fix(opencode): fail task on empty subagent output

packages/opencode/src/tool/task.ts

@@ -208,7 +208,9 @@ export const TaskTool = Tool.define(
           },
           parts,
         })
-        return result.parts.findLast((item) => item.type === "text")?.text ?? ""
+        const text = result.parts.findLast((item) => item.type === "text")?.text
+        if (!text?.trim()) return yield* Effect.fail(new Error("Task completed without text output"))
+        return text
       })
 
       const resumeWhenIdle: (input: { userID: MessageID; state: "completed" | "error" }) => Effect.Effect<void> =

packages/opencode/test/tool/task.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect } from "bun:test"
-import { Effect, Exit, Fiber, Layer } from "effect"
+import { Cause, Effect, Exit, Fiber, Layer } from "effect"
 import { Agent } from "../../src/agent/agent"
 import { BackgroundJob } from "@/background/job"
 import { Bus } from "@/bus"
@@ -376,6 +376,39 @@ describe("tool.task", () => {
     }),
   )
 
+  it.instance("execute fails when the subagent returns empty text output", () =>
+    Effect.gen(function* () {
+      const { chat, assistant } = yield* seed()
+      const tool = yield* TaskTool
+      const def = yield* tool.init()
+
+      const exit = yield* def
+        .execute(
+          {
+            description: "inspect bug",
+            prompt: "look into the cache key path",
+            subagent_type: "general",
+          },
+          {
+            sessionID: chat.id,
+            messageID: assistant.id,
+            agent: "build",
+            abort: new AbortController().signal,
+            extra: { promptOps: stubOps({ text: "" }) },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+        .pipe(Effect.exit)
+
+      expect(Exit.isFailure(exit)).toBe(true)
+      if (Exit.isFailure(exit)) {
+        expect(Cause.squash(exit.cause)).toMatchObject({ message: "Task completed without text output" })
+      }
+    }),
+  )
+
   it.instance(
     "execute shapes child permissions for task, todowrite, and primary tools",
     () =>