feat(compaction): preserve original user prompt across compaction

packages/opencode/src/server/routes/session.ts

@@ -535,6 +535,7 @@ export const SessionRoutes = lazy(() =>
             modelID: body.modelID,
           },
           auto: body.auto,
+          prompt: SessionCompaction.lastPrompt(msgs),
         })
         await SessionPrompt.loop({ sessionID })
         return c.json(true)

packages/opencode/src/session/compaction.ts

@@ -228,6 +228,23 @@ When constructing the summary, try to stick to this template:
     return "continue"
   }
 
+  export function lastPrompt(msgs: MessageV2.WithParts[]) {
+    const match = msgs.findLast((m) => m.info.role === "user" && m.parts.some((p) => p.type === "text" && !p.synthetic))
+    if (!match) return undefined
+    const text = match.parts
+      .filter((p): p is MessageV2.TextPart => p.type === "text" && !p.synthetic)
+      .map((p) => p.text)
+      .join("\n")
+    return text || undefined
+  }
+
+  const PROMPT_MAX = 2500
+
+  export function truncate(text: string, max = PROMPT_MAX) {
+    if (text.length <= max) return text
+    return text.slice(0, max) + "..."
+  }
+
   export const create = fn(
     z.object({
       sessionID: Identifier.schema("session"),
@@ -237,8 +254,10 @@ When constructing the summary, try to stick to this template:
         modelID: z.string(),
       }),
       auto: z.boolean(),
+      prompt: z.string().optional(),
     }),
     async (input) => {
+      const prompt = input.prompt ? truncate(input.prompt) : undefined
       const msg = await Session.updateMessage({
         id: Identifier.ascending("message"),
         role: "user",
@@ -255,6 +274,7 @@ When constructing the summary, try to stick to this template:
         sessionID: msg.sessionID,
         type: "compaction",
         auto: input.auto,
+        prompt,
       })
     },
   )

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

@@ -196,6 +196,7 @@ export namespace MessageV2 {
   export const CompactionPart = PartBase.extend({
     type: z.literal("compaction"),
     auto: z.boolean(),
+    prompt: z.string().optional(),
   }).meta({
     ref: "CompactionPart",
   })
@@ -573,7 +574,7 @@ export namespace MessageV2 {
           if (part.type === "compaction") {
             userMessage.parts.push({
               type: "text",
-              text: "What did we do so far?",
+              text: part.prompt ? `[Compacted]\n\nOriginal request: ${part.prompt}` : "[Compacted]",
             })
           }
           if (part.type === "subtask") {

packages/opencode/src/session/prompt.ts

@@ -549,6 +549,7 @@ export namespace SessionPrompt {
           agent: lastUser.agent,
           model: lastUser.model,
           auto: true,
+          prompt: SessionCompaction.lastPrompt(msgs),
         })
         continue
       }
@@ -709,6 +710,7 @@ export namespace SessionPrompt {
           agent: lastUser.agent,
           model: lastUser.model,
           auto: true,
+          prompt: SessionCompaction.lastPrompt(msgs),
         })
       }
       continue

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

@@ -1,6 +1,7 @@
 import { describe, expect, test } from "bun:test"
 import path from "path"
 import { SessionCompaction } from "../../src/session/compaction"
+import { MessageV2 } from "../../src/session/message-v2"
 import { Token } from "../../src/util/token"
 import { Instance } from "../../src/project/instance"
 import { Log } from "../../src/util/log"
@@ -227,6 +228,73 @@ describe("session.compaction.isOverflow", () => {
   })
 })
 
+describe("session.compaction.truncate", () => {
+  test("returns text unchanged when within limit", () => {
+    const text = "a".repeat(2500)
+    expect(SessionCompaction.truncate(text)).toBe(text)
+  })
+
+  test("truncates and appends ellipsis when over limit", () => {
+    const text = "a".repeat(2501)
+    const result = SessionCompaction.truncate(text)
+    expect(result).toBe("a".repeat(2500) + "...")
+    expect(result.length).toBe(2503)
+  })
+
+  test("respects custom max", () => {
+    expect(SessionCompaction.truncate("abcdef", 3)).toBe("abc...")
+  })
+
+  test("returns empty string as-is", () => {
+    expect(SessionCompaction.truncate("")).toBe("")
+  })
+})
+
+describe("session.compaction.lastPrompt", () => {
+  const user = (id: string, parts: MessageV2.Part[]) =>
+    ({
+      info: {
+        id,
+        role: "user",
+        sessionID: "s1",
+        time: { created: 0 },
+        agent: "user",
+        model: { providerID: "test", modelID: "test" },
+      },
+      parts,
+    }) as unknown as MessageV2.WithParts
+
+  const part = (id: string, msgID: string, text: string, synthetic?: boolean) =>
+    ({
+      id,
+      messageID: msgID,
+      sessionID: "s1",
+      type: "text",
+      text,
+      ...(synthetic ? { synthetic } : {}),
+    }) as MessageV2.Part
+
+  test("returns last non-synthetic user text", () => {
+    expect(SessionCompaction.lastPrompt([user("m1", [part("p1", "m1", "hello"), part("p2", "m1", "world")])])).toBe(
+      "hello\nworld",
+    )
+  })
+
+  test("skips synthetic text parts", () => {
+    expect(
+      SessionCompaction.lastPrompt([user("m1", [part("p1", "m1", "real"), part("p2", "m1", "synthetic", true)])]),
+    ).toBe("real")
+  })
+
+  test("returns undefined when no user messages", () => {
+    const msg = {
+      info: { id: "m1", role: "assistant", sessionID: "s1", time: { created: 0 } },
+      parts: [],
+    } as unknown as MessageV2.WithParts
+    expect(SessionCompaction.lastPrompt([msg])).toBeUndefined()
+  })
+})
+
 describe("util.token.estimate", () => {
   test("estimates tokens from text (4 chars per token)", () => {
     const text = "x".repeat(4000)

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

@@ -256,13 +256,38 @@ describe("session.message-v2.toModelMessage", () => {
             filename: "img.png",
             data: "https://example.com/img.png",
           },
-          { type: "text", text: "What did we do so far?" },
+          { type: "text", text: "[Compacted]" },
           { type: "text", text: "The following tool was executed by the user" },
         ],
       },
     ])
   })
 
+  test("compaction part with prompt includes original request", () => {
+    const messageID = "m-user"
+
+    const input: MessageV2.WithParts[] = [
+      {
+        info: userInfo(messageID),
+        parts: [
+          {
+            ...basePart(messageID, "p1"),
+            type: "compaction",
+            auto: true,
+            prompt: "fix the login bug",
+          },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    expect(MessageV2.toModelMessages(input, model)).toStrictEqual([
+      {
+        role: "user",
+        content: [{ type: "text", text: "[Compacted]\n\nOriginal request: fix the login bug" }],
+      },
+    ])
+  })
+
   test("converts assistant tool completion into tool-call + tool-result messages with attachments", () => {
     const userID = "m-user"
     const assistantID = "m-assistant"

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -505,6 +505,7 @@ export type CompactionPart = {
   messageID: string
   type: "compaction"
   auto: boolean
+  prompt?: string
 }
 
 export type Part =