fix(opencode): route Vertex multi-regions through REP (#42648)

Co-authored-by: Aiden Cline <rekram1-node@users.noreply.github.com>
Co-authored-by: Filip <34747899+neriousy@users.noreply.github.com>

packages/opencode/src/provider/provider.ts

@@ -98,6 +98,12 @@ function googleVertexAnthropicBaseURL(project: string | undefined, location: str
   return `https://aiplatform.${location}.rep.googleapis.com/v1/projects/${project}/locations/${location}/publishers/anthropic/models`
 }
 
+function googleVertexEndpoint(location: string) {
+  if (location === "global") return "aiplatform.googleapis.com"
+  if (location === "eu" || location === "us") return `aiplatform.${location}.rep.googleapis.com`
+  return `${location}-aiplatform.googleapis.com`
+}
+
 type BundledSDK = {
   languageModel(modelId: string): LanguageModelV3
   chat?: (modelId: string) => LanguageModelV3
@@ -519,11 +525,10 @@ function custom(dep: CustomDep): Record<string, CustomLoader> {
       return {
         autoload: true,
         vars(_options: Record<string, any>) {
-          const endpoint = location === "global" ? "aiplatform.googleapis.com" : `${location}-aiplatform.googleapis.com`
           return {
             ...(project && { GOOGLE_VERTEX_PROJECT: project }),
             GOOGLE_VERTEX_LOCATION: location,
-            GOOGLE_VERTEX_ENDPOINT: endpoint,
+            GOOGLE_VERTEX_ENDPOINT: googleVertexEndpoint(location),
           }
         },
         options: {

packages/opencode/test/provider/provider.test.ts

@@ -1909,6 +1909,38 @@ it.instance("Google Vertex: keeps regional Claude endpoints unchanged", () =>
   }),
 )
 
+it.instance("Google Vertex: uses REP endpoint for Gemini continental multi-regions", () =>
+  Effect.gen(function* () {
+    yield* set("GOOGLE_CLOUD_PROJECT", "test-project")
+    yield* set("VERTEX_LOCATION", "eu")
+    const provider = yield* Provider.Service
+    const model = yield* provider.getModel(
+      ProviderV2.ID.make("google-vertex"),
+      ModelV2.ID.make("gemini-3.5-flash"),
+    )
+    const language = yield* provider.getLanguage(model)
+    expect(languageBaseURL(language)).toBe(
+      "https://aiplatform.eu.rep.googleapis.com/v1beta1/projects/test-project/locations/eu/publishers/google",
+    )
+  }),
+)
+
+it.instance("Google Vertex: keeps regional Gemini endpoints unchanged", () =>
+  Effect.gen(function* () {
+    yield* set("GOOGLE_CLOUD_PROJECT", "test-project")
+    yield* set("VERTEX_LOCATION", "europe-west1")
+    const provider = yield* Provider.Service
+    const model = yield* provider.getModel(
+      ProviderV2.ID.make("google-vertex"),
+      ModelV2.ID.make("gemini-3.5-flash"),
+    )
+    const language = yield* provider.getLanguage(model)
+    expect(languageBaseURL(language)).toBe(
+      "https://europe-west1-aiplatform.googleapis.com/v1beta1/projects/test-project/locations/europe-west1/publishers/google",
+    )
+  }),
+)
+
 it.instance("cloudflare-ai-gateway loads with env variables", () =>
   Effect.gen(function* () {
     yield* set("CLOUDFLARE_ACCOUNT_ID", "test-account")