diff --git a/report_generation/src/report_generation/core.py b/report_generation/src/report_generation/core.py
new file mode 100644
index 0000000..8ba48d2
--- /dev/null
+++ b/report_generation/src/report_generation/core.py
@@ -0,0 +1,154 @@
+from __future__ import annotations
+import json
+import os
+from pathlib import Path
+from datetime import datetime
+from typing import Any, Dict, Union
+import pandas as pd
+
+
+class InputDataError(Exception):
+    """Custom exception raised when input validation fails."""
+
+
+class AnalysisResult:
+    """Data representation of a single analysis result."""
+
+    def __init__(
+        self,
+        run_id: str,
+        step_id: int,
+        epoch_ms: int,
+        monotonic_ns: int,
+        tz_offset_minutes: int,
+        retry_tail_p99: float,
+        band_width: float,
+    ) -> None:
+        self.run_id = run_id
+        self.step_id = step_id
+        self.epoch_ms = epoch_ms
+        self.monotonic_ns = monotonic_ns
+        self.tz_offset_minutes = tz_offset_minutes
+        self.retry_tail_p99 = retry_tail_p99
+        self.band_width = band_width
+
+    @staticmethod
+    def validate_dict(data: Dict[str, Any]) -> None:
+        required_fields = {
+            "run_id": str,
+            "step_id": int,
+            "epoch_ms": int,
+            "monotonic_ns": int,
+            "tz_offset_minutes": int,
+            "retry_tail_p99": (float, int),
+            "band_width": (float, int),
+        }
+        for field, expected_type in required_fields.items():
+            if field not in data:
+                raise InputDataError(f"Missing required field: {field}")
+            if not isinstance(data[field], expected_type):
+                raise InputDataError(
+                    f"Invalid type for field '{field}': expected {expected_type}, got {type(data[field])}"
+                )
+
+
+class StabilityReport:
+    """Structured container for the generated stability report."""
+
+    def __init__(
+        self,
+        config_summary: str,
+        findings: str,
+        metric_comparison: Dict[str, Any],
+        timestamp: datetime,
+    ) -> None:
+        self.config_summary = config_summary
+        self.findings = findings
+        self.metric_comparison = metric_comparison
+        self.timestamp = timestamp
+
+    def to_dict(self) -> Dict[str, Any]:
+        return {
+            "config_summary": self.config_summary,
+            "findings": self.findings,
+            "metric_comparison": self.metric_comparison,
+            "timestamp": self.timestamp.isoformat(),
+        }
+
+
+def generate_report(analysis_results: Union[Dict[str, Any], pd.DataFrame]) -> str:
+    """Erstellt einen Bericht auf Basis der Analyseergebnisse und speichert ihn als Datei.
+
+    Args:
+        analysis_results: Datenstruktur (dict oder pandas.DataFrame) mit aggregierten Kennzahlen.
+
+    Returns:
+        Pfad zur generierten Report-Datei im JSON-Format.
+    """
+
+    # Input validation
+    if isinstance(analysis_results, dict):
+        if "data" in analysis_results:
+            records = analysis_results["data"]
+        elif isinstance(analysis_results.get("run_id"), list):
+            records = [analysis_results]
+        else:
+            records = analysis_results if isinstance(analysis_results, list) else [analysis_results]
+        if not isinstance(records, list):
+            raise InputDataError("Expected a list of records in 'analysis_results'.")
+        for rec in records:
+            AnalysisResult.validate_dict(rec)
+        df = pd.DataFrame(records)
+    elif isinstance(analysis_results, pd.DataFrame):
+        df = analysis_results.copy()
+        required_columns = [
+            "run_id",
+            "step_id",
+            "epoch_ms",
+            "monotonic_ns",
+            "tz_offset_minutes",
+            "retry_tail_p99",
+            "band_width",
+        ]
+        missing = [c for c in required_columns if c not in df.columns]
+        if missing:
+            raise InputDataError(f"Missing columns in DataFrame: {missing}")
+    else:
+        raise InputDataError("analysis_results must be a dict or pandas.DataFrame.")
+
+    # Aggregation and comparison
+    grouped = df.groupby("run_id").agg({
+        "retry_tail_p99": ["median", "std"],
+        "band_width": ["median", "std"],
+    })
+    grouped.columns = ["_".join(col).strip() for col in grouped.columns.values]
+    grouped = grouped.reset_index()
+
+    # Metric comparison dictionary
+    metric_comparison = grouped.set_index("run_id").to_dict(orient="index")
+
+    # Simple findings summary
+    ref_metric = grouped["retry_tail_p99_median"].mean()
+    stable_runs = grouped[grouped["retry_tail_p99_std"] < (0.1 * ref_metric)]["run_id"].tolist()
+    findings = (
+        f"Identified {len(stable_runs)} stable configurations: {', '.join(stable_runs)}. "
+        f"Average retry_tail_p99 median: {ref_metric:.2f}."
+    )
+
+    config_summary = f"Runs analyzed: {', '.join(grouped['run_id'].astype(str))}"
+
+    report = StabilityReport(
+        config_summary=config_summary,
+        findings=findings,
+        metric_comparison=metric_comparison,
+        timestamp=datetime.utcnow(),
+    )
+
+    output_path = Path("output/stability_report.json")
+    output_path.parent.mkdir(parents=True, exist_ok=True)
+
+    with output_path.open("w", encoding="utf-8") as f:
+        json.dump(report.to_dict(), f, indent=2)
+
+    assert output_path.exists(), "Report file should exist after generation."
+    return str(output_path.resolve())