Merge pull request #2378 from code-yeongyu/fix/issue-2232

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 {
-  try {
-    const files = readdirSync(messageDir)
-      .filter((name) => 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
-      }
-    }
-
-    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
-        }
-      } catch {
-        continue
-      }
-    }
-  } catch {
+function convertSessionMessageToStoredMessage(message: SessionMessage): StoredMessage | null {
+  const info = message.info
+  if (!info) {
     return null
   }
 
-  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: string) => name.endsWith(".json"))
+      .sort()
+      .reverse()
+
+    const messages: Array<StoredMessage | null> = []
+
+    for (const file of files) {
+      try {
+        const content = readFileSync(join(messageDir, file), "utf-8")
+        messages.push(JSON.parse(content) as StoredMessage)
+      } catch {
+        continue
+      }
+    }
+
+    return mergeStoredMessages(messages, sessionID)
+  } catch {
+    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

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

@@ -1,63 +1,60 @@
 import type { BackgroundManager } from "../../features/background-agent"
 import {
-  createSystemDirective,
-  SystemDirectiveTypes,
-} from "../../shared/system-directive"
+  clearCompactionAgentConfigCheckpoint,
+  setCompactionAgentConfigCheckpoint,
+} from "../../shared/compaction-agent-config-checkpoint"
+import { log } from "../../shared/logger"
+import { COMPACTION_CONTEXT_PROMPT } from "./compaction-context-prompt"
+import { resolveSessionPromptConfig } from "./session-prompt-config-resolver"
+import { finalizeTrackedAssistantMessage, shouldTreatAssistantPartAsOutput, trackAssistantOutput, type TailMonitorState } from "./tail-monitor"
+import { resolveSessionID } from "./session-id"
+import type { CompactionContextClient, CompactionContextInjector } from "./types"
+import { createRecoveryLogic } from "./recovery"
 
-const COMPACTION_CONTEXT_PROMPT = `${createSystemDirective(SystemDirectiveTypes.COMPACTION_CONTEXT)}
+export function createCompactionContextInjector(options?: {
+  ctx?: CompactionContextClient
+  backgroundManager?: BackgroundManager
+}): CompactionContextInjector {
+  const ctx = options?.ctx
+  const backgroundManager = options?.backgroundManager
+  const tailStates = new Map<string, TailMonitorState>()
 
-When summarizing this session, you MUST include the following sections in your summary:
+  const getTailState = (sessionID: string): TailMonitorState => {
+    const existing = tailStates.get(sessionID)
+    if (existing) {
+      return existing
+    }
 
-## 1. User Requests (As-Is)
-- List all original user requests exactly as they were stated
-- Preserve the user's exact wording and intent
+    const created: TailMonitorState = {
+      currentHasOutput: false,
+      consecutiveNoTextMessages: 0,
+    }
+    tailStates.set(sessionID, created)
+    return created
+  }
 
-## 2. Final Goal
-- What the user ultimately wanted to achieve
-- The end result or deliverable expected
+  const { recoverCheckpointedAgentConfig, maybeWarnAboutNoTextTail } = createRecoveryLogic(ctx, getTailState)
 
-## 3. Work Completed
-- What has been done so far
-- Files created/modified
-- Features implemented
-- Problems solved
+  const capture = async (sessionID: string): Promise<void> => {
+    if (!ctx || !sessionID) {
+      return
+    }
 
-## 4. Remaining Tasks
-- What still needs to be done
-- Pending items from the original request
-- Follow-up tasks identified during the work
+    const promptConfig = await resolveSessionPromptConfig(ctx, sessionID)
+    if (!promptConfig.agent && !promptConfig.model && !promptConfig.tools) {
+      return
+    }
 
-## 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
+    setCompactionAgentConfigCheckpoint(sessionID, promptConfig)
+    log(`[compaction-context-injector] Captured agent checkpoint before compaction`, {
+      sessionID,
+      agent: promptConfig.agent,
+      model: promptConfig.model,
+      hasTools: !!promptConfig.tools,
+    })
+  }
 
-## 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.
-`
-
-export function createCompactionContextInjector(backgroundManager?: BackgroundManager) {
-  return (sessionID?: string): string => {
+  const inject = (sessionID?: string): string => {
     let prompt = COMPACTION_CONTEXT_PROMPT
 
     if (backgroundManager && sessionID) {
@@ -69,4 +66,99 @@ export function createCompactionContextInjector(backgroundManager?: BackgroundMa
 
     return prompt
   }
+
+  const event = async ({ event }: { event: { type: string; properties?: unknown } }): Promise<void> => {
+    const props = event.properties as Record<string, unknown> | undefined
+
+    if (event.type === "session.deleted") {
+      const sessionID = resolveSessionID(props)
+      if (sessionID) {
+        clearCompactionAgentConfigCheckpoint(sessionID)
+        tailStates.delete(sessionID)
+      }
+      return
+    }
+
+    if (event.type === "session.idle") {
+      const sessionID = resolveSessionID(props)
+      if (!sessionID) {
+        return
+      }
+
+      const noTextCount = finalizeTrackedAssistantMessage(getTailState(sessionID))
+      if (noTextCount > 0) {
+        await maybeWarnAboutNoTextTail(sessionID)
+      }
+      return
+    }
+
+    if (event.type === "session.compacted") {
+      const sessionID = resolveSessionID(props)
+      if (!sessionID) {
+        return
+      }
+
+      const tailState = getTailState(sessionID)
+      finalizeTrackedAssistantMessage(tailState)
+      tailState.lastCompactedAt = Date.now()
+      await maybeWarnAboutNoTextTail(sessionID)
+      await recoverCheckpointedAgentConfig(sessionID, "session.compacted")
+      return
+    }
+
+    if (event.type === "message.updated") {
+      const info = props?.info as {
+        id?: string
+        role?: string
+        sessionID?: string
+      } | undefined
+
+      if (!info?.sessionID || info.role !== "assistant" || !info.id) {
+        return
+      }
+
+      const tailState = getTailState(info.sessionID)
+      if (tailState.currentMessageID && tailState.currentMessageID !== info.id) {
+        finalizeTrackedAssistantMessage(tailState)
+        await maybeWarnAboutNoTextTail(info.sessionID)
+      }
+
+      if (tailState.currentMessageID !== info.id) {
+        tailState.currentMessageID = info.id
+        tailState.currentHasOutput = false
+      }
+      return
+    }
+
+    if (event.type === "message.part.delta") {
+      const sessionID = props?.sessionID as string | undefined
+      const messageID = props?.messageID as string | undefined
+      const field = props?.field as string | undefined
+      const delta = props?.delta as string | undefined
+
+      if (!sessionID || field !== "text" || !delta?.trim()) {
+        return
+      }
+
+      trackAssistantOutput(getTailState(sessionID), messageID)
+      return
+    }
+
+    if (event.type === "message.part.updated") {
+      const part = props?.part as {
+        messageID?: string
+        sessionID?: string
+        type?: string
+        text?: string
+      } | undefined
+
+      if (!part?.sessionID || !shouldTreatAssistantPartAsOutput(part)) {
+        return
+      }
+
+      trackAssistantOutput(getTailState(part.sessionID), part.messageID)
+    }
+  }
+
+  return { capture, inject, event }
 }

src/hooks/compaction-context-injector/index.test.ts

@@ -17,6 +17,27 @@ mock.module("../../shared/system-directive", () => ({
 import { createCompactionContextInjector } from "./index"
 import { TaskHistory } from "../../features/background-agent/task-history"
 
+function createMockContext(
+  messageResponses: Array<Array<{ info?: Record<string, unknown> }>>,
+  promptAsyncMock = mock(async () => ({})),
+) {
+  let callIndex = 0
+
+  return {
+    client: {
+      session: {
+        messages: mock(async () => {
+          const response = messageResponses[Math.min(callIndex, messageResponses.length - 1)] ?? []
+          callIndex += 1
+          return { data: response }
+        }),
+        promptAsync: promptAsyncMock,
+      },
+    },
+    directory: "/tmp/test",
+  }
+}
+
 describe("createCompactionContextInjector", () => {
   describe("Agent Verification State preservation", () => {
     it("includes Agent Verification State section in compaction prompt", async () => {
@@ -24,7 +45,7 @@ describe("createCompactionContextInjector", () => {
       const injector = createCompactionContextInjector()
 
       //#when
-      const prompt = injector()
+      const prompt = injector.inject()
 
       //#then
       expect(prompt).toContain("Agent Verification State")
@@ -37,7 +58,7 @@ describe("createCompactionContextInjector", () => {
       const injector = createCompactionContextInjector()
 
       //#when
-      const prompt = injector()
+      const prompt = injector.inject()
 
       //#then
       expect(prompt).toContain("Previous Rejections")
@@ -50,7 +71,7 @@ describe("createCompactionContextInjector", () => {
       const injector = createCompactionContextInjector()
 
       //#when
-      const prompt = injector()
+      const prompt = injector.inject()
 
       //#then
       expect(prompt).toContain("Pending Verifications")
@@ -63,7 +84,7 @@ describe("createCompactionContextInjector", () => {
     const injector = createCompactionContextInjector()
 
     //#when
-    const prompt = injector()
+    const prompt = injector.inject()
 
     //#then
     expect(prompt).toContain("Explicit Constraints (Verbatim Only)")
@@ -77,7 +98,7 @@ describe("createCompactionContextInjector", () => {
       const injector = createCompactionContextInjector()
 
       //#when
-      const prompt = injector()
+      const prompt = injector.inject()
 
       //#then
       expect(prompt).toContain("Delegated Agent Sessions")
@@ -89,10 +110,10 @@ describe("createCompactionContextInjector", () => {
       //#given
       const mockManager = { taskHistory: new TaskHistory() } as any
       mockManager.taskHistory.record("ses_parent", { id: "t1", sessionID: "ses_child", agent: "explore", description: "Find patterns", status: "completed", category: "quick" })
-      const injector = createCompactionContextInjector(mockManager)
+      const injector = createCompactionContextInjector({ backgroundManager: mockManager })
 
       //#when
-      const prompt = injector("ses_parent")
+      const prompt = injector.inject("ses_parent")
 
       //#then
       expect(prompt).toContain("Active/Recent Delegated Sessions")
@@ -104,13 +125,152 @@ describe("createCompactionContextInjector", () => {
     it("does not inject task history section when no entries exist", async () => {
       //#given
       const mockManager = { taskHistory: new TaskHistory() } as any
-      const injector = createCompactionContextInjector(mockManager)
+      const injector = createCompactionContextInjector({ backgroundManager: mockManager })
 
       //#when
-      const prompt = injector("ses_empty")
+      const prompt = injector.inject("ses_empty")
 
       //#then
       expect(prompt).not.toContain("Active/Recent Delegated Sessions")
     })
   })
+
+  describe("agent checkpoint recovery", () => {
+    it("re-injects checkpointed agent config after compaction when latest agent is lost", async () => {
+      //#given
+      const promptAsyncMock = mock(async () => ({}))
+      const ctx = createMockContext(
+        [
+          [
+            {
+              info: {
+                role: "user",
+                agent: "atlas",
+                model: { providerID: "openai", modelID: "gpt-5" },
+                tools: { bash: "allow" },
+              },
+            },
+          ],
+          [
+            {
+              info: {
+                role: "user",
+                agent: "compaction",
+                model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+              },
+            },
+          ],
+          [
+            {
+              info: {
+                role: "user",
+                agent: "atlas",
+                model: { providerID: "openai", modelID: "gpt-5" },
+              },
+            },
+          ],
+        ],
+        promptAsyncMock,
+      )
+      const injector = createCompactionContextInjector({ ctx })
+
+      //#when
+      await injector.capture("ses_checkpoint")
+      await injector.event({
+        event: { type: "session.compacted", properties: { sessionID: "ses_checkpoint" } },
+      })
+
+      //#then
+      expect(promptAsyncMock).toHaveBeenCalledWith({
+        path: { id: "ses_checkpoint" },
+        body: {
+          noReply: true,
+          agent: "atlas",
+          model: { providerID: "openai", modelID: "gpt-5" },
+          tools: { bash: true },
+          parts: [
+            {
+              type: "text",
+              text: expect.stringContaining("restore checkpointed session agent configuration"),
+            },
+          ],
+        },
+        query: { directory: "/tmp/test" },
+      })
+    })
+
+    it("recovers after five consecutive assistant messages with no text", async () => {
+      //#given
+      const promptAsyncMock = mock(async () => ({}))
+      const ctx = createMockContext(
+        [
+          [
+            {
+              info: {
+                role: "user",
+                agent: "atlas",
+                model: { providerID: "openai", modelID: "gpt-5" },
+              },
+            },
+          ],
+          [
+            {
+              info: {
+                role: "user",
+                agent: "atlas",
+                model: { providerID: "openai", modelID: "gpt-5" },
+              },
+            },
+          ],
+          [
+            {
+              info: {
+                role: "user",
+                agent: "atlas",
+                model: { providerID: "openai", modelID: "gpt-5" },
+              },
+            },
+          ],
+        ],
+        promptAsyncMock,
+      )
+      const injector = createCompactionContextInjector({ ctx })
+
+      await injector.capture("ses_no_text_tail")
+      await injector.event({
+        event: { type: "session.compacted", properties: { sessionID: "ses_no_text_tail" } },
+      })
+
+      //#when
+      for (let index = 1; index <= 5; index++) {
+        await injector.event({
+          event: {
+            type: "message.updated",
+            properties: {
+              info: {
+                id: `msg_${index}`,
+                role: "assistant",
+                sessionID: "ses_no_text_tail",
+              },
+            },
+          },
+        })
+      }
+      await injector.event({
+        event: { type: "session.idle", properties: { sessionID: "ses_no_text_tail" } },
+      })
+
+      //#then
+      expect(promptAsyncMock).toHaveBeenCalledTimes(1)
+      expect(promptAsyncMock).toHaveBeenCalledWith(
+        expect.objectContaining({
+          path: { id: "ses_no_text_tail" },
+          body: expect.objectContaining({
+            noReply: true,
+            agent: "atlas",
+          }),
+        }),
+      )
+    })
+  })
 })

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

@@ -0,0 +1,76 @@
+import type { CompactionAgentConfigCheckpoint } from "../../shared/compaction-agent-config-checkpoint"
+
+export type RecoveryPromptConfig = CompactionAgentConfigCheckpoint & {
+  agent: string
+}
+
+function isCompactionAgent(agent: string | undefined): boolean {
+  return agent?.trim().toLowerCase() === "compaction"
+}
+
+function matchesExpectedModel(
+  actualModel: CompactionAgentConfigCheckpoint["model"],
+  expectedModel: CompactionAgentConfigCheckpoint["model"],
+): boolean {
+  if (!expectedModel) {
+    return true
+  }
+
+  return (
+    actualModel?.providerID === expectedModel.providerID &&
+    actualModel.modelID === expectedModel.modelID
+  )
+}
+
+function matchesExpectedTools(
+  actualTools: CompactionAgentConfigCheckpoint["tools"],
+  expectedTools: CompactionAgentConfigCheckpoint["tools"],
+): boolean {
+  if (!expectedTools) {
+    return true
+  }
+
+  if (!actualTools) {
+    return false
+  }
+
+  const expectedEntries = Object.entries(expectedTools)
+  if (expectedEntries.length !== Object.keys(actualTools).length) {
+    return false
+  }
+
+  return expectedEntries.every(
+    ([toolName, isAllowed]) => actualTools[toolName] === isAllowed,
+  )
+}
+
+export function createExpectedRecoveryPromptConfig(
+  checkpoint: Pick<RecoveryPromptConfig, "agent"> & CompactionAgentConfigCheckpoint,
+  currentPromptConfig: CompactionAgentConfigCheckpoint,
+): RecoveryPromptConfig {
+  const model = checkpoint.model ?? currentPromptConfig.model
+  const tools = checkpoint.tools ?? currentPromptConfig.tools
+
+  return {
+    agent: checkpoint.agent,
+    ...(model ? { model } : {}),
+    ...(tools ? { tools } : {}),
+  }
+}
+
+export function isPromptConfigRecovered(
+  actualPromptConfig: CompactionAgentConfigCheckpoint,
+  expectedPromptConfig: RecoveryPromptConfig,
+): boolean {
+  const actualAgent = actualPromptConfig.agent
+  const agentMatches =
+    typeof actualAgent === "string" &&
+    !isCompactionAgent(actualAgent) &&
+    actualAgent.toLowerCase() === expectedPromptConfig.agent.toLowerCase()
+
+  return (
+    agentMatches &&
+    matchesExpectedModel(actualPromptConfig.model, expectedPromptConfig.model) &&
+    matchesExpectedTools(actualPromptConfig.tools, expectedPromptConfig.tools)
+  )
+}

src/hooks/compaction-context-injector/recovery.test.ts

@@ -0,0 +1,294 @@
+/// <reference path="../../../bun-test.d.ts" />
+
+import { describe, expect, it } from "bun:test"
+import { createCompactionContextInjector } from "./index"
+
+type SessionMessageResponse = Array<{
+  info?: Record<string, unknown>
+}>
+
+type PromptAsyncInput = {
+  path: { id: string }
+  body: {
+    noReply?: boolean
+    agent?: string
+    model?: { providerID: string; modelID: string }
+    tools?: Record<string, boolean>
+    parts: Array<{ type: "text"; text: string }>
+  }
+  query?: { directory: string }
+}
+
+function createPromptAsyncRecorder(): {
+  calls: PromptAsyncInput[]
+  promptAsync: (input: PromptAsyncInput) => Promise<Record<string, never>>
+} {
+  const calls: PromptAsyncInput[] = []
+
+  return {
+    calls,
+    promptAsync: async (input: PromptAsyncInput) => {
+      calls.push(input)
+      return {}
+    },
+  }
+}
+
+function createMockContext(
+  messageResponses: SessionMessageResponse[],
+  promptAsync: (input: PromptAsyncInput) => Promise<Record<string, never>>,
+) {
+  let callIndex = 0
+
+  return {
+    client: {
+      session: {
+        messages: async () => {
+          const response =
+            messageResponses[Math.min(callIndex, messageResponses.length - 1)] ?? []
+          callIndex += 1
+          return { data: response }
+        },
+        promptAsync,
+      },
+    },
+    directory: "/tmp/test",
+  }
+}
+
+function createAssistantMessageUpdatedEvent(sessionID: string, messageID: string) {
+  return {
+    event: {
+      type: "message.updated",
+      properties: {
+        info: {
+          id: messageID,
+          role: "assistant",
+          sessionID,
+        },
+      },
+    },
+  } as const
+}
+
+function createMeaningfulPartUpdatedEvent(
+  sessionID: string,
+  messageID: string,
+  type: "reasoning" | "tool_use",
+) {
+  return {
+    event: {
+      type: "message.part.updated",
+      properties: {
+        part: {
+          messageID,
+          sessionID,
+          type,
+          ...(type === "reasoning" ? { text: "thinking" } : {}),
+        },
+      },
+    },
+  } as const
+}
+
+describe("createCompactionContextInjector recovery", () => {
+  it("re-injects after compaction when agent and model match but tools are missing", async () => {
+    //#given
+    const promptAsyncRecorder = createPromptAsyncRecorder()
+    const ctx = createMockContext(
+      [
+        [
+          {
+            info: {
+              role: "user",
+              agent: "atlas",
+              model: { providerID: "openai", modelID: "gpt-5" },
+              tools: { bash: true },
+            },
+          },
+        ],
+        [
+          {
+            info: {
+              role: "user",
+              agent: "atlas",
+              model: { providerID: "openai", modelID: "gpt-5" },
+            },
+          },
+        ],
+        [
+          {
+            info: {
+              role: "user",
+              agent: "atlas",
+              model: { providerID: "openai", modelID: "gpt-5" },
+            },
+          },
+        ],
+        [
+          {
+            info: {
+              role: "user",
+              agent: "atlas",
+              model: { providerID: "openai", modelID: "gpt-5" },
+              tools: { bash: true },
+            },
+          },
+        ],
+      ],
+      promptAsyncRecorder.promptAsync,
+    )
+    const injector = createCompactionContextInjector({ ctx })
+
+    //#when
+    await injector.capture("ses_missing_tools")
+    await injector.event({
+      event: { type: "session.compacted", properties: { sessionID: "ses_missing_tools" } },
+    })
+
+    //#then
+    expect(promptAsyncRecorder.calls.length).toBe(1)
+    expect(promptAsyncRecorder.calls[0]?.body.agent).toBe("atlas")
+    expect(promptAsyncRecorder.calls[0]?.body.model).toEqual({
+      providerID: "openai",
+      modelID: "gpt-5",
+    })
+    expect(promptAsyncRecorder.calls[0]?.body.tools).toEqual({ bash: true })
+  })
+
+  it("retries recovery when the recovered prompt config still mismatches expected model or tools", async () => {
+    //#given
+    const promptAsyncRecorder = createPromptAsyncRecorder()
+    const mismatchResponse = [
+      {
+        info: {
+          role: "user",
+          agent: "atlas",
+          model: { providerID: "openai", modelID: "gpt-4.1" },
+        },
+      },
+    ]
+    const ctx = createMockContext(
+      [
+        [
+          {
+            info: {
+              role: "user",
+              agent: "atlas",
+              model: { providerID: "openai", modelID: "gpt-5" },
+              tools: { bash: true },
+            },
+          },
+        ],
+        mismatchResponse,
+        mismatchResponse,
+        mismatchResponse,
+        mismatchResponse,
+        mismatchResponse,
+        mismatchResponse,
+      ],
+      promptAsyncRecorder.promptAsync,
+    )
+    const injector = createCompactionContextInjector({ ctx })
+
+    //#when
+    await injector.capture("ses_retry_incomplete_recovery")
+    await injector.event({
+      event: {
+        type: "session.compacted",
+        properties: { sessionID: "ses_retry_incomplete_recovery" },
+      },
+    })
+    await injector.event({
+      event: {
+        type: "session.compacted",
+        properties: { sessionID: "ses_retry_incomplete_recovery" },
+      },
+    })
+
+    //#then
+    expect(promptAsyncRecorder.calls.length).toBe(2)
+  })
+
+  it("does not treat reasoning-only assistant messages as a no-text tail", async () => {
+    //#given
+    const promptAsyncRecorder = createPromptAsyncRecorder()
+    const matchingPromptConfig = [
+      {
+        info: {
+          role: "user",
+          agent: "atlas",
+          model: { providerID: "openai", modelID: "gpt-5" },
+          tools: { bash: true },
+        },
+      },
+    ]
+    const ctx = createMockContext(
+      [matchingPromptConfig, matchingPromptConfig, matchingPromptConfig],
+      promptAsyncRecorder.promptAsync,
+    )
+    const injector = createCompactionContextInjector({ ctx })
+    const sessionID = "ses_reasoning_tail"
+
+    await injector.capture(sessionID)
+    await injector.event({
+      event: { type: "session.compacted", properties: { sessionID } },
+    })
+
+    //#when
+    for (let index = 1; index <= 5; index++) {
+      const messageID = `msg_reasoning_${index}`
+      await injector.event(createAssistantMessageUpdatedEvent(sessionID, messageID))
+      await injector.event(
+        createMeaningfulPartUpdatedEvent(sessionID, messageID, "reasoning"),
+      )
+      await injector.event({
+        event: { type: "session.idle", properties: { sessionID } },
+      })
+    }
+
+    //#then
+    expect(promptAsyncRecorder.calls.length).toBe(0)
+  })
+
+  it("does not treat tool_use-only assistant messages as a no-text tail", async () => {
+    //#given
+    const promptAsyncRecorder = createPromptAsyncRecorder()
+    const matchingPromptConfig = [
+      {
+        info: {
+          role: "user",
+          agent: "atlas",
+          model: { providerID: "openai", modelID: "gpt-5" },
+          tools: { bash: true },
+        },
+      },
+    ]
+    const ctx = createMockContext(
+      [matchingPromptConfig, matchingPromptConfig, matchingPromptConfig],
+      promptAsyncRecorder.promptAsync,
+    )
+    const injector = createCompactionContextInjector({ ctx })
+    const sessionID = "ses_tool_use_tail"
+
+    await injector.capture(sessionID)
+    await injector.event({
+      event: { type: "session.compacted", properties: { sessionID } },
+    })
+
+    //#when
+    for (let index = 1; index <= 5; index++) {
+      const messageID = `msg_tool_use_${index}`
+      await injector.event(createAssistantMessageUpdatedEvent(sessionID, messageID))
+      await injector.event(
+        createMeaningfulPartUpdatedEvent(sessionID, messageID, "tool_use"),
+      )
+      await injector.event({
+        event: { type: "session.idle", properties: { sessionID } },
+      })
+    }
+
+    //#then
+    expect(promptAsyncRecorder.calls.length).toBe(0)
+  })
+})

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

@@ -0,0 +1,143 @@
+import { updateSessionAgent } from "../../features/claude-code-session-state"
+import {
+  getCompactionAgentConfigCheckpoint,
+} from "../../shared/compaction-agent-config-checkpoint"
+import { createInternalAgentTextPart } from "../../shared/internal-initiator-marker"
+import { log } from "../../shared/logger"
+import { setSessionModel } from "../../shared/session-model-state"
+import { setSessionTools } from "../../shared/session-tools-store"
+import {
+  createExpectedRecoveryPromptConfig,
+  isPromptConfigRecovered,
+} from "./recovery-prompt-config"
+import {
+  resolveLatestSessionPromptConfig,
+  resolveSessionPromptConfig,
+} from "./session-prompt-config-resolver"
+import { AGENT_RECOVERY_PROMPT, NO_TEXT_TAIL_THRESHOLD, RECOVERY_COOLDOWN_MS, RECENT_COMPACTION_WINDOW_MS } from "./constants"
+import type { CompactionContextClient } from "./types"
+import type { TailMonitorState } from "./tail-monitor"
+
+export function createRecoveryLogic(
+  ctx: CompactionContextClient | undefined,
+  getTailState: (sessionID: string) => TailMonitorState,
+) {
+  const recoverCheckpointedAgentConfig = async (
+    sessionID: string,
+    reason: "session.compacted" | "no-text-tail",
+  ): Promise<boolean> => {
+    if (!ctx) {
+      return false
+    }
+
+    const checkpoint = getCompactionAgentConfigCheckpoint(sessionID)
+    if (!checkpoint?.agent) {
+      return false
+    }
+    const checkpointWithAgent = {
+      ...checkpoint,
+      agent: checkpoint.agent,
+    }
+
+    const tailState = getTailState(sessionID)
+    const now = Date.now()
+    if (tailState.lastRecoveryAt && now - tailState.lastRecoveryAt < RECOVERY_COOLDOWN_MS) {
+      return false
+    }
+
+    const currentPromptConfig = await resolveSessionPromptConfig(ctx, sessionID)
+    const expectedPromptConfig = createExpectedRecoveryPromptConfig(
+      checkpointWithAgent,
+      currentPromptConfig,
+    )
+    const model = expectedPromptConfig.model
+    const tools = expectedPromptConfig.tools
+
+    if (reason === "session.compacted") {
+      const latestPromptConfig = await resolveLatestSessionPromptConfig(ctx, sessionID)
+      if (isPromptConfigRecovered(latestPromptConfig, expectedPromptConfig)) {
+        return false
+      }
+    }
+
+    try {
+      await ctx.client.session.promptAsync({
+        path: { id: sessionID },
+        body: {
+          noReply: true,
+          agent: expectedPromptConfig.agent,
+          ...(model ? { model } : {}),
+          ...(tools ? { tools } : {}),
+          parts: [createInternalAgentTextPart(AGENT_RECOVERY_PROMPT)],
+        },
+        query: { directory: ctx.directory },
+      })
+
+      const recoveredPromptConfig = await resolveLatestSessionPromptConfig(ctx, sessionID)
+      if (!isPromptConfigRecovered(recoveredPromptConfig, expectedPromptConfig)) {
+        log(`[compaction-context-injector] Re-injected agent config but recovery is still incomplete`, {
+          sessionID,
+          reason,
+          agent: expectedPromptConfig.agent,
+          model,
+          hasTools: !!tools,
+          recoveredPromptConfig,
+        })
+        return false
+      }
+
+      updateSessionAgent(sessionID, expectedPromptConfig.agent)
+      if (model) {
+        setSessionModel(sessionID, model)
+      }
+      if (tools) {
+        setSessionTools(sessionID, tools)
+      }
+
+      tailState.lastRecoveryAt = now
+      tailState.consecutiveNoTextMessages = 0
+
+      log(`[compaction-context-injector] Re-injected checkpointed agent config`, {
+        sessionID,
+        reason,
+        agent: expectedPromptConfig.agent,
+        model,
+      })
+
+      return true
+    } catch (error) {
+      log(`[compaction-context-injector] Failed to re-inject checkpointed agent config`, {
+        sessionID,
+        reason,
+        error: String(error),
+      })
+      return false
+    }
+  }
+
+  const maybeWarnAboutNoTextTail = async (sessionID: string): Promise<void> => {
+    const tailState = getTailState(sessionID)
+    if (tailState.consecutiveNoTextMessages < NO_TEXT_TAIL_THRESHOLD) {
+      return
+    }
+
+    const recentlyCompacted =
+      tailState.lastCompactedAt !== undefined &&
+      Date.now() - tailState.lastCompactedAt < RECENT_COMPACTION_WINDOW_MS
+
+    log(`[compaction-context-injector] Detected consecutive assistant messages with no text`, {
+      sessionID,
+      consecutiveNoTextMessages: tailState.consecutiveNoTextMessages,
+      recentlyCompacted,
+    })
+
+    if (recentlyCompacted) {
+      await recoverCheckpointedAgentConfig(sessionID, "no-text-tail")
+    }
+  }
+
+  return {
+    recoverCheckpointedAgentConfig,
+    maybeWarnAboutNoTextTail,
+  }
+}

src/hooks/compaction-context-injector/session-id.ts

@@ -0,0 +1,8 @@
+export function isCompactionAgent(agent: string | undefined): boolean {
+  return agent?.trim().toLowerCase() === "compaction"
+}
+
+export function resolveSessionID(props?: Record<string, unknown>): string | undefined {
+  return (props?.sessionID ??
+    (props?.info as { id?: string } | undefined)?.id) as string | undefined
+}

src/hooks/compaction-context-injector/session-prompt-config-resolver.ts

@@ -0,0 +1,131 @@
+import { getSessionAgent } from "../../features/claude-code-session-state"
+import type { CompactionAgentConfigCheckpoint } from "../../shared/compaction-agent-config-checkpoint"
+import { log } from "../../shared/logger"
+import { normalizeSDKResponse } from "../../shared/normalize-sdk-response"
+import { normalizePromptTools } from "../../shared/prompt-tools"
+import { getSessionModel } from "../../shared/session-model-state"
+import { getSessionTools } from "../../shared/session-tools-store"
+
+type SessionMessage = {
+  info?: {
+    agent?: string
+    model?: {
+      providerID?: string
+      modelID?: string
+    }
+    providerID?: string
+    modelID?: string
+    tools?: Record<string, boolean | "allow" | "deny" | "ask">
+  }
+}
+
+type ResolverContext = {
+  client: {
+    session: {
+      messages: (input: { path: { id: string } }) => Promise<unknown>
+    }
+  }
+  directory: string
+}
+
+function isCompactionAgent(agent: string | undefined): boolean {
+  return agent?.trim().toLowerCase() === "compaction"
+}
+
+function resolveModel(
+  info: SessionMessage["info"],
+): CompactionAgentConfigCheckpoint["model"] | undefined {
+  const providerID = info?.model?.providerID ?? info?.providerID
+  const modelID = info?.model?.modelID ?? info?.modelID
+
+  if (!providerID || !modelID) {
+    return undefined
+  }
+
+  return { providerID, modelID }
+}
+
+export async function resolveSessionPromptConfig(
+  ctx: ResolverContext,
+  sessionID: string,
+): Promise<CompactionAgentConfigCheckpoint> {
+  const promptConfig: CompactionAgentConfigCheckpoint = {
+    agent: getSessionAgent(sessionID),
+    model: getSessionModel(sessionID),
+    tools: getSessionTools(sessionID),
+  }
+
+  try {
+    const response = await ctx.client.session.messages({ path: { id: sessionID } })
+    const messages = normalizeSDKResponse(response, [] as SessionMessage[], {
+      preferResponseOnMissingData: true,
+    })
+
+    for (let index = messages.length - 1; index >= 0; index--) {
+      const info = messages[index].info
+
+      if (!promptConfig.agent && info?.agent && !isCompactionAgent(info.agent)) {
+        promptConfig.agent = info.agent
+      }
+
+      if (!promptConfig.model) {
+        const model = resolveModel(info)
+        if (model) {
+          promptConfig.model = model
+        }
+      }
+
+      if (!promptConfig.tools) {
+        const tools = normalizePromptTools(info?.tools)
+        if (tools) {
+          promptConfig.tools = tools
+        }
+      }
+
+      if (promptConfig.agent && promptConfig.model && promptConfig.tools) {
+        break
+      }
+    }
+  } catch (error) {
+    log("[compaction-context-injector] Failed to resolve prompt config from messages", {
+      sessionID,
+      directory: ctx.directory,
+      error: String(error),
+    })
+  }
+
+  return promptConfig
+}
+
+export async function resolveLatestSessionPromptConfig(
+  ctx: ResolverContext,
+  sessionID: string,
+): Promise<CompactionAgentConfigCheckpoint> {
+  try {
+    const response = await ctx.client.session.messages({ path: { id: sessionID } })
+    const messages = normalizeSDKResponse(response, [] as SessionMessage[], {
+      preferResponseOnMissingData: true,
+    })
+    const latestInfo = messages.at(-1)?.info
+
+    if (!latestInfo) {
+      return {}
+    }
+
+    const model = resolveModel(latestInfo)
+    const tools = normalizePromptTools(latestInfo.tools)
+
+    return {
+      ...(latestInfo.agent ? { agent: latestInfo.agent } : {}),
+      ...(model ? { model } : {}),
+      ...(tools ? { tools } : {}),
+    }
+  } catch (error) {
+    log("[compaction-context-injector] Failed to resolve latest prompt config", {
+      sessionID,
+      directory: ctx.directory,
+      error: String(error),
+    })
+    return {}
+  }
+}

src/hooks/compaction-context-injector/tail-monitor.ts

@@ -0,0 +1,52 @@
+const MEANINGFUL_ASSISTANT_PART_TYPES = new Set([
+  "reasoning",
+  "tool",
+  "tool_use",
+])
+
+export type TailMonitorState = {
+  currentMessageID?: string
+  currentHasOutput: boolean
+  consecutiveNoTextMessages: number
+  lastCompactedAt?: number
+  lastRecoveryAt?: number
+}
+
+export function finalizeTrackedAssistantMessage(
+  state: TailMonitorState,
+): number {
+  if (!state.currentMessageID) {
+    return state.consecutiveNoTextMessages
+  }
+
+  state.consecutiveNoTextMessages = state.currentHasOutput
+    ? 0
+    : state.consecutiveNoTextMessages + 1
+  state.currentMessageID = undefined
+  state.currentHasOutput = false
+
+  return state.consecutiveNoTextMessages
+}
+
+export function shouldTreatAssistantPartAsOutput(part: {
+  type?: string
+  text?: string
+}): boolean {
+  if (part.type === "text") {
+    return !!part.text?.trim()
+  }
+
+  return typeof part.type === "string" && MEANINGFUL_ASSISTANT_PART_TYPES.has(part.type)
+}
+
+export function trackAssistantOutput(
+  state: TailMonitorState,
+  messageID?: string,
+): void {
+  if (messageID && !state.currentMessageID) {
+    state.currentMessageID = messageID
+  }
+
+  state.currentHasOutput = true
+  state.consecutiveNoTextMessages = 0
+}

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

@@ -0,0 +1,25 @@
+export interface CompactionContextInjector {
+  capture: (sessionID: string) => Promise<void>
+  inject: (sessionID?: string) => string
+  event: (input: { event: { type: string; properties?: unknown } }) => Promise<void>
+}
+
+export type CompactionContextClient = {
+  client: {
+    session: {
+      messages: (input: { path: { id: string } }) => Promise<unknown>
+      promptAsync: (input: {
+        path: { id: string }
+        body: {
+          noReply?: boolean
+          agent?: string
+          model?: { providerID: string; modelID: string }
+          tools?: Record<string, boolean>
+          parts: Array<{ type: "text"; text: string }>
+        }
+        query?: { directory: string }
+      }) => Promise<unknown>
+    }
+  }
+  directory: string
+}

src/index.test.ts

@@ -2,6 +2,10 @@ import { describe, expect, it, mock } from "bun:test"
 
 describe("experimental.session.compacting handler", () => {
   function createCompactingHandler(hooks: {
+    compactionContextInjector?: {
+      capture: (sessionID: string) => Promise<void>
+      inject: (sessionID: string) => string
+    }
     compactionTodoPreserver?: { capture: (sessionID: string) => Promise<void> }
     claudeCodeHooks?: {
       "experimental.session.compacting"?: (
@@ -9,19 +13,19 @@ describe("experimental.session.compacting handler", () => {
         output: { context: string[] },
       ) => Promise<void>
     }
-    compactionContextInjector?: (sessionID: string) => string
   }) {
     return async (
       _input: { sessionID: string },
       output: { context: string[] },
     ): Promise<void> => {
+      await hooks.compactionContextInjector?.capture(_input.sessionID)
       await hooks.compactionTodoPreserver?.capture(_input.sessionID)
       await hooks.claudeCodeHooks?.["experimental.session.compacting"]?.(
         _input,
         output,
       )
       if (hooks.compactionContextInjector) {
-        output.context.push(hooks.compactionContextInjector(_input.sessionID))
+        output.context.push(hooks.compactionContextInjector.inject(_input.sessionID))
       }
     }
   }
@@ -33,6 +37,15 @@ describe("experimental.session.compacting handler", () => {
     const callOrder: string[] = []
 
     const handler = createCompactingHandler({
+      compactionContextInjector: {
+        capture: mock(async () => {
+          callOrder.push("checkpointCapture")
+        }),
+        inject: mock((sessionID: string) => {
+          callOrder.push("contextInjector")
+          return `context-for-${sessionID}`
+        }),
+      },
       compactionTodoPreserver: {
         capture: mock(async () => { callOrder.push("capture") }),
       },
@@ -41,16 +54,12 @@ describe("experimental.session.compacting handler", () => {
           callOrder.push("preCompact")
         }),
       },
-      compactionContextInjector: mock((sessionID: string) => {
-        callOrder.push("contextInjector")
-        return `context-for-${sessionID}`
-      }),
     })
 
     const output = { context: [] as string[] }
     await handler({ sessionID: "ses_test" }, output)
 
-    expect(callOrder).toEqual(["capture", "preCompact", "contextInjector"])
+    expect(callOrder).toEqual(["checkpointCapture", "capture", "preCompact", "contextInjector"])
     expect(output.context).toEqual(["context-for-ses_test"])
   })
 
@@ -77,17 +86,22 @@ describe("experimental.session.compacting handler", () => {
   //#then handler completes without error and other hooks still run
   it("handles null claudeCodeHooks gracefully", async () => {
     const captureMock = mock(async () => {})
+    const checkpointCaptureMock = mock(async () => {})
     const contextMock = mock(() => "injected-context")
 
     const handler = createCompactingHandler({
+      compactionContextInjector: {
+        capture: checkpointCaptureMock,
+        inject: contextMock,
+      },
       compactionTodoPreserver: { capture: captureMock },
       claudeCodeHooks: undefined,
-      compactionContextInjector: contextMock,
     })
 
     const output = { context: [] as string[] }
     await handler({ sessionID: "ses_test" }, output)
 
+    expect(checkpointCaptureMock).toHaveBeenCalledWith("ses_test")
     expect(captureMock).toHaveBeenCalledWith("ses_test")
     expect(contextMock).toHaveBeenCalledWith("ses_test")
     expect(output.context).toEqual(["injected-context"])

src/index.ts

@@ -83,13 +83,14 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       _input: { sessionID: string },
       output: { context: string[] },
     ): Promise<void> => {
+      await hooks.compactionContextInjector?.capture(_input.sessionID)
       await hooks.compactionTodoPreserver?.capture(_input.sessionID)
       await hooks.claudeCodeHooks?.["experimental.session.compacting"]?.(
         _input,
         output,
       )
       if (hooks.compactionContextInjector) {
-        output.context.push(hooks.compactionContextInjector(_input.sessionID))
+        output.context.push(hooks.compactionContextInjector.inject(_input.sessionID))
       }
     },
   }

src/plugin/event.ts

@@ -184,6 +184,7 @@ export function createEventHandler(args: {
     await Promise.resolve(hooks.interactiveBashSession?.event?.(input as EventInput));
     await Promise.resolve(hooks.ralphLoop?.event?.(input));
     await Promise.resolve(hooks.stopContinuationGuard?.event?.(input));
+    await Promise.resolve(hooks.compactionContextInjector?.event?.(input));
     await Promise.resolve(hooks.compactionTodoPreserver?.event?.(input));
     await Promise.resolve(hooks.writeExistingFileGuard?.event?.(input));
     await Promise.resolve(hooks.atlasHook?.handler?.(input));

src/plugin/hooks/create-continuation-hooks.ts

@@ -56,7 +56,8 @@ export function createContinuationHooks(args: {
     : null
 
   const compactionContextInjector = isHookEnabled("compaction-context-injector")
-    ? safeHook("compaction-context-injector", () => createCompactionContextInjector(backgroundManager))
+    ? safeHook("compaction-context-injector", () =>
+        createCompactionContextInjector({ ctx, backgroundManager }))
     : null
 
   const compactionTodoPreserver = isHookEnabled("compaction-todo-preserver")

src/shared/compaction-agent-config-checkpoint.ts

@@ -0,0 +1,42 @@
+export type CompactionAgentConfigCheckpoint = {
+  agent?: string
+  model?: { providerID: string; modelID: string }
+  tools?: Record<string, boolean>
+}
+
+const checkpoints = new Map<string, CompactionAgentConfigCheckpoint>()
+
+function cloneCheckpoint(
+  checkpoint: CompactionAgentConfigCheckpoint,
+): CompactionAgentConfigCheckpoint {
+  return {
+    ...(checkpoint.agent ? { agent: checkpoint.agent } : {}),
+    ...(checkpoint.model
+      ? {
+          model: {
+            providerID: checkpoint.model.providerID,
+            modelID: checkpoint.model.modelID,
+          },
+        }
+      : {}),
+    ...(checkpoint.tools ? { tools: { ...checkpoint.tools } } : {}),
+  }
+}
+
+export function setCompactionAgentConfigCheckpoint(
+  sessionID: string,
+  checkpoint: CompactionAgentConfigCheckpoint,
+): void {
+  checkpoints.set(sessionID, cloneCheckpoint(checkpoint))
+}
+
+export function getCompactionAgentConfigCheckpoint(
+  sessionID: string,
+): CompactionAgentConfigCheckpoint | undefined {
+  const checkpoint = checkpoints.get(sessionID)
+  return checkpoint ? cloneCheckpoint(checkpoint) : undefined
+}
+
+export function clearCompactionAgentConfigCheckpoint(sessionID: string): void {
+  checkpoints.delete(sessionID)
+}