fix(session): simplify compaction failure message

packages/opencode/src/session/prompt.ts

@@ -82,6 +82,28 @@ const STRUCTURED_OUTPUT_SYSTEM_PROMPT = `IMPORTANT: The user has requested struc
 
 const log = Log.create({ service: "session.prompt" })
 const elog = EffectLogger.create({ service: "session.prompt" })
+const COMPACTION_WITHOUT_TAIL = "none"
+
+function latestAutoCompactionBoundary(messages: MessageV2.WithParts[]) {
+  const users = new Map(
+    messages.flatMap((message) => {
+      if (message.info.role !== "user") return []
+      const part = message.parts.find((item): item is MessageV2.CompactionPart => item.type === "compaction" && item.auto)
+      return part ? [[message.info.id, part] as const] : []
+    }),
+  )
+  const latest = messages
+    .flatMap((message) => {
+      if (message.info.role !== "assistant") return []
+      if (!message.info.summary || !message.info.finish || message.info.error) return []
+      if (!users.has(message.info.parentID)) return []
+      return [message.info]
+    })
+    .sort((a, b) => a.id.localeCompare(b.id))
+    .at(-1)
+  if (!latest) return
+  return users.get(latest.parentID)?.tail_start_id ?? COMPACTION_WITHOUT_TAIL
+}
 
 type ReferencePromptMetadata = {
   name: string
@@ -1646,8 +1668,66 @@ NOTE: At any point in time through this workflow you should feel free to ask the
         const slog = elog.with({ sessionID })
         let structured: unknown
         let step = 0
+        let pendingAutoCompactionBoundary = latestAutoCompactionBoundary(yield* MessageV2.filterCompactedEffect(sessionID))
+        let awaitingAutoCompactionProgress = false
         const session = yield* sessions.get(sessionID).pipe(Effect.orDie)
 
+        const requestAutoCompaction = Effect.fn("SessionPrompt.requestAutoCompaction")(function* (input: {
+          lastUser: MessageV2.User
+          model: Provider.Model
+          messages: MessageV2.WithParts[]
+          overflow?: boolean
+          assistant?: MessageV2.Assistant
+        }) {
+          const boundary = latestAutoCompactionBoundary(input.messages)
+          // A completed auto-compaction only counts as progress when it advances
+          // the retained tail boundary. Otherwise another auto-trigger would loop.
+          if (!awaitingAutoCompactionProgress || boundary !== pendingAutoCompactionBoundary) {
+            pendingAutoCompactionBoundary = boundary
+            awaitingAutoCompactionProgress = true
+            yield* compaction.create({
+              sessionID,
+              agent: input.lastUser.agent,
+              model: input.lastUser.model,
+              auto: true,
+              overflow: input.overflow,
+            })
+            return true
+          }
+
+          const error = new MessageV2.ContextOverflowError({
+            message:
+              "Automatic compaction could not reduce this session enough to continue. Please start a new session.",
+          }).toObject()
+          yield* slog.info("auto compaction blocked", { boundary })
+          if (input.assistant) {
+            input.assistant.error = error
+            input.assistant.finish = "error"
+            input.assistant.time.completed ??= Date.now()
+            yield* sessions.updateMessage(input.assistant)
+          } else {
+            yield* sessions.updateMessage({
+              id: MessageID.ascending(),
+              parentID: input.lastUser.id,
+              role: "assistant",
+              mode: input.lastUser.agent,
+              agent: input.lastUser.agent,
+              variant: input.lastUser.model.variant,
+              path: { cwd: ctx.directory, root: ctx.worktree },
+              cost: 0,
+              tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+              modelID: input.model.id,
+              providerID: input.model.providerID,
+              time: { created: Date.now(), completed: Date.now() },
+              sessionID,
+              finish: "error",
+              error,
+            })
+          }
+          yield* bus.publish(Session.Event.Error, { sessionID, error })
+          return false
+        })
+
         while (true) {
           yield* status.set(sessionID, { type: "busy" })
           yield* slog.info("loop", { step })
@@ -1712,8 +1792,8 @@ NOTE: At any point in time through this workflow you should feel free to ask the
             lastFinished.summary !== true &&
             (yield* compaction.isOverflow({ tokens: lastFinished.tokens, model }))
           ) {
-            yield* compaction.create({ sessionID, agent: lastUser.agent, model: lastUser.model, auto: true })
-            continue
+            if (yield* requestAutoCompaction({ lastUser, model, messages: msgs })) continue
+            break
           }
 
           const agent = yield* agents.get(lastUser.agent)
@@ -1852,13 +1932,14 @@ NOTE: At any point in time through this workflow you should feel free to ask the
 
             if (result === "stop") return "break" as const
             if (result === "compact") {
-              yield* compaction.create({
-                sessionID,
-                agent: lastUser.agent,
-                model: lastUser.model,
-                auto: true,
+              const created = yield* requestAutoCompaction({
+                lastUser,
+                model,
+                messages: msgs,
+                assistant: handle.message,
                 overflow: !handle.message.finish,
               })
+              if (!created) return "break" as const
             }
             return "continue" as const
           }).pipe(

packages/opencode/test/session/prompt.test.ts

@@ -163,7 +163,37 @@ const blockingProcessor = Layer.succeed(
   }),
 )
 
-function makeHttp(input?: { processor?: "blocking" }) {
+const compactLoopProcessor = Layer.effect(
+  SessionProcessor.Service,
+  Effect.gen(function* () {
+    const sessions = yield* Session.Service
+    return SessionProcessor.Service.of({
+      create: (input) =>
+        Effect.succeed({
+          message: input.assistantMessage,
+          updateToolCall: () => Effect.succeed(undefined),
+          completeToolCall: () => Effect.void,
+          process: () =>
+            Effect.gen(function* () {
+              if (!input.assistantMessage.summary) return "compact" as const
+              input.assistantMessage.finish = "stop"
+              input.assistantMessage.time.completed = Date.now()
+              yield* sessions.updateMessage(input.assistantMessage)
+              yield* sessions.updatePart({
+                id: PartID.ascending(),
+                messageID: input.assistantMessage.id,
+                sessionID: input.assistantMessage.sessionID,
+                type: "text",
+                text: "summary",
+              })
+              return "continue" as const
+            }),
+        }),
+    })
+  }),
+)
+
+function makeHttp(input?: { processor?: "blocking" | "compact-loop" }) {
   const deps = Layer.mergeAll(
     Session.defaultLayer,
     Snapshot.defaultLayer,
@@ -199,15 +229,21 @@ function makeHttp(input?: { processor?: "blocking" }) {
     Layer.provideMerge(deps),
   )
   const trunc = Truncate.layer.pipe(Layer.provideMerge(deps))
-  const proc =
-    input?.processor === "blocking"
-      ? blockingProcessor
-      : SessionProcessor.layer.pipe(
+  const proc = (() => {
+    switch (input?.processor) {
+      case "blocking":
+        return blockingProcessor
+      case "compact-loop":
+        return compactLoopProcessor.pipe(Layer.provideMerge(deps))
+      default:
+        return SessionProcessor.layer.pipe(
           Layer.provide(summary),
           Layer.provide(Image.defaultLayer),
           Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
           Layer.provideMerge(deps),
         )
+    }
+  })()
   const compact = SessionCompaction.layer.pipe(
     Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
     Layer.provideMerge(proc),
@@ -235,6 +271,7 @@ function makeHttp(input?: { processor?: "blocking" }) {
 
 const it = testEffect(makeHttp())
 const race = testEffect(makeHttp({ processor: "blocking" }))
+const compactLoop = testEffect(makeHttp({ processor: "compact-loop" }))
 const unix = process.platform !== "win32" ? it.instance : it.instance.skip
 
 // Config that registers a custom "test" provider with a "test-model" model
@@ -477,6 +514,34 @@ it.instance(
   { git: true },
 )
 
+compactLoop.instance(
+  "loop stops when auto-compaction does not make progress",
+  () =>
+    Effect.gen(function* () {
+      yield* useServerConfig(providerCfg)
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const chat = yield* sessions.create({ title: "Pinned" })
+      yield* prompt.prompt({
+        sessionID: chat.id,
+        agent: "build",
+        noReply: true,
+        parts: [{ type: "text", text: "keep going until compaction fails" }],
+      })
+
+      const result = yield* prompt.loop({ sessionID: chat.id })
+      expect(result.info.role).toBe("assistant")
+      if (result.info.role === "assistant") {
+        expect(result.info.finish).toBe("error")
+        expect(result.info.error?.name).toBe("ContextOverflowError")
+      }
+
+      const messages = yield* sessions.messages({ sessionID: chat.id })
+      expect(messages.flatMap((message) => message.parts).filter((part) => part.type === "compaction")).toHaveLength(2)
+    }),
+  { git: true },
+)
+
 it.instance(
   "prompt emits v2 prompted and synthetic events",
   () =>