diff --git a/tests/tools/test_browser_env_loopback_no_proxy.py b/tests/tools/test_browser_env_loopback_no_proxy.py
new file mode 100644
index 0000000000..a05777c0da
--- /dev/null
+++ b/tests/tools/test_browser_env_loopback_no_proxy.py
@@ -0,0 +1,93 @@
+"""Browser child envs must bypass proxies for loopback hosts (#110565).
+
+websockets>=14 defaults to ``proxy=True`` and resolves proxies via
+``urllib.request.getproxies()``, which reads the macOS/Windows *system* proxy
+config even with no ``*_proxy`` env vars set — so a local CDP WebSocket
+(``ws://127.0.0.1:<port>/devtools/...``) gets routed into the system proxy and
+the handshake fails with "did not receive a valid HTTP response".
+``_build_browser_env`` therefore appends the loopback hosts to NO_PROXY/no_proxy
+for every browser subprocess (append, never overwrite operator entries).
+"""
+
+import pytest
+
+import tools.browser_tool as bt
+
+
+class TestEnsureLoopbackNoProxy:
+    def test_empty_env_sets_both_casings(self):
+        env = {}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["NO_PROXY"] == "127.0.0.1,localhost,::1"
+        assert env["no_proxy"] == "127.0.0.1,localhost,::1"
+
+    def test_appends_without_dropping_operator_entries(self):
+        env = {"NO_PROXY": "corp.example.com,.internal"}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["NO_PROXY"] == "corp.example.com,.internal,127.0.0.1,localhost,::1"
+
+    def test_lowercase_value_appended_too(self):
+        env = {"no_proxy": "10.0.0.0/8"}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["no_proxy"] == "10.0.0.0/8,127.0.0.1,localhost,::1"
+
+    def test_idempotent_when_loopback_already_present(self):
+        env = {"NO_PROXY": "127.0.0.1,localhost,::1"}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["NO_PROXY"] == "127.0.0.1,localhost,::1"
+
+    def test_partial_overlap_only_adds_missing(self):
+        env = {"NO_PROXY": "localhost"}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["NO_PROXY"] == "localhost,127.0.0.1,::1"
+
+    def test_unrelated_keys_untouched(self):
+        env = {"PATH": "/usr/bin", "NO_PROXY": "x.example"}
+        bt._ensure_loopback_no_proxy(env)
+        assert env["PATH"] == "/usr/bin"
+        assert set(env) == {"PATH", "NO_PROXY", "no_proxy"}
+
+
+class TestBuildBrowserEnvLoopback:
+    @pytest.fixture
+    def stub_sanitized_env(self, monkeypatch):
+        """Replace the credential-scrub layer with a fixed dict so the test sees
+        exactly what _build_browser_env adds on top."""
+        import tools.environments.local as local
+        holder = {}
+
+        def _fake(inherit_credentials=False):
+            return dict(holder)
+
+        monkeypatch.setattr(local, "hermes_subprocess_env", _fake)
+        return holder
+
+    def test_sets_loopback_no_proxy_when_scrubbed_env_has_none(self, stub_sanitized_env, monkeypatch):
+        for key in ("NO_PROXY", "no_proxy"):
+            monkeypatch.delenv(key, raising=False)
+        monkeypatch.delenv("BROWSER_USE_API_KEY", raising=False)
+
+        env = bt._build_browser_env()
+
+        assert env["NO_PROXY"] == "127.0.0.1,localhost,::1"
+        assert env["no_proxy"] == "127.0.0.1,localhost,::1"
+
+    def test_keeps_operator_loopback_entries(self, stub_sanitized_env, monkeypatch):
+        monkeypatch.delenv("NO_PROXY", raising=False)
+        monkeypatch.delenv("no_proxy", raising=False)
+        stub_sanitized_env["NO_PROXY"] = "git.internal"
+        monkeypatch.delenv("BROWSER_USE_API_KEY", raising=False)
+
+        env = bt._build_browser_env()
+
+        assert env["NO_PROXY"] == "git.internal,127.0.0.1,localhost,::1"
+
+    def test_passthrough_keys_still_readded(self, stub_sanitized_env, monkeypatch):
+        monkeypatch.delenv("NO_PROXY", raising=False)
+        monkeypatch.delenv("no_proxy", raising=False)
+        monkeypatch.setenv("BROWSER_USE_API_KEY", "test-key")
+
+        env = bt._build_browser_env()
+
+        assert env["BROWSER_USE_API_KEY"] == "test-key"
+        assert env["NO_PROXY"] == "127.0.0.1,localhost,::1"
diff --git a/tools/browser_tool.py b/tools/browser_tool.py
index e7c00ab88a..cc9c639343 100644
--- a/tools/browser_tool.py
+++ b/tools/browser_tool.py
@@ -34,6 +34,24 @@ _BROWSER_PASSTHROUGH_KEYS: tuple[str, ...] = (
     "FIRECRAWL_API_KEY", "FIRECRAWL_API_URL", "FIRECRAWL_BROWSER_TTL",
 )
 
+# Loopback hosts that must never be proxied: the browser backends dial local CDP
+# endpoints (ws://127.0.0.1:<port>/devtools/...), and websockets>=14 defaults to
+# proxy=True with proxies resolved via urllib.request.getproxies() — which reads the
+# macOS/Windows *system* proxy config even with no *_proxy env vars set. Without an
+# explicit NO_PROXY the CDP handshake is routed into the system proxy and fails with
+# "did not receive a valid HTTP response" (#110565). See #14372 for the env-var flavor.
+_LOOPBACK_NO_PROXY_ENTRIES: tuple[str, ...] = ("127.0.0.1", "localhost", "::1")
+
+
+def _ensure_loopback_no_proxy(env: dict) -> None:
+    """Append the loopback hosts to NO_PROXY/no_proxy (both casings) without dropping
+    operator-provided entries."""
+    for key in ("NO_PROXY", "no_proxy"):
+        entries = [part.strip() for part in env.get(key, "").split(",") if part.strip()]
+        missing = [host for host in _LOOPBACK_NO_PROXY_ENTRIES if host not in entries]
+        if missing:
+            env[key] = ",".join(entries + missing)
+
 
 def _build_browser_env() -> dict:
     """Credential-scrubbed env for an agent-browser subprocess (deferred import: test
@@ -51,6 +69,7 @@ def _build_browser_env() -> dict:
             value = None  # multiplex, no scope bound: no key rather than a sibling profile's
         if value is not None:
             env[key] = value
+    _ensure_loopback_no_proxy(env)
     return env