fix(opencode): serialize orphaned compaction history (#40800)

Co-authored-by: Aiden Cline <63023139+rekram1-node@users.noreply.github.com>

packages/opencode/src/session/compaction.ts

@@ -49,6 +49,42 @@ type CompletedCompaction = {
   summary: string | undefined
 }
 
+const truncate = (value: string) =>
+  value.length <= TOOL_OUTPUT_MAX_CHARS ? value : `${value.slice(0, TOOL_OUTPUT_MAX_CHARS)}\n[truncated]`
+
+const serialize = (message: SessionV1.WithParts) => {
+  if (message.info.role === "user") {
+    const text = message.parts
+      .filter((part): part is SessionV1.TextPart => part.type === "text" && !part.ignored)
+      .map((part) => part.text)
+      .filter(Boolean)
+      .join("\n")
+    const files = message.parts.flatMap((part) =>
+      part.type === "file" ? [`[Attached ${part.mime}: ${part.filename ?? "file"}]`] : [],
+    )
+    return [...(text ? [`[User]: ${text}`] : []), ...files].join("\n")
+  }
+  return message.parts
+    .flatMap((part) => {
+      if (part.type === "text") return part.text ? [`[Assistant]: ${part.text}`] : []
+      if (part.type === "reasoning") return part.text ? [`[Assistant reasoning]: ${part.text}`] : []
+      if (part.type !== "tool") return []
+      const call = `[Assistant tool call]: ${part.tool}(${JSON.stringify(part.state.input)})`
+      if (part.state.status === "completed") {
+        const attachments = (part.state.attachments ?? []).map(
+          (item) => `[Attached ${item.mime}: ${item.filename ?? "file"}]`,
+        )
+        const output = part.state.time.compacted
+          ? "[Old tool result content cleared]"
+          : truncate([part.state.output, ...attachments].join("\n"))
+        return [call, `[Tool result]: ${output}`]
+      }
+      if (part.state.status === "error") return [call, `[Tool error]: ${part.state.error}`]
+      return [call]
+    })
+    .join("\n")
+}
+
 function summaryText(message: SessionV1.WithParts) {
   const text = message.parts
     .filter((part): part is SessionV1.TextPart => part.type === "text")
@@ -348,10 +384,7 @@ const layer = Layer.effect(
       const nextPrompt = compacting.prompt ?? buildPrompt({ previousSummary, context: compacting.context })
       const msgs = structuredClone(selected.head)
       yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: msgs })
-      const modelMessages = yield* MessageV2.toModelMessagesEffect(msgs, model, {
-        stripMedia: true,
-        toolOutputMaxChars: TOOL_OUTPUT_MAX_CHARS,
-      })
+      const conversation = msgs.map(serialize).filter(Boolean).join("\n\n")
       const ctx = yield* InstanceState.context
       const msg: SessionV1.Assistant = {
         id: MessageID.ascending(),
@@ -392,10 +425,16 @@ const layer = Layer.effect(
         tools: {},
         system: [],
         messages: [
-          ...modelMessages,
           {
             role: "user",
-            content: [{ type: "text", text: nextPrompt }],
+            content: [
+              {
+                type: "text",
+                text: [nextPrompt, "The following is the conversation history:", conversation]
+                  .filter(Boolean)
+                  .join("\n\n"),
+              },
+            ],
           },
         ],
         model,

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

@@ -1362,10 +1362,10 @@ describe("session.compaction.process", () => {
     "summarizes only the head while keeping recent tail out of summary input",
     () => {
       const stub = llm()
-      let captured = ""
+      let messages: LLM.StreamInput["messages"] = []
       stub.push(
         reply("summary", (input) => {
-          captured = JSON.stringify(input.messages)
+          messages = input.messages
         }),
       )
       return Effect.gen(function* () {
@@ -1386,7 +1386,10 @@ describe("session.compaction.process", () => {
           auto: false,
         })
 
-        expect(captured).toContain("older context")
+        const captured = JSON.stringify(messages)
+        expect(messages).toHaveLength(1)
+        expect(messages[0]?.role).toBe("user")
+        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?")
@@ -1437,6 +1440,74 @@ describe("session.compaction.process", () => {
     { git: true },
   )
 
+  itCompaction.instance(
+    "serializes repeated compaction history as one user message",
+    () => {
+      const stub = llm()
+      let captured: LLM.StreamInput["messages"] = []
+      stub.push(
+        reply("summary two", (input) => {
+          captured = input.messages
+        }),
+      )
+
+      return Effect.gen(function* () {
+        const ssn = yield* SessionNs.Service
+        const test = yield* TestInstance
+        const session = yield* ssn.create({})
+        const turn = yield* createUserMessage(session.id, "original request")
+        const kept = yield* createAssistantMessage(session.id, turn.id, test.directory)
+        yield* ssn.updatePart({
+          id: PartID.ascending(),
+          messageID: kept.id,
+          sessionID: session.id,
+          type: "tool",
+          callID: "read-call",
+          tool: "read",
+          state: {
+            status: "completed",
+            input: { filePath: "src/index.ts" },
+            output: "file contents",
+            title: "src/index.ts",
+            metadata: {},
+            time: { start: Date.now(), end: Date.now() },
+          },
+        })
+
+        const previous = yield* ssn.updateMessage({
+          id: MessageID.ascending(),
+          role: "user",
+          model: ref,
+          sessionID: session.id,
+          agent: "build",
+          time: { created: Date.now() },
+        })
+        yield* ssn.updatePart({
+          id: PartID.ascending(),
+          messageID: previous.id,
+          sessionID: session.id,
+          type: "compaction",
+          auto: false,
+          tail_start_id: kept.id,
+        })
+        yield* createSummaryAssistantMessage(session.id, previous.id, test.directory, "summary one")
+        yield* createCompactionMarker(session.id)
+
+        const msgs = MessageV2.filterCompacted(yield* MessageV2.stream(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).toHaveLength(1)
+        expect(captured[0]?.role).toBe("user")
+        expect(JSON.stringify(captured)).toContain('[Assistant tool call]: read({\\"filePath\\":\\"src/index.ts\\"})')
+        expect(JSON.stringify(captured)).toContain("[Tool result]: file contents")
+        expect(JSON.stringify(captured)).not.toContain('\\"role\\":\\"assistant\\"')
+      }).pipe(withCompaction({ llm: stub.llmLayer, config: cfg({ tail_turns: 0 }) }))
+    },
+    { git: true },
+  )
+
   itCompaction.instance("keeps recent pre-compaction turns across repeated compactions", () => {
     const stub = llm()
     stub.push(reply("summary one"))