diff --git a/src/renderer/src/lib/launch-agent-background-session.ts b/src/renderer/src/lib/launch-agent-background-session.ts
index 6fd30efde21..947cabb317b 100644
--- a/src/renderer/src/lib/launch-agent-background-session.ts
+++ b/src/renderer/src/lib/launch-agent-background-session.ts
@@ -11,7 +11,7 @@ import {
   resolveTuiAgentLaunchArgs,
   resolveTuiAgentLaunchEnv
 } from '../../../shared/tui-agent-launch-defaults'
-import { TUI_AGENT_CONFIG } from '../../../shared/tui-agent-config'
+import { requireTuiAgentConfig } from '../../../shared/require-tui-agent-config'
 import { resolveAgentBackgroundLaunchHost } from '@/lib/agent-background-session-launch-host'
 import { makePaneKey } from '../../../shared/stable-pane-id'
 import {
@@ -62,7 +62,7 @@ export async function launchAgentBackgroundSession(
     worktreePath: worktree.path,
     repo
   })
-  const preflight = TUI_AGENT_CONFIG[agent].preflightTrust
+  const preflight = requireTuiAgentConfig(agent).preflightTrust
   if (preflight && worktree.path && window.api.agentTrust?.markTrusted) {
     try {
       await window.api.agentTrust.markTrusted({
@@ -82,7 +82,7 @@ export async function launchAgentBackgroundSession(
   })
   const trimmedPrompt = prompt?.trim() ?? ''
   const hasPrompt = trimmedPrompt.length > 0
-  const isFollowupPath = TUI_AGENT_CONFIG[agent].promptInjectionMode === 'stdin-after-start'
+  const isFollowupPath = requireTuiAgentConfig(agent).promptInjectionMode === 'stdin-after-start'
 
   const pasteDraftAfterLaunch = hasPrompt && isFollowupPath ? trimmedPrompt : null
   const startupPlan = buildAgentStartupPlan({
diff --git a/src/shared/require-tui-agent-config.test.ts b/src/shared/require-tui-agent-config.test.ts
new file mode 100644
index 00000000000..a399fffd954
--- /dev/null
+++ b/src/shared/require-tui-agent-config.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest'
+import type { TuiAgent } from './tui-agent'
+import { requireTuiAgentConfig } from './require-tui-agent-config'
+
+describe('requireTuiAgentConfig', () => {
+  it('returns the config for a known agent', () => {
+    expect(requireTuiAgentConfig('codex').preflightTrust).toBe('codex')
+  })
+
+  it('names the unknown id instead of throwing a property-of-undefined error', () => {
+    // A custom-agent id persisted by a branch build, read back by one without the feature.
+    const stale = 'custom-agent:codex:b2e1ff6f-8932-413a-9133-edffa44e0ee9' as TuiAgent
+    expect(() => requireTuiAgentConfig(stale)).toThrow(
+      `Unknown agent "${stale}". This version of Orca has no such agent — pick a different agent and try again.`
+    )
+  })
+
+  it('rejects a prototype key that Object.hasOwn keeps off the config', () => {
+    expect(() => requireTuiAgentConfig('toString' as TuiAgent)).toThrow(/Unknown agent "toString"/)
+  })
+})
diff --git a/src/shared/require-tui-agent-config.ts b/src/shared/require-tui-agent-config.ts
new file mode 100644
index 00000000000..78f3f14b6df
--- /dev/null
+++ b/src/shared/require-tui-agent-config.ts
@@ -0,0 +1,15 @@
+import type { TuiAgent } from './tui-agent'
+import { isTuiAgent, TUI_AGENT_CONFIG, type TuiAgentConfig } from './tui-agent-config'
+
+/** Why: agent ids persist in automations and settings, so they outlive the build that
+ * wrote them — an id a branch build understood reads back as unknown here. Name the id
+ * instead of letting a bare `TUI_AGENT_CONFIG[agent].x` lookup fail with the unreadable
+ * "Cannot read properties of undefined (reading 'preflightTrust')". */
+export function requireTuiAgentConfig(agent: TuiAgent): TuiAgentConfig {
+  if (!isTuiAgent(agent)) {
+    throw new Error(
+      `Unknown agent "${String(agent)}". This version of Orca has no such agent — pick a different agent and try again.`
+    )
+  }
+  return TUI_AGENT_CONFIG[agent]
+}