diff --git a/sanity_check_tool/src/sanity_check_tool/core.py b/sanity_check_tool/src/sanity_check_tool/core.py
new file mode 100644
index 0000000..10dafa6
--- /dev/null
+++ b/sanity_check_tool/src/sanity_check_tool/core.py
@@ -0,0 +1,81 @@
+import json
+from typing import Dict, Any
+
+
+class InputValidationError(Exception):
+    """Raised when input data does not conform to expected structure."""
+
+
+class SanityResults:
+    """Simple result container for sanity check outcomes."""
+
+    def __init__(self, missing_write_pairs: int, broken_corr_ids: int, empty_fields: int) -> None:
+        assert missing_write_pairs >= 0, "missing_write_pairs must be >= 0"
+        assert broken_corr_ids >= 0, "broken_corr_ids must be >= 0"
+        assert empty_fields >= 0, "empty_fields must be >= 0"
+        self.missing_write_pairs = missing_write_pairs
+        self.broken_corr_ids = broken_corr_ids
+        self.empty_fields = empty_fields
+
+    def to_dict(self) -> Dict[str, int]:
+        return {
+            "missing_write_pairs": self.missing_write_pairs,
+            "broken_corr_ids": self.broken_corr_ids,
+            "empty_fields": self.empty_fields,
+        }
+
+
+_DEF_REQUIRED_FIELDS = {"write_pre", "write_post", "corr_id"}
+
+
+def perform_sanity_check(run_summary: Dict[str, Any]) -> Dict[str, int]:
+    """Überprüft die Run-Summary auf Konsistenzprobleme.
+
+    Args:
+        run_summary: Aggregierte Laufdaten, typischerweise aus einer JSON-Datei geladen.
+
+    Returns:
+        Dictionary mit den Ergebnissen der Sanity-Prüfung.
+    """
+    if not isinstance(run_summary, dict):
+        raise InputValidationError("run_summary must be a dictionary.")
+    entries = run_summary.get("entries")
+    if not isinstance(entries, list):
+        raise InputValidationError("run_summary must contain a list under key 'entries'.")
+
+    missing_write_pairs = 0
+    broken_corr_ids = 0
+    empty_fields = 0
+
+    seen_ids = set()
+
+    for entry in entries:
+        if not isinstance(entry, dict):
+            continue
+
+        # Check missing write_pre/write_post
+        has_pre = entry.get("write_pre") is not None
+        has_post = entry.get("write_post") is not None
+        if has_pre ^ has_post:
+            missing_write_pairs += 1
+
+        # Check broken correlation chains
+        corr_id = entry.get("corr_id")
+        if corr_id in seen_ids:
+            broken_corr_ids += 1
+        elif corr_id is not None:
+            seen_ids.add(corr_id)
+
+        # Check empty mandatory fields
+        for f in _DEF_REQUIRED_FIELDS:
+            v = entry.get(f)
+            if v in (None, ""):
+                empty_fields += 1
+
+    results = SanityResults(
+        missing_write_pairs=missing_write_pairs,
+        broken_corr_ids=broken_corr_ids,
+        empty_fields=empty_fields,
+    )
+
+    return results.to_dict()