feat: support configuring a default_agent across all API/user surfaces (#5843)

Co-authored-by: observerw <observerw@users.noreply.github.com>

packages/opencode/src/cli/cmd/github.ts

@@ -762,7 +762,7 @@ export const GithubRunCommand = cmd({
             providerID,
             modelID,
           },
-          agent: "build",
+          // agent is omitted - server will use default_agent from config or fall back to "build"
           parts: [
             {
               id: Identifier.ascending("part"),

packages/opencode/src/cli/cmd/run.ts

@@ -10,6 +10,7 @@ import { select } from "@clack/prompts"
 import { createOpencodeClient, type OpencodeClient } from "@opencode-ai/sdk/v2"
 import { Server } from "../../server/server"
 import { Provider } from "../../provider/provider"
+import { Agent } from "../../agent/agent"
 
 const TOOL: Record<string, [string, string]> = {
   todowrite: ["Todo", UI.Style.TEXT_WARNING_BOLD],
@@ -223,10 +224,33 @@ export const RunCommand = cmd({
         }
       })()
 
+      // Validate agent if specified
+      const resolvedAgent = await (async () => {
+        if (!args.agent) return undefined
+        const agent = await Agent.get(args.agent)
+        if (!agent) {
+          UI.println(
+            UI.Style.TEXT_WARNING_BOLD + "!",
+            UI.Style.TEXT_NORMAL,
+            `agent "${args.agent}" not found. Falling back to default agent`,
+          )
+          return undefined
+        }
+        if (agent.mode === "subagent") {
+          UI.println(
+            UI.Style.TEXT_WARNING_BOLD + "!",
+            UI.Style.TEXT_NORMAL,
+            `agent "${args.agent}" is a subagent, not a primary agent. Falling back to default agent`,
+          )
+          return undefined
+        }
+        return args.agent
+      })()
+
       if (args.command) {
         await sdk.session.command({
           sessionID,
-          agent: args.agent || "build",
+          agent: resolvedAgent,
           model: args.model,
           command: args.command,
           arguments: message,
@@ -235,7 +259,7 @@ export const RunCommand = cmd({
         const modelParam = args.model ? Provider.parseModel(args.model) : undefined
         await sdk.session.prompt({
           sessionID,
-          agent: args.agent || "build",
+          agent: resolvedAgent,
           model: modelParam,
           parts: [...fileParts, { type: "text", text: message }],
         })

packages/opencode/src/cli/cmd/tui/context/local.tsx

@@ -56,7 +56,7 @@ export const { use: useLocal, provider: LocalProvider } = createSimpleContext({
       const [agentStore, setAgentStore] = createStore<{
         current: string
       }>({
-        current: agents()[0].name,
+        current: agents().find((x) => x.default)?.name ?? agents()[0].name,
       })
       const { theme } = useTheme()
       const colors = createMemo(() => [