fix(compaction): adjust instructions and structure to be more clear to smaller models like dsv4 flash (#42045)

Co-authored-by: akenra <37288280+akenra@users.noreply.github.com>

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

@@ -4,12 +4,32 @@ import { SessionCompaction } from "@opencode-ai/core/session/compaction"
 test("compaction prompt preserves detailed work state and relevant files", () => {
   const prompt = SessionCompaction.buildPrompt({ context: ["conversation history"] })
 
+  expect(prompt).toStartWith(
+    "Here is the conversation so far:\n\n<conversation>\nconversation history\n</conversation>",
+  )
+  expect(prompt.indexOf("</conversation>")).toBeLessThan(prompt.indexOf("Create a new anchored summary"))
+  expect(prompt).toContain("conversation history in the <conversation> tags above")
   expect(prompt).toContain("## Work State\n### Completed")
   expect(prompt).toContain("### Active")
   expect(prompt).toContain("### Blocked")
   expect(prompt).toContain("## Relevant Files")
 })
 
+test("compaction prompt gives update instructions for a prior summary", () => {
+  const prompt = SessionCompaction.buildPrompt({
+    context: ["new conversation"],
+    previousSummary: "existing summary",
+  })
+
+  expect(prompt.indexOf("<conversation>")).toBeLessThan(prompt.indexOf("<prior-summary>"))
+  expect(prompt.indexOf("</prior-summary>")).toBeLessThan(prompt.indexOf("The <prior-summary> summarizes"))
+  expect(prompt).toContain(
+    "Carry forward objectives, constraints, user directives, decisions, and parallel workstreams from the <prior-summary>",
+  )
+  expect(prompt).toContain('Move completed work from "Active" to "Completed".')
+  expect(prompt).toContain('Update "Objective" and "Next Move" to reflect the current work state.')
+})
+
 test("compaction describes tool media without embedding base64", () => {
   const base64 = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB"
   const serialized = SessionCompaction.serializeToolContent([