diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index dca8085c5b2e..1ee108c0c3d0 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -655,9 +655,10 @@ export namespace SessionPrompt {
       // Build system prompt, adding structured output instruction if needed
       const skills = await SystemPrompt.skills(agent)
       const system = [
-        ...(await SystemPrompt.environment(model)),
-        ...(skills ? [skills] : []),
         ...(await InstructionPrompt.system()),
+        // Put the most stable instructions first to maximize prompt-cache reuse.
+        ...(await SystemPrompt.environment(model, session.time.created)),
+        ...(skills ? [skills] : []),
       ]
       const format = lastUser.format ?? { type: "text" }
       if (format.type === "json_schema") {
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index ca324652d9dc..902b3e48b48c 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -25,7 +25,7 @@ export namespace SystemPrompt {
     return [PROMPT_DEFAULT]
   }
 
-  export async function environment(model: Provider.Model) {
+  export async function environment(model: Provider.Model, sessionCreatedAt: number) {
     const project = Instance.project
     return [
       [
@@ -36,7 +36,8 @@ export namespace SystemPrompt {
         `  Workspace root folder: ${Instance.worktree}`,
         `  Is directory a git repo: ${project.vcs === "git" ? "yes" : "no"}`,
         `  Platform: ${process.platform}`,
-        `  Today's date: ${new Date().toDateString()}`,
+        // Keep the session day stable so long-lived sessions do not churn the prompt at midnight.
+        `  Today's date: ${new Date(sessionCreatedAt).toDateString()}`,
         `</env>`,
         `<directories>`,
         `  ${
diff --git a/packages/opencode/test/session/system.test.ts b/packages/opencode/test/session/system.test.ts
index 47f5f6fc25dd..7767b0d83933 100644
--- a/packages/opencode/test/session/system.test.ts
+++ b/packages/opencode/test/session/system.test.ts
@@ -6,6 +6,30 @@ import { SystemPrompt } from "../../src/session/system"
 import { tmpdir } from "../fixture/fixture"
 
 describe("session.system", () => {
+  test("environment uses the session creation day", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const environment = await SystemPrompt.environment(
+          {
+            providerID: "openai",
+            api: {
+              id: "gpt-5.2",
+            },
+          } as any,
+          Date.UTC(2026, 2, 22, 23, 59, 59),
+        )
+
+        expect(environment).toHaveLength(1)
+        expect(environment[0]).toContain("Today's date: Sun Mar 22 2026")
+      },
+    })
+  })
+
   test("skills output is sorted by name and stable across calls", async () => {
     await using tmp = await tmpdir({
       git: true,