diff --git a/ra_aid/prompts.py b/ra_aid/prompts.py
index dc21772..07a0e6f 100644
--- a/ra_aid/prompts.py
+++ b/ra_aid/prompts.py
@@ -196,6 +196,8 @@ Instructions:
 4. Use delete_key_facts to remove any key facts that no longer apply.
 5. Do not add features not explicitly required.
 6. Only create or modify files directly related to this task.
+7. Use file_str_replace and write_file_tool for simple file modifications.
+8. Delegate to run_programming_task for more complex programming tasks.
 
 Testing:
 
diff --git a/tests/ra_aid/tools/test_file_str_replace.py b/tests/ra_aid/tools/test_file_str_replace.py
new file mode 100644
index 0000000..181e076
--- /dev/null
+++ b/tests/ra_aid/tools/test_file_str_replace.py
@@ -0,0 +1,174 @@
+import os
+import pytest
+from pathlib import Path
+from unittest.mock import patch, mock_open
+from ra_aid.tools.file_str_replace import file_str_replace
+
+@pytest.fixture
+def temp_test_dir(tmp_path):
+    """Create a temporary test directory."""
+    test_dir = tmp_path / "test_replace_dir"
+    test_dir.mkdir(exist_ok=True)
+    return test_dir
+
+def test_basic_replacement(temp_test_dir):
+    """Test basic string replacement functionality."""
+    test_file = temp_test_dir / "test.txt"
+    initial_content = "Hello world! This is a test."
+    test_file.write_text(initial_content)
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "world",
+        "new_str": "universe"
+    })
+    
+    assert result["success"] is True
+    assert test_file.read_text() == "Hello universe! This is a test."
+    assert "Successfully replaced" in result["message"]
+
+def test_file_not_found():
+    """Test handling of non-existent file."""
+    result = file_str_replace.invoke({
+        "filepath": "nonexistent.txt",
+        "old_str": "test",
+        "new_str": "replacement"
+    })
+    
+    assert result["success"] is False
+    assert "File not found" in result["message"]
+
+def test_string_not_found(temp_test_dir):
+    """Test handling of string not present in file."""
+    test_file = temp_test_dir / "test.txt"
+    test_file.write_text("Hello world!")
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "nonexistent",
+        "new_str": "replacement"
+    })
+    
+    assert result["success"] is False
+    assert "String not found" in result["message"]
+
+def test_multiple_occurrences(temp_test_dir):
+    """Test handling of multiple string occurrences."""
+    test_file = temp_test_dir / "test.txt"
+    test_file.write_text("test test test")
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "test",
+        "new_str": "replacement"
+    })
+    
+    assert result["success"] is False
+    assert "appears" in result["message"]
+    assert "must be unique" in result["message"]
+
+def test_empty_strings(temp_test_dir):
+    """Test handling of empty strings."""
+    test_file = temp_test_dir / "test.txt"
+    test_file.write_text("Hello world!")
+    
+    # Test empty old string
+    result1 = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "",
+        "new_str": "replacement"
+    })
+    assert result1["success"] is False
+    
+    # Test empty new string
+    result2 = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "world",
+        "new_str": ""
+    })
+    assert result2["success"] is True
+    assert test_file.read_text() == "Hello !"
+
+def test_special_characters(temp_test_dir):
+    """Test handling of special characters."""
+    test_file = temp_test_dir / "test.txt"
+    initial_content = "Hello\nworld!\t\r\nSpecial chars: $@#%"
+    test_file.write_text(initial_content)
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "Special chars: $@#%",
+        "new_str": "Replaced!"
+    })
+    
+    assert result["success"] is True
+    assert "Special chars: $@#%" not in test_file.read_text()
+    assert "Replaced!" in test_file.read_text()
+
+@patch('pathlib.Path.read_text')
+def test_io_error(mock_read_text, temp_test_dir):
+    """Test handling of IO errors during read."""
+    # Create and write to file first
+    test_file = temp_test_dir / "test.txt"
+    test_file.write_text("some test content")
+    
+    # Then mock read_text to raise error
+    mock_read_text.side_effect = IOError("Failed to read file")
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "test",
+        "new_str": "replacement"
+    })
+    
+    assert result["success"] is False
+    assert "Failed to read file" in result["message"]
+
+def test_permission_error(temp_test_dir):
+    """Test handling of permission errors."""
+    test_file = temp_test_dir / "readonly.txt"
+    test_file.write_text("test content")
+    os.chmod(test_file, 0o444)  # Make file read-only
+    
+    try:
+        result = file_str_replace.invoke({
+            "filepath": str(test_file),
+            "old_str": "test",
+            "new_str": "replacement"
+        })
+        
+        assert result["success"] is False
+        assert "Permission" in result["message"] or "Error" in result["message"]
+    finally:
+        os.chmod(test_file, 0o644)  # Restore permissions for cleanup
+
+def test_unicode_strings(temp_test_dir):
+    """Test handling of Unicode strings."""
+    test_file = temp_test_dir / "unicode.txt"
+    initial_content = "Hello 世界! Unicode テスト"
+    test_file.write_text(initial_content)
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": "世界",
+        "new_str": "ワールド"
+    })
+    
+    assert result["success"] is True
+    assert "世界" not in test_file.read_text()
+    assert "ワールド" in test_file.read_text()
+
+def test_long_string_truncation(temp_test_dir):
+    """Test handling and truncation of very long strings."""
+    test_file = temp_test_dir / "test.txt"
+    long_string = "x" * 100
+    test_file.write_text(f"prefix {long_string} suffix")
+    
+    result = file_str_replace.invoke({
+        "filepath": str(test_file),
+        "old_str": long_string,
+        "new_str": "replaced"
+    })
+    
+    assert result["success"] is True
+    assert test_file.read_text() == "prefix replaced suffix"