diff --git a/src/adapters/http/http-execution-v3.ts b/src/adapters/http/http-execution-v3.ts
index bf563b8..190d642 100644
--- a/src/adapters/http/http-execution-v3.ts
+++ b/src/adapters/http/http-execution-v3.ts
@@ -468,6 +468,16 @@ export function createContractHttpExecutor(
       // A missing credential never downgrades into an anonymous request.
       return finish(unauthenticated("NOT_STARTED", isCommand), "NOT_STARTED");
     }
+    if (
+      Object.keys(patch.headers).some(
+        (name) => name.toLowerCase() === "idempotency-key",
+      )
+    ) {
+      return finish(
+        violation("UNEXPECTED_IDEMPOTENCY_KEY", "REQUEST", "NOT_STARTED"),
+        "NOT_STARTED",
+      );
+    }
 
     const headers: Record<string, string> = {
       Accept: "application/json",
diff --git a/tests/unit/http-execution-v3.test.ts b/tests/unit/http-execution-v3.test.ts
index 76e9ad9..5882adf 100644
--- a/tests/unit/http-execution-v3.test.ts
+++ b/tests/unit/http-execution-v3.test.ts
@@ -144,6 +144,52 @@ describe("descriptor-driven HTTP execution lifetime", () => {
     },
   );
 
+  it.each([
+    {
+      label: "query with the canonical reserved header",
+      operation: installed,
+      input: { limit: 20 },
+      context: { scope },
+      headerName: "Idempotency-Key",
+    },
+    {
+      label: "valid KEYED command with a case-variant reserved header",
+      operation: createInstalled,
+      input: { name: "created" },
+      context: { scope, intent: mutationIntent() },
+      headerName: "iDeMpOtEnCy-KeY",
+    },
+  ])(
+    "rejects a credential patch for $label before fetch",
+    async ({ operation, input, context, headerName }) => {
+      const attachCredentials = vi.fn(() => ({
+        kind: "READY" as const,
+        headers: { [headerName]: "credential-owned-key" },
+        credentials: "omit" as const,
+      }));
+      const fetcher = vi.fn();
+      const executor = createContractHttpExecutor({
+        baseUrl: "https://api.example/",
+        maxRetryAttempts: 0,
+        attachCredentials,
+        fetcher,
+      });
+
+      await expect(
+        executor.execute(operation, input, context),
+      ).resolves.toMatchObject({
+        kind: "CONTRACT_VIOLATION",
+        violation: {
+          kind: "UNEXPECTED_IDEMPOTENCY_KEY",
+          operation: "REQUEST",
+        },
+        effect: "NOT_STARTED",
+      });
+      expect(attachCredentials).toHaveBeenCalledOnce();
+      expect(fetcher).not.toHaveBeenCalled();
+    },
+  );
+
   it("reuses one supplied idempotency key across every physical retry", async () => {
     const observedKeys: Array<string | null> = [];
     let attempt = 0;