
+++ b/bincio/dev.py
@@ -68,7 +68,18 @@ def _merge_all_users(data: Path) -> None:
     _write_root_manifest(data)
 
 
-def _start_serve(data: Path, api_port: int, site: Path) -> None:
+def _local_ip() -> str:
+    """Return the machine's primary LAN IP (best-effort)."""
+    import socket
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
+            s.connect(("8.8.8.8", 80))
+            return s.getsockname()[0]
+    except Exception:
+        return "127.0.0.1"
+
+
+def _start_serve(data: Path, api_port: int, site: Path, api_host: str = "127.0.0.1") -> None:
     """Start bincio serve in a background thread."""
     import uvicorn
     import bincio.serve.server as srv
@@ -78,7 +89,7 @@ def _start_serve(data: Path, api_port: int, site: Path) -> None:
 
     config = uvicorn.Config(
         srv.app,
-        host="127.0.0.1",
+        host=api_host,
         port=api_port,
         log_level="warning",   # quiet — astro dev output takes priority
     )
@@ -147,11 +158,14 @@ def _watch_data(data: Path) -> None:
 @click.option("--site-dir", default=None, help="Astro project directory (default: ./site)")
 @click.option("--port", default=4321, show_default=True, help="Astro dev server port")
 @click.option("--api-port", default=4041, show_default=True, help="bincio serve API port")
+@click.option("--api-host", default="127.0.0.1", show_default=True,
+              help="Host for bincio serve. Use 0.0.0.0 to expose on the local network for mobile testing.")
 def dev(
     data_dir: Optional[str],
     site_dir: Optional[str],
     port: int,
     api_port: int,
+    api_host: str,
 ) -> None:
     """Start the local dev environment: bincio serve + astro dev.
 
@@ -169,11 +183,16 @@ def dev(
 
     has_auth = (data / "instance.db").exists()
 
+    lan_ip = _local_ip() if api_host == "0.0.0.0" else api_host
+    mobile_url = f"http://{lan_ip}:{api_port}" if api_host == "0.0.0.0" else None
+
     console.print(f"[bold]bincio dev[/bold]")
     console.print(f"  Data:    [cyan]{data}[/cyan]")
     console.print(f"  Site:    [cyan]{site}[/cyan]")
     if has_auth:
-        console.print(f"  API:     [cyan]http://127.0.0.1:{api_port}[/cyan]")
+        console.print(f"  API:     [cyan]http://{api_host}:{api_port}[/cyan]")
+        if mobile_url:
+            console.print(f"  Mobile:  [bold cyan]{mobile_url}[/bold cyan]  ← set this as instance URL in the app")
     else:
         console.print(f"  Auth:    [yellow]none[/yellow] (single-user, no instance.db)")
     console.print(f"  Browser: [cyan]http://localhost:{port}[/cyan]")
@@ -196,8 +215,8 @@ def dev(
 
     # Start bincio serve only when instance.db exists (auth / write API)
     if has_auth:
-        console.print(f"Starting [cyan]bincio serve[/cyan] on port {api_port}…")
-        t = threading.Thread(target=_start_serve, args=(data, api_port, site), daemon=True)
+        console.print(f"Starting [cyan]bincio serve[/cyan] on {api_host}:{api_port}…")
+        t = threading.Thread(target=_start_serve, args=(data, api_port, site, api_host), daemon=True)
         t.start()
 
     # Watch data dir for sidecar/activity changes → auto-merge
diff --git a/scripts/dev_test.py b/scripts/dev_test.py
index 7d559e9..2ed6f01 100755
--- a/scripts/dev_test.py
+++ b/scripts/dev_test.py
@@ -11,6 +11,7 @@ Run from the project root:
 Options:
     --fresh    Wipe DATA_DIR before starting (default: reuse if it exists)
     --no-dev   Stop after extract (skip `bincio dev`)
+    --mobile   Bind API to 0.0.0.0 for mobile app testing on the same WiFi
 
 Credentials:  dave / testpass  and  brut / testpass
 URL:          http://localhost:4321
@@ -139,7 +140,7 @@ def prepare_serve() -> None:
 
 # ── 4. Hand off to bincio dev ─────────────────────────────────────────────────
 
-def start_dev() -> None:
+def start_dev(mobile: bool = False) -> None:
     section("Starting bincio dev")
     print()
     print("  \033[1mCredentials\033[0m")
@@ -150,11 +151,12 @@ def start_dev() -> None:
     print()
     print("  Press Ctrl+C to stop.\n")
 
+    cmd = ["uv", "run", "bincio", "dev", "--data-dir", str(DATA_DIR)]
+    if mobile:
+        cmd += ["--api-host", "0.0.0.0"]
+
     try:
-        subprocess.run(
-            ["uv", "run", "bincio", "dev", "--data-dir", str(DATA_DIR)],
-            cwd=PROJECT_DIR,
-        )
+        subprocess.run(cmd, cwd=PROJECT_DIR)
     except KeyboardInterrupt:
         pass
 
@@ -166,6 +168,7 @@ def main() -> None:
                                      formatter_class=argparse.RawDescriptionHelpFormatter)
     parser.add_argument("--fresh",  action="store_true", help="Wipe DATA_DIR before starting")
     parser.add_argument("--no-dev", action="store_true", help="Stop after extract, skip bincio dev")
+    parser.add_argument("--mobile", action="store_true", help="Bind API to 0.0.0.0 for local mobile testing")
     args = parser.parse_args()
 
     print(f"\033[1mbincio dev test\033[0m  →  {DATA_DIR}")
@@ -181,7 +184,7 @@ def main() -> None:
     prepare_serve()
 
     if not args.no_dev:
-        start_dev()
+        start_dev(mobile=args.mobile)
     else:
         print(f"\n\033[32mDone.\033[0m  Data ready at {DATA_DIR}")
         print(f"Run:  uv run bincio dev --data-dir {DATA_DIR}\n")