diff --git a/src/commands/onboard-custom.ts b/src/commands/onboard-custom.ts
index 2ab5de416a..f9e8ae84b6 100644
--- a/src/commands/onboard-custom.ts
+++ b/src/commands/onboard-custom.ts
@@ -313,8 +313,14 @@ async function requestAnthropicVerification(params: {
   apiKey: string;
   modelId: string;
 }): Promise<VerificationResult> {
+  // Use a base URL with /v1 injected for this raw fetch only. The rest of the app uses the
+  // Anthropic client, which appends /v1 itself; config should store the base URL
+  // without /v1 to avoid /v1/v1/messages at runtime. See docs/gateway/configuration-reference.md.
+  const baseUrlForRequest = /\/v1\/?$/.test(params.baseUrl.trim())
+    ? params.baseUrl.trim()
+    : params.baseUrl.trim().replace(/\/?$/, "") + "/v1";
   const endpoint = resolveVerificationEndpoint({
-    baseUrl: params.baseUrl,
+    baseUrl: baseUrlForRequest,
     modelId: params.modelId,
     endpointPath: "messages",
   });