diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 99aefe46b..af9cf2c31 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -969,6 +969,24 @@ export namespace SessionPrompt {
 
       await Plugin.trigger("experimental.chat.messages.transform", {}, { messages: msgs })
 
+      // altimate_change start — upstream_fix: carry the date in the trailing user message
+      // so it stays out of the cache-controlled system prefix (see session/system.ts).
+      const lastUserForDate = msgs.findLast((m) => m.info.role === "user")
+      if (lastUserForDate) {
+        lastUserForDate.parts = [
+          ...lastUserForDate.parts,
+          {
+            type: "text" as const,
+            id: PartID.ascending(),
+            sessionID,
+            messageID: lastUserForDate.info.id,
+            text: `\n\n${SystemPrompt.currentDate()}`,
+            synthetic: true,
+          },
+        ]
+      }
+      // altimate_change end
+
       // Build system prompt, adding structured output instruction if needed
       const skills = await SystemPrompt.skills(agent)
       // altimate_change start - unified context-aware injection for memory + training
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 475d595c1..f468d4b64 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -75,7 +75,12 @@ 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()}`,
+        // altimate_change start — upstream_fix: move volatile date out of cached system prefix
+        // The date changes daily but environment() is the first entry in the system[]
+        // array, which applyCaching() marks with cacheControl. A session crossing midnight
+        // within the cache TTL would otherwise invalidate the whole system prefix. The date
+        // is appended to the trailing user message instead (see session/prompt.ts).
+        // altimate_change end
         `</env>`,
         `<directories>`,
         `  ${
@@ -91,6 +96,12 @@ export namespace SystemPrompt {
     ]
   }
 
+  // altimate_change start — upstream_fix: date carried outside the cached system prefix
+  export function currentDate() {
+    return `Today's date is ${new Date().toDateString()}.`
+  }
+  // altimate_change end
+
   export async function skills(agent: Agent.Info) {
     if (PermissionNext.disabled(["skill"], agent.permission).has("skill")) return
 
diff --git a/packages/opencode/test/session/system.test.ts b/packages/opencode/test/session/system.test.ts
index a6568b947..802eda4a3 100644
--- a/packages/opencode/test/session/system.test.ts
+++ b/packages/opencode/test/session/system.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, test } from "bun:test"
+import { describe, expect, setSystemTime, test } from "bun:test"
 import path from "path"
 import { Agent } from "../../src/agent/agent"
 import { Instance } from "../../src/project/instance"
@@ -160,4 +160,27 @@ description: ${description}
       process.env.OPENCODE_TEST_HOME = home
     }
   })
+
+  test("environment() keeps the volatile date out of the cached system prefix", async () => {
+    // Freeze the clock so the captured `today` and the `new Date()` inside
+    // currentDate() read the same instant — otherwise the assertion can race
+    // across midnight.
+    setSystemTime(new Date("2026-06-17T12:00:00.000Z"))
+    try {
+      await using tmp = await tmpdir({ git: true })
+      await Instance.provide({
+        directory: tmp.path,
+        fn: async () => {
+          const today = new Date().toDateString()
+          const [env] = await SystemPrompt.environment(makeModel({ apiId: "claude-3-7-sonnet" }))
+          expect(env).toMatch(/<env>/)
+          expect(env).not.toMatch(/Today's date/)
+          expect(env).not.toContain(today)
+          expect(SystemPrompt.currentDate()).toContain(today)
+        },
+      })
+    } finally {
+      setSystemTime()
+    }
+  })
 })