fix(core): limit v2 subagent nesting depth

packages/opencode/src/config/config.ts

@@ -1044,6 +1044,12 @@ export namespace Config {
             .array(z.string())
             .optional()
             .describe("Tools that should only be available to primary agents."),
+          subagent_depth: z
+            .number()
+            .int()
+            .min(0)
+            .optional()
+            .describe("Maximum subagent nesting depth. Defaults to 1."),
           continue_loop_on_deny: z.boolean().optional().describe("Continue the agent loop when a tool call is denied"),
           mcp_timeout: z
             .number()

packages/opencode/src/tool/task.ts

@@ -41,6 +41,22 @@ export const TaskTool = Tool.define(
     const run = Effect.fn("TaskTool.execute")(function* (params: z.infer<typeof parameters>, ctx: Tool.Context) {
       const cfg = yield* config.get()
 
+      const parent = yield* sessions.get(ctx.sessionID)
+      let current = parent
+      let depth = 0
+      while (current.parentID) {
+        depth++
+        current = yield* sessions.get(current.parentID)
+      }
+      const limit = cfg.experimental?.subagent_depth ?? 1
+      if (depth >= limit) {
+        return yield* Effect.fail(
+          new Error(
+            `Subagent depth limit reached (${limit}). Increase "experimental.subagent_depth" to allow nested subagents.`,
+          ),
+        )
+      }
+
       if (!ctx.extra?.bypassAgentCheck) {
         yield* ctx.ask({
           permission: id,

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

@@ -313,6 +313,89 @@ describe("tool.task", () => {
     ),
   )
 
+  it.live("prevents subagents from launching subagents by default", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const { chat, assistant } = yield* seed()
+        const child = yield* sessions.create({ parentID: chat.id, title: "child" })
+        const nested = yield* sessions.updateMessage({
+          ...assistant,
+          id: MessageID.ascending(),
+          parentID: MessageID.ascending(),
+          sessionID: child.id,
+        })
+        const tool = yield* TaskTool
+        const def = yield* tool.init()
+        let asked = false
+
+        const exit = yield* def
+          .execute(
+            {
+              description: "inspect bug",
+              prompt: "look into the cache key path",
+              subagent_type: "general",
+            },
+            {
+              sessionID: child.id,
+              messageID: nested.id,
+              agent: "general",
+              abort: new AbortController().signal,
+              extra: { promptOps: stubOps() },
+              messages: [],
+              metadata: () => Effect.void,
+              ask: () => Effect.sync(() => (asked = true)),
+            },
+          )
+          .pipe(Effect.exit)
+
+        expect(exit._tag).toBe("Failure")
+        expect(asked).toBe(false)
+        expect(yield* sessions.children(child.id)).toHaveLength(0)
+      }),
+    ),
+  )
+
+  it.live("allows nested subagents up to the configured depth", () =>
+    provideTmpdirInstance(
+      () =>
+        Effect.gen(function* () {
+          const sessions = yield* Session.Service
+          const { chat, assistant } = yield* seed()
+          const child = yield* sessions.create({ parentID: chat.id, title: "child" })
+          const nested = yield* sessions.updateMessage({
+            ...assistant,
+            id: MessageID.ascending(),
+            parentID: MessageID.ascending(),
+            sessionID: child.id,
+          })
+          const tool = yield* TaskTool
+          const def = yield* tool.init()
+
+          const result = yield* def.execute(
+            {
+              description: "inspect bug",
+              prompt: "look into the cache key path",
+              subagent_type: "general",
+            },
+            {
+              sessionID: child.id,
+              messageID: nested.id,
+              agent: "general",
+              abort: new AbortController().signal,
+              extra: { promptOps: stubOps() },
+              messages: [],
+              metadata: () => Effect.void,
+              ask: () => Effect.void,
+            },
+          )
+
+          expect((yield* sessions.get(result.metadata.sessionId)).parentID).toBe(child.id)
+        }),
+      { config: { experimental: { subagent_depth: 2 } } },
+    ),
+  )
+
   it.live("execute shapes child permissions for task, todowrite, and primary tools", () =>
     provideTmpdirInstance(
       () =>

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -1623,6 +1623,10 @@ export type Config = {
      * Tools that should only be available to primary agents.
      */
     primary_tools?: Array<string>
+    /**
+     * Maximum subagent nesting depth. Defaults to 1.
+     */
+    subagent_depth?: number
     /**
      * Continue the agent loop when a tool call is denied
      */