diff --git a/src/constants.ts b/src/constants.ts
index b9f151e3..ca74925f 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -655,7 +655,9 @@ export function isValidModelId(value: string): boolean {
   return (
     modelId.length > 0 &&
     modelId.length <= 120 &&
-    /^[A-Za-z0-9][A-Za-z0-9._:/@+-]*$/u.test(modelId) &&
+    // Leading @ for Cloudflare Workers AI ids (@cf/...); interior @ for
+    // Vertex AI @-versioned ids (e.g. claude-sonnet-4-5@20250929).
+    /^[@A-Za-z0-9][A-Za-z0-9._:/@+-]*$/u.test(modelId) &&
     !modelId.includes("://")
   );
 }
diff --git a/test/constants.test.ts b/test/constants.test.ts
index 52d85bff..40db95d5 100644
--- a/test/constants.test.ts
+++ b/test/constants.test.ts
@@ -38,6 +38,12 @@ describe("isValidModelId", () => {
     expect(isValidModelId("nvidia/nemotron-3-super-120b-a12b")).toBe(true);
   });
 
+  test("accepts Cloudflare Workers AI model ids with leading '@'", () => {
+    expect(isValidModelId("@cf/meta/llama-3-8b-instruct")).toBe(true);
+    expect(isValidModelId("@cf/moonshotai/kimi-k2.7-code")).toBe(true);
+    expect(isValidModelId("@cf/qwen/qwen1.5-14b-chat-awq")).toBe(true);
+  });
+
   test("rejects empty, whitespace-only, and over-long ids", () => {
     expect(isValidModelId("")).toBe(false);
     expect(isValidModelId("   ")).toBe(false);
@@ -53,10 +59,11 @@ describe("isValidModelId", () => {
     expect(isValidModelId("claude-haiku-4-5@20251001")).toBe(true);
   });
 
-  test("rejects ids starting with a non-alphanumeric character", () => {
+  test("rejects ids starting with a disallowed non-alphanumeric character", () => {
     expect(isValidModelId("-leading-dash")).toBe(false);
     expect(isValidModelId("/leading-slash")).toBe(false);
-    expect(isValidModelId("@leading-at")).toBe(false);
+    // A leading "@" is intentionally allowed for Cloudflare Workers AI ids
+    // (covered above), so it is not rejected here.
   });
 
   test("normalizeModelId trims surrounding whitespace", () => {