diff --git a/admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx b/admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx
index 0e9f759..e2697bf 100644
--- a/admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx
+++ b/admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx
@@ -43,6 +43,9 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
   const [drawingRegion, setDrawingRegion] = useState(false)
   const [dragStart, setDragStart] = useState<{ x: number; y: number } | null>(null)
   const [dragEnd, setDragEnd] = useState<{ x: number; y: number } | null>(null)
+  const [isGroundTruth, setIsGroundTruth] = useState(false)
+  const [gtSaving, setGtSaving] = useState(false)
+  const [gtMessage, setGtMessage] = useState('')
 
   const leftPanelRef = useRef<HTMLDivElement>(null)
   const rightPanelRef = useRef<HTMLDivElement>(null)
@@ -86,6 +89,10 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
           : `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/image/original`,
       })
 
+      // Check if session has ground truth reference
+      const gt = data.ground_truth
+      setIsGroundTruth(!!gt?.build_grid_reference)
+
       // Load existing validation data
       const valResp = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/reconstruction/validation`)
       if (valResp.ok) {
@@ -196,6 +203,31 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
     }
   }
 
+  // Mark/update ground truth reference
+  const handleMarkGroundTruth = async () => {
+    if (!sessionId) return
+    setGtSaving(true)
+    setGtMessage('')
+    try {
+      const resp = await fetch(
+        `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/mark-ground-truth`,
+        { method: 'POST' }
+      )
+      if (!resp.ok) {
+        const body = await resp.text().catch(() => '')
+        throw new Error(`Ground Truth fehlgeschlagen (${resp.status}): ${body}`)
+      }
+      const data = await resp.json()
+      setIsGroundTruth(true)
+      setGtMessage(`Ground Truth gespeichert (${data.cells_saved} Zellen)`)
+      setTimeout(() => setGtMessage(''), 5000)
+    } catch (e) {
+      setGtMessage(e instanceof Error ? e.message : String(e))
+    } finally {
+      setGtSaving(false)
+    }
+  }
+
   // Handle manual region drawing on reconstruction
   const handleReconMouseDown = (e: React.MouseEvent<HTMLDivElement>) => {
     if (!drawingRegion) return
@@ -570,8 +602,20 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
         <div className="text-sm text-gray-500 dark:text-gray-400">
           {status === 'saved' && <span className="text-green-600 dark:text-green-400">Validierung gespeichert</span>}
           {status === 'saving' && <span>Speichere...</span>}
+          {gtMessage && (
+            <span className={gtMessage.includes('fehlgeschlagen') ? 'text-red-500' : 'text-amber-600 dark:text-amber-400'}>
+              {gtMessage}
+            </span>
+          )}
         </div>
         <div className="flex items-center gap-3">
+          <button
+            onClick={handleMarkGroundTruth}
+            disabled={gtSaving || status === 'saving'}
+            className="px-4 py-2 text-sm bg-amber-600 text-white rounded hover:bg-amber-700 disabled:opacity-50"
+          >
+            {gtSaving ? 'Speichere...' : isGroundTruth ? 'Ground Truth aktualisieren' : 'Als Ground Truth markieren'}
+          </button>
           <button
             onClick={handleSave}
             disabled={status === 'saving'}
diff --git a/klausur-service/backend/grid_editor_api.py b/klausur-service/backend/grid_editor_api.py
index 36ef379..aba9c91 100644
--- a/klausur-service/backend/grid_editor_api.py
+++ b/klausur-service/backend/grid_editor_api.py
@@ -745,42 +745,38 @@ def _filter_footer_words(
 
 
 # ---------------------------------------------------------------------------
-# Endpoints
+# Core computation (used by build-grid endpoint and regression tests)
 # ---------------------------------------------------------------------------
 
-@router.post("/sessions/{session_id}/build-grid")
-async def build_grid(session_id: str):
-    """Build a structured, zone-aware grid from existing Kombi word results.
+async def _build_grid_core(session_id: str, session: dict) -> dict:
+    """Core grid building logic — pure computation, no HTTP or DB side effects.
 
-    Requires that paddle-kombi or rapid-kombi has already been run on the session.
-    Uses the image for box detection and the word positions for grid structuring.
+    Args:
+        session_id: Session identifier (for logging and image loading).
+        session: Full session dict from get_session_db().
 
-    Returns a StructuredGrid with zones, each containing their own
-    columns, rows, and cells — ready for the frontend Excel-like editor.
+    Returns:
+        StructuredGrid result dict.
+
+    Raises:
+        ValueError: If session data is incomplete.
     """
     t0 = time.time()
 
-    # 1. Load session and word results
-    session = await get_session_db(session_id)
-    if not session:
-        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
-
+    # 1. Validate and load word results
     word_result = session.get("word_result")
     if not word_result or not word_result.get("cells"):
-        raise HTTPException(
-            status_code=400,
-            detail="No word results found. Run paddle-kombi or rapid-kombi first.",
-        )
+        raise ValueError("No word results found. Run paddle-kombi or rapid-kombi first.")
 
     img_w = word_result.get("image_width", 0)
     img_h = word_result.get("image_height", 0)
     if not img_w or not img_h:
-        raise HTTPException(status_code=400, detail="Missing image dimensions in word_result")
+        raise ValueError("Missing image dimensions in word_result")
 
     # 2. Flatten all word boxes from cells
     all_words = _flatten_word_boxes(word_result["cells"])
     if not all_words:
-        raise HTTPException(status_code=400, detail="No word boxes found in cells")
+        raise ValueError("No word boxes found in cells")
 
     logger.info("build-grid session %s: %d words from %d cells",
                 session_id, len(all_words), len(word_result["cells"]))
@@ -1313,14 +1309,45 @@ async def build_grid(session_id: str):
         "duration_seconds": round(duration, 2),
     }
 
-    # 7. Persist to DB
+    return result
+
+
+# ---------------------------------------------------------------------------
+# Endpoints
+# ---------------------------------------------------------------------------
+
+@router.post("/sessions/{session_id}/build-grid")
+async def build_grid(session_id: str):
+    """Build a structured, zone-aware grid from existing Kombi word results.
+
+    Requires that paddle-kombi or rapid-kombi has already been run on the session.
+    Uses the image for box detection and the word positions for grid structuring.
+
+    Returns a StructuredGrid with zones, each containing their own
+    columns, rows, and cells — ready for the frontend Excel-like editor.
+    """
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    try:
+        result = await _build_grid_core(session_id, session)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+
+    # Persist to DB
     await update_session_db(session_id, grid_editor_result=result)
 
     logger.info(
         "build-grid session %s: %d zones, %d cols, %d rows, %d cells, "
         "%d boxes in %.2fs",
-        session_id, len(zones_data), total_columns, total_rows,
-        total_cells, boxes_detected, duration,
+        session_id,
+        len(result.get("zones", [])),
+        result.get("summary", {}).get("total_columns", 0),
+        result.get("summary", {}).get("total_rows", 0),
+        result.get("summary", {}).get("total_cells", 0),
+        result.get("boxes_detected", 0),
+        result.get("duration_seconds", 0),
     )
 
     return result
diff --git a/klausur-service/backend/ocr_pipeline_api.py b/klausur-service/backend/ocr_pipeline_api.py
index 642d122..56eda09 100644
--- a/klausur-service/backend/ocr_pipeline_api.py
+++ b/klausur-service/backend/ocr_pipeline_api.py
@@ -47,6 +47,7 @@ from ocr_pipeline_ocr_merge import (
 )
 from ocr_pipeline_postprocess import router as _postprocess_router
 from ocr_pipeline_auto import router as _auto_router
+from ocr_pipeline_regression import router as _regression_router
 
 # ---------------------------------------------------------------------------
 # Composite router (used by main.py)
@@ -59,3 +60,4 @@ router.include_router(_words_router)
 router.include_router(_ocr_merge_router)
 router.include_router(_postprocess_router)
 router.include_router(_auto_router)
+router.include_router(_regression_router)
diff --git a/klausur-service/backend/ocr_pipeline_regression.py b/klausur-service/backend/ocr_pipeline_regression.py
new file mode 100644
index 0000000..015df26
--- /dev/null
+++ b/klausur-service/backend/ocr_pipeline_regression.py
@@ -0,0 +1,367 @@
+"""
+OCR Pipeline Regression Tests — Ground Truth comparison system.
+
+Allows marking sessions as "ground truth" and re-running build_grid()
+to detect regressions after code changes.
+
+Lizenz: Apache 2.0
+DATENSCHUTZ: Alle Verarbeitung erfolgt lokal.
+"""
+
+import logging
+from datetime import datetime, timezone
+from typing import Any, Dict, List, Optional
+
+from fastapi import APIRouter, HTTPException
+
+from grid_editor_api import _build_grid_core
+from ocr_pipeline_session_store import (
+    get_session_db,
+    list_ground_truth_sessions_db,
+    update_session_db,
+)
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/v1/ocr-pipeline", tags=["regression"])
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def _extract_cells_for_comparison(grid_result: dict) -> List[Dict[str, Any]]:
+    """Extract a flat list of cells from a grid_editor_result for comparison.
+
+    Only keeps fields relevant for comparison: cell_id, row_index, col_index,
+    col_type, text.  Ignores confidence, bbox, word_boxes, duration, is_bold.
+    """
+    cells = []
+    for zone in grid_result.get("zones", []):
+        for cell in zone.get("cells", []):
+            cells.append({
+                "cell_id": cell.get("cell_id", ""),
+                "row_index": cell.get("row_index"),
+                "col_index": cell.get("col_index"),
+                "col_type": cell.get("col_type", ""),
+                "text": cell.get("text", ""),
+            })
+    return cells
+
+
+def _build_reference_snapshot(grid_result: dict) -> dict:
+    """Build a ground-truth reference snapshot from a grid_editor_result."""
+    cells = _extract_cells_for_comparison(grid_result)
+
+    total_zones = len(grid_result.get("zones", []))
+    total_columns = sum(len(z.get("columns", [])) for z in grid_result.get("zones", []))
+    total_rows = sum(len(z.get("rows", [])) for z in grid_result.get("zones", []))
+
+    return {
+        "saved_at": datetime.now(timezone.utc).isoformat(),
+        "version": 1,
+        "summary": {
+            "total_zones": total_zones,
+            "total_columns": total_columns,
+            "total_rows": total_rows,
+            "total_cells": len(cells),
+        },
+        "cells": cells,
+    }
+
+
+def compare_grids(reference: dict, current: dict) -> dict:
+    """Compare a reference grid snapshot with a newly computed one.
+
+    Returns a diff report with:
+      - status: "pass" or "fail"
+      - structural_diffs: changes in zone/row/column counts
+      - cell_diffs: list of individual cell changes
+    """
+    ref_summary = reference.get("summary", {})
+    cur_summary = current.get("summary", {})
+
+    structural_diffs = []
+    for key in ("total_zones", "total_columns", "total_rows", "total_cells"):
+        ref_val = ref_summary.get(key, 0)
+        cur_val = cur_summary.get(key, 0)
+        if ref_val != cur_val:
+            structural_diffs.append({
+                "field": key,
+                "reference": ref_val,
+                "current": cur_val,
+            })
+
+    # Build cell lookup by cell_id
+    ref_cells = {c["cell_id"]: c for c in reference.get("cells", [])}
+    cur_cells = {c["cell_id"]: c for c in current.get("cells", [])}
+
+    cell_diffs: List[Dict[str, Any]] = []
+
+    # Check for missing cells (in reference but not in current)
+    for cell_id in ref_cells:
+        if cell_id not in cur_cells:
+            cell_diffs.append({
+                "type": "cell_missing",
+                "cell_id": cell_id,
+                "reference_text": ref_cells[cell_id].get("text", ""),
+            })
+
+    # Check for added cells (in current but not in reference)
+    for cell_id in cur_cells:
+        if cell_id not in ref_cells:
+            cell_diffs.append({
+                "type": "cell_added",
+                "cell_id": cell_id,
+                "current_text": cur_cells[cell_id].get("text", ""),
+            })
+
+    # Check for changes in shared cells
+    for cell_id in ref_cells:
+        if cell_id not in cur_cells:
+            continue
+        ref_cell = ref_cells[cell_id]
+        cur_cell = cur_cells[cell_id]
+
+        if ref_cell.get("text", "") != cur_cell.get("text", ""):
+            cell_diffs.append({
+                "type": "text_change",
+                "cell_id": cell_id,
+                "reference": ref_cell.get("text", ""),
+                "current": cur_cell.get("text", ""),
+            })
+
+        if ref_cell.get("col_type", "") != cur_cell.get("col_type", ""):
+            cell_diffs.append({
+                "type": "col_type_change",
+                "cell_id": cell_id,
+                "reference": ref_cell.get("col_type", ""),
+                "current": cur_cell.get("col_type", ""),
+            })
+
+    status = "pass" if not structural_diffs and not cell_diffs else "fail"
+
+    return {
+        "status": status,
+        "structural_diffs": structural_diffs,
+        "cell_diffs": cell_diffs,
+        "summary": {
+            "structural_changes": len(structural_diffs),
+            "cells_missing": sum(1 for d in cell_diffs if d["type"] == "cell_missing"),
+            "cells_added": sum(1 for d in cell_diffs if d["type"] == "cell_added"),
+            "text_changes": sum(1 for d in cell_diffs if d["type"] == "text_change"),
+            "col_type_changes": sum(1 for d in cell_diffs if d["type"] == "col_type_change"),
+        },
+    }
+
+
+# ---------------------------------------------------------------------------
+# Endpoints
+# ---------------------------------------------------------------------------
+
+@router.post("/sessions/{session_id}/mark-ground-truth")
+async def mark_ground_truth(session_id: str):
+    """Save the current build-grid result as ground-truth reference."""
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    grid_result = session.get("grid_editor_result")
+    if not grid_result or not grid_result.get("zones"):
+        raise HTTPException(
+            status_code=400,
+            detail="No grid_editor_result found. Run build-grid first.",
+        )
+
+    reference = _build_reference_snapshot(grid_result)
+
+    # Merge into existing ground_truth JSONB
+    gt = session.get("ground_truth") or {}
+    gt["build_grid_reference"] = reference
+    await update_session_db(session_id, ground_truth=gt)
+
+    logger.info(
+        "Ground truth marked for session %s: %d cells",
+        session_id, len(reference["cells"]),
+    )
+
+    return {
+        "status": "ok",
+        "session_id": session_id,
+        "cells_saved": len(reference["cells"]),
+        "summary": reference["summary"],
+    }
+
+
+@router.delete("/sessions/{session_id}/mark-ground-truth")
+async def unmark_ground_truth(session_id: str):
+    """Remove the ground-truth reference from a session."""
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    gt = session.get("ground_truth") or {}
+    if "build_grid_reference" not in gt:
+        raise HTTPException(status_code=404, detail="No ground truth reference found")
+
+    del gt["build_grid_reference"]
+    await update_session_db(session_id, ground_truth=gt)
+
+    logger.info("Ground truth removed for session %s", session_id)
+    return {"status": "ok", "session_id": session_id}
+
+
+@router.get("/ground-truth-sessions")
+async def list_ground_truth_sessions():
+    """List all sessions that have a ground-truth reference."""
+    sessions = await list_ground_truth_sessions_db()
+
+    result = []
+    for s in sessions:
+        gt = s.get("ground_truth") or {}
+        ref = gt.get("build_grid_reference", {})
+        result.append({
+            "session_id": s["id"],
+            "name": s.get("name", ""),
+            "filename": s.get("filename", ""),
+            "saved_at": ref.get("saved_at"),
+            "summary": ref.get("summary", {}),
+        })
+
+    return {"sessions": result, "count": len(result)}
+
+
+@router.post("/sessions/{session_id}/regression/run")
+async def run_single_regression(session_id: str):
+    """Re-run build_grid for a single session and compare to ground truth."""
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    gt = session.get("ground_truth") or {}
+    reference = gt.get("build_grid_reference")
+    if not reference:
+        raise HTTPException(
+            status_code=400,
+            detail="No ground truth reference found for this session",
+        )
+
+    # Re-compute grid without persisting
+    try:
+        new_result = await _build_grid_core(session_id, session)
+    except (ValueError, Exception) as e:
+        return {
+            "session_id": session_id,
+            "name": session.get("name", ""),
+            "status": "error",
+            "error": str(e),
+        }
+
+    new_snapshot = _build_reference_snapshot(new_result)
+    diff = compare_grids(reference, new_snapshot)
+
+    logger.info(
+        "Regression test session %s: %s (%d structural, %d cell diffs)",
+        session_id, diff["status"],
+        diff["summary"]["structural_changes"],
+        sum(v for k, v in diff["summary"].items() if k != "structural_changes"),
+    )
+
+    return {
+        "session_id": session_id,
+        "name": session.get("name", ""),
+        "status": diff["status"],
+        "diff": diff,
+        "reference_summary": reference.get("summary", {}),
+        "current_summary": new_snapshot.get("summary", {}),
+    }
+
+
+@router.post("/regression/run")
+async def run_all_regressions():
+    """Re-run build_grid for ALL ground-truth sessions and compare."""
+    sessions = await list_ground_truth_sessions_db()
+
+    if not sessions:
+        return {
+            "status": "pass",
+            "message": "No ground truth sessions found",
+            "results": [],
+            "summary": {"total": 0, "passed": 0, "failed": 0, "errors": 0},
+        }
+
+    results = []
+    passed = 0
+    failed = 0
+    errors = 0
+
+    for s in sessions:
+        session_id = s["id"]
+        gt = s.get("ground_truth") or {}
+        reference = gt.get("build_grid_reference")
+        if not reference:
+            continue
+
+        # Re-load full session (list query may not include all JSONB fields)
+        full_session = await get_session_db(session_id)
+        if not full_session:
+            results.append({
+                "session_id": session_id,
+                "name": s.get("name", ""),
+                "status": "error",
+                "error": "Session not found during re-load",
+            })
+            errors += 1
+            continue
+
+        try:
+            new_result = await _build_grid_core(session_id, full_session)
+        except (ValueError, Exception) as e:
+            results.append({
+                "session_id": session_id,
+                "name": s.get("name", ""),
+                "status": "error",
+                "error": str(e),
+            })
+            errors += 1
+            continue
+
+        new_snapshot = _build_reference_snapshot(new_result)
+        diff = compare_grids(reference, new_snapshot)
+
+        entry = {
+            "session_id": session_id,
+            "name": s.get("name", ""),
+            "status": diff["status"],
+            "diff_summary": diff["summary"],
+            "reference_summary": reference.get("summary", {}),
+            "current_summary": new_snapshot.get("summary", {}),
+        }
+
+        # Include full diffs only for failures (keep response compact)
+        if diff["status"] == "fail":
+            entry["structural_diffs"] = diff["structural_diffs"]
+            entry["cell_diffs"] = diff["cell_diffs"]
+            failed += 1
+        else:
+            passed += 1
+
+        results.append(entry)
+
+    overall = "pass" if failed == 0 and errors == 0 else "fail"
+
+    logger.info(
+        "Regression suite: %s — %d passed, %d failed, %d errors (of %d)",
+        overall, passed, failed, errors, len(results),
+    )
+
+    return {
+        "status": overall,
+        "results": results,
+        "summary": {
+            "total": len(results),
+            "passed": passed,
+            "failed": failed,
+            "errors": errors,
+        },
+    }
diff --git a/klausur-service/backend/ocr_pipeline_session_store.py b/klausur-service/backend/ocr_pipeline_session_store.py
index 8c34cc7..1fed396 100644
--- a/klausur-service/backend/ocr_pipeline_session_store.py
+++ b/klausur-service/backend/ocr_pipeline_session_store.py
@@ -270,6 +270,26 @@ async def get_sub_sessions(parent_session_id: str) -> List[Dict[str, Any]]:
         return [_row_to_dict(row) for row in rows]
 
 
+async def list_ground_truth_sessions_db() -> List[Dict[str, Any]]:
+    """List sessions that have a build_grid_reference in ground_truth."""
+    pool = await get_pool()
+    async with pool.acquire() as conn:
+        rows = await conn.fetch("""
+            SELECT id, name, filename, status, current_step,
+                   document_category, doc_type,
+                   ground_truth,
+                   parent_session_id, box_index,
+                   created_at, updated_at
+            FROM ocr_pipeline_sessions
+            WHERE ground_truth IS NOT NULL
+              AND ground_truth::text LIKE '%build_grid_reference%'
+              AND parent_session_id IS NULL
+            ORDER BY created_at DESC
+        """)
+
+        return [_row_to_dict(row) for row in rows]
+
+
 async def delete_session_db(session_id: str) -> bool:
     """Delete a session."""
     pool = await get_pool()