feat(compaction): serialize compaction tail (#26830)

packages/opencode/src/session/compaction.ts

@@ -79,12 +79,10 @@ Rules:
 type Turn = {
   start: number
   end: number
-  id: MessageID
 }
 
 type Tail = {
   start: number
-  id: MessageID
 }
 
 type CompletedCompaction = {
@@ -121,19 +119,41 @@ function completedCompactions(messages: MessageV2.WithParts[]) {
   })
 }
 
-function buildPrompt(input: { previousSummary?: string; context: string[] }) {
+function buildPrompt(input: { previousSummary?: string; context: string[]; tail?: string }) {
+  const source = input.tail
+    ? "the conversation history above and the serialized recent conversation tail below"
+    : "the conversation history above"
   const anchor = input.previousSummary
     ? [
-        "Update the anchored summary below using the conversation history above.",
+        `Update the anchored summary below using ${source}.`,
         "Preserve still-true details, remove stale details, and merge in the new facts.",
         "<previous-summary>",
         input.previousSummary,
         "</previous-summary>",
       ].join("\n")
-    : "Create a new anchored summary from the conversation history above."
-  return [anchor, SUMMARY_TEMPLATE, ...input.context].join("\n\n")
+    : `Create a new anchored summary from ${source}.`
+  const tail = input.tail
+    ? [
+        "Fold this serialized recent conversation tail into the summary; it is not provider message history.",
+        "<recent-conversation-tail>",
+        input.tail,
+        "</recent-conversation-tail>",
+      ].join("\n")
+    : undefined
+  return [anchor, ...(tail ? [tail] : []), SUMMARY_TEMPLATE, ...input.context].join("\n\n")
 }
 
+const serialize = Effect.fn("SessionCompaction.serialize")(function* (input: {
+  messages: MessageV2.WithParts[]
+  model: Provider.Model
+}) {
+  const messages = yield* MessageV2.toModelMessagesEffect(input.messages, input.model, {
+    stripMedia: true,
+    toolOutputMaxChars: TOOL_OUTPUT_MAX_CHARS,
+  })
+  return messages.length ? JSON.stringify(messages, null, 2) : undefined
+})
+
 function preserveRecentBudget(input: { cfg: Config.Info; model: Provider.Model }) {
   return (
     input.cfg.compaction?.preserve_recent_tokens ??
@@ -150,7 +170,6 @@ function turns(messages: MessageV2.WithParts[]) {
     result.push({
       start: i,
       end: messages.length,
-      id: msg.info.id,
     })
   }
   for (let i = 0; i < result.length - 1; i++) {
@@ -177,7 +196,6 @@ function splitTurn(input: {
       if (size > input.budget) continue
       return {
         start,
-        id: input.messages[start]!.info.id,
       } satisfies Tail
     }
     return undefined
@@ -244,8 +262,7 @@ export const layer: Layer.Layer<
       messages: MessageV2.WithParts[]
       model: Provider.Model
     }) {
-      const msgs = yield* MessageV2.toModelMessagesEffect(input.messages, input.model)
-      return Token.estimate(JSON.stringify(msgs))
+      return Token.estimate((yield* serialize(input)) ?? "")
     })
 
     const select = Effect.fn("SessionCompaction.select")(function* (input: {
@@ -254,10 +271,10 @@ export const layer: Layer.Layer<
       model: Provider.Model
     }) {
       const limit = input.cfg.compaction?.tail_turns ?? DEFAULT_TAIL_TURNS
-      if (limit <= 0) return { head: input.messages, tail_start_id: undefined }
+      if (limit <= 0) return { head: input.messages, tail: [] }
       const budget = preserveRecentBudget({ cfg: input.cfg, model: input.model })
       const all = turns(input.messages)
-      if (!all.length) return { head: input.messages, tail_start_id: undefined }
+      if (!all.length) return { head: input.messages, tail: [] }
       const recent = all.slice(-limit)
       const sizes = yield* Effect.forEach(
         recent,
@@ -276,7 +293,7 @@ export const layer: Layer.Layer<
         const size = sizes[i]
         if (total + size <= budget) {
           total += size
-          keep = { start: turn.start, id: turn.id }
+          keep = { start: turn.start }
           continue
         }
         const remaining = budget - total
@@ -292,10 +309,10 @@ export const layer: Layer.Layer<
         break
       }
 
-      if (!keep || keep.start === 0) return { head: input.messages, tail_start_id: undefined }
+      if (!keep) return { head: input.messages, tail: [] }
       return {
         head: input.messages.slice(0, keep.start),
-        tail_start_id: keep.id,
+        tail: input.messages.slice(keep.start),
       }
     })
 
@@ -406,7 +423,10 @@ export const layer: Layer.Layer<
         { sessionID: input.sessionID },
         { context: [], prompt: undefined },
       )
-      const nextPrompt = compacting.prompt ?? buildPrompt({ previousSummary, context: compacting.context })
+      const tailMessages = structuredClone(selected.tail)
+      yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: tailMessages })
+      const tail = yield* serialize({ messages: tailMessages, model })
+      const nextPrompt = compacting.prompt ?? buildPrompt({ previousSummary, context: compacting.context, tail })
       const msgs = structuredClone(selected.head)
       yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: msgs })
       const modelMessages = yield* MessageV2.toModelMessagesEffect(msgs, model, {
@@ -473,13 +493,6 @@ export const layer: Layer.Layer<
         return "stop"
       }
 
-      if (compactionPart && selected.tail_start_id && compactionPart.tail_start_id !== selected.tail_start_id) {
-        yield* session.updatePart({
-          ...compactionPart,
-          tail_start_id: selected.tail_start_id,
-        })
-      }
-
       if (result === "continue" && input.auto) {
         if (replay) {
           const original = replay.info
@@ -575,7 +588,6 @@ export const layer: Layer.Layer<
             sessionID: input.sessionID,
             timestamp: DateTime.makeUnsafe(Date.now()),
             text: summary ?? "",
-            include: selected.tail_start_id,
           })
         }
         yield* bus.publish(Event.Compacted, { sessionID: input.sessionID })