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,