diff --git a/src/main/java/org/prebid/server/bidder/cpmstar/CpmStarBidder.java b/src/main/java/org/prebid/server/bidder/cpmstar/CpmStarBidder.java
index b3dc01cee97..a5b5687c1b4 100644
--- a/src/main/java/org/prebid/server/bidder/cpmstar/CpmStarBidder.java
+++ b/src/main/java/org/prebid/server/bidder/cpmstar/CpmStarBidder.java
@@ -1,6 +1,8 @@
 package org.prebid.server.bidder.cpmstar;
 
 import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.iab.openrtb.request.BidRequest;
 import com.iab.openrtb.request.Imp;
 import com.iab.openrtb.response.Bid;
@@ -79,7 +81,17 @@ private Imp createImp(ExtImpCpmStar extImpCpmStar, Imp imp) {
         if (extImpCpmStar == null) {
             throw new PreBidException("imp id=%s: bidder.ext is null".formatted(imp.getId()));
         }
-        return imp.toBuilder().ext(mapper.mapper().valueToTree(extImpCpmStar)).build();
+
+        final ObjectNode impExt = imp.getExt() != null
+                ? imp.getExt().deepCopy()
+                : mapper.mapper().createObjectNode();
+
+        final JsonNode bidderNode = impExt.remove("bidder");
+        if (bidderNode != null && bidderNode.isObject()) {
+            bidderNode.fields().forEachRemaining(entry -> impExt.set(entry.getKey(), entry.getValue()));
+        }
+
+        return imp.toBuilder().ext(impExt).build();
     }
 
     @Override
diff --git a/src/test/java/org/prebid/server/bidder/cpmstar/CpmStarBidderTest.java b/src/test/java/org/prebid/server/bidder/cpmstar/CpmStarBidderTest.java
index a5f77bc0394..80ee546bfe3 100644
--- a/src/test/java/org/prebid/server/bidder/cpmstar/CpmStarBidderTest.java
+++ b/src/test/java/org/prebid/server/bidder/cpmstar/CpmStarBidderTest.java
@@ -1,6 +1,7 @@
 package org.prebid.server.bidder.cpmstar;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.iab.openrtb.request.Audio;
 import com.iab.openrtb.request.Banner;
 import com.iab.openrtb.request.BidRequest;
@@ -87,6 +88,41 @@ public void makeHttpRequestsShouldSkipInvalidImpressionAndAddError() {
         assertThat(result.getValue()).isEmpty();
     }
 
+    @Test
+    public void makeHttpRequestsShouldPreserveExistingExtFieldsWhenFlatteningBidder() throws JsonProcessingException {
+        // given:
+        final ObjectNode ext = mapper.createObjectNode();
+        ext.set("bidder", mapper.valueToTree(ExtImpCpmStar.of(12, 123)));
+        ext.put("gpid", "abc-123");
+
+        final Imp imp = Imp.builder()
+                .id("123")
+                .banner(Banner.builder().w(300).h(250).build())
+                .ext(ext)
+                .build();
+
+        final BidRequest bidRequest = BidRequest.builder()
+                .imp(singletonList(imp))
+                .build();
+
+        final CpmStarBidder target = new CpmStarBidder("https://test.endpoint.com", jacksonMapper);
+
+        // when
+        final Result<List<HttpRequest<BidRequest>>> result = target.makeHttpRequests(bidRequest);
+
+        // then
+        assertThat(result.getErrors()).isEmpty();
+
+        final BidRequest outgoing = result.getValue().getFirst().getPayload();
+        final ObjectNode outgoingExt = outgoing.getImp().getFirst().getExt();
+
+        assertThat(outgoingExt.has("gpid")).isTrue();
+        assertThat(outgoingExt.get("gpid").asText()).isEqualTo("abc-123");
+        assertThat(outgoingExt.has("bidder")).isFalse();
+        assertThat(outgoingExt.get("placementId").asInt()).isEqualTo(12);
+        assertThat(outgoingExt.get("subpoolId").asInt()).isEqualTo(123);
+    }
+
     @Test
     public void makeBidsShouldReturnErrorIfResponseBodyCouldNotBeParsed() {
         // given
diff --git a/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-auction-cpmstar-request.json b/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-auction-cpmstar-request.json
index 9afb14a3c23..793cea67d5b 100644
--- a/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-auction-cpmstar-request.json
+++ b/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-auction-cpmstar-request.json
@@ -21,4 +21,4 @@
       "gdpr": 0
     }
   }
-}
\ No newline at end of file
+}
diff --git a/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-cpmstar-bid-request.json b/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-cpmstar-bid-request.json
index 99f805d86bc..b1f5b42ff89 100644
--- a/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-cpmstar-bid-request.json
+++ b/src/test/resources/org/prebid/server/it/openrtb2/cpmstar/test-cpmstar-bid-request.json
@@ -9,14 +9,12 @@
         "h": 250
       },
       "ext": {
+        "tid": "${json-unit.any-string}",
         "placementId": 1234,
         "subpoolId": 1234
       }
     }
   ],
-  "source": {
-    "tid": "${json-unit.any-string}"
-  },
   "site": {
     "domain": "www.example.com",
     "page": "http://www.example.com",
@@ -36,6 +34,9 @@
   "cur": [
     "USD"
   ],
+  "source": {
+    "tid": "${json-unit.any-string}"
+  },
   "regs": {
     "ext": {
       "gdpr": 0
@@ -52,3 +53,4 @@
     }
   }
 }
+