diff --git a/lib/addons/prebid/analytics.test.ts b/lib/addons/prebid/analytics.test.ts
index 284207c..a28749e 100644
--- a/lib/addons/prebid/analytics.test.ts
+++ b/lib/addons/prebid/analytics.test.ts
@@ -684,6 +684,119 @@ describe("OptablePrebidAnalytics", () => {
       expect(storedAuction).toBeDefined();
     });
 
+    it("should extract splitTestAssignment from bidsReceived", async () => {
+      const event = {
+        auctionId: "auction-split-test",
+        timeout: 3000,
+        bidderRequests: [
+          {
+            bidderCode: "bidder1",
+            bidderRequestId: "req-1",
+            ortb2: {
+              site: { domain: "example.com" },
+              user: {
+                eids: [
+                  {
+                    inserter: "optable.co",
+                    matcher: "matcher1",
+                    source: "source1",
+                  },
+                ],
+              },
+            },
+            bids: [
+              {
+                bidId: "bid-1",
+                adUnitCode: "ad-unit-1",
+                adUnitId: "ad-id-1",
+                transactionId: "trans-1",
+                src: "client",
+              },
+            ],
+          },
+        ],
+        bidsReceived: [
+          {
+            requestId: "bid-1",
+            cpm: 1.5,
+            width: 300,
+            height: 250,
+            currency: "USD",
+            adUnitCode: "ad-unit-1",
+            ortb2Imp: {
+              ext: {
+                optable: {
+                  splitTestAssignment: "treatment",
+                },
+              },
+            },
+          },
+        ],
+        noBids: [],
+        timeoutBids: [],
+      };
+
+      await analytics.trackAuctionEnd(event);
+
+      // Get the stored auction and check the bid has splitTestAssignment
+      const storedAuction = analytics["auctions"].get("auction-split-test");
+      expect(storedAuction).toBeDefined();
+
+      // The splitTestAssignment should be extracted and merged in toWitness
+      const payload = await analytics.toWitness(storedAuction.auctionEnd, null);
+      expect(payload.bidderRequests[0].bids[0].splitTestAssignment).toBe("treatment");
+    });
+
+    it("should handle missing splitTestAssignment gracefully", async () => {
+      const event = {
+        auctionId: "auction-no-split-test",
+        timeout: 3000,
+        bidderRequests: [
+          {
+            bidderCode: "bidder1",
+            bidderRequestId: "req-1",
+            ortb2: {
+              site: { domain: "example.com" },
+              user: {
+                eids: [],
+              },
+            },
+            bids: [
+              {
+                bidId: "bid-1",
+                adUnitCode: "ad-unit-1",
+                adUnitId: "ad-id-1",
+                transactionId: "trans-1",
+                src: "client",
+              },
+            ],
+          },
+        ],
+        bidsReceived: [
+          {
+            requestId: "bid-1",
+            cpm: 1.5,
+            width: 300,
+            height: 250,
+            currency: "USD",
+            adUnitCode: "ad-unit-1",
+            // No ortb2Imp field
+          },
+        ],
+        noBids: [],
+        timeoutBids: [],
+      };
+
+      await analytics.trackAuctionEnd(event);
+
+      // Should not throw error and splitTestAssignment should be undefined
+      const storedAuction = analytics["auctions"].get("auction-no-split-test");
+      expect(storedAuction).toBeDefined();
+
+      const payload = await analytics.toWitness(storedAuction.auctionEnd, null);
+      expect(payload.bidderRequests[0].bids[0].splitTestAssignment).toBeUndefined();
+    });
+
     it("should handle noBids and update status", async () => {
       const event = {
         auctionId: "auction-no-bids",
diff --git a/lib/addons/prebid/analytics.ts b/lib/addons/prebid/analytics.ts
index 22a8d2c..cb9e6c5 100644
--- a/lib/addons/prebid/analytics.ts
+++ b/lib/addons/prebid/analytics.ts
@@ -244,6 +244,7 @@ class OptablePrebidAnalytics {
               transactionId: string;
               src: string;
               floorData?: { floorMin: number };
+              ortb2Imp?: { ext?: { optable?: { splitTestAssignment?: string } } };
             }) => ({
               bidId: b.bidId,
               bidderRequestId,
@@ -252,6 +253,7 @@ class OptablePrebidAnalytics {
               transactionId: b.transactionId,
               src: b.src,
               floorMin: b.floorData?.floorMin,
+              splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
               status: STATUS.REQUESTED,
             })
           ),
@@ -291,6 +293,7 @@ class OptablePrebidAnalytics {
           cpm: b.cpm,
           size: `${b.width}x${b.height}`,
           currency: b.currency,
+          splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
         });
       } else {
         // Create new bid object for this response
@@ -305,6 +308,7 @@ class OptablePrebidAnalytics {
           size: `${b.width}x${b.height}`,
           currency: b.currency,
           status: STATUS.RECEIVED,
+          splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
         };
         br.bids.push(bidObj);
         bidIndex[bidId] = bidObj;
@@ -349,7 +353,7 @@ class OptablePrebidAnalytics {
       this.sendToWitnessAPI("optable.prebid.auction", payload);
     }, this.config.bidWinTimeout);
 
-    // Store the processed auction
+    // Store the auction data
     this.auctions.set(auctionId, { auctionEnd: event, createdAt, missed, auctionEndTimeoutId });
 
     // Clean up old auctions
@@ -433,6 +437,7 @@ class OptablePrebidAnalytics {
     let totalBids = 0;
     let device = null;
 
+    // Process bidder requests
     const requests = bidderRequests.map((br: any) => {
       const { bidderCode, bidderRequestId, bids = [] } = br;
       const domain = br.ortb2.site?.domain ?? "unknown";
@@ -461,6 +466,7 @@ class OptablePrebidAnalytics {
             transactionId: string;
             src: string;
             floorData?: { floorMin: number };
+            ortb2Imp?: { ext?: { optable?: { splitTestAssignment?: string } } };
           }) => ({
             bidId: b.bidId,
             bidderRequestId,
@@ -469,12 +475,24 @@ class OptablePrebidAnalytics {
             transactionId: b.transactionId,
             src: b.src,
             floorMin: b.floorData?.floorMin,
+            splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
             status: STATUS.REQUESTED,
           })
         ),
       };
     });
 
+    // Merge splitTestAssignment from bidsReceived into the requests
+    const bidsReceivedMap = new Map<string, any>(bidsReceived.map((b: any) => [b.requestId, b]));
+    requests.forEach((request: any) => {
+      request.bids.forEach((bid: any) => {
+        const bidReceived = bidsReceivedMap.get(bid.bidId);
+        if (bidReceived?.ortb2Imp?.ext?.optable?.splitTestAssignment) {
+          bid.splitTestAssignment = bidReceived.ortb2Imp.ext.optable.splitTestAssignment;
+        }
+      });
+    });
+
     const witnessData: WitnessProperties = {
       bidderRequests: requests.map((br: any) => {
         br.optableMatchers.forEach((m: unknown) => oMatchersSet.add(m));
diff --git a/lib/addons/prototypes/analytics.js b/lib/addons/prototypes/analytics.js
index 91d02dd..e4cfb5e 100644
--- a/lib/addons/prototypes/analytics.js
+++ b/lib/addons/prototypes/analytics.js
@@ -141,6 +141,7 @@ class OptablePrebidAnalytics {
             transactionId: b.transactionId,
             src: b.src,
             floorMin: b.floorData?.floorMin,
+            splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
             status: STATUS.REQUESTED,
           })),
         };
@@ -178,6 +179,7 @@ class OptablePrebidAnalytics {
           cpm: b.cpm,
           size: `${b.width}x${b.height}`,
           currency: b.currency,
+          splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
         });
       } else {
         // Create new bid object for this response
@@ -192,6 +194,7 @@ class OptablePrebidAnalytics {
           size: `${b.width}x${b.height}`,
           currency: b.currency,
           status: STATUS.RECEIVED,
+          splitTestAssignment: b.ortb2Imp?.ext?.optable?.splitTestAssignment,
         };
         br.bids.push(bidObj);
         bidIndex[bidId] = bidObj;
@@ -255,6 +258,7 @@ class OptablePrebidAnalytics {
                 cpm: b.cpm,
                 size: b.size,
                 bidId: b.bidId,
+                splitTestAssignment: b.splitTestAssignment,
               };
             }),
           };