fix(core): limit subagent nesting depth (#37124)

Co-authored-by: Dax <mail@thdxr.com>

packages/core/src/v1/config/config.ts

@@ -81,6 +81,9 @@ export const Info = Schema.Struct({
     description:
       "Default agent to use when none is specified. Must be a primary agent. Falls back to 'build' if not set or if the specified agent is invalid.",
   }),
+  subagent_depth: Schema.optional(NonNegativeInt).annotate({
+    description: "Maximum subagent nesting depth. Defaults to 1, which prevents subagents from launching subagents.",
+  }),
   username: Schema.optional(Schema.String).annotate({
     description: "Custom username to display in conversations instead of system username",
   }),

packages/opencode/src/tool/task.ts

@@ -101,6 +101,21 @@ export const TaskTool = Tool.define(
         )
       }
 
+      const parent = yield* sessions.get(ctx.sessionID)
+      let current = parent
+      let depth = 0
+      while (current.parentID) {
+        depth++
+        current = yield* sessions.get(current.parentID)
+      }
+      if (depth >= (cfg.subagent_depth ?? 1)) {
+        return yield* Effect.fail(
+          new Error(
+            `Subagent depth limit reached (${cfg.subagent_depth ?? 1}). Increase "subagent_depth" to allow nested subagents.`,
+          ),
+        )
+      }
+
       if (!ctx.extra?.bypassAgentCheck) {
         yield* ctx.ask({
           permission: id,
@@ -121,7 +136,6 @@ export const TaskTool = Tool.define(
       const session = params.task_id
         ? yield* sessions.get(SessionID.make(params.task_id)).pipe(Effect.catchCause(() => Effect.succeed(undefined)))
         : undefined
-      const parent = yield* sessions.get(ctx.sessionID)
       const childPermission = deriveSubagentSessionPermission({
         parentSessionPermission: parent.permission ?? [],
         subagent: next,

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

@@ -388,6 +388,86 @@ describe("tool.task", () => {
     }),
   )
 
+  it.instance("prevents subagents from launching subagents by default", () =>
+    Effect.gen(function* () {
+      const sessions = yield* Session.Service
+      const { chat, assistant } = yield* seed()
+      const child = yield* sessions.create({ parentID: chat.id, title: "child" })
+      const nestedAssistant = 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: nestedAssistant.id,
+            agent: "general",
+            abort: new AbortController().signal,
+            extra: { promptOps: stubOps() },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.sync(() => (asked = true)),
+          },
+        )
+        .pipe(Effect.exit)
+
+      expect(Exit.isFailure(exit)).toBe(true)
+      expect(asked).toBe(false)
+      expect(yield* sessions.children(child.id)).toHaveLength(0)
+    }),
+  )
+
+  it.instance(
+    "allows nested subagents up to the configured depth",
+    () =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const { chat, assistant } = yield* seed()
+        const child = yield* sessions.create({ parentID: chat.id, title: "child" })
+        const nestedAssistant = 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: nestedAssistant.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: { subagent_depth: 2 } },
+  )
+
   it.instance(
     "execute shapes child permissions for task, todowrite, and primary tools",
     () =>

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

@@ -1928,6 +1928,7 @@ export type Config = {
   model?: string
   small_model?: string
   default_agent?: string
+  subagent_depth?: number
   username?: string
   mode?: {
     build?: AgentConfig