diff --git a/.gitmodules b/.gitmodules
index 80d831a02..2b4152ccf 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -17,4 +17,4 @@
 [submodule "tools/flagd-api-testkit/test-harness"]
 	path = tools/flagd-api-testkit/test-harness
 	url = https://github.com/open-feature/test-harness.git
-	branch = v3.1.0
+	branch = v3.5.0
diff --git a/tools/flagd-api-testkit/src/main/java/dev/openfeature/contrib/tools/flagd/api/testkit/EvaluationSteps.java b/tools/flagd-api-testkit/src/main/java/dev/openfeature/contrib/tools/flagd/api/testkit/EvaluationSteps.java
index 1bfceba29..c69ba5502 100644
--- a/tools/flagd-api-testkit/src/main/java/dev/openfeature/contrib/tools/flagd/api/testkit/EvaluationSteps.java
+++ b/tools/flagd-api-testkit/src/main/java/dev/openfeature/contrib/tools/flagd/api/testkit/EvaluationSteps.java
@@ -77,6 +77,14 @@ public void flagEvaluatedWithDetails() {
                     .errorCode(ErrorCode.TYPE_MISMATCH)
                     .errorMessage(e.getMessage())
                     .build();
+        } catch (dev.openfeature.sdk.exceptions.OpenFeatureError e) {
+            // Mirror the OpenFeature SDK client behaviour: on any provider error, return the
+            // caller-supplied default value together with the error code and message.
+            state.evaluation = dev.openfeature.sdk.ProviderEvaluation.builder()
+                    .value(state.defaultValue)
+                    .errorCode(e.getErrorCode())
+                    .errorMessage(e.getMessage())
+                    .build();
         }
     }
 
diff --git a/tools/flagd-api-testkit/test-harness b/tools/flagd-api-testkit/test-harness
index 854e80122..ff2fbe6c6 160000
--- a/tools/flagd-api-testkit/test-harness
+++ b/tools/flagd-api-testkit/test-harness
@@ -1 +1 @@
-Subproject commit 854e801228317a6a2eb61aca045e1e77325302b1
+Subproject commit ff2fbe6c6584953cb2753ae9188d1cee14f7f57f