diff --git a/tests/test_normalize_code_blocks.py b/tests/test_normalize_code_blocks.py
new file mode 100644
index 00000000..ed323eb0
--- /dev/null
+++ b/tests/test_normalize_code_blocks.py
@@ -0,0 +1,139 @@
+#!/usr/bin/env python3
+"""Tests for normalize-code-blocks.py — training data code block indentation fix (#750)."""
+
+import json
+import os
+import sys
+import tempfile
+import textwrap
+from pathlib import Path
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "scripts"))
+from normalize_code_blocks import normalize_code_block, process_line, CODE_BLOCK_RE
+
+
+class TestNormalizeCodeBlock:
+    def test_basic_dedent(self):
+        block = "```python\n    from fastapi import FastAPI\n    app = FastAPI()\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert "    from fastapi" not in result
+        assert "from fastapi" in result
+
+    def test_preserves_language_tag(self):
+        block = "```python\n    x = 1\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert result.startswith("```python")
+
+    def test_empty_block_unchanged(self):
+        block = "```python\n   \n   \n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert result == block
+
+    def test_multiple_blocks(self):
+        text = 'First: ```python\n    x = 1\n``` and second: ```python\n    y = 2\n```'
+        result = CODE_BLOCK_RE.sub(normalize_code_block, text)
+        assert "    x = 1" not in result
+        assert "    y = 2" not in result
+        assert "x = 1" in result
+        assert "y = 2" in result
+
+    def test_bash_block(self):
+        block = "```bash\n    echo hello\n    ls -la\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert "    echo" not in result
+        assert "echo hello" in result
+
+    def test_unlabeled_block(self):
+        block = "```\n    some code\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert "    some code" not in result
+
+    def test_mixed_indentation(self):
+        block = "```python\n    def foo():\n        return 42\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        lines = result.split("\n")
+        # First code line should not have leading spaces from embedding
+        code_lines = [l for l in lines if l.strip() and not l.startswith("```")]
+        assert code_lines[0].startswith("def")
+
+    def test_strips_leading_trailing_blanks(self):
+        block = "```python\n\n    x = 1\n\n```"
+        result = CODE_BLOCK_RE.sub(normalize_code_block, block)
+        assert "\n\n" not in result.split("```python")[1].split("```")[0]
+
+
+class TestProcessLine:
+    def test_valid_jsonl_with_code(self):
+        obj = {"prompt": "write code", "response": "```python\n    x = 1\n```"}
+        line = json.dumps(obj)
+        fixed, n = process_line(line)
+        parsed = json.loads(fixed)
+        assert n == 1
+        assert "    x = 1" not in parsed["response"]
+
+    def test_no_code_blocks(self):
+        obj = {"text": "hello world"}
+        line = json.dumps(obj)
+        fixed, n = process_line(line)
+        assert n == 0
+        assert json.loads(fixed)["text"] == "hello world"
+
+    def test_invalid_jsonl(self):
+        line = "not valid json {{{"
+        fixed, n = process_line(line)
+        assert n == 0
+        assert fixed == line
+
+    def test_nested_code_blocks(self):
+        obj = {
+            "messages": [
+                {"role": "user", "content": "write code"},
+                {"role": "assistant", "content": "```python\n    def f():\n        pass\n```"}
+            ]
+        }
+        line = json.dumps(obj)
+        fixed, n = process_line(line)
+        assert n == 1
+        parsed = json.loads(fixed)
+        assert "    def f" not in parsed["messages"][1]["content"]
+
+    def test_multiple_fields_with_code(self):
+        obj = {
+            "terse": "```python\n    x = 1\n```",
+            "rich": "```python\n    y = 2\n```"
+        }
+        line = json.dumps(obj)
+        fixed, n = process_line(line)
+        parsed = json.loads(fixed)
+        assert n == 2
+        assert "    x = 1" not in parsed["terse"]
+        assert "    y = 2" not in parsed["rich"]
+
+
+class TestEndToEnd:
+    def test_file_processing(self):
+        with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False) as f:
+            f.write(json.dumps({"r": "```python\n    x = 1\n```"}) + "\n")
+            f.write(json.dumps({"r": "no code here"}) + "\n")
+            f.write(json.dumps({"r": "```python\n    def g():\n        return 99\n```"}) + "\n")
+            f.flush()
+
+            # Process using the script logic
+            lines = Path(f.name).read_text().splitlines(keepends=True)
+            fixed = []
+            total = 0
+            for line in lines:
+                fl, n = process_line(line)
+                fixed.append(fl)
+                total += n
+
+        os.unlink(f.name)
+        assert total == 2
+        # Verify first line is fixed
+        first = json.loads(fixed[0])
+        assert "    x = 1" not in first["r"]
+
+
+if __name__ == "__main__":
+    import unittest
+    unittest.main()