fix(plugin): verify event hook compatibility with v1.4.0

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -1,5 +1,5 @@
 import { describe, test, expect, beforeEach, afterEach } from "bun:test"
-import { mkdtempSync, writeFileSync, rmSync } from "node:fs"
+import { mkdtempSync, mkdirSync, writeFileSync, rmSync } from "node:fs"
 import { join } from "node:path"
 import { tmpdir } from "node:os"
 import {
@@ -11,6 +11,7 @@ import {
   clearCompactionAgentConfigCheckpoint,
   setCompactionAgentConfigCheckpoint,
 } from "../../shared/compaction-agent-config-checkpoint"
+import { PART_STORAGE } from "../../shared"
 
 describe("isCompactionAgent", () => {
   describe("#given agent name variations", () => {
@@ -73,6 +74,7 @@ describe("findNearestMessageExcludingCompaction", () => {
 
   afterEach(() => {
     rmSync(tempDir, { force: true, recursive: true })
+    rmSync(join(PART_STORAGE, "msg_test_background_compaction_marker"), { force: true, recursive: true })
     clearCompactionAgentConfigCheckpoint("ses_checkpoint")
   })
 
@@ -116,6 +118,30 @@ describe("findNearestMessageExcludingCompaction", () => {
       expect(result?.agent).toBe("sisyphus")
     })
 
+    test("skips JSON messages whose part storage contains a compaction marker", () => {
+      // given
+      const compactionMessageID = "msg_test_background_compaction_marker"
+      const partDir = join(PART_STORAGE, compactionMessageID)
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify({
+        id: compactionMessageID,
+        agent: "atlas",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+      }))
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify({
+        id: "msg_001",
+        agent: "sisyphus",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+      }))
+      mkdirSync(partDir, { recursive: true })
+      writeFileSync(join(partDir, "prt_0001.json"), JSON.stringify({ type: "compaction" }))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result?.agent).toBe("sisyphus")
+    })
+
     test("falls back to partial agent/model match", () => {
       // given
       const messageWithAgentOnly = {
@@ -256,4 +282,28 @@ describe("resolvePromptContextFromSessionMessages", () => {
       tools: { bash: true },
     })
   })
+
+  test("skips SDK messages that only exist to mark compaction", () => {
+    // given
+    const messages = [
+      {
+        id: "msg_compaction",
+        info: { agent: "atlas", model: { providerID: "openai", modelID: "gpt-5" } },
+        parts: [{ type: "compaction" }],
+      },
+      { info: { agent: "sisyphus" } },
+      { info: { model: { providerID: "anthropic", modelID: "claude-opus-4-1" } } },
+      { info: { tools: { bash: true } } },
+    ]
+
+    // when
+    const result = resolvePromptContextFromSessionMessages(messages)
+
+    // then
+    expect(result).toEqual({
+      agent: "sisyphus",
+      model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+      tools: { bash: true },
+    })
+  })
 })

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

@@ -2,8 +2,16 @@ 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"
+import {
+  hasCompactionPartInStorage,
+  isCompactionAgent,
+  isCompactionMessage,
+} from "../../shared/compaction-marker"
+
+export { isCompactionAgent } from "../../shared/compaction-marker"
 
 type SessionMessage = {
+  id?: string
   info?: {
     agent?: string
     model?: {
@@ -15,10 +23,7 @@ type SessionMessage = {
     modelID?: string
     tools?: StoredMessage["tools"]
   }
-}
-
-export function isCompactionAgent(agent: string | undefined): boolean {
-  return agent?.trim().toLowerCase() === "compaction"
+  parts?: Array<{ type?: string }>
 }
 
 function hasFullAgentAndModel(message: StoredMessage): boolean {
@@ -35,6 +40,10 @@ function hasPartialAgentOrModel(message: StoredMessage): boolean {
 }
 
 function convertSessionMessageToStoredMessage(message: SessionMessage): StoredMessage | null {
+  if (isCompactionMessage(message)) {
+    return null
+  }
+
   const info = message.info
   if (!info) {
     return null
@@ -138,7 +147,11 @@ export function findNearestMessageExcludingCompaction(
     for (const file of files) {
       try {
         const content = readFileSync(join(messageDir, file), "utf-8")
-        messages.push(JSON.parse(content) as StoredMessage)
+        const parsed = JSON.parse(content) as StoredMessage & { id?: string }
+        if (hasCompactionPartInStorage(parsed.id) || isCompactionAgent(parsed.agent)) {
+          continue
+        }
+        messages.push(parsed)
       } catch {
         continue
       }