diff --git a/mobile/src/utils/slashCommandHelpers.test.ts b/mobile/src/utils/slashCommandHelpers.test.ts
index 8593b23647..fd38609c95 100644
--- a/mobile/src/utils/slashCommandHelpers.test.ts
+++ b/mobile/src/utils/slashCommandHelpers.test.ts
@@ -54,6 +54,7 @@ describe("buildMobileCompactionPayload", () => {
     const payload = buildMobileCompactionPayload(parsed, baseOptions);
 
     expect(payload.messageText).toContain("approximately 615 words");
+
     expect(payload.messageText).toContain(parsed.continueMessage);
     expect(payload.metadata.type).toBe("compaction-request");
     expect(payload.metadata.rawCommand).toContain("/compact -t 800 -m anthropic:claude-opus-4-1");
diff --git a/mobile/src/utils/slashCommandHelpers.ts b/mobile/src/utils/slashCommandHelpers.ts
index ce9ad9df12..a9a0e9f9dd 100644
--- a/mobile/src/utils/slashCommandHelpers.ts
+++ b/mobile/src/utils/slashCommandHelpers.ts
@@ -2,14 +2,17 @@ import type { MuxFrontendMetadata } from "@/common/types/message";
 import type { ParsedCommand, SlashSuggestion } from "@/browser/utils/slashCommands/types";
 import type { InferClientInputs } from "@orpc/client";
 import type { ORPCClient } from "../orpc/client";
+import {
+  DEFAULT_COMPACTION_WORD_TARGET,
+  WORDS_TO_TOKENS_RATIO,
+  buildCompactionPrompt,
+} from "@/common/constants/ui";
 
 type SendMessageOptions = NonNullable<
   InferClientInputs<ORPCClient>["workspace"]["sendMessage"]["options"]
 >;
 
 export const MOBILE_HIDDEN_COMMANDS = new Set(["telemetry", "vim"]);
-const WORDS_PER_TOKEN = 1.3;
-const DEFAULT_WORD_TARGET = 2000;
 
 export function extractRootCommand(replacement: string): string | null {
   if (typeof replacement !== "string") {
@@ -44,12 +47,10 @@ export function buildMobileCompactionPayload(
   baseOptions: SendMessageOptions
 ): MobileCompactionPayload {
   const targetWords = parsed.maxOutputTokens
-    ? Math.round(parsed.maxOutputTokens / WORDS_PER_TOKEN)
-    : DEFAULT_WORD_TARGET;
+    ? Math.round(parsed.maxOutputTokens / WORDS_TO_TOKENS_RATIO)
+    : DEFAULT_COMPACTION_WORD_TARGET;
 
-  let messageText =
-    `Summarize this conversation into a compact form for a new Assistant to continue helping the user. ` +
-    `Use approximately ${targetWords} words.`;
+  let messageText = buildCompactionPrompt(targetWords);
 
   if (parsed.continueMessage) {
     messageText += `\n\nThe user wants to continue with: ${parsed.continueMessage}`;
diff --git a/src/browser/utils/chatCommands.test.ts b/src/browser/utils/chatCommands.test.ts
index b102f2c0be..9e4fd2c8fa 100644
--- a/src/browser/utils/chatCommands.test.ts
+++ b/src/browser/utils/chatCommands.test.ts
@@ -128,18 +128,6 @@ describe("prepareCompactionMessage", () => {
     expect(metadata.parsed.continueMessage?.model).toBe(sendMessageOptions.model);
   });
 
-  test("generates correct prompt text with strict summary instructions", () => {
-    const sendMessageOptions = createBaseOptions();
-    const { messageText } = prepareCompactionMessage({
-      workspaceId: "ws-1",
-      maxOutputTokens: 4096,
-      sendMessageOptions,
-    });
-
-    expect(messageText).toContain("Focus entirely on the summary");
-    expect(messageText).toContain("Do not suggest next steps or future actions");
-  });
-
   test("does not create continueMessage when no text or images provided", () => {
     const sendMessageOptions = createBaseOptions();
     const { metadata } = prepareCompactionMessage({
diff --git a/src/browser/utils/chatCommands.ts b/src/browser/utils/chatCommands.ts
index 2632049593..1283e6646b 100644
--- a/src/browser/utils/chatCommands.ts
+++ b/src/browser/utils/chatCommands.ts
@@ -25,7 +25,11 @@ import { resolveCompactionModel } from "@/browser/utils/messages/compactionModel
 import type { ImageAttachment } from "../components/ImageAttachments";
 import { dispatchWorkspaceSwitch } from "./workspaceEvents";
 import { getRuntimeKey, copyWorkspaceStorage } from "@/common/constants/storage";
-import { DEFAULT_COMPACTION_WORD_TARGET, WORDS_TO_TOKENS_RATIO } from "@/common/constants/ui";
+import {
+  DEFAULT_COMPACTION_WORD_TARGET,
+  WORDS_TO_TOKENS_RATIO,
+  buildCompactionPrompt,
+} from "@/common/constants/ui";
 
 // ============================================================================
 // Workspace Creation
@@ -593,7 +597,7 @@ export function prepareCompactionMessage(options: CompactionOptions): {
     : DEFAULT_COMPACTION_WORD_TARGET;
 
   // Build compaction message with optional continue context
-  let messageText = `Summarize this conversation into a compact form for a new Assistant to continue helping the user. Focus entirely on the summary of what has happened. Do not suggest next steps or future actions. Use approximately ${targetWords} words.`;
+  let messageText = buildCompactionPrompt(targetWords);
 
   if (options.continueMessage) {
     messageText += `\n\nThe user wants to continue with: ${options.continueMessage.text}`;
diff --git a/src/common/constants/ui.ts b/src/common/constants/ui.ts
index 5ce8a4de6a..d56c881167 100644
--- a/src/common/constants/ui.ts
+++ b/src/common/constants/ui.ts
@@ -40,6 +40,31 @@ export const DEFAULT_COMPACTION_WORD_TARGET = 2000;
  */
 export const WORDS_TO_TOKENS_RATIO = 1.3;
 
+/**
+ * Build the compaction prompt for a given word target.
+ * Shared across desktop and mobile clients.
+ */
+export function buildCompactionPrompt(targetWords: number): string {
+  return `Summarize this conversation for a new Assistant to continue helping the user.
+
+Your summary must be approximately ${targetWords} words.
+
+Include:
+- The user's overall goal and current task
+- Key decisions made and their rationale
+- Current state of the work (what's done, what's in progress)
+- Important technical details (file paths, function names, configurations)
+- Any errors encountered and how they were resolved
+- Unresolved issues or blockers
+
+Do not include:
+- Suggestions for next steps
+- Conversational filler or pleasantries
+- Redundant information
+
+Write in a factual, dense style. Every sentence should convey essential context.`;
+}
+
 /**
  * Force-compact this many percentage points after threshold.
  * Gives user a buffer zone between warning and force-compaction.
diff --git a/src/node/services/mock/scenarios/slashCommands.ts b/src/node/services/mock/scenarios/slashCommands.ts
index 0defa15d86..bfa107b83b 100644
--- a/src/node/services/mock/scenarios/slashCommands.ts
+++ b/src/node/services/mock/scenarios/slashCommands.ts
@@ -1,13 +1,13 @@
 import type { ScenarioTurn } from "@/node/services/mock/scenarioTypes";
 import { KNOWN_MODELS } from "@/common/constants/knownModels";
 import { STREAM_BASE_DELAY } from "@/node/services/mock/scenarioTypes";
+import { buildCompactionPrompt } from "@/common/constants/ui";
 
 export const SLASH_COMMAND_PROMPTS = {
   MODEL_STATUS: "Please confirm which model is currently active for this conversation.",
 } as const;
 
-export const COMPACTION_MESSAGE =
-  "Summarize this conversation into a compact form for a new Assistant to continue helping the user. Focus entirely on the summary of what has happened. Do not suggest next steps or future actions. Use approximately 385 words.";
+export const COMPACTION_MESSAGE = buildCompactionPrompt(385);
 
 export const COMPACT_SUMMARY_TEXT =
   "Compact summary: The assistant read project files, listed directory contents, created and inspected test.txt, then confirmed the contents remained 'hello'. Technical details preserved.";