diff --git a/tests/test_diagnostic_metadata.py b/tests/test_diagnostic_metadata.py
new file mode 100644
index 00000000..c46a322c
--- /dev/null
+++ b/tests/test_diagnostic_metadata.py
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""Tests for build diagnostic report metadata ($30 bounty, issue #3).
+
+Validates commit ID tracking, module result summaries, logd path handling,
+and deterministic output from build_diagnostic_report().
+"""
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+import build  # noqa: E402
+
+
+class TestDiagnosticMetadata(unittest.TestCase):
+    """Validates build_diagnostic_report output structure and determinism."""
+
+    # ── report metadata ─────────────────────────────────────────────
+
+    def test_report_includes_commit_id(self):
+        report = build.build_diagnostic_report([], "abc123")
+        self.assertIn("commit", report)
+        self.assertEqual(report["commit"], "abc123")
+
+    def test_report_includes_module_result_summaries(self):
+        results = [
+            ("mod_a", True, 1.2, "output A", None),
+            ("mod_b", False, 0.8, "output B", None),
+        ]
+        report = build.build_diagnostic_report(results, "commit-1")
+        self.assertEqual(report["total_modules"], 2)
+        self.assertEqual(report["passed"], 1)
+        self.assertEqual(report["failed"], 1)
+        self.assertEqual(len(report["modules"]), 2)
+        # Module entries have expected keys
+        for mod in report["modules"]:
+            self.assertIn("name", mod)
+            self.assertIn("status", mod)
+            self.assertIn("elapsed_seconds", mod)
+
+    def test_module_status_pass_fail(self):
+        results = [
+            ("pass_mod", True, 0.5, "ok", None),
+            ("fail_mod", False, 1.0, "error", "artifact.bin"),
+        ]
+        report = build.build_diagnostic_report(results, "c2")
+        mods = {m["name"]: m for m in report["modules"]}
+        self.assertEqual(mods["pass_mod"]["status"], "PASS")
+        self.assertEqual(mods["fail_mod"]["status"], "FAIL")
+        self.assertEqual(mods["fail_mod"]["artifact"], "artifact.bin")
+
+    # ── logd path handling ─────────────────────────────────────────
+
+    def test_report_includes_diagnostic_logd_key(self):
+        report = build.build_diagnostic_report([], "test")
+        self.assertIn("diagnostic_logd", report)
+
+    def test_logd_relpaths_appear_in_pr_note(self):
+        report = build.build_diagnostic_report(
+            [], "c3", logd_relpaths=["diagnostic/build-abc.logd"]
+        )
+        self.assertIn("build-abc.logd", report["pr_note"])
+
+    # ── determinism ─────────────────────────────────────────────────
+
+    def test_empty_results_are_deterministic(self):
+        report1 = build.build_diagnostic_report([], "same-commit")
+        report2 = build.build_diagnostic_report([], "same-commit")
+        self.assertEqual(report1["total_modules"], report2["total_modules"])
+        self.assertEqual(report1["passed"], report2["passed"])
+        self.assertEqual(report1["commit"], report2["commit"])
+
+    def test_same_inputs_produce_same_output(self):
+        results = [("m", True, 0.1, "out", None)]
+        r1 = build.build_diagnostic_report(results, "c4")
+        r2 = build.build_diagnostic_report(results, "c4")
+        self.assertEqual(r1["passed"], r2["passed"])
+        self.assertEqual(r1["total_modules"], r2["total_modules"])
+
+    # ── no external dependencies ────────────────────────────────────
+
+    def test_build_diagnostic_report_no_network(self):
+        """Report generation does not require network access."""
+        report = build.build_diagnostic_report(
+            [("isolated", True, 0.0, "", None)],
+            "no-net-commit",
+        )
+        self.assertEqual(report["total_modules"], 1)
+
+    def test_build_diagnostic_report_no_file_io(self):
+        """Report generation is pure in-memory (no disk writes)."""
+        import tempfile
+        report = build.build_diagnostic_report([], "mem-only")
+        self.assertIsInstance(report, dict)
+        self.assertIn("generated_at", report)
+
+
+if __name__ == "__main__":
+    unittest.main()