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/opencode/test/session/compaction.test.ts

@@ -365,6 +365,20 @@ function autocontinue(enabled: boolean) {
   })
 }
 
+function compactionContext(context: string) {
+  return Layer.mock(Plugin.Service)({
+    trigger: <Name extends string, Input, Output>(name: Name, _input: Input, output: Output) => {
+      if (name !== "experimental.session.compacting") return Effect.succeed(output)
+      return Effect.sync(() => {
+        ;(output as { context: string[] }).context.push(context)
+        return output
+      })
+    },
+    list: () => Effect.succeed([]),
+    init: () => Effect.void,
+  })
+}
+
 describe("session.compaction.isOverflow", () => {
   it.live(
     "returns true when token count exceeds usable context",
@@ -1389,11 +1403,21 @@ describe("session.compaction.process", () => {
         const captured = JSON.stringify(messages)
         expect(messages).toHaveLength(1)
         expect(messages[0]?.role).toBe("user")
+        expect(captured).toContain("Here is the conversation so far:")
+        expect(captured).toContain("<conversation>")
+        expect(captured.indexOf("[User]: older context")).toBeLessThan(
+          captured.indexOf("Create a new anchored summary"),
+        )
         expect(captured).toContain("[User]: older context")
         expect(captured).not.toContain("keep this turn")
         expect(captured).not.toContain("and this one too")
         expect(captured).not.toContain("What did we do so far?")
-      }).pipe(withCompaction({ llm: stub.llmLayer }))
+      }).pipe(
+        withCompaction({
+          llm: stub.llmLayer,
+          config: cfg({ tail_turns: 2, preserve_recent_tokens: 10_000 }),
+        }),
+      )
     },
     { git: true },
   )
@@ -1430,9 +1454,11 @@ describe("session.compaction.process", () => {
         expect(parent).toBeTruthy()
         yield* SessionCompaction.use.process({ parentID: parent!, messages: msgs, sessionID: session.id, auto: false })
 
-        expect(captured).toContain("<previous-summary>")
+        expect(captured).toContain("<prior-summary>")
         expect(captured).toContain("summary one")
         expect(captured.match(/summary one/g)?.length).toBe(1)
+        expect(captured.indexOf("latest turn")).toBeLessThan(captured.indexOf("<prior-summary>"))
+        expect(captured).toContain("summary of the conversation before the <conversation> above")
         expect(captured).toContain("## Important Details")
         expect(captured).toContain("## Work State")
       }).pipe(withCompaction({ llm: stub.llmLayer }))
@@ -1440,6 +1466,49 @@ describe("session.compaction.process", () => {
     { git: true },
   )
 
+  itCompaction.instance(
+    "keeps plugin context outside the serialized conversation",
+    () => {
+      const stub = llm()
+      let captured = ""
+      stub.push(
+        reply("summary", (input) => {
+          captured = JSON.stringify(input.messages)
+        }),
+      )
+
+      return Effect.gen(function* () {
+        const ssn = yield* SessionNs.Service
+        const session = yield* ssn.create({})
+        yield* createUserMessage(session.id, "older context")
+        yield* createUserMessage(session.id, "keep this turn")
+        yield* createUserMessage(session.id, "and this one too")
+        yield* createCompactionMarker(session.id)
+
+        const msgs = yield* ssn.messages({ sessionID: session.id })
+        const parent = msgs.at(-1)?.info.id
+        expect(parent).toBeTruthy()
+        yield* SessionCompaction.use.process({
+          parentID: parent!,
+          messages: msgs,
+          sessionID: session.id,
+          auto: false,
+        })
+
+        expect(captured).toContain("Prioritize unresolved migration details")
+        expect(captured.indexOf("</conversation>")).toBeLessThan(
+          captured.indexOf("Prioritize unresolved migration details"),
+        )
+      }).pipe(
+        withCompaction({
+          llm: stub.llmLayer,
+          plugin: compactionContext("Prioritize unresolved migration details"),
+        }),
+      )
+    },
+    { git: true },
+  )
+
   itCompaction.instance(
     "serializes repeated compaction history as one user message",
     () => {