fix: fallback to first primary agent when default_agent invalid

packages/opencode/src/config/config.ts

@@ -32,6 +32,15 @@ export namespace Config {
     return merged
   }
 
+  function firstPrimaryAgent(agents: Record<string, Agent>): string {
+    for (const [name, config] of Object.entries(agents)) {
+      if (config.mode !== "subagent" && !config.disable) {
+        return name
+      }
+    }
+    return "build"
+  }
+
   export const state = Instance.state(async () => {
     const auth = await Auth.all()
     let result = await global()
@@ -128,14 +137,11 @@ export namespace Config {
 
     if (!result.keybinds) result.keybinds = Info.shape.keybinds.parse({})
 
-    // Set defaultAgent if not specified
-    if (!result.default_agent) result.default_agent = "build"
-
-    // Validate that the default agent is not disabled
-    if (result.agent && result.agent[result.default_agent]?.disable) {
-      throw new Error(
-        `Cannot disable the default agent '${result.default_agent}'. Please either enable it or specify a different default agent in your configuration.`,
-      )
+    if (!result.default_agent || !(result.default_agent in result.agent)) {
+      log.warn(`default_agent not found in agents, selecting first primary agent`)
+      log.debug(`available agents: ${Object.keys(result.agent).join(", ")}`)
+      result.default_agent = firstPrimaryAgent(result.agent)
+      log.debug(`selected fallback agent: ${result.default_agent}`)
     }
 
     return {

packages/opencode/test/config/config.test.ts

@@ -502,18 +502,23 @@ test("deduplicates duplicate plugins from global and local configs", async () =>
   })
 })
 
-test("sets default agent to 'build' when not specified", async () => {
+test("uses specified default_agent when provided", async () => {
   await using tmp = await tmpdir({
     init: async (dir) => {
       await Bun.write(
         path.join(dir, "opencode.json"),
         JSON.stringify({
           $schema: "https://opencode.ai/config.json",
+          default_agent: "custom",
           agent: {
             build: {
               model: "test/model",
               description: "build agent",
             },
+            custom: {
+              model: "test/model",
+              description: "custom agent",
+            },
           },
         }),
       )
@@ -523,27 +528,29 @@ test("sets default agent to 'build' when not specified", async () => {
     directory: tmp.path,
     fn: async () => {
       const config = await Config.get()
-      expect(config.default_agent).toBe("build")
+      expect(config.default_agent).toBe("custom")
     },
   })
 })
 
-test("uses specified defaultAgent when provided", async () => {
+test("falls back when default_agent is disabled", async () => {
   await using tmp = await tmpdir({
     init: async (dir) => {
       await Bun.write(
         path.join(dir, "opencode.json"),
         JSON.stringify({
           $schema: "https://opencode.ai/config.json",
-          defaultAgent: "custom",
+          default_agent: "build",
           agent: {
             build: {
               model: "test/model",
               description: "build agent",
+              disable: true,
             },
             custom: {
               model: "test/model",
               description: "custom agent",
+              mode: "primary",
             },
           },
         }),
@@ -559,31 +566,23 @@ test("uses specified defaultAgent when provided", async () => {
   })
 })
 
-test("throws error when defaultAgent is disabled", async () => {
+test("falls back when configured default_agent does not exist", async () => {
   await using tmp = await tmpdir({
     init: async (dir) => {
       await Bun.write(
         path.join(dir, "opencode.json"),
         JSON.stringify({
           $schema: "https://opencode.ai/config.json",
-          defaultAgent: "build",
-          agent: {
-            build: {
-              model: "test/model",
-              description: "build agent",
-              disable: true,
-            },
-          },
+          default_agent: "nonexistent-agent",
         }),
       )
     },
   })
   await Instance.provide({
     directory: tmp.path,
     fn: async () => {
-      expect(Config.get()).rejects.toThrow(
-        "Cannot disable the default agent 'build'. Please either enable it or specify a different default agent in your configuration.",
-      )
+      const config = await Config.get()
+      expect(config.default_agent).toBe("build")
     },
   })
 })