From 7127fe053fe47758f7e4663cf3a00501f6f8e797 Mon Sep 17 00:00:00 2001
From: 0xDevNinja <manmit0x@gmail.com>
Date: Wed, 17 Jun 2026 12:22:56 +0530
Subject: [PATCH] fix(session): anchor system prompt date to session-start time

SystemPrompt.environment() was calling new Date().toDateString() on every
LLM request. applyCaching() marks the first two system messages with
cacheControl, so the date lived inside the cached prefix. Any session
crossing local midnight triggered a full cache miss and re-billed the
entire system block as a write instead of a read.

Pass the session's creation timestamp to environment() and derive
Today's date from that instead. The cached system prefix now contains a
date that is stable for the lifetime of the session, so the cache-busting
midnight rollover no longer occurs.

Fixes #32622
---
 packages/opencode/src/session/prompt.ts | 2 +-
 packages/opencode/src/session/system.ts | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index b3f85c813f20..bd1d453c971e 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1326,7 +1326,7 @@ export const layer = Layer.effect(
 
             const [skills, env, instructions, modelMsgs] = yield* Effect.all([
               sys.skills(agent),
-              sys.environment(model),
+              sys.environment(model, session.time.created),
               instruction.system().pipe(Effect.orDie),
               MessageV2.toModelMessagesEffect(msgs, model),
             ])
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 74401779d353..dd4f4fcc7f2d 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -39,7 +39,7 @@ export function provider(model: Provider.Model) {
 }
 
 export interface Interface {
-  readonly environment: (model: Provider.Model) => Effect.Effect<string[]>
+  readonly environment: (model: Provider.Model, sessionCreated?: number) => Effect.Effect<string[]>
   readonly skills: (agent: Agent.Info) => Effect.Effect<string | undefined>
 }
 
@@ -52,7 +52,7 @@ export const layer = Layer.effect(
     const locations = yield* LocationServiceMap
 
     return Service.of({
-      environment: Effect.fn("SystemPrompt.environment")(function* (model: Provider.Model) {
+      environment: Effect.fn("SystemPrompt.environment")(function* (model: Provider.Model, sessionCreated?: number) {
         const ctx = yield* InstanceState.context
         const references = yield* Effect.gen(function* () {
           yield* (yield* PluginBoot.Service).wait()
@@ -67,7 +67,7 @@ export const layer = Layer.effect(
             `  Workspace root folder: ${ctx.worktree}`,
             `  Is directory a git repo: ${ctx.project.vcs === "git" ? "yes" : "no"}`,
             `  Platform: ${process.platform}`,
-            `  Today's date: ${new Date().toDateString()}`,
+            `  Today's date: ${new Date(sessionCreated ?? Date.now()).toDateString()}`,
             `</env>`,
           ].join("\n"),
           references.length === 0