diff --git a/tests/tools/test_approval.py b/tests/tools/test_approval.py
index 3863bc01d..fb1ef72cd 100644
--- a/tests/tools/test_approval.py
+++ b/tests/tools/test_approval.py
@@ -389,6 +389,57 @@ class TestTeePattern:
         assert key is None
 
 
+class TestHermesConfigWriteProtection:
+    """Terminal-side pairing for the file_tools write_file/patch deny on
+    ~/.hermes/config.yaml (#14639). config.yaml IS the security policy
+    (approvals.mode/yolo live there, mtime-keyed cache reloads mid-session),
+    so a write_file deny without terminal-side coverage is unpaired theater.
+    These pin every terminal write idiom against the config file."""
+
+    def test_redirect_overwrite(self):
+        dangerous, key, desc = detect_dangerous_command("echo 'approvals:' > ~/.hermes/config.yaml")
+        assert dangerous is True
+        assert key is not None
+
+    def test_append(self):
+        dangerous, key, desc = detect_dangerous_command("echo '  mode: off' >> ~/.hermes/config.yaml")
+        assert dangerous is True
+
+    def test_tee(self):
+        dangerous, key, desc = detect_dangerous_command("echo x | tee ~/.hermes/config.yaml")
+        assert dangerous is True
+
+    def test_cp_over_config(self):
+        dangerous, key, desc = detect_dangerous_command("cp /tmp/evil.yaml ~/.hermes/config.yaml")
+        assert dangerous is True
+
+    def test_sed_in_place(self):
+        # The gap the pairing closes: sed -i mutates the file directly,
+        # bypassing the redirection/tee patterns.
+        dangerous, key, desc = detect_dangerous_command("sed -i 's/manual/off/' ~/.hermes/config.yaml")
+        assert dangerous is True
+        assert "hermes config" in desc.lower() or "in-place" in desc.lower()
+
+    def test_sed_in_place_long_flag(self):
+        dangerous, key, desc = detect_dangerous_command("sed --in-place 's/manual/off/' ~/.hermes/config.yaml")
+        assert dangerous is True
+
+    def test_custom_hermes_home(self):
+        dangerous, key, desc = detect_dangerous_command("echo x | tee $HERMES_HOME/config.yaml")
+        assert dangerous is True
+
+    def test_read_is_safe(self):
+        # Reading config is not a write — must not trip.
+        dangerous, key, desc = detect_dangerous_command("cat ~/.hermes/config.yaml")
+        assert dangerous is False
+
+    def test_normal_yaml_write_safe(self):
+        # A non-Hermes config.yaml in a project dir is handled by the project
+        # patterns, but a plain temp write must not false-positive.
+        dangerous, key, desc = detect_dangerous_command("echo data > /tmp/scratch.txt")
+        assert dangerous is False
+
+
 class TestFindExecFullPathRm:
     """Detect find -exec with full-path rm bypasses."""
 
diff --git a/tools/approval.py b/tools/approval.py
index 1dbb6eb6e..47f4a5f44 100644
--- a/tools/approval.py
+++ b/tools/approval.py
@@ -128,6 +128,20 @@ _HERMES_ENV_PATH = (
     r'(?:\$hermes_home|\$\{hermes_home\})/)'
     r'\.env\b'
 )
+# ~/.hermes/config.yaml IS the security policy: approvals.mode, yolo, and the
+# permanent-approval allowlist live here, and the config cache is mtime-keyed
+# so a write takes effect mid-session (the agent could flip approvals.mode=off
+# and immediately bypass the gate). Pair the write_file/patch deny (file_tools
+# _check_sensitive_path) with terminal-side coverage so `sed -i`, `tee`, `>`,
+# `cp`, etc. targeting it are gated too — otherwise the deny is unpaired
+# theater. Mirrors _HERMES_ENV_PATH; matches the HERMES_HOME override form as
+# well as ~/.hermes/.
+_HERMES_CONFIG_PATH = (
+    r'(?:~\/\.hermes/|'
+    r'(?:\$home|\$\{home\})/\.hermes/|'
+    r'(?:\$hermes_home|\$\{hermes_home\})/)'
+    r'config\.yaml\b'
+)
 _PROJECT_ENV_PATH = r'(?:(?:/|\.{1,2}/)?(?:[^\s/"\'`]+/)*\.env(?:\.[^/\s"\'`]+)*)'
 _PROJECT_CONFIG_PATH = r'(?:(?:/|\.{1,2}/)?(?:[^\s/"\'`]+/)*config\.yaml)'
 _SHELL_RC_FILES = (
@@ -153,6 +167,7 @@ _SENSITIVE_WRITE_TARGET = (
     rf'(?:{_SYSTEM_CONFIG_PATH}|/dev/sd|'
     rf'{_SSH_SENSITIVE_PATH}|'
     rf'{_HERMES_ENV_PATH}|'
+    rf'{_HERMES_CONFIG_PATH}|'
     rf'{_SHELL_RC_FILES}|'
     rf'{_CREDENTIAL_FILES})'
 )
@@ -391,6 +406,12 @@ DANGEROUS_PATTERNS = [
     (rf'\b(cp|mv|install)\b.*\s["\']?{_PROJECT_SENSITIVE_WRITE_TARGET}["\']?{_COMMAND_TAIL}', "overwrite project env/config file"),
     (rf'\bsed\s+-[^\s]*i.*\s{_SYSTEM_CONFIG_PATH}', "in-place edit of system config"),
     (rf'\bsed\s+--in-place\b.*\s{_SYSTEM_CONFIG_PATH}', "in-place edit of system config (long flag)"),
+    # In-place edit of a Hermes-managed security file (~/.hermes/config.yaml or
+    # .env). sed -i bypasses the redirection/tee patterns above because it
+    # mutates the file directly. Pairs the file_tools write_file/patch deny so
+    # the terminal side is not an open door. See #14639.
+    (rf'\bsed\s+-[^\s]*i.*(?:{_HERMES_CONFIG_PATH}|{_HERMES_ENV_PATH})', "in-place edit of Hermes config/env"),
+    (rf'\bsed\s+--in-place\b.*(?:{_HERMES_CONFIG_PATH}|{_HERMES_ENV_PATH})', "in-place edit of Hermes config/env (long flag)"),
     # Script execution via heredoc — bypasses the -e/-c flag patterns above.
     # `python3 << 'EOF'` feeds arbitrary code via stdin without -c/-e flags.
     (r'\b(python[23]?|perl|ruby|node)\s+<<', "script execution via heredoc"),