fix(compaction): budget retained tail with media

4 files changed, 83 insertions, 29 deletions

diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index b4e6268b1..97e96ccbf 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -1009,13 +1009,15 @@ export const Info = z
           .int()
           .min(0)
           .optional()
-          .describe("Number of recent real user turns to keep verbatim during compaction (default: 2)"),
+          .describe(
+            "Number of recent user turns, including their following assistant/tool responses, to keep verbatim during compaction (default: 2)",
+          ),
         tail_tokens: z
           .number()
           .int()
           .min(0)
           .optional()
-          .describe("Token budget for retained recent turns during compaction"),
+          .describe("Token budget for retained recent turn spans during compaction"),
         reserved: z
           .number()
           .int()
diff --git a/packages/opencode/src/session/compaction.ts b/packages/opencode/src/session/compaction.ts
index 325eb5a3d..ff2b316c4 100644
--- a/packages/opencode/src/session/compaction.ts
+++ b/packages/opencode/src/session/compaction.ts
@@ -2,7 +2,7 @@ import { BusEvent } from "@/bus/bus-event"
 import { Bus } from "@/bus"
 import * as Session from "./session"
 import { SessionID, MessageID, PartID } from "./schema"
-import { Provider, ProviderTransform } from "../provider"
+import { Provider } from "../provider"
 import { MessageV2 } from "./message-v2"
 import z from "zod"
 import { Token } from "../util"
@@ -17,7 +17,7 @@ import { Effect, Layer, Context } from "effect"
 import { InstanceState } from "@/effect"
 import { makeRuntime } from "@/effect/run-service"
 import { fn } from "@/util/fn"
-import { isOverflow as overflow } from "./overflow"
+import { isOverflow as overflow, usable } from "./overflow"
 
 export namespace SessionCompaction {
   const log = Log.create({ service: "session.compaction" })
@@ -43,13 +43,6 @@ export namespace SessionCompaction {
     id: MessageID
   }
 
-  function usable(input: { cfg: Config.Info; model: Provider.Model }) {
-    const reserved = input.cfg.compaction?.reserved ?? Math.min(20_000, ProviderTransform.maxOutputTokens(input.model))
-    return input.model.limit.input
-      ? Math.max(0, input.model.limit.input - reserved)
-      : Math.max(0, input.model.limit.context - ProviderTransform.maxOutputTokens(input.model))
-  }
-
   function tailBudget(input: { cfg: Config.Info; model: Provider.Model }) {
     return (
       input.cfg.compaction?.tail_tokens ??
@@ -131,7 +124,7 @@ export namespace SessionCompaction {
         messages: MessageV2.WithParts[]
         model: Provider.Model
       }) {
-        const msgs = yield* MessageV2.toModelMessagesEffect(input.messages, input.model, { stripMedia: true })
+        const msgs = yield* MessageV2.toModelMessagesEffect(input.messages, input.model)
         return Token.estimate(JSON.stringify(msgs))
       })
 
@@ -282,14 +275,7 @@ export namespace SessionCompaction {
           { sessionID: input.sessionID },
           { context: [], prompt: undefined },
         )
-        const defaultPrompt = `Summarize the older conversation history so another agent can continue the work with the retained recent turns.
-The most recent conversation turns will remain verbatim outside this summary, so focus on older context that is still needed to understand and continue the work.
-Include what we did, what we're doing, which files we're working on, and what we're going to do next.
-The summary that you construct will be used so that another agent can read it and continue the work.
-Do not call any tools. Respond only with the summary text.
-Respond in the same language as the user's messages in the conversation.
-
-When constructing the summary, try to stick to this template:
+        const defaultPrompt = `When constructing the summary, try to stick to this template:
 ---
 ## Goal
 
diff --git a/packages/opencode/src/session/overflow.ts b/packages/opencode/src/session/overflow.ts
index 6f48a760d..477b5815b 100644
--- a/packages/opencode/src/session/overflow.ts
+++ b/packages/opencode/src/session/overflow.ts
@@ -5,18 +5,22 @@ import type { MessageV2 } from "./message-v2"
 
 const COMPACTION_BUFFER = 20_000
 
+export function usable(input: { cfg: Config.Info; model: Provider.Model }) {
+  const context = input.model.limit.context
+  if (context === 0) return 0
+
+  const reserved =
+    input.cfg.compaction?.reserved ?? Math.min(COMPACTION_BUFFER, ProviderTransform.maxOutputTokens(input.model))
+  return input.model.limit.input
+    ? Math.max(0, input.model.limit.input - reserved)
+    : Math.max(0, context - ProviderTransform.maxOutputTokens(input.model))
+}
+
 export function isOverflow(input: { cfg: Config.Info; tokens: MessageV2.Assistant["tokens"]; model: Provider.Model }) {
   if (input.cfg.compaction?.auto === false) return false
-  const context = input.model.limit.context
-  if (context === 0) return false
+  if (input.model.limit.context === 0) return false
 
   const count =
     input.tokens.total || input.tokens.input + input.tokens.output + input.tokens.cache.read + input.tokens.cache.write
-
-  const reserved =
-    input.cfg.compaction?.reserved ?? Math.min(COMPACTION_BUFFER, ProviderTransform.maxOutputTokens(input.model))
-  const usable = input.model.limit.input
-    ? input.model.limit.input - reserved
-    : context - ProviderTransform.maxOutputTokens(input.model)
-  return count >= usable
+  return count >= usable(input)
 }
diff --git a/packages/opencode/test/session/compaction.test.ts b/packages/opencode/test/session/compaction.test.ts
index 85bc6e54a..015a1653a 100644
--- a/packages/opencode/test/session/compaction.test.ts
+++ b/packages/opencode/test/session/compaction.test.ts
@@ -1044,6 +1044,68 @@ describe("session.compaction.process", () => {
     })
   })
 
+  test("falls back to full summary when retained tail media exceeds tail budget", async () => {
+    await using tmp = await tmpdir({ git: true })
+    const stub = llm()
+    let captured = ""
+    stub.push(
+      reply("summary", (input) => {
+        captured = JSON.stringify(input.messages)
+      }),
+    )
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const session = await svc.create({})
+        await user(session.id, "older")
+        const recent = await user(session.id, "recent image turn")
+        await svc.updatePart({
+          id: PartID.ascending(),
+          messageID: recent.id,
+          sessionID: session.id,
+          type: "file",
+          mime: "image/png",
+          filename: "big.png",
+          url: `data:image/png;base64,${"a".repeat(4_000)}`,
+        })
+        await SessionCompaction.create({
+          sessionID: session.id,
+          agent: "build",
+          model: ref,
+          auto: false,
+        })
+
+        const rt = liveRuntime(stub.layer, wide(), cfg({ tail_turns: 1, tail_tokens: 100 }))
+        try {
+          const msgs = await svc.messages({ sessionID: session.id })
+          const parent = msgs.at(-1)?.info.id
+          expect(parent).toBeTruthy()
+          await rt.runPromise(
+            SessionCompaction.Service.use((svc) =>
+              svc.process({
+                parentID: parent!,
+                messages: msgs,
+                sessionID: session.id,
+                auto: false,
+              }),
+            ),
+          )
+
+          const part = (await svc.messages({ sessionID: session.id }))
+            .at(-2)
+            ?.parts.find((item) => item.type === "compaction")
+
+          expect(part?.type).toBe("compaction")
+          if (part?.type === "compaction") expect(part.tail_start_id).toBeUndefined()
+          expect(captured).toContain("recent image turn")
+          expect(captured).toContain("Attached image/png: big.png")
+        } finally {
+          await rt.dispose()
+        }
+      },
+    })
+  })
+
   test("allows plugins to disable synthetic continue prompt", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({