diff --git a/.changeset/static-environment-section.md b/.changeset/static-environment-section.md
new file mode 100644
index 00000000..498348c3
--- /dev/null
+++ b/.changeset/static-environment-section.md
@@ -0,0 +1,6 @@
+---
+"@perstack/runtime": patch
+"perstack": patch
+---
+
+Remove dynamic content from system prompt to improve cross-run prefix caching
diff --git a/packages/runtime/src/messages/instruction-message.ts b/packages/runtime/src/messages/instruction-message.ts
index 602aab98..c14c109e 100644
--- a/packages/runtime/src/messages/instruction-message.ts
+++ b/packages/runtime/src/messages/instruction-message.ts
@@ -1,12 +1,10 @@
+import os from "node:os"
 import { createId } from "@paralleldrive/cuid2"
 import { type Expert, type InstructionMessage, isCoordinatorExpert } from "@perstack/core"
 import { dedent } from "ts-dedent"
 
-function getEnvironmentSection(startedAt: number): string {
-  const lines = [
-    `- Current time: ${new Date(startedAt).toISOString()}`,
-    `- Working directory: ${process.cwd()}`,
-  ]
+function getEnvironmentSection(): string {
+  const lines = [`- Platform: ${os.platform()} ${os.release()} (${os.arch()})`]
   if (process.env.PERSTACK_SANDBOX === "1") {
     lines.push(
       "- Sandbox: This is an isolated container environment (Ubuntu). You can freely install packages with `sudo apt-get install` and run arbitrary commands without affecting the host system.",
@@ -15,18 +13,18 @@ function getEnvironmentSection(startedAt: number): string {
   return `Environment:\n${lines.join("\n")}`
 }
 
-function getDelegateMetaInstruction(startedAt: number): string {
+function getDelegateMetaInstruction(): string {
   return dedent`
     Before starting work, investigate the workspace and understand the current state. Then use the todo tool to create a plan of action. Work through the todos step by step, marking each completed as you go.
 
     When the task is complete, call attemptCompletion with a result parameter containing your final response.
     When you cannot help, call attemptCompletion without a result.
 
-    ${getEnvironmentSection(startedAt)}
+    ${getEnvironmentSection()}
   `
 }
 
-function getCoordinatorMetaInstruction(startedAt: number): string {
+function getCoordinatorMetaInstruction(): string {
   return dedent`
     Your role:
     - Act as the coordinator for the given task: define the task, set goals, and delegate.
@@ -62,14 +60,14 @@ function getCoordinatorMetaInstruction(startedAt: number): string {
     When the task is complete, call attemptCompletion with a result parameter containing your final response.
     When you cannot help, call attemptCompletion without a result.
 
-    ${getEnvironmentSection(startedAt)}
+    ${getEnvironmentSection()}
   `
 }
 
-export function createInstructionMessage(expert: Expert, startedAt: number): InstructionMessage {
+export function createInstructionMessage(expert: Expert): InstructionMessage {
   const metaInstruction = isCoordinatorExpert(expert.name)
-    ? getCoordinatorMetaInstruction(startedAt)
-    : getDelegateMetaInstruction(startedAt)
+    ? getCoordinatorMetaInstruction()
+    : getDelegateMetaInstruction()
 
   const preamble = dedent`
     You are Perstack, an AI expert that tackles tasks requested by users by utilizing all available tools.
diff --git a/packages/runtime/src/messages/message.test.ts b/packages/runtime/src/messages/message.test.ts
index 43b5a35f..93b1b4ed 100644
--- a/packages/runtime/src/messages/message.test.ts
+++ b/packages/runtime/src/messages/message.test.ts
@@ -551,8 +551,6 @@ describe("@perstack/messages: message", () => {
 })
 
 describe("@perstack/messages: instruction-message", () => {
-  const startedAt = 1700000000000
-
   describe("createInstructionMessage", () => {
     it("creates instruction message with preamble and expert instruction as separate parts", () => {
       const expert = {
@@ -566,7 +564,7 @@ describe("@perstack/messages: instruction-message", () => {
         runtime: ["local" as const],
         minRuntimeVersion: "v1.0" as const,
       }
-      const result = createInstructionMessage(expert, startedAt)
+      const result = createInstructionMessage(expert)
       expect(result.type).toBe("instructionMessage")
       expect(result.cache).toBeUndefined()
       expect(result.contents).toHaveLength(2)
@@ -600,7 +598,7 @@ describe("@perstack/messages: instruction-message", () => {
         runtime: ["local" as const],
         minRuntimeVersion: "v1.0" as const,
       }
-      const result = createInstructionMessage(expert, startedAt)
+      const result = createInstructionMessage(expert)
       expect(result.contents).toHaveLength(3)
       expect(result.contents[2].text).toContain("Always use this skill carefully.")
       expect(result.contents[2].text).toContain('"test-skill" skill rules:')
@@ -628,11 +626,11 @@ describe("@perstack/messages: instruction-message", () => {
         runtime: ["local" as const],
         minRuntimeVersion: "v1.0" as const,
       }
-      const result = createInstructionMessage(expert, startedAt)
+      const result = createInstructionMessage(expert)
       expect(result.contents).toHaveLength(2)
     })
 
-    it("uses startedAt for timestamp in instruction", () => {
+    it("includes platform info in environment section", () => {
       const expert = {
         key: "test-expert",
         name: "Test Expert",
@@ -644,8 +642,8 @@ describe("@perstack/messages: instruction-message", () => {
         runtime: ["local" as const],
         minRuntimeVersion: "v1.0" as const,
       }
-      const result = createInstructionMessage(expert, startedAt)
-      expect(result.contents[0].text).toContain("2023-11-14T22:13:20.000Z")
+      const result = createInstructionMessage(expert)
+      expect(result.contents[0].text).toContain("Platform:")
     })
   })
 })
diff --git a/packages/runtime/src/state-machine/states/init.ts b/packages/runtime/src/state-machine/states/init.ts
index 05022436..1af4cca0 100644
--- a/packages/runtime/src/state-machine/states/init.ts
+++ b/packages/runtime/src/state-machine/states/init.ts
@@ -34,7 +34,7 @@ export async function initLogic({
       return startRun(setting, checkpoint, {
         initialCheckpoint: checkpoint,
         inputMessages: [
-          createInstructionMessage(expert, setting.startedAt),
+          createInstructionMessage(expert),
           createUserMessage([{ type: "textPart", text: setting.input.text }]),
         ],
         model: setting.model,