diff --git a/src/features/background-agent/manager.test.ts b/src/features/background-agent/manager.test.ts
index 7d751ea6a..7d0fcdcf2 100644
--- a/src/features/background-agent/manager.test.ts
+++ b/src/features/background-agent/manager.test.ts
@@ -1731,15 +1731,22 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
      }
    }
 
-    function createMockClientWithSessionChain(
-      sessions: Record<string, { directory: string; parentID?: string }>
+  function createMockClientWithSessionChain(
+      sessions: Record<string, { directory: string; parentID?: string }>,
+      options?: { sessionLookupError?: Error }
     ) {
       return {
         session: {
           create: async (_args?: any) => ({ data: { id: `ses_${crypto.randomUUID()}` } }),
-          get: async ({ path }: { path: { id: string } }) => ({
-            data: sessions[path.id] ?? { directory: "/test/dir" },
-          }),
+          get: async ({ path }: { path: { id: string } }) => {
+            if (options?.sessionLookupError) {
+              throw options.sessionLookupError
+            }
+
+            return {
+              data: sessions[path.id] ?? { directory: "/test/dir" },
+            }
+          },
           prompt: async () => ({}),
           promptAsync: async () => ({}),
           messages: async () => ({ data: [] }),
@@ -2058,6 +2065,37 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
       // then
       await expect(result).rejects.toThrow("background_task.maxDescendants=1")
     })
+
+    test("should fail closed when session lineage lookup fails", async () => {
+      // given
+      manager.shutdown()
+      manager = new BackgroundManager(
+        {
+          client: createMockClientWithSessionChain(
+            {
+              "session-root": { directory: "/test/dir" },
+            },
+            { sessionLookupError: new Error("session lookup failed") }
+          ),
+          directory: tmpdir(),
+        } as unknown as PluginInput,
+        { maxDescendants: 1 },
+      )
+
+      const input = {
+        description: "Test task",
+        prompt: "Do something",
+        agent: "test-agent",
+        parentSessionID: "session-root",
+        parentMessageID: "parent-message",
+      }
+
+      // when
+      const result = manager.launch(input)
+
+      // then
+      await expect(result).rejects.toThrow("background_task.maxDescendants cannot be enforced safely")
+    })
   })
 
   describe("pending task can be cancelled", () => {
diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index ba4ff816d..92db3513d 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -54,7 +54,7 @@ import { removeTaskToastTracking } from "./remove-task-toast-tracking"
 import {
   createSubagentDepthLimitError,
   createSubagentDescendantLimitError,
-  getMaxRootDescendants,
+  getMaxRootSessionSpawnBudget,
   getMaxSubagentDepth,
   resolveSubagentSpawnContext,
   type SubagentSpawnContext,
@@ -165,13 +165,13 @@ export class BackgroundManager {
       })
     }
 
-    const maxDescendants = getMaxRootDescendants(this.config)
+    const maxRootSessionSpawnBudget = getMaxRootSessionSpawnBudget(this.config)
     const descendantCount = this.rootDescendantCounts.get(spawnContext.rootSessionID) ?? 0
-    if (descendantCount >= maxDescendants) {
+    if (descendantCount >= maxRootSessionSpawnBudget) {
       throw createSubagentDescendantLimitError({
         rootSessionID: spawnContext.rootSessionID,
         descendantCount,
-        maxDescendants,
+        maxDescendants: maxRootSessionSpawnBudget,
       })
     }
 
diff --git a/src/features/background-agent/subagent-spawn-limits.ts b/src/features/background-agent/subagent-spawn-limits.ts
index 2b0068102..c33ad7b21 100644
--- a/src/features/background-agent/subagent-spawn-limits.ts
+++ b/src/features/background-agent/subagent-spawn-limits.ts
@@ -2,7 +2,7 @@ import type { BackgroundTaskConfig } from "../../config/schema"
 import type { OpencodeClient } from "./constants"
 
 export const DEFAULT_MAX_SUBAGENT_DEPTH = 3
-export const DEFAULT_MAX_ROOT_DESCENDANTS = 50
+export const DEFAULT_MAX_ROOT_SESSION_SPAWN_BUDGET = 50
 
 export interface SubagentSpawnContext {
   rootSessionID: string
@@ -14,8 +14,8 @@ export function getMaxSubagentDepth(config?: BackgroundTaskConfig): number {
   return config?.maxDepth ?? DEFAULT_MAX_SUBAGENT_DEPTH
 }
 
-export function getMaxRootDescendants(config?: BackgroundTaskConfig): number {
-  return config?.maxDescendants ?? DEFAULT_MAX_ROOT_DESCENDANTS
+export function getMaxRootSessionSpawnBudget(config?: BackgroundTaskConfig): number {
+  return config?.maxDescendants ?? DEFAULT_MAX_ROOT_SESSION_SPAWN_BUDGET
 }
 
 export async function resolveSubagentSpawnContext(
@@ -34,11 +34,19 @@ export async function resolveSubagentSpawnContext(
 
     visitedSessionIDs.add(currentSessionID)
 
-    const session = await client.session.get({
-      path: { id: currentSessionID },
-    }).catch(() => null)
+    let nextParentSessionID: string | undefined
+    try {
+      const session = await client.session.get({
+        path: { id: currentSessionID },
+      })
+      nextParentSessionID = session.data?.parentID
+    } catch (error) {
+      const reason = error instanceof Error ? error.message : String(error)
+      throw new Error(
+        `Subagent spawn blocked: failed to resolve session lineage for ${parentSessionID}, so background_task.maxDescendants cannot be enforced safely. ${reason}`
+      )
+    }
 
-    const nextParentSessionID = session?.data?.parentID
     if (!nextParentSessionID) {
       rootSessionID = currentSessionID
       break