refactor: deduplicate compaction prompt between system and user messages

Move the authoritative structured summary template into the system prompt
(compaction.txt) and simplify the inline defaultPrompt in compaction.ts to
a short user-side reminder. This eliminates the duplicated section headings
and template structure that existed in both places.

Author: jwcrystal

packages/opencode/src/agent/prompt/compaction.txt

@@ -1,14 +1,25 @@
-You are a helpful AI assistant tasked with summarizing conversations.
+You are summarizing a conversation for continuation by another agent.
 
-When asked to summarize, provide a detailed but concise summary of the conversation.
-Focus on information that would be helpful for continuing the conversation, including:
-- What was done
-- What is currently being worked on
-- Which files are being modified
-- What needs to be done next
-- Key user requests, constraints, or preferences that should persist
-- Important technical decisions and why they were made
+Produce a structured summary with these sections:
 
-Your summary should be comprehensive enough to provide context but concise enough to be quickly understood.
+## Goal
+[What goal(s) is the user trying to accomplish?]
 
-Do not respond to any questions in the conversation, only output the summary.
+## Instructions
+[Important instructions from the user. Include plan/spec details if any.]
+
+## Discoveries
+[Notable findings that would help the next agent continue]
+
+## Accomplished
+[What has been completed, what is in progress, what remains]
+
+## Relevant files
+[Structured list of files read, edited, or created]
+
+## Unresolved issues
+[Any errors or blockers that need attention]
+
+Be thorough but concise. Focus on information critical for continuation.
+Do not respond to questions in the conversation. Output only the summary.
+Do not call any tools.

packages/opencode/src/session/compaction.ts

@@ -21,7 +21,7 @@ import { InstanceState } from "@/effect/instance-state"
 import { isOverflow as overflow } from "./overflow"
 import { SessionCompactionPolicy } from "./compaction-policy"
 
-  export namespace SessionCompaction {
+export namespace SessionCompaction {
   const log = Log.create({ service: "session.compaction" })
 
   const COMPACTION_MAX_RETRIES = 2
@@ -68,6 +68,7 @@ import { SessionCompactionPolicy } from "./compaction-policy"
         if (part.state.time.compacted) continue
         for (const att of part.state.attachments ?? []) {
           if (MessageV2.isMedia(att.mime)) continue
+          if (att.mime !== "application/x-directory" && !att.mime.startsWith("text/")) continue
           if (result.length >= POST_COMPACT_MAX_ATTACHMENTS) return result
           result.push(att)
         }
@@ -241,34 +242,11 @@ import { SessionCompactionPolicy } from "./compaction-policy"
           { sessionID: input.sessionID },
           { context: [], prompt: undefined },
         )
-        const defaultPrompt = `Provide a detailed prompt for continuing our conversation above.
-Focus on information that would be helpful for continuing the conversation, including 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.
-
-When constructing the summary, try to stick to this template:
----
-## Goal
-
-[What goal(s) is the user trying to accomplish?]
-
-## Instructions
-
-- [What important instructions did the user give you that are relevant]
-- [If there is a plan or spec, include information about it so next agent can continue using it]
-
-## Discoveries
-
-[What notable things were learned during this conversation that would be useful for the next agent to know when continuing the work]
-
-## Accomplished
-
-[What work has been completed, what work is still in progress, and what work is left?]
-
-## Relevant files / directories
-
-[Construct a structured list of relevant files that have been read, edited, or created that pertain to the task at hand. If all the files in a directory are relevant, include the path to the directory.]
----`
+        const defaultPrompt = [
+          "Summarize the conversation for another agent continuing this work.",
+          "Focus on goals, constraints, discoveries, progress, relevant files, and unresolved issues.",
+          "Do not call tools. Output only the summary.",
+        ].join("\n")
 
         const prompt = compacting.prompt ?? [defaultPrompt, ...compacting.context].join("\n\n")
         const ctx = yield* InstanceState.context

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

@@ -21,8 +21,7 @@ import { LLM } from "../../src/session/llm"
 import { MessageV2 } from "../../src/session/message-v2"
 import { AppFileSystem } from "../../src/filesystem"
 import { SessionCompaction } from "../../src/session/compaction"
-import { Instruction } from "../../src/instruction"
-import { Session } from "../../src/session"
+import { Instruction } from "../../src/session/instruction"
 import { SessionCompactionPolicy } from "../../src/session/compaction-policy"
 import { SessionProcessor } from "../../src/session/processor"
 import { SessionPrompt } from "../../src/session/prompt"
@@ -175,7 +174,7 @@ function makeHttp() {
       Layer.provideMerge(proc),
       Layer.provideMerge(registry),
       Layer.provideMerge(trunc),
-      Layer.provide(Instruction.defaultLayer),
+      Layer.provideMerge(Instruction.defaultLayer),
       Layer.provideMerge(deps),
     ),
   )
@@ -231,6 +230,36 @@ function providerCfg(url: string) {
   }
 }
 
+function compactCfg(url: string) {
+  return {
+    ...providerCfg(url),
+    compaction: {
+      max_failures: 3,
+      max_retries: 2,
+      max_output_tokens: 20,
+      post_budget: 100,
+      restore_attachments: true,
+    },
+    provider: {
+      ...cfg.provider,
+      test: {
+        ...cfg.provider.test,
+        options: {
+          ...cfg.provider.test.options,
+          baseURL: url,
+        },
+        models: {
+          ...cfg.provider.test.models,
+          "test-model": {
+            ...cfg.provider.test.models["test-model"],
+            limit: { context: 80, output: 10 },
+          },
+        },
+      },
+    },
+  }
+}
+
 const user = Effect.fn("test.user")(function* (sessionID: SessionID, text: string) {
   const session = yield* Session.Service
   const msg = yield* session.updateMessage({
@@ -280,6 +309,37 @@ const seed = Effect.fn("test.seed")(function* (sessionID: SessionID, opts?: { fi
   return { user: msg, assistant }
 })
 
+const attach = Effect.fn("test.attach")(function* (sessionID: SessionID, messageID: MessageID, url: string) {
+  const session = yield* Session.Service
+  yield* session.updatePart({
+    id: PartID.ascending(),
+    messageID,
+    sessionID,
+    type: "tool",
+    callID: "call-1",
+    tool: "read",
+    state: {
+      status: "completed",
+      input: {},
+      output: "sensitive-old-tool-output",
+      title: "done",
+      metadata: {},
+      time: { start: Date.now(), end: Date.now() },
+      attachments: [
+        {
+          id: PartID.ascending(),
+          messageID,
+          sessionID,
+          type: "file",
+          mime: "application/json",
+          filename: "note.json",
+          url,
+        },
+      ],
+    },
+  })
+})
+
 const addSubtask = (sessionID: SessionID, messageID: MessageID, model = ref) =>
   Effect.gen(function* () {
     const session = yield* Session.Service
@@ -456,6 +516,57 @@ it.live("loop continues when finish is tool-calls", () =>
   ),
 )
 
+it.live("loop auto-compacts through prompt flow and skips unsupported restored attachments", () =>
+  provideTmpdirServer(
+    Effect.fnUntraced(function* ({ llm }) {
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const chat = yield* sessions.create({
+        title: "Compaction",
+        permission: [{ permission: "*", pattern: "*", action: "allow" }],
+      })
+      const prev = yield* seed(chat.id, { finish: "stop" })
+      yield* attach(chat.id, prev.assistant.id, "https://example.com/note.json")
+      yield* prompt.prompt({
+        sessionID: chat.id,
+        agent: "build",
+        noReply: true,
+        parts: [{ type: "text", text: "continue" }],
+      })
+
+      yield* llm.text("too large", { usage: { input: 61, output: 10 } })
+      yield* llm.text("compact summary", { usage: { input: 20, output: 8 } })
+      yield* llm.text("resumed", { usage: { input: 10, output: 3 } })
+
+      const result = yield* prompt.loop({ sessionID: chat.id })
+      expect(yield* llm.calls).toBe(3)
+
+      expect(result.info.role).toBe("assistant")
+      expect(result.parts.some((part) => part.type === "text" && part.text === "resumed")).toBe(true)
+
+      const msgs = yield* sessions.messages({ sessionID: chat.id })
+      const summary = msgs.find((msg) => msg.info.role === "assistant" && msg.info.summary)
+      expect(summary?.parts.some((part) => part.type === "text" && part.text === "compact summary")).toBe(true)
+
+      const next = msgs.findLast(
+        (msg) =>
+          msg.info.role === "user" &&
+          msg.parts.some((part) => part.type === "text" && part.synthetic),
+      )
+      expect(next?.parts.some((part) => part.type === "file")).toBe(false)
+
+      const inputs = yield* llm.inputs
+      expect(inputs).toHaveLength(3)
+      expect(JSON.stringify(inputs[1]?.messages)).toContain("Summarize the conversation")
+      expect(JSON.stringify(inputs[1]?.messages)).toContain("sensitive-old-tool-output")
+      expect(JSON.stringify(inputs[2]?.messages)).toContain("compact summary")
+      expect(JSON.stringify(inputs[2]?.messages)).not.toContain("note.json")
+      expect(JSON.stringify(inputs[2]?.messages)).not.toContain("sensitive-old-tool-output")
+    }),
+    { git: true, config: compactCfg },
+  ),
+)
+
 it.live("failed subtask preserves metadata on error tool state", () =>
   provideTmpdirServer(
     Effect.fnUntraced(function* ({ llm }) {

packages/opencode/test/session/snapshot-tool-race.test.ts

@@ -40,6 +40,7 @@ import { Plugin } from "../../src/plugin"
 import { Provider as ProviderSvc } from "../../src/provider/provider"
 import { SessionCompaction } from "../../src/session/compaction"
 import { Instruction } from "../../src/session/instruction"
+import { SessionCompactionPolicy } from "../../src/session/compaction-policy"
 import { SessionProcessor } from "../../src/session/processor"
 import { SessionStatus } from "../../src/session/status"
 import { Shell } from "../../src/shell/shell"
@@ -118,6 +119,7 @@ function makeHttp() {
     Plugin.defaultLayer,
     Config.defaultLayer,
     ProviderSvc.defaultLayer,
+    SessionCompactionPolicy.defaultLayer,
     filetime,
     lsp,
     mcp,