diff --git a/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt b/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt
index 6f48632df0..0f2f153f75 100644
--- a/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt
+++ b/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt
@@ -19,13 +19,13 @@ import software.amazon.smithy.model.shapes.SetShape
 import software.amazon.smithy.model.shapes.Shape
 import software.amazon.smithy.model.shapes.ShapeId
 import software.amazon.smithy.model.shapes.ShortShape
-import software.amazon.smithy.model.shapes.UnionShape
 import software.amazon.smithy.model.traits.LengthTrait
 import software.amazon.smithy.model.traits.RangeTrait
 import software.amazon.smithy.model.traits.RequiredTrait
 import software.amazon.smithy.model.traits.StreamingTrait
 import software.amazon.smithy.model.traits.Trait
 import software.amazon.smithy.model.traits.UniqueItemsTrait
+import software.amazon.smithy.rust.codegen.core.smithy.traits.SyntheticEventStreamUnionTrait
 import software.amazon.smithy.rust.codegen.core.util.expectTrait
 import software.amazon.smithy.rust.codegen.core.util.getTrait
 import software.amazon.smithy.rust.codegen.core.util.hasTrait
@@ -213,15 +213,24 @@ fun validateUnsupportedConstraints(
 
     // 3. Constraint traits in event streams are used. Their semantics are unclear.
     // TODO(https://github.com/awslabs/smithy/issues/1388)
-    val unsupportedConstraintOnShapeReachableViaAnEventStreamSet = walker
+    val eventStreamShapes = walker
         .walkShapes(service)
         .asSequence()
-        .filterIsInstance<UnionShape>()
-        .filter { it.hasTrait<StreamingTrait>() }
+        .filter { it.hasTrait<SyntheticEventStreamUnionTrait>() }
+    val unsupportedConstraintOnNonErrorShapeReachableViaAnEventStreamSet = eventStreamShapes
         .flatMap { walker.walkShapes(it) }
         .filterMapShapesToTraits(allConstraintTraits)
         .map { (shape, trait) -> UnsupportedConstraintOnShapeReachableViaAnEventStream(shape, trait) }
         .toSet()
+    val eventStreamErrors = eventStreamShapes.map { it.expectTrait<SyntheticEventStreamUnionTrait>() }.map { it.errorMembers }
+    val unsupportedConstraintErrorShapeReachableViaAnEventStreamSet = eventStreamErrors
+        .flatMap { it }
+        .flatMap { walker.walkShapes(it) }
+        .filterMapShapesToTraits(allConstraintTraits)
+        .map { (shape, trait) -> UnsupportedConstraintOnShapeReachableViaAnEventStream(shape, trait) }
+        .toSet()
+    val unsupportedConstraintShapeReachableViaAnEventStreamSet =
+        unsupportedConstraintOnNonErrorShapeReachableViaAnEventStreamSet + unsupportedConstraintErrorShapeReachableViaAnEventStreamSet
 
     // 4. Length trait on blob shapes is used. It has not been implemented yet.
     // TODO(https://github.com/awslabs/smithy-rs/issues/1401)
@@ -261,7 +270,7 @@ fun validateUnsupportedConstraints(
     val messages =
         unsupportedConstraintOnMemberShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
             unsupportedLengthTraitOnStreamingBlobShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
-            unsupportedConstraintOnShapeReachableViaAnEventStreamSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
+            unsupportedConstraintShapeReachableViaAnEventStreamSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
             unsupportedLengthTraitOnBlobShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
             unsupportedRangeTraitOnShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
             unsupportedUniqueItemsTraitOnShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) }
diff --git a/codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt b/codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt
index 6cc4f14230..a6dce78264 100644
--- a/codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt
+++ b/codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt
@@ -5,6 +5,7 @@
 
 package software.amazon.smithy.rust.codegen.server.smithy
 
+import io.kotest.inspectors.forOne
 import io.kotest.inspectors.forSome
 import io.kotest.inspectors.shouldForAll
 import io.kotest.matchers.collections.shouldHaveAtLeastSize
@@ -14,6 +15,7 @@ import io.kotest.matchers.string.shouldContain
 import org.junit.jupiter.api.Test
 import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.shapes.ServiceShape
+import software.amazon.smithy.rust.codegen.core.smithy.transformers.EventStreamNormalizer
 import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
 import software.amazon.smithy.rust.codegen.core.util.lookup
 import java.util.logging.Level
@@ -130,24 +132,39 @@ internal class ValidateUnsupportedConstraintsAreNotUsedTest {
 
             @streaming
             union EventStream {
-                message: Message
+                message: Message,
+                error: Error
             }
 
             structure Message {
                 lengthString: LengthString
             }
 
+            structure Error {
+                @required
+                message: String
+            }
+
             @length(min: 1)
             string LengthString
             """.asSmithyModel()
-        val validationResult = validateModel(model)
+        val validationResult = validateModel(EventStreamNormalizer.transform(model))
 
-        validationResult.messages shouldHaveSize 1
-        validationResult.messages[0].message shouldContain
-            """
-            The string shape `test#LengthString` has the constraint trait `smithy.api#length` attached.
-            This shape is also part of an event stream; it is unclear what the semantics for constrained shapes in event streams are.
-            """.trimIndent().replace("\n", " ")
+        validationResult.messages shouldHaveSize 2
+        validationResult.messages.forOne {
+            it.message shouldContain
+                """
+                The string shape `test#LengthString` has the constraint trait `smithy.api#length` attached.
+                This shape is also part of an event stream; it is unclear what the semantics for constrained shapes in event streams are.
+                """.trimIndent().replace("\n", " ")
+        }
+        validationResult.messages.forOne {
+            it.message shouldContain
+                """
+                The member shape `test#Error${"$"}message` has the constraint trait `smithy.api#required` attached.
+                This shape is also part of an event stream; it is unclear what the semantics for constrained shapes in event streams are.
+                """.trimIndent().replace("\n", " ")
+        }
     }
 
     @Test