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

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

packages/opencode/test/agent/plan-mode-subagent-bypass.test.ts

@@ -0,0 +1,141 @@
+/**
+ * Reproducer for opencode issue #26514:
+ *
+ * In Plan Mode (the `plan` agent), the main agent's edit/write tools are
+ * blocked by the plan agent's permission ruleset (`edit: { "*": "deny" }`).
+ * However, when the plan agent spawns a subagent via the `task` tool, the
+ * subagent retains full file modification capabilities — a security bypass.
+ *
+ * This test replicates the permission ruleset that would govern a
+ * `general` subagent when launched from a `plan` parent session, mirroring
+ * the logic in `src/tool/task.ts` (filtered parent permissions ++ runtime
+ * subagent agent permissions, evaluated as in `session/prompt.ts`).
+ *
+ * The expected (secure) behavior is that the subagent inherits the plan
+ * mode read-only restriction and `edit`/`write` resolve to `deny`. On
+ * origin/dev this assertion fails because the parent **agent** permissions
+ * are not propagated to the subagent — only the parent **session**
+ * permissions are passed through, and Plan Mode's restrictions live on the
+ * agent, not the session.
+ */
+import { test, expect, afterEach } from "bun:test"
+import { Effect } from "effect"
+import { disposeAllInstances, provideInstance, tmpdir } from "../fixture/fixture"
+import { WithInstance } from "../../src/project/with-instance"
+import { Agent } from "../../src/agent/agent"
+import { deriveSubagentSessionPermission } from "../../src/agent/subagent-permissions"
+import { Permission } from "../../src/permission"
+
+afterEach(async () => {
+  await disposeAllInstances()
+})
+
+function load<A>(dir: string, fn: (svc: Agent.Interface) => Effect.Effect<A>) {
+  return Effect.runPromise(provideInstance(dir)(Agent.Service.use(fn)).pipe(Effect.provide(Agent.defaultLayer)))
+}
+
+// `deriveSubagentSessionPermission` is imported from production. The test
+// exercises the actual helper that task.ts uses to build the subagent's
+// session permission, so any regression in that helper trips this test.
+
+test("[#26514] subagent spawned from plan mode inherits read-only restriction (edit denied)", async () => {
+  await using tmp = await tmpdir()
+  await WithInstance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const planAgent = await load(tmp.path, (svc) => svc.get("plan"))
+      const generalAgent = await load(tmp.path, (svc) => svc.get("general"))
+
+      expect(planAgent).toBeDefined()
+      expect(generalAgent).toBeDefined()
+      // Sanity: the plan agent itself blocks edit. (Note: `write` and
+      // `apply_patch` route through the `edit` permission at the runtime
+      // tool layer — see Permission.disabled / EDIT_TOOLS.)
+      expect(Permission.evaluate("edit", "/some/file.ts", planAgent!.permission).action).toBe("deny")
+
+      // Simulate the plan-mode parent session: in real flow the plan
+      // session's `permission` field is empty (Plan Mode lives on the agent
+      // ruleset, not the session). So we pass [] through as the parent
+      // session permission, exactly like the actual code path.
+      const parentSessionPermission: Permission.Ruleset = []
+
+      const subagentSessionPermission = deriveSubagentSessionPermission({
+        parentSessionPermission,
+        parentAgent: planAgent,
+        subagent: generalAgent!,
+      })
+
+      // Mirror the runtime evaluation in session/prompt.ts (~line 410, 639):
+      //   ruleset: Permission.merge(agent.permission, session.permission ?? [])
+      const effective = Permission.merge(generalAgent!.permission, subagentSessionPermission)
+
+      expect(Permission.evaluate("edit", "/some/file.ts", effective).action).toBe("deny")
+      expect(Permission.evaluate("edit", "/another/path/index.tsx", effective).action).toBe("deny")
+    },
+  })
+})
+
+test("[#26514] explore subagent launched from plan mode also stays read-only", async () => {
+  // Sibling check: even though `explore` is intrinsically read-only, the
+  // bug surface is the same. Including this case to document that the fix
+  // should propagate the parent **agent** permissions, not just deny edit
+  // when the subagent happens to already deny it.
+  await using tmp = await tmpdir()
+  await WithInstance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const planAgent = await load(tmp.path, (svc) => svc.get("plan"))
+      const explore = await load(tmp.path, (svc) => svc.get("explore"))
+      expect(planAgent).toBeDefined()
+      expect(explore).toBeDefined()
+
+      const parentSessionPermission: Permission.Ruleset = []
+      const subagentSessionPermission = deriveSubagentSessionPermission({
+        parentSessionPermission,
+        parentAgent: planAgent,
+        subagent: explore!,
+      })
+      const effective = Permission.merge(explore!.permission, subagentSessionPermission)
+
+      // Already deny — sanity check.
+      expect(Permission.evaluate("edit", "/x.ts", effective).action).toBe("deny")
+    },
+  })
+})
+
+test("[#26514] custom user subagent launched from plan mode bypasses Plan Mode read-only", async () => {
+  // The most damaging case: a user-defined subagent with default
+  // permissions (allow-by-default, like `general`). The subagent must NOT
+  // be able to edit when the parent agent is `plan`.
+  await using tmp = await tmpdir({
+    config: {
+      agent: {
+        my_subagent: {
+          description: "A user-defined subagent",
+          mode: "subagent",
+        },
+      },
+    },
+  })
+  await WithInstance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const planAgent = await load(tmp.path, (svc) => svc.get("plan"))
+      const my = await load(tmp.path, (svc) => svc.get("my_subagent"))
+      expect(planAgent).toBeDefined()
+      expect(my).toBeDefined()
+
+      const parentSessionPermission: Permission.Ruleset = []
+      const subagentSessionPermission = deriveSubagentSessionPermission({
+        parentSessionPermission,
+        parentAgent: planAgent,
+        subagent: my!,
+      })
+      const effective = Permission.merge(my!.permission, subagentSessionPermission)
+
+      // BUG: on origin/dev edit resolves to "allow" because the plan
+      // agent's `edit: deny *` rule never reaches the subagent.
+      expect(Permission.evaluate("edit", "/some/file.ts", effective).action).toBe("deny")
+    },
+  })
+})