From f6cf0bec940923dde3d58759ee3fe5ada873af24 Mon Sep 17 00:00:00 2001
From: Burak Yigit Kaya <byk@sentry.io>
Date: Tue, 9 Jun 2026 14:35:46 +0000
Subject: [PATCH] fix(session): cache messages across prompt loop to preserve
 prompt cache byte-identity
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filterCompactedEffect(sessionID) reloads all messages from the DB at
the start of every prompt loop iteration. Between reloads, the DB
returns rows with new object identity, breaking Anthropic's prompt cache
which relies on byte-identical prefixes.

Cache messages across loop iterations — first iteration does a full
load, subsequent iterations only append new messages by ID. Full reload
is flagged after subtask, compaction, or overflow operations.
---
 packages/app/vite.js                    |  1 +
 packages/opencode/src/session/prompt.ts | 23 ++++++++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/packages/app/vite.js b/packages/app/vite.js
index e1f851653c62..42997fe8922a 100644
--- a/packages/app/vite.js
+++ b/packages/app/vite.js
@@ -23,6 +23,7 @@ export default [
         resolve: {
           alias: {
             "@": fileURLToPath(new URL("./src", import.meta.url)),
+            "@opencode-ai/core": fileURLToPath(new URL("../core/src", import.meta.url)),
           },
         },
         define: {
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 56ca82e3fcc0..c3b2fdaf0d30 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1218,14 +1218,27 @@ export const layer = Layer.effect(
         let structured: unknown
         let step = 0
         const session = yield* sessions.get(sessionID).pipe(Effect.orDie)
+        let msgs: MessageV2.WithParts[] | undefined
+        let needsFullReload = true
 
         while (true) {
           yield* status.set(sessionID, { type: "busy" })
           yield* Effect.logInfo("loop", { "session.id": sessionID, step })
 
-          let msgs = yield* MessageV2.filterCompactedEffect(sessionID).pipe(
-            Effect.provideService(Database.Service, database),
-          )
+          if (needsFullReload || !msgs) {
+            msgs = yield* MessageV2.filterCompactedEffect(sessionID).pipe(
+              Effect.provideService(Database.Service, database),
+            )
+            needsFullReload = false
+          } else {
+            const fresh = yield* MessageV2.filterCompactedEffect(sessionID).pipe(
+              Effect.provideService(Database.Service, database),
+            )
+            const knownIDs = new Set(msgs.map((m) => m.info.id))
+            for (const m of fresh) {
+              if (!knownIDs.has(m.info.id)) msgs.push(m)
+            }
+          }
 
           const { user: lastUser, assistant: lastAssistant, finished: lastFinished, tasks } = MessageV2.latest(msgs)
 
@@ -1277,6 +1290,7 @@ export const layer = Layer.effect(
 
           if (task?.type === "subtask") {
             yield* handleSubtask({ task, model, lastUser, sessionID, session, msgs })
+            needsFullReload = true
             continue
           }
 
@@ -1289,6 +1303,7 @@ export const layer = Layer.effect(
               overflow: task.overflow,
             })
             if (result === "stop") break
+            needsFullReload = true
             continue
           }
 
@@ -1298,6 +1313,7 @@ export const layer = Layer.effect(
             (yield* compaction.isOverflow({ tokens: lastFinished.tokens, model }))
           ) {
             yield* compaction.create({ sessionID, agent: lastUser.agent, model: lastUser.model, auto: true })
+            needsFullReload = true
             continue
           }
 
@@ -1455,6 +1471,7 @@ export const layer = Layer.effect(
                 auto: true,
                 overflow: !handle.message.finish,
               })
+              needsFullReload = true
             }
             return "continue" as const
           }).pipe(