diff --git a/src/adapters/bidderFactory.ts b/src/adapters/bidderFactory.ts
index 1d97253e9e..2c5eb8607d 100644
--- a/src/adapters/bidderFactory.ts
+++ b/src/adapters/bidderFactory.ts
@@ -279,11 +279,11 @@ export function newBidder<B extends BidderCode>(spec: BidderSpec<B>) {
       const tidGuard = guardTids(bidderRequest);
 
       const adUnitCodesHandled = {};
-      function addBidWithCode(adUnitCode: string, bid: Bid) {
+      function addBidWithCode(adUnitCode: string, bid: Bid, responseMediaType = null) {
         const metrics = useMetrics(bid.metrics);
         metrics.checkpoint('addBidResponse');
         adUnitCodesHandled[adUnitCode] = true;
-        if (metrics.measureTime('addBidResponse.validate', () => isValid(adUnitCode, bid))) {
+        if (metrics.measureTime('addBidResponse.validate', () => isValid(adUnitCode, bid, {responseMediaType}))) {
           addBidResponse(adUnitCode, bid);
         } else {
           addBidResponse.reject(adUnitCode, bid, REJECTION_REASON.INVALID)
@@ -345,7 +345,11 @@ export function newBidder<B extends BidderCode>(spec: BidderSpec<B>) {
             bid.deferBilling = bidRequest.deferBilling;
             bid.deferRendering = bid.deferBilling && (bidResponse.deferRendering ?? typeof spec.onBidBillable !== 'function');
             const prebidBid: Bid = Object.assign(createBid(bidRequest), bid, pick(bidRequest, Object.keys(TIDS)));
-            addBidWithCode(bidRequest.adUnitCode, prebidBid);
+            // codex agent bot: retain whether mediaType was explicitly set by the adapter response.
+            const responseMediaType = Object.prototype.hasOwnProperty.call(bidResponse, 'mediaType')
+              ? bidResponse.mediaType
+              : null;
+            addBidWithCode(bidRequest.adUnitCode, prebidBid, responseMediaType);
           } else {
             logWarn(`Bidder ${spec.code} made bid for unknown request ID: ${bidResponse.requestId}. Ignoring.`);
             addBidResponse.reject(null, bidResponse, REJECTION_REASON.INVALID_REQUEST_ID);
@@ -647,7 +651,7 @@ function validBidSize(adUnitCode, bid: BannerBid, {index = auctionManager.index}
 }
 
 // Validate the arguments sent to us by the adapter. If this returns false, the bid should be totally ignored.
-export function isValid(adUnitCode: string, bid: Bid, {index = auctionManager.index} = {}) {
+export function isValid(adUnitCode: string, bid: Bid, {index = auctionManager.index, responseMediaType = bid.mediaType} = {}) {
   function hasValidKeys() {
     const bidKeys = Object.keys(bid);
     return COMMON_BID_RESPONSE_KEYS.every(key => bidKeys.includes(key) && ![undefined, null].includes(bid[key]));
@@ -672,6 +676,16 @@ export function isValid(adUnitCode: string, bid: Bid, {index = auctionManager.in
     return false;
   }
 
+  if (responseMediaType != null) {
+    const mediaTypes = index.getMediaTypes(bid);
+    if (mediaTypes && Object.keys(mediaTypes).length > 0) {
+      if (!mediaTypes.hasOwnProperty(responseMediaType)) {
+        logError(errorMessage(`Bid mediaType '${responseMediaType}' is not supported by the ad unit. Allowed: ${Object.keys(mediaTypes).join(', ')}`));
+        return false;
+      }
+    }
+  }
+
   if (FEATURES.NATIVE && bid.mediaType === 'native' && !nativeBidIsValid(bid, {index})) {
     logError(errorMessage('Native bid missing some required properties.'));
     return false;
diff --git a/test/spec/unit/core/bidderFactory_spec.js b/test/spec/unit/core/bidderFactory_spec.js
index ff390b9526..19d6577f8c 100644
--- a/test/spec/unit/core/bidderFactory_spec.js
+++ b/test/spec/unit/core/bidderFactory_spec.js
@@ -1691,6 +1691,62 @@ describe('bidderFactory', () => {
         });
       });
     })
+
+    describe('media type validation', () => {
+      let req;
+
+      function mkResponse(props) {
+        return Object.assign({
+          requestId: req.bidId,
+          cpm: 1,
+          ttl: 60,
+          creativeId: '123',
+          netRevenue: true,
+          currency: 'USD',
+          width: 1,
+          height: 2,
+          mediaType: 'banner',
+        }, props);
+      }
+
+      function checkValid(bid, opts = {}) {
+        return isValid('au', bid, {
+          index: stubAuctionIndex({bidRequests: [req]}),
+          ...opts,
+        });
+      }
+
+      beforeEach(() => {
+        req = {
+          ...MOCK_BIDS_REQUEST.bids[0],
+          mediaTypes: {
+            banner: {
+              sizes: [[1, 2]]
+            }
+          }
+        };
+      });
+
+      it('should reject video bid when ad unit only has banner', () => {
+        expect(checkValid(mkResponse({mediaType: 'video'}))).to.be.false;
+      });
+
+      it('should accept video bid when ad unit has both banner and video', () => {
+        req.mediaTypes = {
+          banner: {sizes: [[1, 2]]},
+          video: {context: 'instream'}
+        };
+        expect(checkValid(mkResponse({mediaType: 'video', vastUrl: 'http://vast.xml'}))).to.be.true;
+      });
+
+      it('should skip media type check when adapter omits mediaType', () => {
+        req.mediaTypes = {
+          video: {context: 'instream'}
+        };
+
+        expect(checkValid(mkResponse({mediaType: 'banner'}), {responseMediaType: null})).to.be.true;
+      });
+    });
   });
 
   describe('gzip compression', () => {