diff --git a/src/hooks/keyword-detector/hook.ts b/src/hooks/keyword-detector/hook.ts
index 1376ded90..65051021e 100644
--- a/src/hooks/keyword-detector/hook.ts
+++ b/src/hooks/keyword-detector/hook.ts
@@ -17,7 +17,7 @@ import {
   removeSystemReminders,
 } from "../../shared/system-directive"
 import type { RalphLoopHook } from "../ralph-loop"
-import { getUltraworkMessage, isNonOmoAgent, isPlannerAgent } from "./constants"
+import { isNonOmoAgent, isPlannerAgent } from "./constants"
 import type { DetectedKeyword } from "./detector"
 import { detectKeywordsWithType, extractPromptText, looksLikeSlashCommand } from "./detector"
 
@@ -113,14 +113,7 @@ export function createKeywordDetectorHook(
         if (defaultMode?.ultrawork && !isNonMainSession && !defaultModeUltraworkInjectedSessions.has(input.sessionID)) {
           defaultModeUltraworkInjectedSessions.add(input.sessionID)
 
-          const ultraworkMessage = getUltraworkMessage(currentAgent, modelID)
-          const textPartIndex = output.parts.findIndex(isRealUserTextPart)
-          if (textPartIndex >= 0) {
-            const originalText = output.parts[textPartIndex].text ?? ""
-            output.parts[textPartIndex].text = `${ultraworkMessage}\n\n---\n\n${originalText}`
-          }
-
-          log(`[keyword-detector] Default ultrawork mode auto-activated`, { sessionID: input.sessionID })
+          log(`[keyword-detector] Default ultrawork mode auto-activated (injected via system prompt)`, { sessionID: input.sessionID })
 
           ctx.client.tui
             .showToast({
diff --git a/src/plugin-interface.ts b/src/plugin-interface.ts
index 5bcc0c364..79543c086 100644
--- a/src/plugin-interface.ts
+++ b/src/plugin-interface.ts
@@ -7,6 +7,7 @@ import { createChatMessageHandler } from "./plugin/chat-message"
 import { createCommandExecuteBeforeHandler } from "./plugin/command-execute-before"
 import { createMessagesTransformHandler } from "./plugin/messages-transform"
 import { createSystemTransformHandler } from "./plugin/system-transform"
+import { getUltraworkMessage } from "./hooks/keyword-detector/ultrawork"
 import { createEventHandler } from "./plugin/event"
 import { createToolExecuteAfterHandler } from "./plugin/tool-execute-after"
 import { createToolExecuteBeforeHandler } from "./plugin/tool-execute-before"
@@ -58,7 +59,10 @@ export function createPluginInterface(args: {
       hooks,
     }),
 
-    "experimental.chat.system.transform": createSystemTransformHandler(),
+    "experimental.chat.system.transform": createSystemTransformHandler(
+      pluginConfig.default_mode,
+      getUltraworkMessage,
+    ),
 
     config: managers.configHandler,
 
diff --git a/src/plugin/system-transform.ts b/src/plugin/system-transform.ts
index d6a04820c..39356a380 100644
--- a/src/plugin/system-transform.ts
+++ b/src/plugin/system-transform.ts
@@ -1,6 +1,25 @@
-export function createSystemTransformHandler(): (
+import type { DefaultModeConfig } from "../config/schema/default-mode"
+
+const ULTRAWORK_MODE_TAG = "<ultrawork-mode>"
+
+export function createSystemTransformHandler(
+  defaultMode?: DefaultModeConfig,
+  getUltraworkMessage?: (agentName?: string, modelID?: string) => string,
+): (
   input: { sessionID?: string; model: { id: string; providerID: string; [key: string]: unknown } },
   output: { system: string[] },
 ) => Promise<void> {
-  return async (): Promise<void> => {}
+  return async (input, output): Promise<void> => {
+    if (!defaultMode?.ultrawork || !getUltraworkMessage) return
+
+    // Avoid re-injecting if the ultrawork prompt is already in the system prompt
+    // (e.g. after compaction the system prompt is rebuilt and this hook fires again)
+    if (output.system.some((part) => part.includes(ULTRAWORK_MODE_TAG))) return
+
+    const modelID = input.model?.id
+    const ultraworkMessage = getUltraworkMessage("sisyphus", modelID)
+    if (!ultraworkMessage) return
+
+    output.system.push(ultraworkMessage)
+  }
 }