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

Author: nexxeln

packages/opencode/src/config/config.ts

@@ -273,10 +273,10 @@ export const Info = Schema.Struct({
       }),
       tail_turns: Schema.optional(NonNegativeInt).annotate({
         description:
-          "Number of recent user turns, including their following assistant/tool responses, to keep verbatim during compaction (default: 2)",
+          "Number of recent user turns, including their following assistant/tool responses, to serialize into the compaction summary (default: 2)",
       }),
       preserve_recent_tokens: Schema.optional(NonNegativeInt).annotate({
-        description: "Maximum number of tokens from recent turns to preserve verbatim after compaction",
+        description: "Maximum number of tokens from recent turns to serialize into the compaction summary",
       }),
       reserved: Schema.optional(NonNegativeInt).annotate({
         description: "Token buffer for compaction. Leaves enough window to avoid overflow during compaction.",

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 })

packages/opencode/src/session/message-v2.ts

@@ -840,12 +840,13 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
         return part.metadata?.anthropic?.signature != null
       })
       for (const part of msg.parts) {
+        if (msg.info.summary && part.type !== "text") continue
         if (part.type === "text") {
           const text = part.text === "" && hasSignedReasoning ? " " : part.text
           assistantMessage.parts.push({
             type: "text",
             text,
-            ...(differentModel ? {} : { providerMetadata: part.metadata }),
+            ...(differentModel || msg.info.summary ? {} : { providerMetadata: part.metadata }),
           })
         }
         if (part.type === "step-start")
@@ -1071,53 +1072,16 @@ export function get(input: { sessionID: SessionID; messageID: MessageID }): With
 export function filterCompacted(msgs: Iterable<WithParts>) {
   const result = [] as WithParts[]
   const completed = new Set<string>()
-  let retain: MessageID | undefined
   for (const msg of msgs) {
     result.push(msg)
-    if (retain) {
-      if (msg.info.id === retain) break
-      continue
-    }
     if (msg.info.role === "user" && completed.has(msg.info.id)) {
-      const part = msg.parts.find((item): item is CompactionPart => item.type === "compaction")
-      if (!part) continue
-      if (!part.tail_start_id) break
-      retain = part.tail_start_id
-      if (msg.info.id === retain) break
+      if (msg.parts.some((item): item is CompactionPart => item.type === "compaction")) break
       continue
     }
-    if (msg.info.role === "user" && completed.has(msg.info.id) && msg.parts.some((part) => part.type === "compaction"))
-      break
     if (msg.info.role === "assistant" && msg.info.summary && msg.info.finish && !msg.info.error)
       completed.add(msg.info.parentID)
   }
   result.reverse()
-  const compactionIndex = result.findLastIndex(
-    (msg) =>
-      msg.info.role === "user" &&
-      msg.parts.some((item): item is CompactionPart => item.type === "compaction" && item.tail_start_id !== undefined),
-  )
-  const compaction = result[compactionIndex]
-  const part = compaction?.parts.find(
-    (item): item is CompactionPart => item.type === "compaction" && item.tail_start_id !== undefined,
-  )
-  const summaryIndex = compaction
-    ? result.findIndex(
-        (msg, index) =>
-          index > compactionIndex &&
-          msg.info.role === "assistant" &&
-          msg.info.summary &&
-          msg.info.parentID === compaction.info.id,
-      )
-    : -1
-  const tailIndex = part?.tail_start_id ? result.findIndex((msg) => msg.info.id === part.tail_start_id) : -1
-  if (tailIndex >= 0 && tailIndex < compactionIndex && summaryIndex > compactionIndex) {
-    return [
-      ...result.slice(compactionIndex, summaryIndex + 1),
-      ...result.slice(tailIndex, compactionIndex),
-      ...result.slice(summaryIndex + 1),
-    ]
-  }
   return result
 }
 

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

@@ -926,12 +926,12 @@ describe("session.compaction.process", () => {
   )
 
   itCompaction.instance(
-    "persists tail_start_id for retained recent turns",
+    "does not persist tail_start_id for serialized recent turns",
     Effect.gen(function* () {
       const ssn = yield* SessionNs.Service
       const session = yield* ssn.create({})
       yield* createUserMessage(session.id, "first")
-      const keep = yield* createUserMessage(session.id, "second")
+      yield* createUserMessage(session.id, "second")
       yield* createUserMessage(session.id, "third")
       yield* createSummaryCompaction(session.id)
 
@@ -947,18 +947,18 @@ describe("session.compaction.process", () => {
 
       const part = yield* readCompactionPart(session.id)
       expect(part?.type).toBe("compaction")
-      expect(part?.tail_start_id).toBe(keep.id)
+      expect(part?.tail_start_id).toBeUndefined()
     }).pipe(withCompaction({ config: cfg({ tail_turns: 2, preserve_recent_tokens: 10_000 }) })),
   )
 
   itCompaction.instance(
-    "shrinks retained tail to fit preserve token budget",
+    "does not persist tail_start_id when shrinking serialized tail",
     Effect.gen(function* () {
       const ssn = yield* SessionNs.Service
       const session = yield* ssn.create({})
       yield* createUserMessage(session.id, "first")
       yield* createUserMessage(session.id, "x".repeat(2_000))
-      const keep = yield* createUserMessage(session.id, "tiny")
+      yield* createUserMessage(session.id, "tiny")
       yield* createSummaryCompaction(session.id)
 
       const msgs = yield* ssn.messages({ sessionID: session.id })
@@ -973,7 +973,7 @@ describe("session.compaction.process", () => {
 
       const part = yield* readCompactionPart(session.id)
       expect(part?.type).toBe("compaction")
-      expect(part?.tail_start_id).toBe(keep.id)
+      expect(part?.tail_start_id).toBeUndefined()
     }).pipe(withCompaction({ config: cfg({ tail_turns: 2, preserve_recent_tokens: 100 }) })),
   )
 
@@ -1005,7 +1005,7 @@ describe("session.compaction.process", () => {
   )
 
   itCompaction.instance(
-    "falls back to full summary when retained tail media exceeds preserve token budget",
+    "serializes retained tail media as text in the summary input",
     () => {
       const stub = llm()
       let captured = ""
@@ -1078,15 +1078,16 @@ describe("session.compaction.process", () => {
 
         const part = yield* readCompactionPart(session.id)
         expect(part?.type).toBe("compaction")
-        expect(part?.tail_start_id).toBe(keep.id)
+        expect(part?.tail_start_id).toBeUndefined()
         expect(captured).toContain("zzzz")
-        expect(captured).not.toContain("keep tail")
+        expect(captured).toContain("keep tail")
 
         const filtered = MessageV2.filterCompacted(MessageV2.stream(session.id))
-        expect(filtered.map((msg) => msg.info.id).slice(0, 3)).toEqual([parent!, expect.any(String), keep.id])
+        expect(filtered.map((msg) => msg.info.id)).toEqual([parent!, expect.any(String)])
         expect(filtered[1]?.info.role).toBe("assistant")
         expect(filtered[1]?.info.role === "assistant" ? filtered[1].info.summary : false).toBe(true)
         expect(filtered.map((msg) => msg.info.id)).not.toContain(large.id)
+        expect(filtered.map((msg) => msg.info.id)).not.toContain(keep.id)
       }).pipe(withCompaction({ llm: stub.layer, config: cfg({ tail_turns: 1, preserve_recent_tokens: 100 }) }))
     },
     { git: true },
@@ -1353,13 +1354,13 @@ describe("session.compaction.process", () => {
   )
 
   itCompaction.instance(
-    "summarizes only the head while keeping recent tail out of summary input",
+    "summarizes the head while serializing recent tail into summary input",
     () => {
       const stub = llm()
-      let captured = ""
+      let captured: LLM.StreamInput["messages"] = []
       stub.push(
         reply("summary", (input) => {
-          captured = JSON.stringify(input.messages)
+          captured = input.messages
         }),
       )
       return Effect.gen(function* () {
@@ -1380,10 +1381,15 @@ describe("session.compaction.process", () => {
           auto: false,
         })
 
-        expect(captured).toContain("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?")
+        const head = JSON.stringify(captured.slice(0, -1))
+        const prompt = JSON.stringify(captured.at(-1))
+        expect(head).toContain("older context")
+        expect(head).not.toContain("keep this turn")
+        expect(head).not.toContain("and this one too")
+        expect(prompt).toContain("keep this turn")
+        expect(prompt).toContain("and this one too")
+        expect(prompt).toContain("recent-conversation-tail")
+        expect(prompt).not.toContain("What did we do so far?")
       }).pipe(withCompaction({ llm: stub.layer }))
     },
     { git: true },
@@ -1431,7 +1437,7 @@ describe("session.compaction.process", () => {
     { git: true },
   )
 
-  itCompaction.instance("keeps recent pre-compaction turns across repeated compactions", () => {
+  itCompaction.instance("does not replay recent pre-compaction turns across repeated compactions", () => {
     const stub = llm()
     stub.push(reply("summary one"))
     stub.push(reply("summary two"))
@@ -1462,8 +1468,8 @@ describe("session.compaction.process", () => {
 
       expect(ids).not.toContain(u1.id)
       expect(ids).not.toContain(u2.id)
-      expect(ids).toContain(u3.id)
-      expect(ids).toContain(u4.id)
+      expect(ids).not.toContain(u3.id)
+      expect(ids).not.toContain(u4.id)
       expect(filtered.some((msg) => msg.info.role === "assistant" && msg.info.summary)).toBe(true)
       expect(
         filtered.some((msg) => msg.info.role === "user" && msg.parts.some((part) => part.type === "compaction")),
@@ -1472,7 +1478,7 @@ describe("session.compaction.process", () => {
   })
 
   itCompaction.instance(
-    "ignores previous summaries when sizing the retained tail",
+    "ignores previous summaries when sizing the serialized tail",
     Effect.gen(function* () {
       const ssn = yield* SessionNs.Service
       const test = yield* TestInstance
@@ -1511,7 +1517,7 @@ describe("session.compaction.process", () => {
 
       const part = yield* readCompactionPart(session.id)
       expect(part?.type).toBe("compaction")
-      expect(part?.tail_start_id).toBe(keep.id)
+      expect(part?.tail_start_id).toBeUndefined()
     }).pipe(withCompaction({ config: cfg({ tail_turns: 2, preserve_recent_tokens: 500 }) })),
   )
 })