diff --git a/shell_runner.py b/shell_runner.py
index a822beb..d66c466 100644
--- a/shell_runner.py
+++ b/shell_runner.py
@@ -1,14 +1,16 @@
 # shell_runner.py
 import subprocess, shutil
 from pathlib import Path
+from typing import Callable, Optional
 
 TIMEOUT_SECONDS = 60
 
-def run_powershell(script: str, base_dir: str) -> str:
+def run_powershell(script: str, base_dir: str, qa_callback: Optional[Callable[[str], str]] = None) -> str:
     """
     Run a PowerShell script with working directory set to base_dir.
     Returns a string combining stdout, stderr, and exit code.
-    Raises nothing - all errors are captured into the return string.
+    If qa_callback is provided and the command fails or has stderr, 
+    the callback is called with the stderr content and its result is appended.
     """
     safe_dir = str(base_dir).replace("'", "''")
     full_script = f"Set-Location -LiteralPath '{safe_dir}'\n{script}"
@@ -25,6 +27,13 @@ def run_powershell(script: str, base_dir: str) -> str:
         if r.stdout.strip(): parts.append(f"STDOUT:\n{r.stdout.strip()}")
         if r.stderr.strip(): parts.append(f"STDERR:\n{r.stderr.strip()}")
         parts.append(f"EXIT CODE: {r.returncode}")
+        
+        # QA Interceptor logic
+        if (r.returncode != 0 or r.stderr.strip()) and qa_callback:
+            qa_analysis = qa_callback(r.stderr.strip())
+            if qa_analysis:
+                parts.append(f"\nQA ANALYSIS:\n{qa_analysis}")
+                
         return "\n".join(parts)
     except subprocess.TimeoutExpired: return f"ERROR: timed out after {TIMEOUT_SECONDS}s"
     except Exception as e: return f"ERROR: {e}"
diff --git a/tests/test_tier4_interceptor.py b/tests/test_tier4_interceptor.py
new file mode 100644
index 0000000..10e6453
--- /dev/null
+++ b/tests/test_tier4_interceptor.py
@@ -0,0 +1,102 @@
+import pytest
+from unittest.mock import MagicMock, patch
+import subprocess
+from shell_runner import run_powershell
+
+def test_run_powershell_qa_callback_on_failure():
+    """
+    Test that qa_callback is called when a powershell command fails (non-zero exit code).
+    The result of the callback should be appended to the output.
+    """
+    script = "Write-Error 'something went wrong'; exit 1"
+    base_dir = "."
+    
+    # Mocking subprocess.run to simulate failure
+    mock_result = MagicMock()
+    mock_result.stdout = ""
+    mock_result.stderr = "something went wrong"
+    mock_result.returncode = 1
+    
+    qa_callback = MagicMock(return_value="QA ANALYSIS: This looks like a syntax error.")
+    
+    with patch("subprocess.run", return_value=mock_result), \
+         patch("shutil.which", return_value="powershell.exe"):
+        
+        # We expect run_powershell to accept qa_callback
+        output = run_powershell(script, base_dir, qa_callback=qa_callback)
+        
+        # Verify callback was called with stderr
+        qa_callback.assert_called_once_with("something went wrong")
+        
+        # Verify output contains the callback result
+        assert "QA ANALYSIS: This looks like a syntax error." in output
+        assert "STDERR:\nsomething went wrong" in output
+        assert "EXIT CODE: 1" in output
+
+def test_run_powershell_qa_callback_on_stderr_only():
+    """
+    Test that qa_callback is called when a command has stderr even if exit code is 0.
+    """
+    script = "Write-Error 'non-fatal error'"
+    base_dir = "."
+    
+    mock_result = MagicMock()
+    mock_result.stdout = "Success"
+    mock_result.stderr = "non-fatal error"
+    mock_result.returncode = 0
+    
+    qa_callback = MagicMock(return_value="QA ANALYSIS: Ignorable warning.")
+    
+    with patch("subprocess.run", return_value=mock_result), \
+         patch("shutil.which", return_value="powershell.exe"):
+        
+        output = run_powershell(script, base_dir, qa_callback=qa_callback)
+        
+        qa_callback.assert_called_once_with("non-fatal error")
+        assert "QA ANALYSIS: Ignorable warning." in output
+        assert "STDOUT:\nSuccess" in output
+
+def test_run_powershell_no_qa_callback_on_success():
+    """
+    Test that qa_callback is NOT called when the command succeeds without stderr.
+    """
+    script = "Write-Output 'All good'"
+    base_dir = "."
+    
+    mock_result = MagicMock()
+    mock_result.stdout = "All good"
+    mock_result.stderr = ""
+    mock_result.returncode = 0
+    
+    qa_callback = MagicMock()
+    
+    with patch("subprocess.run", return_value=mock_result), \
+         patch("shutil.which", return_value="powershell.exe"):
+        
+        output = run_powershell(script, base_dir, qa_callback=qa_callback)
+        
+        qa_callback.assert_not_called()
+        assert "STDOUT:\nAll good" in output
+        assert "EXIT CODE: 0" in output
+        assert "QA ANALYSIS" not in output
+
+def test_run_powershell_optional_qa_callback():
+    """
+    Test that run_powershell still works without providing a qa_callback.
+    """
+    script = "Write-Error 'error'"
+    base_dir = "."
+    
+    mock_result = MagicMock()
+    mock_result.stdout = ""
+    mock_result.stderr = "error"
+    mock_result.returncode = 1
+    
+    with patch("subprocess.run", return_value=mock_result), \
+         patch("shutil.which", return_value="powershell.exe"):
+        
+        # Should not raise TypeError even if qa_callback is not provided
+        output = run_powershell(script, base_dir)
+        
+        assert "STDERR:\nerror" in output
+        assert "EXIT CODE: 1" in output