diff --git a/.changes/4855cecf-6a96-4622-b7ff-5d09e54f6564.json b/.changes/4855cecf-6a96-4622-b7ff-5d09e54f6564.json
new file mode 100644
index 0000000000..6c781606fe
--- /dev/null
+++ b/.changes/4855cecf-6a96-4622-b7ff-5d09e54f6564.json
@@ -0,0 +1,5 @@
+{
+    "id": "4855cecf-6a96-4622-b7ff-5d09e54f6564",
+    "type": "bugfix",
+    "description": "Overwrite `Content-Length` header rather than appending in `CrtHttpEngine`"
+}
\ No newline at end of file
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index 4d2c857514..8b60cc05ea 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -2,7 +2,7 @@
 kotlin-version = "2.2.0"
 dokka-version = "2.0.0"
 
-aws-kotlin-repo-tools-version = "0.4.55"
+aws-kotlin-repo-tools-version = "0.4.56"
 
 # libs
 coroutines-version = "1.10.2"
diff --git a/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/src/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtil.kt b/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/src/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtil.kt
index 5708f111b0..73f7d605a3 100644
--- a/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/src/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtil.kt
+++ b/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/src/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtil.kt
@@ -68,7 +68,7 @@ internal fun HttpRequest.toCrtRequest(callContext: CoroutineContext): aws.sdk.ko
     }
 
     val contentLength = body.contentLength?.takeIf { it >= 0 }?.toString() ?: headers[CONTENT_LENGTH_HEADER]
-    contentLength?.let { crtHeaders.append(CONTENT_LENGTH_HEADER, it) }
+    contentLength?.let { crtHeaders[CONTENT_LENGTH_HEADER] = it }
 
     return aws.sdk.kotlin.crt.http.HttpRequest(method.name, url.requestRelativePath, crtHeaders.build(), bodyStream)
 }
diff --git a/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/test/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtilTest.kt b/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/test/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtilTest.kt
index 25db47c70f..cb2d338588 100644
--- a/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/test/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtilTest.kt
+++ b/runtime/protocol/http-client-engines/http-client-engine-crt/jvm/test/aws/smithy/kotlin/runtime/http/engine/crt/RequestUtilTest.kt
@@ -4,7 +4,15 @@
  */
 package aws.smithy.kotlin.runtime.http.engine.crt
 
+import aws.smithy.kotlin.runtime.content.ByteStream
+import aws.smithy.kotlin.runtime.http.Headers
+import aws.smithy.kotlin.runtime.http.HttpMethod
+import aws.smithy.kotlin.runtime.http.request.HttpRequest
+import aws.smithy.kotlin.runtime.http.toHttpBody
+import aws.smithy.kotlin.runtime.net.url.Url
+import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertFalse
 import kotlin.test.assertTrue
 
@@ -42,4 +50,19 @@ class RequestUtilTest {
             assertFalse(isRetryable(code, name), "Expected $name to not be retryable!")
         }
     }
+
+    @Test
+    fun testContentLengthHeader() = runTest {
+        val data = "a".repeat(100)
+
+        val request = HttpRequest(
+            HttpMethod.POST,
+            url = Url.parse("https://notarealurl.com"),
+            headers = Headers { set("Content-Length", data.length.toString()) },
+            body = ByteStream.fromString(data).toHttpBody(),
+        )
+
+        val crtRequest = request.toCrtRequest(coroutineContext)
+        assertEquals(listOf(data.length.toString()), crtRequest.headers.getAll("Content-Length"))
+    }
 }
diff --git a/runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/test/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpRequestTest.kt b/runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/test/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpRequestTest.kt
index 16f6773008..760a947308 100644
--- a/runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/test/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpRequestTest.kt
+++ b/runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/test/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpRequestTest.kt
@@ -5,6 +5,7 @@
 
 package aws.smithy.kotlin.runtime.http.engine.okhttp
 
+import aws.smithy.kotlin.runtime.content.ByteStream
 import aws.smithy.kotlin.runtime.http.*
 import aws.smithy.kotlin.runtime.http.engine.internal.HttpClientMetrics
 import aws.smithy.kotlin.runtime.http.request.HttpRequest
@@ -71,6 +72,22 @@ class OkHttpRequestTest {
         assertEquals(listOf("bar", "baz"), actual.headers("FoO"))
     }
 
+    @Test
+    fun itConvertsContentLengthHeader() {
+        val data = "a".repeat(100)
+        val url = Url.parse("https://aws.amazon.com")
+        val headers = Headers {
+            append("Content-Length", data.length.toString())
+        }
+        val request = HttpRequest(HttpMethod.POST, url, headers, ByteStream.fromString(data).toHttpBody())
+
+        val execContext = ExecutionContext()
+        val actual = request.toOkHttpRequest(execContext, EmptyCoroutineContext, testMetrics)
+
+        assertTrue(actual.headers.size >= 1)
+        assertEquals(listOf(data.length.toString()), actual.headers("Content-Length"))
+    }
+
     @Test
     fun itSupportsNonAsciiHeaderValues() {
         val url = Url.parse("https://aws.amazon.com")