fix(compaction): recover agent config after session compaction

src/features/background-agent/compaction-aware-message-resolver.test.ts

@@ -2,7 +2,15 @@ import { describe, test, expect, beforeEach, afterEach } from "bun:test"
 import { mkdtempSync, writeFileSync, rmSync } from "node:fs"
 import { join } from "node:path"
 import { tmpdir } from "node:os"
-import { isCompactionAgent, findNearestMessageExcludingCompaction } from "./compaction-aware-message-resolver"
+import {
+  isCompactionAgent,
+  findNearestMessageExcludingCompaction,
+  resolvePromptContextFromSessionMessages,
+} from "./compaction-aware-message-resolver"
+import {
+  clearCompactionAgentConfigCheckpoint,
+  setCompactionAgentConfigCheckpoint,
+} from "../../shared/compaction-agent-config-checkpoint"
 
 describe("isCompactionAgent", () => {
   describe("#given agent name variations", () => {
@@ -65,6 +73,7 @@ describe("findNearestMessageExcludingCompaction", () => {
 
   afterEach(() => {
     rmSync(tempDir, { force: true, recursive: true })
+    clearCompactionAgentConfigCheckpoint("ses_checkpoint")
   })
 
   describe("#given directory with messages", () => {
@@ -186,5 +195,65 @@ describe("findNearestMessageExcludingCompaction", () => {
       expect(result).not.toBeNull()
       expect(result?.agent).toBe("newer")
     })
+
+    test("merges partial metadata from multiple recent messages", () => {
+      // given
+      writeFileSync(
+        join(tempDir, "003.json"),
+        JSON.stringify({ model: { providerID: "anthropic", modelID: "claude-opus-4-1" } }),
+      )
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify({ agent: "atlas" }))
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify({ tools: { bash: true } }))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).toEqual({
+        agent: "atlas",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+        tools: { bash: true },
+      })
+    })
+
+    test("fills missing metadata from compaction checkpoint", () => {
+      // given
+      setCompactionAgentConfigCheckpoint("ses_checkpoint", {
+        agent: "sisyphus",
+        model: { providerID: "openai", modelID: "gpt-5" },
+      })
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify({ tools: { bash: true } }))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir, "ses_checkpoint")
+
+      // then
+      expect(result).toEqual({
+        agent: "sisyphus",
+        model: { providerID: "openai", modelID: "gpt-5" },
+        tools: { bash: true },
+      })
+    })
+  })
+})
+
+describe("resolvePromptContextFromSessionMessages", () => {
+  test("merges partial prompt context from recent SDK messages", () => {
+    // given
+    const messages = [
+      { info: { agent: "atlas" } },
+      { info: { model: { providerID: "anthropic", modelID: "claude-opus-4-1" } } },
+      { info: { tools: { bash: true } } },
+    ]
+
+    // when
+    const result = resolvePromptContextFromSessionMessages(messages)
+
+    // then
+    expect(result).toEqual({
+      agent: "atlas",
+      model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+      tools: { bash: true },
+    })
   })
 })

src/features/background-agent/compaction-aware-message-resolver.ts

@@ -1,6 +1,21 @@
 import { readdirSync, readFileSync } from "node:fs"
 import { join } from "node:path"
 import type { StoredMessage } from "../hook-message-injector"
+import { getCompactionAgentConfigCheckpoint } from "../../shared/compaction-agent-config-checkpoint"
+
+type SessionMessage = {
+  info?: {
+    agent?: string
+    model?: {
+      providerID?: string
+      modelID?: string
+      variant?: string
+    }
+    providerID?: string
+    modelID?: string
+    tools?: StoredMessage["tools"]
+  }
+}
 
 export function isCompactionAgent(agent: string | undefined): boolean {
   return agent?.trim().toLowerCase() === "compaction"
@@ -16,42 +31,121 @@ function hasFullAgentAndModel(message: StoredMessage): boolean {
 function hasPartialAgentOrModel(message: StoredMessage): boolean {
   const hasAgent = !!message.agent && !isCompactionAgent(message.agent)
   const hasModel = !!message.model?.providerID && !!message.model?.modelID
-  return hasAgent || hasModel
+  return hasAgent || hasModel || !!message.tools
 }
 
-export function findNearestMessageExcludingCompaction(messageDir: string): StoredMessage | null {
+function convertSessionMessageToStoredMessage(message: SessionMessage): StoredMessage | null {
+  const info = message.info
+  if (!info) {
+    return null
+  }
+
+  const providerID = info.model?.providerID ?? info.providerID
+  const modelID = info.model?.modelID ?? info.modelID
+
+  return {
+    ...(info.agent ? { agent: info.agent } : {}),
+    ...(providerID && modelID
+      ? {
+          model: {
+            providerID,
+            modelID,
+            ...(info.model?.variant ? { variant: info.model.variant } : {}),
+          },
+        }
+      : {}),
+    ...(info.tools ? { tools: info.tools } : {}),
+  }
+}
+
+function mergeStoredMessages(
+  messages: Array<StoredMessage | null>,
+  sessionID?: string,
+): StoredMessage | null {
+  const merged: StoredMessage = {}
+
+  for (const message of messages) {
+    if (!message || isCompactionAgent(message.agent)) {
+      continue
+    }
+
+    if (!merged.agent && message.agent) {
+      merged.agent = message.agent
+    }
+
+    if (!merged.model?.providerID && message.model?.providerID && message.model.modelID) {
+      merged.model = {
+        providerID: message.model.providerID,
+        modelID: message.model.modelID,
+        ...(message.model.variant ? { variant: message.model.variant } : {}),
+      }
+    }
+
+    if (!merged.tools && message.tools) {
+      merged.tools = message.tools
+    }
+
+    if (hasFullAgentAndModel(merged) && merged.tools) {
+      break
+    }
+  }
+
+  const checkpoint = sessionID
+    ? getCompactionAgentConfigCheckpoint(sessionID)
+    : undefined
+
+  if (!merged.agent && checkpoint?.agent) {
+    merged.agent = checkpoint.agent
+  }
+
+  if (!merged.model && checkpoint?.model) {
+    merged.model = {
+      providerID: checkpoint.model.providerID,
+      modelID: checkpoint.model.modelID,
+    }
+  }
+
+  if (!merged.tools && checkpoint?.tools) {
+    merged.tools = checkpoint.tools
+  }
+
+  return hasPartialAgentOrModel(merged) ? merged : null
+}
+
+export function resolvePromptContextFromSessionMessages(
+  messages: SessionMessage[],
+  sessionID?: string,
+): StoredMessage | null {
+  const convertedMessages = messages
+    .map(convertSessionMessageToStoredMessage)
+    .reverse()
+
+  return mergeStoredMessages(convertedMessages, sessionID)
+}
+
+export function findNearestMessageExcludingCompaction(
+  messageDir: string,
+  sessionID?: string,
+): StoredMessage | null {
   try {
     const files = readdirSync(messageDir)
-      .filter((name) => name.endsWith(".json"))
+      .filter((name: string) => name.endsWith(".json"))
       .sort()
       .reverse()
 
-    for (const file of files) {
-      try {
-        const content = readFileSync(join(messageDir, file), "utf-8")
-        const parsed = JSON.parse(content) as StoredMessage
-        if (hasFullAgentAndModel(parsed)) {
-          return parsed
-        }
-      } catch {
-        continue
-      }
-    }
+    const messages: Array<StoredMessage | null> = []
 
     for (const file of files) {
       try {
         const content = readFileSync(join(messageDir, file), "utf-8")
-        const parsed = JSON.parse(content) as StoredMessage
-        if (hasPartialAgentOrModel(parsed)) {
-          return parsed
-        }
+        messages.push(JSON.parse(content) as StoredMessage)
       } catch {
         continue
       }
     }
+
+    return mergeStoredMessages(messages, sessionID)
   } catch {
     return null
   }
-
-  return null
 }

src/features/background-agent/manager.ts

@@ -41,7 +41,10 @@ import {
 } from "./error-classifier"
 import { tryFallbackRetry } from "./fallback-retry-handler"
 import { registerManagerForCleanup, unregisterManagerForCleanup } from "./process-cleanup"
-import { isCompactionAgent, findNearestMessageExcludingCompaction } from "./compaction-aware-message-resolver"
+import {
+  findNearestMessageExcludingCompaction,
+  resolvePromptContextFromSessionMessages,
+} from "./compaction-aware-message-resolver"
 import { handleSessionIdleBackgroundEvent } from "./session-idle-event-handler"
 import { MESSAGE_STORAGE } from "../hook-message-injector"
 import { join } from "node:path"
@@ -1323,20 +1326,20 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
               tools?: Record<string, boolean | "allow" | "deny" | "ask">
             }
           }>)
-          for (let i = messages.length - 1; i >= 0; i--) {
-            const info = messages[i].info
-            if (isCompactionAgent(info?.agent)) {
-              continue
-            }
-            const normalizedTools = isRecord(info?.tools)
-              ? normalizePromptTools(info.tools as Record<string, boolean | "allow" | "deny" | "ask">)
+          const promptContext = resolvePromptContextFromSessionMessages(
+            messages,
+            task.parentSessionID,
+          )
+          const normalizedTools = isRecord(promptContext?.tools)
+            ? normalizePromptTools(promptContext.tools)
+            : undefined
+
+          if (promptContext?.agent || promptContext?.model || normalizedTools) {
+            agent = promptContext?.agent ?? task.parentAgent
+            model = promptContext?.model?.providerID && promptContext.model.modelID
+              ? { providerID: promptContext.model.providerID, modelID: promptContext.model.modelID }
               : undefined
-            if (info?.agent || info?.model || (info?.modelID && info?.providerID) || normalizedTools) {
-              agent = info?.agent ?? task.parentAgent
-              model = info?.model ?? (info?.providerID && info?.modelID ? { providerID: info.providerID, modelID: info.modelID } : undefined)
-              tools = normalizedTools ?? tools
-              break
-            }
+            tools = normalizedTools ?? tools
           }
         } catch (error) {
           if (isAbortedSessionError(error)) {
@@ -1346,7 +1349,9 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
             })
           }
           const messageDir = join(MESSAGE_STORAGE, task.parentSessionID)
-          const currentMessage = messageDir ? findNearestMessageExcludingCompaction(messageDir) : null
+          const currentMessage = messageDir
+            ? findNearestMessageExcludingCompaction(messageDir, task.parentSessionID)
+            : null
           agent = currentMessage?.agent ?? task.parentAgent
           model = currentMessage?.model?.providerID && currentMessage?.model?.modelID
             ? { providerID: currentMessage.model.providerID, modelID: currentMessage.model.modelID }

src/hooks/compaction-context-injector/compaction-context-prompt.ts

@@ -0,0 +1,56 @@
+import {
+  createSystemDirective,
+  SystemDirectiveTypes,
+} from "../../shared/system-directive"
+
+export const COMPACTION_CONTEXT_PROMPT = `${createSystemDirective(SystemDirectiveTypes.COMPACTION_CONTEXT)}
+
+When summarizing this session, you MUST include the following sections in your summary:
+
+## 1. User Requests (As-Is)
+- List all original user requests exactly as they were stated
+- Preserve the user's exact wording and intent
+
+## 2. Final Goal
+- What the user ultimately wanted to achieve
+- The end result or deliverable expected
+
+## 3. Work Completed
+- What has been done so far
+- Files created/modified
+- Features implemented
+- Problems solved
+
+## 4. Remaining Tasks
+- What still needs to be done
+- Pending items from the original request
+- Follow-up tasks identified during the work
+
+## 5. Active Working Context (For Seamless Continuation)
+- **Files**: Paths of files currently being edited or frequently referenced
+- **Code in Progress**: Key code snippets, function signatures, or data structures under active development
+- **External References**: Documentation URLs, library APIs, or external resources being consulted
+- **State & Variables**: Important variable names, configuration values, or runtime state relevant to ongoing work
+
+## 6. Explicit Constraints (Verbatim Only)
+- Include ONLY constraints explicitly stated by the user or in existing AGENTS.md context
+- Quote constraints verbatim (do not paraphrase)
+- Do NOT invent, add, or modify constraints
+- If no explicit constraints exist, write "None"
+
+## 7. Agent Verification State (Critical for Reviewers)
+- **Current Agent**: What agent is running (momus, oracle, etc.)
+- **Verification Progress**: Files already verified/validated
+- **Pending Verifications**: Files still needing verification
+- **Previous Rejections**: If reviewer agent, what was rejected and why
+- **Acceptance Status**: Current state of review process
+
+This section is CRITICAL for reviewer agents (momus, oracle) to maintain continuity.
+
+## 8. Delegated Agent Sessions
+- List ALL background agent tasks spawned during this session
+- For each: agent name, category, status, description, and **session_id**
+- **RESUME, DON'T RESTART.** Each listed session retains full context. After compaction, use \`session_id\` to continue existing agent sessions instead of spawning new ones. This saves tokens, preserves learned context, and prevents duplicate work.
+
+This context is critical for maintaining continuity after compaction.
+`

src/hooks/compaction-context-injector/constants.ts

@@ -0,0 +1,5 @@
+export const HOOK_NAME = "compaction-context-injector"
+export const AGENT_RECOVERY_PROMPT = "[restore checkpointed session agent configuration after compaction]"
+export const NO_TEXT_TAIL_THRESHOLD = 5
+export const RECOVERY_COOLDOWN_MS = 60_000
+export const RECENT_COMPACTION_WINDOW_MS = 10 * 60 * 1000