fix(task): subagent inherits parent agent's deny rules (Plan Mode security bypass) (#26597)

Co-authored-by: Developer <temp@example.com>

packages/opencode/src/agent/subagent-permissions.ts

@@ -0,0 +1,33 @@
+import type { Permission } from "../permission"
+import type { Agent } from "./agent"
+
+/**
+ * Build the `permission` ruleset for a subagent's session when it's spawned
+ * via the task tool. Combines:
+ *
+ * 1. The parent **agent's** deny rules — Plan Mode and other agent-level
+ *    restrictions live on the agent ruleset, not on the session, so a
+ *    subagent that only inherited the parent SESSION's permission would
+ *    silently bypass them. (#26514)
+ * 2. The parent **session's** deny rules and external_directory rules —
+ *    same forwarding the original code already did.
+ * 3. Default `todowrite` and `task` denies if the subagent's own ruleset
+ *    doesn't already permit them.
+ */
+export function deriveSubagentSessionPermission(input: {
+  parentSessionPermission: Permission.Ruleset
+  parentAgent: Agent.Info | undefined
+  subagent: Agent.Info
+}): Permission.Ruleset {
+  const canTask = input.subagent.permission.some((rule) => rule.permission === "task")
+  const canTodo = input.subagent.permission.some((rule) => rule.permission === "todowrite")
+  const parentAgentDenies = input.parentAgent?.permission.filter((rule) => rule.action === "deny") ?? []
+  return [
+    ...parentAgentDenies,
+    ...input.parentSessionPermission.filter(
+      (rule) => rule.permission === "external_directory" || rule.action === "deny",
+    ),
+    ...(canTodo ? [] : [{ permission: "todowrite" as const, pattern: "*" as const, action: "deny" as const }]),
+    ...(canTask ? [] : [{ permission: "task" as const, pattern: "*" as const, action: "deny" as const }]),
+  ]
+}

packages/opencode/src/tool/task.ts

@@ -4,6 +4,7 @@ import { Session } from "@/session/session"
 import { SessionID, MessageID } from "../session/schema"
 import { MessageV2 } from "../session/message-v2"
 import { Agent } from "../agent/agent"
+import { deriveSubagentSessionPermission } from "../agent/subagent-permissions"
 import type { SessionPrompt } from "../session/prompt"
 import { Config } from "@/config/config"
 import { Effect, Exit, Schema } from "effect"
@@ -58,41 +59,25 @@ export const TaskTool = Tool.define(
         return yield* Effect.fail(new Error(`Unknown agent type: ${params.subagent_type} is not a valid agent type`))
       }
 
-      const canTask = next.permission.some((rule) => rule.permission === id)
-      const canTodo = next.permission.some((rule) => rule.permission === "todowrite")
-
       const taskID = params.task_id
       const session = taskID
         ? yield* sessions.get(SessionID.make(taskID)).pipe(Effect.catchCause(() => Effect.succeed(undefined)))
         : undefined
       const parent = yield* sessions.get(ctx.sessionID)
+      const parentAgent = parent.agent
+        ? yield* agent.get(parent.agent).pipe(Effect.catchCause(() => Effect.succeed(undefined)))
+        : undefined
       const nextSession =
         session ??
         (yield* sessions.create({
           parentID: ctx.sessionID,
           title: params.description + ` (@${next.name} subagent)`,
           permission: [
-            ...(parent.permission ?? []).filter(
-              (rule) => rule.permission === "external_directory" || rule.action === "deny",
-            ),
-            ...(canTodo
-              ? []
-              : [
-                  {
-                    permission: "todowrite" as const,
-                    pattern: "*" as const,
-                    action: "deny" as const,
-                  },
-                ]),
-            ...(canTask
-              ? []
-              : [
-                  {
-                    permission: id,
-                    pattern: "*" as const,
-                    action: "deny" as const,
-                  },
-                ]),
+            ...deriveSubagentSessionPermission({
+              parentSessionPermission: parent.permission ?? [],
+              parentAgent,
+              subagent: next,
+            }),
             ...(cfg.experimental?.primary_tools?.map((item) => ({
               pattern: "*",
               action: "allow" as const,
@@ -144,8 +129,8 @@ export const TaskTool = Tool.define(
               },
               agent: next.name,
               tools: {
-                ...(canTodo ? {} : { todowrite: false }),
-                ...(canTask ? {} : { task: false }),
+                ...(next.permission.some((rule) => rule.permission === "todowrite") ? {} : { todowrite: false }),
+                ...(next.permission.some((rule) => rule.permission === id) ? {} : { task: false }),
                 ...Object.fromEntries((cfg.experimental?.primary_tools ?? []).map((item) => [item, false])),
               },
               parts,