diff --git a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/KeyLoadAndSignAcceptanceTest.java b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/KeyLoadAndSignAcceptanceTest.java
index 6879f3eef..e5b3be9ce 100644
--- a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/KeyLoadAndSignAcceptanceTest.java
+++ b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/KeyLoadAndSignAcceptanceTest.java
@@ -71,11 +71,10 @@ public void signDataWithKeyLoadedFromUnencryptedFile() {
         .contentType(ContentType.JSON)
         .pathParam("publicKey", keyPair.getPublicKey().toString())
         .body(new JsonObject().put("signingRoot", SIGNING_ROOT.toHexString()).toString())
-        .when()
         .post(SIGN_ENDPOINT)
         .then()
-        .assertThat()
         .statusCode(200)
+        .contentType(ContentType.TEXT)
         .body(equalToIgnoringCase(expectedSignature.toString()));
   }
 
@@ -97,11 +96,10 @@ public void signDataWithKeyLoadedFromKeyStoreFile(KdfFunction kdfFunction) {
         .contentType(ContentType.JSON)
         .pathParam("publicKey", keyPair.getPublicKey().toString())
         .body(new JsonObject().put("signingRoot", SIGNING_ROOT.toHexString()).toString())
-        .when()
         .post(SIGN_ENDPOINT)
         .then()
-        .assertThat()
         .statusCode(200)
+        .contentType(ContentType.TEXT)
         .body(equalToIgnoringCase(expectedSignature.toString()));
   }
 
diff --git a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/PublicKeysAcceptanceTest.java b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/PublicKeysAcceptanceTest.java
index 7550a5f86..fda518d2e 100644
--- a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/PublicKeysAcceptanceTest.java
+++ b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/PublicKeysAcceptanceTest.java
@@ -26,6 +26,7 @@
 import java.io.IOException;
 import java.nio.file.Path;
 
+import io.restassured.http.ContentType;
 import io.restassured.response.ValidatableResponse;
 import org.apache.tuweni.bytes.Bytes;
 import org.junit.jupiter.api.Test;
@@ -88,15 +89,35 @@ public void allLoadedKeysAreReturnedPublicKeyResponse() {
             "", containsInAnyOrder(key1.getPublicKey().toString(), key2.getPublicKey().toString()));
   }
 
+  @Test
+  public void allLoadedKeysAreReturnedPublicKeyResponseWithEmptyAccept() {
+    final BLSKeyPair key1 = createKey(PRIVATE_KEY_1);
+    final BLSKeyPair key2 = createKey(PRIVATE_KEY_2);
+
+    final SignerConfigurationBuilder builder = new SignerConfigurationBuilder();
+    builder.withKeyStoreDirectory(testDirectory);
+    startSigner(builder.build());
+
+    // without openapi filter
+    given()
+        .baseUri(signer.getUrl())
+        .accept("")
+        .get(SIGNER_PUBLIC_KEYS_PATH)
+        .then()
+        .statusCode(200)
+        .contentType(ContentType.JSON)
+        .body(
+            "", containsInAnyOrder(key1.getPublicKey().toString(), key2.getPublicKey().toString()));
+  }
+
   private ValidatableResponse whenGetSignerPublicKeysPathThenAssertThat() {
     return given()
         .baseUri(signer.getUrl())
         .filter(getOpenApiValidationFilter())
-        .when()
         .get(SIGNER_PUBLIC_KEYS_PATH)
         .then()
-        .assertThat()
-        .statusCode(200);
+        .statusCode(200)
+        .contentType(ContentType.JSON);
   }
 
   private BLSKeyPair createKey(final String privateKey) {
diff --git a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/UpcheckAcceptanceTest.java b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/UpcheckAcceptanceTest.java
index 0ff50f23d..f7d535472 100644
--- a/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/UpcheckAcceptanceTest.java
+++ b/acceptance-tests/src/test/java/tech/pegasys/eth2signer/tests/UpcheckAcceptanceTest.java
@@ -17,6 +17,7 @@
 
 import tech.pegasys.eth2signer.dsl.signer.SignerConfigurationBuilder;
 
+import io.restassured.http.ContentType;
 import org.junit.jupiter.api.Test;
 
 public class UpcheckAcceptanceTest extends AcceptanceTestBase {
@@ -28,11 +29,10 @@ void upcheckOnCorrectPortRespondsWithOK() {
     given()
         .baseUri(signer.getUrl())
         .filter(getOpenApiValidationFilter())
-        .when()
         .get("/upcheck")
         .then()
-        .assertThat()
         .statusCode(200)
+        .contentType(ContentType.TEXT)
         .body(equalToIgnoringCase("OK"));
   }
 }
diff --git a/core/src/main/java/tech/pegasys/eth2signer/core/Runner.java b/core/src/main/java/tech/pegasys/eth2signer/core/Runner.java
index 1cbe25312..e5407406c 100644
--- a/core/src/main/java/tech/pegasys/eth2signer/core/Runner.java
+++ b/core/src/main/java/tech/pegasys/eth2signer/core/Runner.java
@@ -137,6 +137,9 @@ private OpenAPI3RouterFactory createOpenApiRouterFactory(
       throws InterruptedException, ExecutionException {
     final LogErrorHandler errorHandler = new LogErrorHandler();
     final OpenAPI3RouterFactory openAPI3RouterFactory = getOpenAPI3RouterFactory(vertx);
+    openAPI3RouterFactory
+        .getOptions()
+        .setMountResponseContentTypeHandler(false); // manually set content-type
 
     openAPI3RouterFactory.addHandlerByOperationId(UPCHECK_OPERATION_ID, new UpcheckHandler());
     openAPI3RouterFactory.addFailureHandlerByOperationId(UPCHECK_OPERATION_ID, errorHandler);
diff --git a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/GetPublicKeysHandler.java b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/GetPublicKeysHandler.java
index 328eeb1de..06f9537de 100644
--- a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/GetPublicKeysHandler.java
+++ b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/GetPublicKeysHandler.java
@@ -12,6 +12,9 @@
  */
 package tech.pegasys.eth2signer.core.http.handlers;
 
+import static com.google.common.net.MediaType.JSON_UTF_8;
+import static io.vertx.core.http.HttpHeaders.CONTENT_TYPE;
+
 import tech.pegasys.eth2signer.core.signing.ArtifactSignerProvider;
 
 import io.vertx.core.Handler;
@@ -29,6 +32,6 @@ public GetPublicKeysHandler(final ArtifactSignerProvider signerProvider) {
   public void handle(final RoutingContext context) {
     final JsonArray jsonArray = new JsonArray();
     signerProvider.availableIdentifiers().forEach(jsonArray::add);
-    context.response().end(jsonArray.encode());
+    context.response().putHeader(CONTENT_TYPE, JSON_UTF_8.toString()).end(jsonArray.encode());
   }
 }
diff --git a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/SignForPublicKeyHandler.java b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/SignForPublicKeyHandler.java
index 96bfdfa48..b9faa2a04 100644
--- a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/SignForPublicKeyHandler.java
+++ b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/SignForPublicKeyHandler.java
@@ -12,6 +12,9 @@
  */
 package tech.pegasys.eth2signer.core.http.handlers;
 
+import static com.google.common.net.MediaType.PLAIN_TEXT_UTF_8;
+import static io.vertx.core.http.HttpHeaders.CONTENT_TYPE;
+
 import tech.pegasys.artemis.bls.BLSSignature;
 import tech.pegasys.eth2signer.core.http.models.SigningRequestBody;
 import tech.pegasys.eth2signer.core.signing.ArtifactSigner;
@@ -49,7 +52,10 @@ public void handle(RoutingContext routingContext) {
 
     final Bytes dataToSign = getDataToSign(params);
     final BLSSignature signature = signer.get().sign(dataToSign);
-    routingContext.response().end(signature.toString());
+    routingContext
+        .response()
+        .putHeader(CONTENT_TYPE, PLAIN_TEXT_UTF_8.toString())
+        .end(signature.toString());
   }
 
   private Bytes getDataToSign(final RequestParameters params) {
diff --git a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/UpcheckHandler.java b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/UpcheckHandler.java
index 233da7a43..1552811a8 100644
--- a/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/UpcheckHandler.java
+++ b/core/src/main/java/tech/pegasys/eth2signer/core/http/handlers/UpcheckHandler.java
@@ -12,6 +12,9 @@
  */
 package tech.pegasys.eth2signer.core.http.handlers;
 
+import static com.google.common.net.MediaType.PLAIN_TEXT_UTF_8;
+import static io.vertx.core.http.HttpHeaders.CONTENT_TYPE;
+
 import io.vertx.core.Handler;
 import io.vertx.ext.web.RoutingContext;
 
@@ -19,6 +22,6 @@ public class UpcheckHandler implements Handler<RoutingContext> {
 
   @Override
   public void handle(RoutingContext routingContext) {
-    routingContext.response().end("OK");
+    routingContext.response().putHeader(CONTENT_TYPE, PLAIN_TEXT_UTF_8.toString()).end("OK");
   }
 }