diff --git a/scripts/check_coverage.py b/scripts/check_coverage.py
index 727057c..29590ce 100755
--- a/scripts/check_coverage.py
+++ b/scripts/check_coverage.py
@@ -8,6 +8,32 @@ import json
 from pathlib import Path
 
 
+def _normalize_report_path(path: str, cwd: Path) -> str:
+    """Return a stable repository-relative path for a coverage report entry."""
+
+    candidate = Path(path)
+    if candidate.is_absolute():
+        try:
+            return candidate.relative_to(cwd).as_posix()
+        except ValueError:
+            return candidate.as_posix()
+    return candidate.as_posix()
+
+
+def _production_files(root: Path, cwd: Path) -> set[str]:
+    """List production Python files that must appear in the coverage report."""
+
+    required: set[str] = set()
+    for path in root.rglob("*.py"):
+        if path.name == "__init__.py" or "__pycache__" in path.parts:
+            continue
+        try:
+            required.add(path.relative_to(cwd).as_posix())
+        except ValueError:
+            required.add(path.as_posix())
+    return required
+
+
 def main() -> int:
     """Check each production file against a minimum coverage percentage."""
 
@@ -19,24 +45,35 @@ def main() -> int:
 
     data = json.loads(Path(args.coverage_json).read_text(encoding="utf-8"))
     files = data.get("files") if isinstance(data, dict) else {}
-    violations: list[tuple[float, str]] = []
+    cwd = Path.cwd().resolve()
+    root = Path(args.root)
+    root_path = (root if root.is_absolute() else cwd / root).resolve()
+    root_prefix = root_path.relative_to(cwd).as_posix() if root_path.is_relative_to(cwd) else root_path.as_posix()
+    covered_paths: set[str] = set()
+    violations: list[str] = []
+
     for path, payload in sorted(files.items()):
-        if not path.startswith(f"{args.root}/"):
+        normalized_path = _normalize_report_path(path, cwd)
+        if not normalized_path.startswith(f"{root_prefix}/"):
             continue
         summary = payload.get("summary") if isinstance(payload, dict) else {}
         percent = summary.get("percent_covered") if isinstance(summary, dict) else None
         if not isinstance(percent, (int, float)):
+            violations.append(f"{normalized_path}: coverage percent missing")
             continue
+        covered_paths.add(normalized_path)
         if float(percent) < args.threshold:
-            violations.append((float(percent), path))
+            violations.append(f"{normalized_path}: {float(percent):.2f}% < {args.threshold:.2f}%")
+
+    for path in sorted(_production_files(root_path, cwd) - covered_paths):
+        violations.append(f"{path}: missing from coverage report")
 
     if violations:
-        for percent, path in sorted(violations):
-            print(f"{path}: {percent:.2f}% < {args.threshold:.2f}%")
+        for violation in sorted(violations):
+            print(violation)
         return 1
     return 0
 
 
 if __name__ == "__main__":
     raise SystemExit(main())
-
diff --git a/tests/test_check_coverage_contract.py b/tests/test_check_coverage_contract.py
new file mode 100644
index 0000000..46c8ac1
--- /dev/null
+++ b/tests/test_check_coverage_contract.py
@@ -0,0 +1,83 @@
+"""Tests for Atlasbot's per-file coverage contract script."""
+
+from __future__ import annotations
+
+import json
+import subprocess
+import sys
+from pathlib import Path
+
+
+SCRIPT = Path(__file__).resolve().parents[1] / "scripts" / "check_coverage.py"
+
+
+def _run_check(tmp_path: Path, coverage_payload: dict) -> subprocess.CompletedProcess[str]:
+    """Run the coverage script against a temporary Atlasbot source tree."""
+
+    coverage_path = tmp_path / "coverage.json"
+    coverage_path.write_text(json.dumps(coverage_payload), encoding="utf-8")
+    return subprocess.run(
+        [sys.executable, str(SCRIPT), str(coverage_path), "--root", "atlasbot", "--threshold", "95"],
+        cwd=tmp_path,
+        text=True,
+        capture_output=True,
+        check=False,
+    )
+
+
+def test_missing_source_file_fails_coverage_contract(tmp_path: Path) -> None:
+    """Every non-init production source file must appear in the coverage report."""
+
+    source_root = tmp_path / "atlasbot"
+    source_root.mkdir()
+    (source_root / "__init__.py").write_text("", encoding="utf-8")
+    (source_root / "covered.py").write_text("value = 1\n", encoding="utf-8")
+    (source_root / "missing.py").write_text("value = 2\n", encoding="utf-8")
+
+    result = _run_check(
+        tmp_path,
+        {"files": {"atlasbot/covered.py": {"summary": {"percent_covered": 100.0}}}},
+    )
+
+    assert result.returncode == 1
+    assert "atlasbot/missing.py: missing from coverage report" in result.stdout
+    assert "atlasbot/__init__.py" not in result.stdout
+
+
+def test_low_or_malformed_file_coverage_fails_contract(tmp_path: Path) -> None:
+    """Covered files still fail if their per-file percentage is bad or missing."""
+
+    source_root = tmp_path / "atlasbot"
+    source_root.mkdir()
+    (source_root / "low.py").write_text("value = 1\n", encoding="utf-8")
+    (source_root / "malformed.py").write_text("value = 2\n", encoding="utf-8")
+
+    result = _run_check(
+        tmp_path,
+        {
+            "files": {
+                "atlasbot/low.py": {"summary": {"percent_covered": 94.9}},
+                "atlasbot/malformed.py": {"summary": {}},
+            }
+        },
+    )
+
+    assert result.returncode == 1
+    assert "atlasbot/low.py: 94.90% < 95.00%" in result.stdout
+    assert "atlasbot/malformed.py: coverage percent missing" in result.stdout
+
+
+def test_complete_per_file_coverage_passes_contract(tmp_path: Path) -> None:
+    """The contract passes when every production file is present above threshold."""
+
+    source_root = tmp_path / "atlasbot"
+    source_root.mkdir()
+    (source_root / "covered.py").write_text("value = 1\n", encoding="utf-8")
+
+    result = _run_check(
+        tmp_path,
+        {"files": {"atlasbot/covered.py": {"summary": {"percent_covered": 95.0}}}},
+    )
+
+    assert result.returncode == 0
+    assert result.stdout == ""