diff --git a/tests/test_diagnostic_redaction.py b/tests/test_diagnostic_redaction.py
new file mode 100644
index 00000000..67b0e4cb
--- /dev/null
+++ b/tests/test_diagnostic_redaction.py
@@ -0,0 +1,118 @@
+#!/usr/bin/env python3
+"""Regression tests for diagnostic artifact redaction (issue #1, $40 bounty).
+
+Validates that diagnostic metadata does not leak local paths, usernames,
+or machine names, and that artifact paths use repository-relative format.
+"""
+from __future__ import annotations
+
+import json
+import os
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+# Make build.py importable from repo root
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+import build  # noqa: E402
+
+
+class TestDiagnosticRedaction(unittest.TestCase):
+    """Validates that diagnostic metadata contains no path/user leakage."""
+
+    def setUp(self):
+        self.home = str(Path.home())
+        self.username = os.environ.get("USER", os.environ.get("USERNAME", ""))
+        self.repo_root = str(Path(__file__).resolve().parents[1])
+        self.temp = tempfile.gettempdir()
+
+    # ── path normalisation ───────────────────────────────────────────
+
+    def test_diagnostic_paths_under_diagnostic_dir(self):
+        """diagnostic_paths_for_commit returns paths under diagnostic/."""
+        logd_path, metadata_path, _ = build.diagnostic_paths_for_commit()
+
+        logd_str = str(logd_path).replace("\\", "/")
+        meta_str = str(metadata_path).replace("\\", "/")
+        self.assertIn("diagnostic/", logd_str,
+                      f"logd path should be under diagnostic/, got {logd_path}")
+        self.assertIn("diagnostic/", meta_str,
+                      f"metadata path should be under diagnostic/, got {metadata_path}")
+
+    # ── no path leakage ──────────────────────────────────────────────
+
+    def test_report_does_not_leak_home_directory(self):
+        """Metadata JSON must not contain the user's home directory path."""
+        report = build.build_diagnostic_report([],"test-commit")
+        json_str = json.dumps(report)
+        self.assertNotIn(self.home, json_str,
+                         "diagnostic report leaked HOME path")
+        # Also check Path.home() expansion for Windows
+        home_lower = self.home.lower().replace("\\", "/")
+        json_lower = json_str.lower().replace("\\", "/")
+        self.assertNotIn(home_lower, json_lower)
+
+    def test_report_does_not_leak_temp_directory(self):
+        """Metadata must not contain temp directory paths."""
+        report = build.build_diagnostic_report([],"test-commit")
+        json_str = json.dumps(report)
+        # Strip drive letter for Windows comparison
+        temp_clean = self.temp.replace("\\", "/").split(":")[-1].lstrip("/")
+        json_clean = json_str.replace("\\", "/")
+        # Only assert if temp path is distinct enough
+        if len(temp_clean) > 4:
+            self.assertNotIn(temp_clean, json_clean,
+                             f"diagnostic report leaked TEMP: {temp_clean}")
+
+    def test_report_does_not_leak_username(self):
+        """Metadata must not contain the current username."""
+        if not self.username or len(self.username) < 2:
+            self.skipTest("no username to check")
+        report = build.build_diagnostic_report([],"test-commit")
+        json_str = json.dumps(report).lower()
+        self.assertNotIn(self.username.lower(), json_str,
+                         f"diagnostic report leaked username: {self.username}")
+
+    def test_report_does_not_leak_repo_absolute_path(self):
+        """Metadata must not contain the absolute repository root path."""
+        report = build.build_diagnostic_report([],"test-commit")
+        json_str = json.dumps(report)
+        self.assertNotIn(self.repo_root, json_str,
+                         "diagnostic report leaked repo absolute path")
+
+    # ── artifact pairing ─────────────────────────────────────────────
+
+    def test_report_includes_logd_reference(self):
+        """diagnostic report references the encrypted logd artifact."""
+        report = build.build_diagnostic_report([],"test-commit")
+        self.assertIn("diagnostic_logd", report,
+                      "report must include diagnostic_logd key")
+
+    def test_empty_results_yields_report(self):
+        """Report with empty results still produces valid structure."""
+        report = build.build_diagnostic_report([], "test-commit")
+        self.assertEqual(report["commit"], "test-commit")
+        self.assertEqual(report["passed"], 0)
+        self.assertEqual(report["total_modules"], 0)
+
+    # ── cross-platform determinism ───────────────────────────────────
+
+    def test_paths_are_strings(self):
+        """All path values in report are plain strings, not Path objects."""
+        report = build.build_diagnostic_report([],"test-commit")
+        for key, value in report.items():
+            if key.endswith("_path") or key.endswith("_logd"):
+                self.assertIsInstance(value, (str, list, type(None)),
+                                      f"{key} should be str/list/None, got {type(value)}")
+
+    def test_commit_id_present(self):
+        """Report must include the commit ID provided."""
+        report = build.build_diagnostic_report([],"abc123def456")
+        self.assertIn("commit", report)
+        self.assertEqual(report["commit"], "abc123def456")
+
+
+if __name__ == "__main__":
+    unittest.main()