diff --git a/browser_cli/auth.py b/browser_cli/auth.py
index 97f956e..94dca13 100644
--- a/browser_cli/auth.py
+++ b/browser_cli/auth.py
@@ -174,7 +174,7 @@ def verify(pub_hex: str, nonce: bytes, msg: dict, sig_hex: str) -> bool:
         message = nonce + hashlib.sha256(canonical_payload(msg)).digest()
         pub_key.verify(bytes.fromhex(sig_hex), message)
         return True
-    except (InvalidSignature, Exception):
+    except (InvalidSignature, ValueError):
         return False
 
 
diff --git a/browser_cli/client.py b/browser_cli/client.py
index 6014cbf..24844fb 100644
--- a/browser_cli/client.py
+++ b/browser_cli/client.py
@@ -19,6 +19,7 @@ from pathlib import Path
 from typing import Any
 
 from browser_cli.platform import endpoint_for_alias, is_windows, registry_path
+from browser_cli.version_manager import MAX_MSG_BYTES as _MAX_MSG_BYTES
 from browser_cli.registry import load_registry
 
 try:
@@ -72,7 +73,7 @@ def _load_remotes() -> dict[str, dict[str, str]]:
 
 def _is_valid_key_spec(s: str) -> bool:
     """Return True if s looks like a usable key spec: 'agent', 'agent:<sel>', or a file path."""
-    return s == "agent" or s.startswith("agent:") or (not s.startswith("<") and "/" in s or Path(s).suffix in {".pem", ".key"})
+    return s == "agent" or s.startswith("agent:") or (not s.startswith("<") and ("/" in s or Path(s).suffix in {".pem", ".key"}))
 
 
 def save_remote_key(endpoint: str, key_spec: str) -> None:
@@ -327,22 +328,22 @@ def send_command(command: str, args: dict | None = None, profile: str | None = N
             msg["_route"] = route_profile
     else:
         private_key = None
-    payload = json.dumps(msg).encode("utf-8")
-    framed = struct.pack("<I", len(payload)) + payload
 
     try:
         if remote_endpoint:
             response = _send_remote(remote_endpoint, msg, private_key)
         elif is_windows():
+            payload = json.dumps(msg).encode("utf-8")
             sock_path = _resolve_socket(profile)
             with PipeClient(sock_path, family="AF_PIPE") as conn:
                 conn.send_bytes(payload)
                 response = conn.recv_bytes()
         else:
+            payload = json.dumps(msg).encode("utf-8")
             sock_path = _resolve_socket(profile)
             with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as sock:
                 sock.connect(sock_path)
-                sock.sendall(framed)
+                sock.sendall(struct.pack("<I", len(payload)) + payload)
                 response = _recv_all(sock)
     except (FileNotFoundError, ConnectionRefusedError, OSError):
         if remote_endpoint:
@@ -368,9 +369,6 @@ def send_command(command: str, args: dict | None = None, profile: str | None = N
     return result.get("data")
 
 
-_MAX_MSG_BYTES = 32 * 1024 * 1024
-
-
 def _recv_all(sock: socket.socket) -> bytes:
     raw_len = _recv_exact(sock, 4)
     msg_len = struct.unpack("<I", raw_len)[0]
diff --git a/browser_cli/commands/__init__.py b/browser_cli/commands/__init__.py
index e69de29..ff8eae7 100644
--- a/browser_cli/commands/__init__.py
+++ b/browser_cli/commands/__init__.py
@@ -0,0 +1,40 @@
+import click
+from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
+from rich.console import Console
+
+_console = Console()
+
+
+def _handle(command, args=None, profile=None):
+    try:
+        return send_command(command, args or {}, profile=profile)
+    except BrowserNotConnected as e:
+        _console.print(f"[red]Error:[/red] {e}")
+        raise SystemExit(1)
+    except RuntimeError as e:
+        _console.print(f"[red]Browser error:[/red] {e}")
+        raise SystemExit(1)
+
+
+def _handle_multi(command, args=None, profile=None, remote=None):
+    try:
+        if remote:
+            return send_command(command, args or {}, profile=profile, remote=remote)
+        return send_command(command, args or {}, profile=profile)
+    except (BrowserNotConnected, RuntimeError):
+        return None
+
+
+def _multi_browser_targets():
+    root = click.get_current_context().find_root()
+    if root.obj.get("browser_explicit"):
+        return []
+    remote = root.obj.get("remote")
+    key = root.obj.get("key")
+    if remote:
+        targets = remote_browser_targets(remote, key=key)
+    else:
+        targets = active_browser_targets(key=key)
+    if len(targets) <= 1 and not any(target.remote for target in targets):
+        return []
+    return targets
diff --git a/browser_cli/commands/cookies.py b/browser_cli/commands/cookies.py
index 26c7105..eea7365 100644
--- a/browser_cli/commands/cookies.py
+++ b/browser_cli/commands/cookies.py
@@ -1,22 +1,11 @@
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None):
-    try:
-        return send_command(command, args or {})
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("cookies")
 def cookies_group():
     """Manage browser cookies."""
diff --git a/browser_cli/commands/dom.py b/browser_cli/commands/dom.py
index 9ab07a2..0d85aeb 100644
--- a/browser_cli/commands/dom.py
+++ b/browser_cli/commands/dom.py
@@ -1,5 +1,5 @@
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 from rich.table import Table
 import json
@@ -7,17 +7,6 @@ import json
 console = Console()
 
 
-def _handle(command, args=None):
-    try:
-        return send_command(command, args or {})
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("dom")
 def dom_group():
     """Query and interact with page DOM elements."""
diff --git a/browser_cli/commands/extract.py b/browser_cli/commands/extract.py
index 6918f9f..3107c0f 100644
--- a/browser_cli/commands/extract.py
+++ b/browser_cli/commands/extract.py
@@ -3,7 +3,7 @@ import re
 from html.parser import HTMLParser
 
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 from rich.table import Table
 
@@ -423,17 +423,6 @@ def _convert_html_to_markdown(html):
     return _clean_markdown_output(markdown)
 
 
-def _handle(command, args=None):
-    try:
-        return send_command(command, args or {})
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("extract")
 def extract_group():
     """Extract content from the active tab."""
diff --git a/browser_cli/commands/groups.py b/browser_cli/commands/groups.py
index c0b1b42..bb760c4 100644
--- a/browser_cli/commands/groups.py
+++ b/browser_cli/commands/groups.py
@@ -1,46 +1,11 @@
 import click
-from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
+from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None, profile=None):
-    try:
-        return send_command(command, args or {}, profile=profile)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
-def _handle_multi(command, args=None, profile=None, remote=None):
-    try:
-        if remote:
-            return send_command(command, args or {}, profile=profile, remote=remote)
-        return send_command(command, args or {}, profile=profile)
-    except (BrowserNotConnected, RuntimeError):
-        return None
-
-
-def _multi_browser_targets():
-    root = click.get_current_context().find_root()
-    if root.obj.get("browser_explicit"):
-        return []
-    remote = root.obj.get("remote")
-    key = root.obj.get("key")
-    if remote:
-        targets = remote_browser_targets(remote, key=key)
-    else:
-        targets = active_browser_targets(key=key)
-    if len(targets) <= 1 and not any(target.remote for target in targets):
-        return []
-    return targets
-
-
 def _print_groups(groups: list[dict], *, show_browser: bool = False) -> None:
     if not groups:
         console.print("[yellow]No groups found[/yellow]")
diff --git a/browser_cli/commands/navigate.py b/browser_cli/commands/navigate.py
index 70bcbed..ef31b1f 100644
--- a/browser_cli/commands/navigate.py
+++ b/browser_cli/commands/navigate.py
@@ -1,21 +1,10 @@
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 
 console = Console()
 
 
-def _handle(command, args):
-    try:
-        return send_command(command, args)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("nav")
 def nav_group():
     """Navigate — open URLs, reload, go back/forward, focus tabs."""
diff --git a/browser_cli/commands/page.py b/browser_cli/commands/page.py
index 2579cc4..99cd4de 100644
--- a/browser_cli/commands/page.py
+++ b/browser_cli/commands/page.py
@@ -1,22 +1,11 @@
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None):
-    try:
-        return send_command(command, args or {})
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("page")
 def page_group():
     """Inspect current page metadata."""
diff --git a/browser_cli/commands/search.py b/browser_cli/commands/search.py
index 40fb08f..f0aaeaf 100644
--- a/browser_cli/commands/search.py
+++ b/browser_cli/commands/search.py
@@ -1,6 +1,6 @@
 import click
 from urllib.parse import quote_plus
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 
 console = Console()
@@ -71,14 +71,7 @@ def _build_command(engine_key: str, help_text: str) -> click.Command:
     def _cmd(query, bg, window, group):
         terms = " ".join(query)
         url = ENGINES[engine_key].format(query=quote_plus(terms))
-        try:
-            send_command("navigate.open", {"url": url, "background": bg, "window": window, "group": group})
-        except BrowserNotConnected as e:
-            console.print(f"[red]Error:[/red] {e}")
-            raise SystemExit(1)
-        except RuntimeError as e:
-            console.print(f"[red]Browser error:[/red] {e}")
-            raise SystemExit(1)
+        _handle("navigate.open", {"url": url, "background": bg, "window": window, "group": group})
         suffix = f" in group '{group}'" if group else (f" in window '{window}'" if window else "")
         display = _DISPLAY_NAMES.get(engine_key, engine_key.capitalize())
         console.print(f"[green]Searching[/green] [cyan]{display}[/cyan]: {terms}{suffix}")
diff --git a/browser_cli/commands/serve.py b/browser_cli/commands/serve.py
index d471c33..8833409 100644
--- a/browser_cli/commands/serve.py
+++ b/browser_cli/commands/serve.py
@@ -1,25 +1,20 @@
-import re, threading, secrets, socket, struct, click, json, sys, os
+import re, threading, secrets, socket, struct, click, json, sys
+from datetime import datetime
 from pathlib import Path
-from browser_cli.version_manager import PROTOCOL_MIN_CLIENT, parse_version, get_installed_version
-from browser_cli.compat import adapt_request, adapt_response
+
+from rich.console import Console
+
+from browser_cli.client import _recv_exact, _recv_all
+from browser_cli.compat import adapt_auth, adapt_request, adapt_response
+from browser_cli.version_manager import PROTOCOL_MIN_CLIENT, MAX_MSG_BYTES, parse_version, get_installed_version
 
 _UA_PATTERN = re.compile(r"^browser-cli/\d")
-
 _CONN_LIMIT = threading.BoundedSemaphore(64)
-_MAX_MSG_BYTES = 32 * 1024 * 1024
-from rich.console import Console
-from datetime import datetime
-
 console = Console()
 
-def _recv_exact(sock:socket.socket, n:int) -> bytes:
-  buf = b""
-  while len(buf) < n:
-    chunk = sock.recv(n - len(buf))
-    if not chunk:
-      raise ConnectionError("Connection closed")
-    buf += chunk
-  return buf
+
+def _framed_send(sock: socket.socket, data: bytes) -> None:
+  sock.sendall(struct.pack("<I", len(data)) + data)
 
 def _log(addr:tuple, command:str, profile:str|None, status:str, error:str|None=None) -> None:
   ts = datetime.now().strftime("%H:%M:%S")
@@ -37,14 +32,21 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
   def _send_error(msg_id, msg:str) -> None:
     err = json.dumps({"id": msg_id, "success": False, "error": msg}).encode()
     try:
-      client_sock.sendall(struct.pack("<I", len(err)) + err)
+      _framed_send(client_sock, err)
+    except OSError:
+      pass
+
+  def _send_ok(msg_id, payload) -> None:
+    out = json.dumps({"id": msg_id, "success": True, "data": payload}).encode()
+    try:
+      _framed_send(client_sock, out)
     except OSError:
       pass
 
   try:
     header = _recv_exact(client_sock, 4)
     msg_len = struct.unpack("<I", header)[0]
-    if msg_len > _MAX_MSG_BYTES:
+    if msg_len > MAX_MSG_BYTES:
       _send_error(None, f"message too large ({msg_len} bytes)")
       return
     payload = _recv_exact(client_sock, msg_len)
@@ -58,25 +60,26 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
     _log(addr, "?", None, "ERROR", "invalid JSON")
     return
 
-  msg_id = msg.get("id")
-  command = msg.get("command", "?")
-
   # ── user-agent + version check ────────────────────────────────────────────
+  msg_id = msg.get("id")
   ua = msg.get("user_agent") or ""
   if not _UA_PATTERN.match(ua):
     _send_error(msg_id, "forbidden: client required")
-    _log(addr, command, None, "DENIED", f"bad user-agent: {ua!r}")
+    _log(addr, msg.get("command", "?"), None, "DENIED", f"bad user-agent: {ua!r}")
     return
   client_ver = "0"
   try:
     client_ver = ua.split("/", 1)[1]
     if parse_version(client_ver) < parse_version(PROTOCOL_MIN_CLIENT):
       _send_error(msg_id, f"client version {client_ver} is too old; please upgrade to >= {PROTOCOL_MIN_CLIENT}")
-      _log(addr, command, None, "DENIED", f"client {client_ver} < min {PROTOCOL_MIN_CLIENT}")
+      _log(addr, msg.get("command", "?"), None, "DENIED", f"client {client_ver} < min {PROTOCOL_MIN_CLIENT}")
       return
   except (IndexError, ValueError):
     pass
 
+  msg = adapt_auth(msg, client_ver)
+  command = msg.get("command", "?")
+
   # ── auth ──────────────────────────────────────────────────────────────────
   if auth_keys is not None:
     pub = msg.get("pubkey") or ""
@@ -101,8 +104,7 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
       {"profile": target.profile, "displayName": target.display_name}
       for target in active_browser_targets(include_remotes=False)
     ]
-    data = json.dumps({"id": msg_id, "success": True, "data": targets}).encode()
-    client_sock.sendall(struct.pack("<I", len(data)) + data)
+    _send_ok(msg_id, targets)
     _log(addr, command, None, "OK")
     return
 
@@ -113,8 +115,7 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
       return
     from browser_cli.auth import load_authorized_keys_with_names
     entries = [{"pubkey": pk, "name": name} for pk, name in load_authorized_keys_with_names(auth_keys_path)]
-    data = json.dumps({"id": msg_id, "success": True, "data": entries}).encode()
-    client_sock.sendall(struct.pack("<I", len(data)) + data)
+    _send_ok(msg_id, entries)
     _log(addr, command, None, "OK")
     return
 
@@ -127,13 +128,12 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
     args = msg.get("args") or {}
     pubkey = str(args.get("pubkey") or "")
     name = str(args.get("name") or "")
-    if len(pubkey) != 64:
-      _send_error(msg_id, "invalid pubkey: expected 64 hex characters")
+    if not re.fullmatch(r"[0-9a-f]{64}", pubkey):
+      _send_error(msg_id, "invalid pubkey: expected 64 lowercase hex characters")
       _log(addr, command, None, "ERROR", "invalid pubkey")
       return
     added = add_authorized_key(auth_keys_path, pubkey, name)
-    data = json.dumps({"id": msg_id, "success": True, "data": {"added": added}}).encode()
-    client_sock.sendall(struct.pack("<I", len(data)) + data)
+    _send_ok(msg_id, {"added": added})
     _log(addr, command, None, "OK" if added else "ALREADY_TRUSTED")
     return
 
@@ -158,20 +158,18 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, auth
       from multiprocessing.connection import Client as PipeClient
       with PipeClient(sock_path, family="AF_PIPE") as pipe:
         pipe.send_bytes(clean_payload)
-        resp = pipe.recv_bytes()
-      resp = adapt_response(resp, command, client_ver)
-      client_sock.sendall(struct.pack("<I", len(resp)) + resp)
+        resp_payload = pipe.recv_bytes()
+      resp_payload = adapt_response(resp_payload, command, client_ver)
+      _framed_send(client_sock, resp_payload)
     else:
       with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as local:
         local.connect(sock_path)
         local.sendall(clean_header + clean_payload)
-        resp_header = _recv_exact(local, 4)
-        resp_len = struct.unpack("<I", resp_header)[0]
-        resp_payload = _recv_exact(local, resp_len)
+        resp_payload = _recv_all(local)
       resp_payload = adapt_response(resp_payload, command, client_ver)
-      client_sock.sendall(struct.pack("<I", len(resp_payload)) + resp_payload)
+      _framed_send(client_sock, resp_payload)
 
-    resp_data = json.loads(resp_payload if not is_windows() else resp)
+    resp_data = json.loads(resp_payload)
     if resp_data.get("success", True):
       _log(addr, command, resolved_profile, "OK")
     else:
@@ -201,7 +199,7 @@ def _handle_client(client_sock:socket.socket, addr:tuple, profile:str|None, auth
         "min_client_version": PROTOCOL_MIN_CLIENT,
       }).encode()
       try:
-        client_sock.sendall(struct.pack("<I", len(challenge)) + challenge)
+        _framed_send(client_sock, challenge)
       except OSError:
         return
       _proxy_request(client_sock, addr, profile, auth_keys, auth_keys_path, nonce)
diff --git a/browser_cli/commands/session.py b/browser_cli/commands/session.py
index e3d4836..f94be6b 100644
--- a/browser_cli/commands/session.py
+++ b/browser_cli/commands/session.py
@@ -1,45 +1,10 @@
 import click
-from browser_cli.client import active_browser_targets, remote_browser_targets, send_command, BrowserNotConnected
+from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
 from rich.console import Console
 
 console = Console()
 
 
-def _handle(command, args=None, profile=None):
-    try:
-        return send_command(command, args or {}, profile=profile)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
-def _handle_multi(command, args=None, profile=None, remote=None):
-    try:
-        if remote:
-            return send_command(command, args or {}, profile=profile, remote=remote)
-        return send_command(command, args or {}, profile=profile)
-    except (BrowserNotConnected, RuntimeError):
-        return None
-
-
-def _multi_browser_targets():
-    root = click.get_current_context().find_root()
-    if root.obj.get("browser_explicit"):
-        return []
-    remote = root.obj.get("remote")
-    key = root.obj.get("key")
-    if remote:
-        targets = remote_browser_targets(remote, key=key)
-    else:
-        targets = active_browser_targets(key=key)
-    if len(targets) <= 1 and not any(target.remote for target in targets):
-        return []
-    return targets
-
-
 @click.group("session")
 def session_group():
     """Save and restore browser sessions."""
diff --git a/browser_cli/commands/storage.py b/browser_cli/commands/storage.py
index dc576de..7404684 100644
--- a/browser_cli/commands/storage.py
+++ b/browser_cli/commands/storage.py
@@ -1,22 +1,11 @@
 import json
 import click
-from browser_cli.client import send_command, BrowserNotConnected
+from browser_cli.commands import _handle
 from rich.console import Console
 
 console = Console()
 
 
-def _handle(command, args=None):
-    try:
-        return send_command(command, args or {})
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
 @click.group("storage")
 def storage_group():
     """Read and write the page's localStorage / sessionStorage."""
diff --git a/browser_cli/commands/tabs.py b/browser_cli/commands/tabs.py
index 7890f20..3a4d2d0 100644
--- a/browser_cli/commands/tabs.py
+++ b/browser_cli/commands/tabs.py
@@ -1,48 +1,13 @@
 import base64
 import binascii
 import click
-from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
+from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None, profile=None):
-    try:
-        return send_command(command, args or {}, profile=profile)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
-def _handle_multi(command, args=None, profile=None, remote=None):
-    try:
-        if remote:
-            return send_command(command, args or {}, profile=profile, remote=remote)
-        return send_command(command, args or {}, profile=profile)
-    except (BrowserNotConnected, RuntimeError):
-        return None
-
-
-def _multi_browser_targets():
-    root = click.get_current_context().find_root()
-    if root.obj.get("browser_explicit"):
-        return []
-    remote = root.obj.get("remote")
-    key = root.obj.get("key")
-    if remote:
-        targets = remote_browser_targets(remote, key=key)
-    else:
-        targets = active_browser_targets(key=key)
-    if len(targets) <= 1 and not any(target.remote for target in targets):
-        return []
-    return targets
-
-
 def _print_tabs(tabs: list[dict], *, show_browser: bool = False) -> None:
     if not tabs:
         console.print("[yellow]No tabs found[/yellow]")
diff --git a/browser_cli/commands/windows.py b/browser_cli/commands/windows.py
index a4e8bf9..3d814e3 100644
--- a/browser_cli/commands/windows.py
+++ b/browser_cli/commands/windows.py
@@ -1,46 +1,11 @@
 import click
-from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
+from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
 
-def _handle(command, args=None, profile=None):
-    try:
-        return send_command(command, args or {}, profile=profile)
-    except BrowserNotConnected as e:
-        console.print(f"[red]Error:[/red] {e}")
-        raise SystemExit(1)
-    except RuntimeError as e:
-        console.print(f"[red]Browser error:[/red] {e}")
-        raise SystemExit(1)
-
-
-def _handle_multi(command, args=None, profile=None, remote=None):
-    try:
-        if remote:
-            return send_command(command, args or {}, profile=profile, remote=remote)
-        return send_command(command, args or {}, profile=profile)
-    except (BrowserNotConnected, RuntimeError):
-        return None
-
-
-def _multi_browser_targets():
-    root = click.get_current_context().find_root()
-    if root.obj.get("browser_explicit"):
-        return []
-    remote = root.obj.get("remote")
-    key = root.obj.get("key")
-    if remote:
-        targets = remote_browser_targets(remote, key=key)
-    else:
-        targets = active_browser_targets(key=key)
-    if len(targets) <= 1 and not any(target.remote for target in targets):
-        return []
-    return targets
-
-
 def _print_windows(windows: list[dict], *, show_browser: bool = False) -> None:
     if not windows:
         console.print("[yellow]No windows found[/yellow]")
diff --git a/browser_cli/compat.py b/browser_cli/compat.py
deleted file mode 100644
index 2851b27..0000000
--- a/browser_cli/compat.py
+++ /dev/null
@@ -1,49 +0,0 @@
-"""
-Stripe-style version compatibility layer for browser-cli serve.
-
-When a behaviour-breaking change ships in a new server version, add one entry
-to _COMPAT below:
-
-    ("X.Y.Z", request_fn, response_fn)
-
-- ``request_fn(msg: dict) -> dict``
-      Upgrade an incoming client message from a client older than X.Y.Z to the
-      current format before forwarding it to the native host.
-- ``response_fn(resp: bytes, command: str) -> bytes``
-      Downgrade a native-host response to the format a client older than X.Y.Z
-      expects before sending it back.
-
-Either function may be ``None`` when only one direction needs adapting.
-
-Entries must stay in ascending version order.  ``adapt_request`` walks forward
-(oldest change first); ``adapt_response`` walks backward (newest change first)
-so the transformations compose correctly.
-
-Current baseline: 0.9.1  — no shims needed yet.
-"""
-
-from __future__ import annotations
-from typing import Callable
-from browser_cli.version_manager import parse_version
-
-_COMPAT: list[tuple[str, Callable[[dict], dict] | None, Callable[[bytes, str], bytes] | None]] = [
-    # ("1.0.0", _req_1_0_0, _resp_1_0_0),
-]
-
-
-def adapt_request(msg: dict, client_version: str) -> dict:
-    """Upgrade a client message to the current server format."""
-    cv = parse_version(client_version)
-    for version, req_fn, _ in _COMPAT:
-        if cv < parse_version(version) and req_fn is not None:
-            msg = req_fn(msg)
-    return msg
-
-
-def adapt_response(resp: bytes, command: str, client_version: str) -> bytes:
-    """Downgrade a server response to the format the client expects."""
-    cv = parse_version(client_version)
-    for version, _, resp_fn in reversed(_COMPAT):
-        if cv < parse_version(version) and resp_fn is not None:
-            resp = resp_fn(resp, command)
-    return resp
diff --git a/browser_cli/compat/__init__.py b/browser_cli/compat/__init__.py
new file mode 100644
index 0000000..a32d711
--- /dev/null
+++ b/browser_cli/compat/__init__.py
@@ -0,0 +1,4 @@
+from browser_cli.compat.commands import adapt_request, adapt_response
+from browser_cli.compat.auth import adapt_auth
+
+__all__ = ["adapt_auth", "adapt_request", "adapt_response"]
diff --git a/browser_cli/compat/auth.py b/browser_cli/compat/auth.py
new file mode 100644
index 0000000..c7a6eb0
--- /dev/null
+++ b/browser_cli/compat/auth.py
@@ -0,0 +1,44 @@
+"""
+Auth-field normalizers — applied to the raw incoming message *before* the
+auth check runs.  Protocol fields (pubkey, sig, …) are still present here.
+
+Add one entry per breaking auth-field change:
+    ("X.Y.Z", transformer_fn)
+
+Entries must stay in ascending version order.
+"""
+from __future__ import annotations
+from typing import Callable
+from browser_cli.version_manager import parse_version
+
+
+# ── v0.9.3 ────────────────────────────────────────────────────────────────────
+
+def _auth_0_9_3(msg: dict) -> dict:
+    """pubkey validation tightened to lowercase hex; normalize for older clients."""
+    changed: dict = {}
+    pk = msg.get("pubkey")
+    if isinstance(pk, str) and pk:
+        changed["pubkey"] = pk.lower()
+    if msg.get("command") == "browser-cli.auth.trust":
+        args = msg.get("args") or {}
+        trust_pk = args.get("pubkey")
+        if isinstance(trust_pk, str) and trust_pk:
+            changed["args"] = {**args, "pubkey": trust_pk.lower()}
+    return {**msg, **changed} if changed else msg
+
+
+# ── registry ──────────────────────────────────────────────────────────────────
+
+_AUTH_COMPAT: list[tuple[str, Callable[[dict], dict]]] = [
+    ("0.9.3", _auth_0_9_3),
+]
+
+
+def adapt_auth(msg: dict, client_version: str) -> dict:
+    """Apply all auth normalizers needed to bring msg up to the current format."""
+    cv = parse_version(client_version)
+    for version, fn in _AUTH_COMPAT:
+        if cv < parse_version(version):
+            msg = fn(msg)
+    return msg
diff --git a/browser_cli/compat/commands.py b/browser_cli/compat/commands.py
new file mode 100644
index 0000000..b798c0b
--- /dev/null
+++ b/browser_cli/compat/commands.py
@@ -0,0 +1,43 @@
+"""
+Command-format shims — applied to clean_msg (protocol fields already stripped)
+before forwarding to the native host, and to responses before sending back.
+
+Add one entry per breaking command-format change:
+    ("X.Y.Z", request_fn, response_fn)
+
+- request_fn(msg: dict) -> dict   or None
+- response_fn(resp: bytes, command: str) -> bytes   or None
+
+Entries must stay in ascending version order.
+adapt_request walks forward (oldest first); adapt_response walks backward.
+
+Current baseline: 0.9.3 — no command-format shims needed yet.
+"""
+from __future__ import annotations
+from typing import Callable
+from browser_cli.version_manager import parse_version
+
+
+# ── registry ──────────────────────────────────────────────────────────────────
+
+_COMPAT: list[tuple[str, Callable[[dict], dict] | None, Callable[[bytes, str], bytes] | None]] = [
+    # ("1.0.0", _req_1_0_0, _resp_1_0_0),
+]
+
+
+def adapt_request(msg: dict, client_version: str) -> dict:
+    """Upgrade a client message to the current browser command format."""
+    cv = parse_version(client_version)
+    for version, req_fn, _ in _COMPAT:
+        if cv < parse_version(version) and req_fn is not None:
+            msg = req_fn(msg)
+    return msg
+
+
+def adapt_response(resp: bytes, command: str, client_version: str) -> bytes:
+    """Downgrade a native-host response to the format the client expects."""
+    cv = parse_version(client_version)
+    for version, _, resp_fn in reversed(_COMPAT):
+        if cv < parse_version(version) and resp_fn is not None:
+            resp = resp_fn(resp, command)
+    return resp
diff --git a/browser_cli/native_host.py b/browser_cli/native_host.py
index c10d486..186385d 100644
--- a/browser_cli/native_host.py
+++ b/browser_cli/native_host.py
@@ -19,6 +19,7 @@ from multiprocessing.connection import Listener
 from pathlib import Path
 
 from browser_cli.platform import DEFAULT_ALIAS, endpoint_for_alias, is_windows, registry_path, runtime_dir
+from browser_cli.version_manager import MAX_MSG_BYTES as _MAX_MSG_BYTES
 from browser_cli.registry import update_registry
 
 SOCKET_PATH: str = ""          # set after hello handshake
@@ -278,6 +279,8 @@ def _recv_all(conn: socket.socket) -> bytes | None:
     if raw_len is None:
         return None
     msg_len = struct.unpack("<I", raw_len)[0]
+    if msg_len > _MAX_MSG_BYTES:
+        return None
     return _recv_exact(conn, msg_len)
 
 
diff --git a/browser_cli/version_manager.py b/browser_cli/version_manager.py
index e8876ce..be8653a 100644
--- a/browser_cli/version_manager.py
+++ b/browser_cli/version_manager.py
@@ -1,6 +1,7 @@
 from importlib.metadata import version as _pkg_version
 
 PROTOCOL_MIN_CLIENT = "0.9.0"
+MAX_MSG_BYTES = 32 * 1024 * 1024
 
 
 def parse_version(v: str) -> tuple[int, ...]:
diff --git a/pyproject.toml b/pyproject.toml
index 34f6d9c..1fc348c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "browser-cli"
-version = "0.9.2"
+version = "0.9.3"
 description = "Control your real running browser from the terminal via a browser extension"
 requires-python = ">=3.10"
 dependencies = [
diff --git a/tests/test_auth.py b/tests/test_auth.py
new file mode 100644
index 0000000..1ffc010
--- /dev/null
+++ b/tests/test_auth.py
@@ -0,0 +1,160 @@
+import json
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from browser_cli.auth import (
+    add_authorized_key,
+    canonical_payload,
+    generate_keypair,
+    load_authorized_keys,
+    load_authorized_keys_with_names,
+    load_private_key,
+    new_nonce,
+    sign,
+    verify,
+)
+from browser_cli.client import _is_valid_key_spec
+
+
+class TestGenerateKeypair:
+    def test_returns_pem_and_hex(self):
+        pem, pub_hex = generate_keypair()
+        assert pem.startswith(b"-----BEGIN PRIVATE KEY-----")
+        assert len(pub_hex) == 64
+
+    def test_each_call_unique(self):
+        _, pub1 = generate_keypair()
+        _, pub2 = generate_keypair()
+        assert pub1 != pub2
+
+
+class TestCanonicalPayload:
+    def test_strips_pubkey_and_sig(self):
+        msg = {"command": "tabs.list", "id": "x", "pubkey": "abc", "sig": "def"}
+        data = json.loads(canonical_payload(msg))
+        assert "pubkey" not in data
+        assert "sig" not in data
+
+    def test_keys_sorted(self):
+        msg = {"z": 1, "a": 2, "m": 3}
+        payload = canonical_payload(msg).decode()
+        assert payload.index('"a"') < payload.index('"m"') < payload.index('"z"')
+
+    def test_deterministic(self):
+        msg = {"b": 2, "a": 1}
+        assert canonical_payload(msg) == canonical_payload(msg)
+
+
+@pytest.fixture()
+def keypair(tmp_path):
+    pem, pub_hex = generate_keypair()
+    key_path = tmp_path / "client.key.pem"
+    key_path.write_bytes(pem)
+    priv = load_private_key(key_path)
+    return priv, pub_hex
+
+
+class TestSignVerify:
+    def test_valid_signature_verifies(self, keypair):
+        priv, pub_hex = keypair
+        nonce = bytes.fromhex(new_nonce())
+        msg = {"command": "tabs.list", "id": "uuid-1", "args": {}}
+        sig = sign(priv, nonce, msg).hex()
+        assert verify(pub_hex, nonce, msg, sig) is True
+
+    def test_tampered_sig_fails(self, keypair):
+        priv, pub_hex = keypair
+        nonce = bytes.fromhex(new_nonce())
+        msg = {"command": "tabs.list", "id": "x"}
+        sign(priv, nonce, msg)
+        assert verify(pub_hex, nonce, msg, "00" * 64) is False
+
+    def test_wrong_pubkey_fails(self, keypair):
+        priv, _ = keypair
+        _, other_pub = generate_keypair()
+        nonce = bytes.fromhex(new_nonce())
+        msg = {"command": "tabs.list"}
+        sig = sign(priv, nonce, msg).hex()
+        assert verify(other_pub, nonce, msg, sig) is False
+
+    def test_wrong_nonce_fails(self, keypair):
+        priv, pub_hex = keypair
+        nonce = bytes.fromhex(new_nonce())
+        msg = {"command": "tabs.list"}
+        sig = sign(priv, nonce, msg).hex()
+        other_nonce = bytes.fromhex(new_nonce())
+        assert verify(pub_hex, other_nonce, msg, sig) is False
+
+    def test_garbage_pub_hex_returns_false_not_exception(self):
+        assert verify("not-hex!!!!", b"nonce", {}, "00" * 64) is False
+
+    def test_truncated_sig_hex_returns_false_not_exception(self, keypair):
+        _, pub_hex = keypair
+        assert verify(pub_hex, b"nonce", {}, "aabb") is False
+
+    def test_wrong_length_pubkey_returns_false_not_exception(self):
+        assert verify("aabbcc", b"nonce", {}, "00" * 64) is False
+
+
+class TestAuthorizedKeys:
+    def test_add_and_load(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, pub = generate_keypair()
+        assert add_authorized_key(path, pub, "alice") is True
+        assert pub in load_authorized_keys(path)
+
+    def test_add_duplicate_returns_false(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, pub = generate_keypair()
+        add_authorized_key(path, pub)
+        assert add_authorized_key(path, pub) is False
+
+    def test_load_with_names(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, pub1 = generate_keypair()
+        _, pub2 = generate_keypair()
+        add_authorized_key(path, pub1, "alice")
+        add_authorized_key(path, pub2)
+        entries = load_authorized_keys_with_names(path)
+        assert (pub1, "alice") in entries
+        assert (pub2, "") in entries
+
+    def test_ignores_comment_lines(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        path.write_text("# this is a comment\n")
+        assert load_authorized_keys(path) == []
+
+    def test_returns_empty_for_missing_file(self, tmp_path):
+        assert load_authorized_keys(tmp_path / "nofile") == []
+
+
+class TestIsValidKeySpec:
+    def test_agent_bare(self):
+        assert _is_valid_key_spec("agent") is True
+
+    def test_agent_with_selector(self):
+        assert _is_valid_key_spec("agent:cardno:000012345678") is True
+
+    def test_absolute_pem_path(self):
+        assert _is_valid_key_spec("/home/user/.config/browser-cli/client.key.pem") is True
+
+    def test_dot_key_extension(self):
+        assert _is_valid_key_spec("/tmp/mykey.key") is True
+
+    def test_angled_bracket_pem_rejected(self):
+        # regression: operator precedence bug allowed "<garbage>.pem" to pass
+        assert _is_valid_key_spec("<garbage>.pem") is False
+
+    def test_angled_bracket_key_rejected(self):
+        assert _is_valid_key_spec("<garbage>.key") is False
+
+    def test_serialized_object_rejected(self):
+        assert _is_valid_key_spec("<AgentKey(blob=b'...', comment='test')>.pem") is False
+
+    def test_empty_string_rejected(self):
+        assert _is_valid_key_spec("") is False
+
+    def test_bare_filename_no_slash_no_ext_rejected(self):
+        assert _is_valid_key_spec("mykey") is False
diff --git a/tests/test_cli.py b/tests/test_cli.py
index ef4cb9c..b4c585f 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -262,12 +262,12 @@ def test_tabs_list_multi_browser_shows_browser_column():
         return [{"id": 1 if profile == "default" else 2, "windowId": 1, "active": True, "title": profile, "url": "https://example.com"}]
 
     with patch(
-        "browser_cli.commands.tabs.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "550e8400-e29b-41d4-a716-446655440000", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
-    ), patch("browser_cli.commands.tabs.send_command", side_effect=fake_send_command):
+    ), patch("browser_cli.commands.send_command", side_effect=fake_send_command):
         result = CliRunner().invoke(main, ["tabs", "list"])
 
     assert result.exit_code == 0
@@ -278,13 +278,13 @@ def test_tabs_list_multi_browser_shows_browser_column():
 
 def test_tabs_list_with_remote_uses_only_remote_targets():
     with patch(
-        "browser_cli.commands.tabs.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         side_effect=AssertionError("local targets should not be used for explicit remote"),
     ), patch(
-        "browser_cli.commands.tabs.remote_browser_targets",
+        "browser_cli.commands.remote_browser_targets",
         return_value=[BrowserTarget("work", "remote-host:work", "", remote="remote-host:8765")],
     ), patch(
-        "browser_cli.commands.tabs.send_command",
+        "browser_cli.commands.send_command",
         return_value=[{"id": 1, "windowId": 1, "active": True, "title": "Remote", "url": "https://example.com"}],
     ) as send_command:
         result = CliRunner().invoke(main, ["--remote", "remote-host:8765", "tabs", "list"])
@@ -297,13 +297,13 @@ def test_tabs_list_with_remote_uses_only_remote_targets():
 
 def test_tabs_list_with_explicit_browser_does_not_show_browser_column():
     with patch(
-        "browser_cli.commands.tabs.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
     ), patch(
-        "browser_cli.commands.tabs.send_command",
+        "browser_cli.commands.send_command",
         return_value=[{"id": 1, "windowId": 1, "active": True, "title": "Example", "url": "https://example.com"}],
     ) as send_command:
         result = CliRunner().invoke(main, ["--browser", "work", "tabs", "list"])
@@ -322,12 +322,12 @@ def test_tabs_count_multi_browser_shows_total():
         return counts[profile]
 
     with patch(
-        "browser_cli.commands.tabs.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
-    ), patch("browser_cli.commands.tabs.send_command", side_effect=fake_send_command):
+    ), patch("browser_cli.commands.send_command", side_effect=fake_send_command):
         result = CliRunner().invoke(main, ["tabs", "count", "github"])
 
     assert result.exit_code == 0
@@ -344,12 +344,12 @@ def test_group_count_multi_browser_shows_total():
         return counts[profile]
 
     with patch(
-        "browser_cli.commands.groups.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
-    ), patch("browser_cli.commands.groups.send_command", side_effect=fake_send_command):
+    ), patch("browser_cli.commands.send_command", side_effect=fake_send_command):
         result = CliRunner().invoke(main, ["groups", "count"])
 
     assert result.exit_code == 0
@@ -360,7 +360,7 @@ def test_group_count_multi_browser_shows_total():
 
 def test_group_list_leaves_unnamed_group_cell_empty():
     with patch(
-        "browser_cli.commands.groups.send_command",
+        "browser_cli.commands.send_command",
         return_value=[{"id": 42, "title": "", "color": "grey", "collapsed": False, "tabCount": 1}],
     ):
         result = CliRunner().invoke(main, ["groups", "list"])
@@ -372,7 +372,7 @@ def test_group_list_leaves_unnamed_group_cell_empty():
 
 
 def test_tabs_move_accepts_right_short_alias():
-    with patch("browser_cli.commands.tabs.send_command") as send_command:
+    with patch("browser_cli.commands.send_command") as send_command:
         result = CliRunner().invoke(main, ["tabs", "move", "12", "-r"])
 
     assert result.exit_code == 0
@@ -384,7 +384,7 @@ def test_tabs_move_accepts_right_short_alias():
 
 
 def test_groups_move_accepts_left_short_alias():
-    with patch("browser_cli.commands.groups.send_command") as send_command:
+    with patch("browser_cli.commands.send_command") as send_command:
         result = CliRunner().invoke(main, ["groups", "move", "research", "-l"])
 
     assert result.exit_code == 0
@@ -399,12 +399,12 @@ def test_windows_list_multi_browser_shows_browser_column():
         return [{"id": 1, "alias": profile, "focused": True, "tabCount": 2, "state": "normal"}]
 
     with patch(
-        "browser_cli.commands.windows.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
-    ), patch("browser_cli.commands.windows.send_command", side_effect=fake_send_command):
+    ), patch("browser_cli.commands.send_command", side_effect=fake_send_command):
         result = CliRunner().invoke(main, ["windows", "list"])
 
     assert result.exit_code == 0
@@ -420,12 +420,12 @@ def test_session_list_multi_browser_shows_browser_column():
         return [{"name": f"{profile}-session", "tabs": 2, "savedAt": 1712707200000}]
 
     with patch(
-        "browser_cli.commands.session.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
-    ), patch("browser_cli.commands.session.send_command", side_effect=fake_send_command):
+    ), patch("browser_cli.commands.send_command", side_effect=fake_send_command):
         result = CliRunner().invoke(main, ["session", "list"])
 
     assert result.exit_code == 0
@@ -438,13 +438,13 @@ def test_session_list_multi_browser_shows_browser_column():
 
 def test_session_list_with_explicit_browser_does_not_show_browser_column():
     with patch(
-        "browser_cli.commands.session.active_browser_targets",
+        "browser_cli.commands.active_browser_targets",
         return_value=[
             BrowserTarget("default", "uuid-1", "/tmp/default.sock"),
             BrowserTarget("work", "work", "/tmp/work.sock"),
         ],
     ), patch(
-        "browser_cli.commands.session.send_command",
+        "browser_cli.commands.send_command",
         return_value=[{"name": "work-session", "tabs": 2, "savedAt": 1712707200000}],
     ) as send_command:
         result = CliRunner().invoke(main, ["--browser", "work", "session", "list"])
@@ -455,7 +455,7 @@ def test_session_list_with_explicit_browser_does_not_show_browser_column():
 
 
 def test_windows_open_passes_url():
-    with patch("browser_cli.commands.windows.send_command", return_value={"id": 7}) as send_command:
+    with patch("browser_cli.commands.send_command", return_value={"id": 7}) as send_command:
         result = CliRunner().invoke(main, ["windows", "open", "https://example.com"])
 
     assert result.exit_code == 0
@@ -463,20 +463,20 @@ def test_windows_open_passes_url():
     send_command.assert_called_once_with("windows.open", {"url": "https://example.com"}, profile=None)
 
 def test_extract_markdown_command():
-    with patch("browser_cli.commands.extract.send_command", return_value="# Title") as send_command:
+    with patch("browser_cli.commands.send_command", return_value="# Title") as send_command:
         result = CliRunner().invoke(main, ["extract", "markdown"])
 
     assert result.exit_code == 0
     assert result.output == "# Title\n"
-    send_command.assert_called_once_with("extract.markdown", {"selector": None})
+    send_command.assert_called_once_with("extract.markdown", {"selector": None}, profile=None)
 
 def test_extract_markdown_command_with_selector():
-    with patch("browser_cli.commands.extract.send_command", return_value="## Post") as send_command:
+    with patch("browser_cli.commands.send_command", return_value="## Post") as send_command:
         result = CliRunner().invoke(main, ["extract", "markdown", "--selector", "article"])
 
     assert result.exit_code == 0
     assert result.output == "## Post\n"
-    send_command.assert_called_once_with("extract.markdown", {"selector": "article"})
+    send_command.assert_called_once_with("extract.markdown", {"selector": "article"}, profile=None)
 
 
 def test_clean_markdown_output_removes_escaped_underscores_and_dashes():
@@ -561,7 +561,7 @@ def test_extract_markdown_command_repairs_malformed_tables_and_code_blocks():
         "Golden Set   │   ▼Promptfoo(Testausführung)   │   ▼Plattformen├ Omnifact└ Le Chat\n"
         "```"
     )
-    with patch("browser_cli.commands.extract.send_command", return_value=raw):
+    with patch("browser_cli.commands.send_command", return_value=raw):
         result = CliRunner().invoke(main, ["extract", "markdown"])
 
     assert result.exit_code == 0
@@ -639,8 +639,8 @@ def test_tabs_list_multi_browser_queries_remote_target():
         remote=endpoint,
     )
 
-    with patch("browser_cli.commands.tabs.active_browser_targets", return_value=[remote_target, BrowserTarget("local", "local", "/tmp/local.sock")]), patch(
-        "browser_cli.commands.tabs.send_command",
+    with patch("browser_cli.commands.active_browser_targets", return_value=[remote_target, BrowserTarget("local", "local", "/tmp/local.sock")]), patch(
+        "browser_cli.commands.send_command",
         return_value=[{"id": 1, "windowId": 1, "active": True, "title": "Remote", "url": "https://example.com"}],
     ) as send_command:
         result = CliRunner().invoke(main, ["tabs", "list"])
diff --git a/tests/test_serve.py b/tests/test_serve.py
new file mode 100644
index 0000000..2649173
--- /dev/null
+++ b/tests/test_serve.py
@@ -0,0 +1,249 @@
+"""Unit tests for the TCP serve layer (challenge-response auth, framing, rejection paths)."""
+import json
+import socket
+import struct
+import threading
+
+import pytest
+
+from browser_cli.auth import generate_keypair, load_private_key, new_nonce, sign
+from browser_cli.client import BrowserNotConnected
+from browser_cli.commands.serve import _handle_client
+
+FAKE_UA = "browser-cli/0.9.3"
+
+
+# ── helpers ────────────────────────────────────────────────────────────────────
+
+def _send_framed(sock: socket.socket, data: bytes) -> None:
+    sock.sendall(struct.pack("<I", len(data)) + data)
+
+
+def _recv_framed(sock: socket.socket) -> dict:
+    raw = b""
+    while len(raw) < 4:
+        chunk = sock.recv(4 - len(raw))
+        if not chunk:
+            raise ConnectionError("socket closed before response header")
+        raw += chunk
+    n = struct.unpack("<I", raw)[0]
+    data = b""
+    while len(data) < n:
+        chunk = sock.recv(n - len(data))
+        if not chunk:
+            raise ConnectionError("socket closed mid-response")
+        data += chunk
+    return json.loads(data)
+
+
+def _spawn(server_sock: socket.socket, auth_keys_path) -> threading.Thread:
+    t = threading.Thread(
+        target=_handle_client,
+        args=(server_sock, ("127.0.0.1", 9999), None, auth_keys_path),
+        daemon=True,
+    )
+    t.start()
+    return t
+
+
+def _pair():
+    return socket.socketpair()
+
+
+def _mock_no_browser(*_args, **_kwargs):
+    raise BrowserNotConnected("no browser")
+
+
+# ── challenge frame ────────────────────────────────────────────────────────────
+
+class TestChallenge:
+    def test_challenge_sent_on_connect(self):
+        client, server = _pair()
+        t = _spawn(server, None)
+        challenge = _recv_framed(client)
+        assert challenge["type"] == "challenge"
+        assert "nonce" in challenge
+        client.close()
+        t.join(timeout=2)
+
+    def test_challenge_includes_version_fields(self):
+        client, server = _pair()
+        t = _spawn(server, None)
+        challenge = _recv_framed(client)
+        assert "server_version" in challenge
+        assert "min_client_version" in challenge
+        client.close()
+        t.join(timeout=2)
+
+
+# ── rejection paths ────────────────────────────────────────────────────────────
+
+class TestRejection:
+    def _connect(self, auth_keys_path):
+        client, server = _pair()
+        t = _spawn(server, auth_keys_path)
+        challenge = _recv_framed(client)
+        return client, t, challenge
+
+    def test_bad_user_agent_rejected(self):
+        client, t, _ = self._connect(None)
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": "curl/7.88"}
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        assert "forbidden" in resp["error"].lower() or "client" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_missing_pubkey_sig_rejected(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, pub = generate_keypair()
+        path.write_text(pub + "\n")
+        client, t, _ = self._connect(path)
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": FAKE_UA}
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        assert "unauthorized" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_untrusted_pubkey_rejected(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, trusted_pub = generate_keypair()
+        path.write_text(trusted_pub + "\n")
+
+        pem, untrusted_pub = generate_keypair()
+        key_path = tmp_path / "other.pem"
+        key_path.write_bytes(pem)
+        priv = load_private_key(key_path)
+
+        client, t, challenge = self._connect(path)
+        nonce = bytes.fromhex(challenge["nonce"])
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": FAKE_UA, "pubkey": untrusted_pub}
+        msg["sig"] = sign(priv, nonce, msg).hex()
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        assert "untrusted" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_bad_signature_rejected(self, tmp_path):
+        path = tmp_path / "authorized_keys"
+        _, pub = generate_keypair()
+        path.write_text(pub + "\n")
+        client, t, _ = self._connect(path)
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": FAKE_UA, "pubkey": pub, "sig": "00" * 64}
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        assert "signature" in resp["error"].lower() or "invalid" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_oversized_message_rejected(self):
+        client, server = _pair()
+        t = _spawn(server, None)
+        _recv_framed(client)  # consume challenge
+        client.sendall(struct.pack("<I", 33 * 1024 * 1024))
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        assert "too large" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_invalid_json_rejected(self):
+        client, server = _pair()
+        t = _spawn(server, None)
+        _recv_framed(client)  # consume challenge
+        bad = b"this is not json {"
+        _send_framed(client, bad)
+        resp = _recv_framed(client)
+        assert resp["success"] is False
+        client.close()
+        t.join(timeout=2)
+
+
+# ── auth success paths ─────────────────────────────────────────────────────────
+
+class TestAuthSuccess:
+    def test_valid_auth_reaches_proxy(self, tmp_path, monkeypatch):
+        """Correct signature → error must be 'browser not connected', not 'unauthorized'."""
+        path = tmp_path / "authorized_keys"
+        pem, pub = generate_keypair()
+        path.write_text(pub + "\n")
+        key_path = tmp_path / "client.key.pem"
+        key_path.write_bytes(pem)
+        priv = load_private_key(key_path)
+
+        monkeypatch.setattr("browser_cli.client._resolve_socket", _mock_no_browser)
+
+        client, server = _pair()
+        t = threading.Thread(
+            target=_handle_client,
+            args=(server, ("127.0.0.1", 9999), None, path),
+            daemon=True,
+        )
+        t.start()
+
+        challenge = _recv_framed(client)
+        nonce = bytes.fromhex(challenge["nonce"])
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": FAKE_UA, "pubkey": pub}
+        msg["sig"] = sign(priv, nonce, msg).hex()
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+
+        assert resp["success"] is False
+        assert "unauthorized" not in resp["error"].lower()
+        assert "browser" in resp["error"].lower() or "connected" in resp["error"].lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_uppercase_pubkey_normalized_by_compat(self, tmp_path, monkeypatch):
+        """Clients < 0.9.3 may send uppercase pubkeys; compat layer normalises before auth."""
+        path = tmp_path / "authorized_keys"
+        pem, pub = generate_keypair()  # pub is lowercase hex
+        path.write_text(pub + "\n")
+        key_path = tmp_path / "client.key.pem"
+        key_path.write_bytes(pem)
+        priv = load_private_key(key_path)
+
+        monkeypatch.setattr("browser_cli.client._resolve_socket", _mock_no_browser)
+
+        client, server = _pair()
+        t = _spawn(server, path)
+
+        challenge = _recv_framed(client)
+        nonce = bytes.fromhex(challenge["nonce"])
+        # old client sends uppercase pubkey
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": "browser-cli/0.9.2", "pubkey": pub.upper()}
+        msg["sig"] = sign(priv, nonce, msg).hex()
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+
+        assert "unauthorized" not in resp.get("error", "").lower()
+        assert "browser" in resp.get("error", "").lower() or "connected" in resp.get("error", "").lower()
+        client.close()
+        t.join(timeout=2)
+
+    def test_no_auth_mode_reaches_proxy(self, monkeypatch):
+        """auth_keys_path=None (--no-auth): no pubkey required, reaches proxy layer."""
+        monkeypatch.setattr("browser_cli.client._resolve_socket", _mock_no_browser)
+
+        client, server = _pair()
+        t = threading.Thread(
+            target=_handle_client,
+            args=(server, ("127.0.0.1", 9999), None, None),
+            daemon=True,
+        )
+        t.start()
+
+        _recv_framed(client)  # challenge
+        msg = {"id": "x", "command": "tabs.list", "args": {}, "user_agent": FAKE_UA}
+        _send_framed(client, json.dumps(msg).encode())
+        resp = _recv_framed(client)
+
+        assert "unauthorized" not in resp.get("error", "").lower()
+        client.close()
+        t.join(timeout=2)
diff --git a/uv.lock b/uv.lock
index a8d6fde..ca8c328 100644
--- a/uv.lock
+++ b/uv.lock
@@ -4,7 +4,7 @@ requires-python = ">=3.10"
 
 [[package]]
 name = "browser-cli"
-version = "0.9.2"
+version = "0.9.3"
 source = { editable = "." }
 dependencies = [
     { name = "click" },