Fix chat provider routing for OpenCode Zen/Go

OpenCode Zen and Go models were being routed to api.openai.com because
their base URLs weren't in the provider map. Add correct base URLs
(opencode.ai/zen and opencode.ai/zen/go) and error on unknown providers
instead of silently falling back to OpenAI.

apps/harness/src/app/api/chat/route.ts

@@ -68,8 +68,14 @@ export async function POST(request: NextRequest) {
   }
 
   const apiKey = creds[0].token;
-  const baseUrl = getBaseUrl(provider, creds[0].baseUrl);
   const useAnthropic = isAnthropicProvider(provider);
+  const baseUrl = useAnthropic ? null : getBaseUrl(provider, creds[0].baseUrl);
+  if (!useAnthropic && !baseUrl) {
+    return Response.json(
+      { error: `No API base URL known for provider: ${provider}. Set a baseUrl on the credential.` },
+      { status: 400 },
+    );
+  }
 
   // Build conversation history in provider format
   const providerMessages: ProviderMessages[] = [];
@@ -125,7 +131,7 @@ export async function POST(request: NextRequest) {
           // Stream from provider
           const events = useAnthropic
             ? streamAnthropic(apiKey, model, SYSTEM_PROMPT, currentMessages, toolsAsAnthropic())
-            : streamOpenAI(apiKey, baseUrl, model, SYSTEM_PROMPT, currentMessages, toolsAsOpenAI());
+            : streamOpenAI(apiKey, baseUrl!, model, SYSTEM_PROMPT, currentMessages, toolsAsOpenAI());
 
           for await (const event of events) {
             if (event.type === "text_delta") {

apps/harness/src/lib/chat-providers.ts

@@ -327,6 +327,8 @@ const PROVIDER_BASE_URLS: Record<string, string> = {
   openai: "https://api.openai.com",
   openrouter: "https://openrouter.ai/api",
   google: "https://generativelanguage.googleapis.com",
+  "opencode-zen": "https://opencode.ai/zen",
+  "opencode-go": "https://opencode.ai/zen/go",
 };
 
 export type ChatProvider = "anthropic" | "openai" | "openrouter" | "google";
@@ -335,6 +337,6 @@ export function isAnthropicProvider(provider: string): boolean {
   return provider === "anthropic";
 }
 
-export function getBaseUrl(provider: string, credentialBaseUrl?: string): string {
-  return credentialBaseUrl || PROVIDER_BASE_URLS[provider] || "https://api.openai.com";
+export function getBaseUrl(provider: string, credentialBaseUrl?: string): string | null {
+  return credentialBaseUrl || PROVIDER_BASE_URLS[provider] || null;
 }