fix(ai): align Vertex Anthropic version headers (#43804)

packages/ai/src/providers/google-vertex-messages.ts

@@ -13,6 +13,7 @@ export type AnthropicProviderOptionsInput = AnthropicMessages.ProviderOptionsInp
 export type AnthropicThinkingInput = AnthropicMessages.ThinkingInput
 
 const VERSION = "vertex-2023-10-16" as const
+const HEADER_VERSION = "2023-06-01" as const
 
 export const id = ProviderID.make("google-vertex")
 
@@ -57,6 +58,7 @@ const route = Route.make({
   endpoint: Endpoint.path(({ request }) => `/${request.model.id}:streamRawPredict`),
   auth: Auth.none,
   framing: AnthropicMessages.framing,
+  headers: () => ({ "anthropic-version": HEADER_VERSION }),
 })
 
 export const routes = [route]

packages/ai/test/provider/google-vertex.test.ts

@@ -96,7 +96,7 @@ describe("Google Vertex providers", () => {
                 "https://aiplatform.eu.rep.googleapis.com/v1/projects/vertex-project/locations/eu/publishers/anthropic/models/claude-sonnet-4-6:streamRawPredict",
               )
               expect(request.headers.get("authorization")).toBe("Bearer vertex-token")
-              expect(request.headers.get("anthropic-version")).toBeNull()
+              expect(request.headers.get("anthropic-version")).toBe("2023-06-01")
               const body = yield* Effect.promise(() => request.json())
               expect(body).toMatchObject({
                 anthropic_version: "vertex-2023-10-16",