diff --git a/src/features/hook-message-injector/injector.test.ts b/src/features/hook-message-injector/injector.test.ts
index fffdf5a7..c66d18d9 100644
--- a/src/features/hook-message-injector/injector.test.ts
+++ b/src/features/hook-message-injector/injector.test.ts
@@ -4,6 +4,8 @@ import {
   findFirstMessageWithAgent,
   findNearestMessageWithFieldsFromSDK,
   findFirstMessageWithAgentFromSDK,
+  generateMessageId,
+  generatePartId,
   injectHookMessage,
 } from "./injector"
 import { isSqliteBackend, resetSqliteBackendCache } from "../../shared/opencode-storage-detection"
@@ -192,6 +194,38 @@ describe("findFirstMessageWithAgentFromSDK", () => {
   })
 })
 
+describe("generateMessageId", () => {
+  it("returns deterministic sequential IDs with fixed format", () => {
+    // given
+    const format = /^msg_\d{12}$/
+
+    // when
+    const firstId = generateMessageId()
+    const secondId = generateMessageId()
+
+    // then
+    expect(firstId).toMatch(format)
+    expect(secondId).toMatch(format)
+    expect(Number(secondId.slice(4))).toBe(Number(firstId.slice(4)) + 1)
+  })
+})
+
+describe("generatePartId", () => {
+  it("returns deterministic sequential IDs with fixed format", () => {
+    // given
+    const format = /^prt_\d{12}$/
+
+    // when
+    const firstId = generatePartId()
+    const secondId = generatePartId()
+
+    // then
+    expect(firstId).toMatch(format)
+    expect(secondId).toMatch(format)
+    expect(Number(secondId.slice(4))).toBe(Number(firstId.slice(4)) + 1)
+  })
+})
+
 describe("injectHookMessage", () => {
   beforeEach(() => {
     vi.clearAllMocks()
diff --git a/src/features/hook-message-injector/injector.ts b/src/features/hook-message-injector/injector.ts
index 8f4e0d57..68520007 100644
--- a/src/features/hook-message-injector/injector.ts
+++ b/src/features/hook-message-injector/injector.ts
@@ -29,6 +29,9 @@ interface SDKMessage {
   }
 }
 
+let messageCounter = 0
+let partCounter = 0
+
 function convertSDKMessageToStoredMessage(msg: SDKMessage): StoredMessage | null {
   const info = msg.info
   if (!info) return null
@@ -204,16 +207,12 @@ export function findFirstMessageWithAgent(messageDir: string): string | null {
   return null
 }
 
-function generateMessageId(): string {
-  const timestamp = Date.now().toString(16)
-  const random = Math.random().toString(36).substring(2, 14)
-  return `msg_${timestamp}${random}`
+export function generateMessageId(): string {
+  return `msg_${String(++messageCounter).padStart(12, "0")}`
 }
 
-function generatePartId(): string {
-  const timestamp = Date.now().toString(16)
-  const random = Math.random().toString(36).substring(2, 10)
-  return `prt_${timestamp}${random}`
+export function generatePartId(): string {
+  return `prt_${String(++partCounter).padStart(12, "0")}`
 }
 
 function getOrCreateMessageDir(sessionID: string): string {