diff --git a/.changeset/tender-ducks-act.md b/.changeset/tender-ducks-act.md
new file mode 100644
index 00000000000..a12392f93de
--- /dev/null
+++ b/.changeset/tender-ducks-act.md
@@ -0,0 +1,5 @@
+---
+"@smithy/core": patch
+---
+
+idempotency token generation for HttpBindingProtocol
diff --git a/packages/core/src/submodules/protocols/HttpBindingProtocol.spec.ts b/packages/core/src/submodules/protocols/HttpBindingProtocol.spec.ts
index 4b3001a695b..a6e37c1b3dd 100644
--- a/packages/core/src/submodules/protocols/HttpBindingProtocol.spec.ts
+++ b/packages/core/src/submodules/protocols/HttpBindingProtocol.spec.ts
@@ -14,6 +14,7 @@ import type {
   SerdeFunctions,
   ShapeDeserializer,
   ShapeSerializer,
+  StaticStructureSchema,
   StringSchema,
   TimestampDefaultSchema,
   TimestampEpochSecondsSchema,
@@ -206,4 +207,41 @@ describe(HttpBindingProtocol.name, () => {
       header: "header-value",
     });
   });
+
+  it("should fill in undefined idempotency tokens", async () => {
+    const protocol = new StringRestProtocol();
+    const request = await protocol.serializeRequest(
+      op(
+        "",
+        "",
+        {
+          http: ["GET", "/{labelToken}/Operation", 200],
+        },
+        [
+          3,
+          "ns",
+          "Struct",
+          0,
+          ["name", "queryToken", "labelToken", "headerToken"],
+          [
+            0,
+            [0, { idempotencyToken: 1, httpQuery: "token" }],
+            [0, { idempotencyToken: 1, httpLabel: 1 }],
+            [0, { idempotencyToken: 1, httpHeader: "header-token" }],
+          ],
+        ] satisfies StaticStructureSchema,
+        "unit"
+      ),
+      {
+        Name: "my-name",
+      },
+      {
+        endpoint: async () => parseUrl("https://localhost/custom"),
+      } as any
+    );
+
+    expect(request.query?.token).toMatch(/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/);
+    expect(request.path).toMatch(/[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}/);
+    expect(request.headers?.["header-token"]).toMatch(/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/);
+  });
 });
diff --git a/packages/core/src/submodules/protocols/HttpBindingProtocol.ts b/packages/core/src/submodules/protocols/HttpBindingProtocol.ts
index 8cfec4482b4..77c423d18fe 100644
--- a/packages/core/src/submodules/protocols/HttpBindingProtocol.ts
+++ b/packages/core/src/submodules/protocols/HttpBindingProtocol.ts
@@ -78,7 +78,7 @@ export abstract class HttpBindingProtocol extends HttpProtocol {
       const memberTraits = memberNs.getMergedTraits() ?? {};
       const inputMemberValue = input[memberName];
 
-      if (inputMemberValue == null) {
+      if (inputMemberValue == null && !memberNs.isIdempotencyToken()) {
         continue;
       }