fix(core): correct compaction history direction (#45698)

.changeset/compaction-history-direction.md

@@ -0,0 +1,5 @@
+---
+"@opencode-ai/core": patch
+---
+
+Correct the compaction prompt to point existing summaries at the conversation history that follows them.

packages/core/src/session/compaction.ts

@@ -215,7 +215,7 @@ const select = (
 export const buildPrompt = (input: { readonly previousSummary?: string; readonly context: readonly string[] }) =>
   [
     input.previousSummary
-      ? `Update the anchored summary below using the conversation history above.\nPreserve still-true details, remove stale details, and merge in the new facts.\n<previous-summary>\n${input.previousSummary}\n</previous-summary>`
+      ? `Update the anchored summary below using the conversation history below.\nPreserve still-true details, remove stale details, and merge in the new facts.\n<previous-summary>\n${input.previousSummary}\n</previous-summary>`
       : "Create a new anchored summary from the conversation history.",
     SUMMARY_TEMPLATE,
     "The following is the conversation history:",

packages/core/test/session-compaction.test.ts

@@ -143,6 +143,13 @@ test("compaction prompt requires the checkpoint headings in order", () => {
   expect(prompt).toContain("Keep every section, even when empty.")
 })
 
+test("compaction points an existing summary to the following history", () => {
+  const prompt = SessionCompaction.buildPrompt({ previousSummary: "Previous summary", context: ["Recent history"] })
+
+  expect(prompt.split("\n", 1)[0]).toBe("Update the anchored summary below using the conversation history below.")
+  expect(prompt).not.toContain("conversation history above")
+})
+
 it.effect("auto compaction reserves a buffer below the prompt ceiling", () =>
   Effect.gen(function* () {
     const compaction = yield* SessionCompaction.Service