diff --git a/tests/tools/test_browser_hardening.py b/tests/tools/test_browser_hardening.py
index 657edad2a20..23ff9c93b9c 100644
--- a/tests/tools/test_browser_hardening.py
+++ b/tests/tools/test_browser_hardening.py
@@ -115,6 +115,29 @@ class TestCommandTimeoutCache:
         mock_read.assert_called_once()
 
 
+class TestSessionInactivityTimeout:
+
+    def test_default_is_300(self, monkeypatch):
+        from tools.browser_tool import _get_session_inactivity_timeout
+        monkeypatch.delenv("BROWSER_INACTIVITY_TIMEOUT", raising=False)
+        with patch("hermes_cli.config.read_raw_config", return_value={}):
+            assert _get_session_inactivity_timeout() == 300
+
+    def test_reads_from_config_over_env(self, monkeypatch):
+        from tools.browser_tool import _get_session_inactivity_timeout
+        monkeypatch.setenv("BROWSER_INACTIVITY_TIMEOUT", "120")
+        cfg = {"browser": {"inactivity_timeout": 900}}
+        with patch("hermes_cli.config.read_raw_config", return_value=cfg):
+            assert _get_session_inactivity_timeout() == 900
+
+    def test_floor_at_30_seconds(self, monkeypatch):
+        from tools.browser_tool import _get_session_inactivity_timeout
+        monkeypatch.setenv("BROWSER_INACTIVITY_TIMEOUT", "120")
+        cfg = {"browser": {"inactivity_timeout": 1}}
+        with patch("hermes_cli.config.read_raw_config", return_value=cfg):
+            assert _get_session_inactivity_timeout() == 30
+
+
 # ---------------------------------------------------------------------------
 # Caching: _discover_homebrew_node_dirs
 # ---------------------------------------------------------------------------
diff --git a/tools/browser_tool.py b/tools/browser_tool.py
index 66684b8ee35..2c56bf9bb7e 100644
--- a/tools/browser_tool.py
+++ b/tools/browser_tool.py
@@ -1177,10 +1177,23 @@ _cleanup_done = False
 # Inactivity Timeout Configuration
 # =============================================================================
 
-# Session inactivity timeout (seconds) - cleanup if no activity for this long
-# Default: 5 minutes. Needs headroom for LLM reasoning between browser commands,
-# especially when subagents are doing multi-step browser tasks.
-BROWSER_SESSION_INACTIVITY_TIMEOUT = env_int("BROWSER_INACTIVITY_TIMEOUT", 300)
+# Session inactivity timeout (seconds) - cleanup if no activity for this long.
+# config.yaml is authoritative; BROWSER_INACTIVITY_TIMEOUT remains a legacy
+# fallback so old deployments keep working if they have not migrated yet.
+def _get_session_inactivity_timeout() -> int:
+    result = env_int("BROWSER_INACTIVITY_TIMEOUT", 300)
+    try:
+        from hermes_cli.config import read_raw_config
+        cfg = read_raw_config()
+        val = cfg_get(cfg, "browser", "inactivity_timeout")
+        if val is not None:
+            result = max(int(val), 30)  # Floor at 30s to avoid instant reaping
+    except Exception as e:
+        logger.debug("Could not read inactivity_timeout from config: %s", e)
+    return result
+
+
+BROWSER_SESSION_INACTIVITY_TIMEOUT = _get_session_inactivity_timeout()
 
 # Track last activity time per session
 _session_last_activity: Dict[str, float] = {}