fix(plugin): normalize event session ids

Handle OpenCode session events that carry the session ID under properties.info.id or properties.info.sessionID so background tasks and continuation hooks do not miss idle/error/delete events.

Add regression coverage for nested session.idle events completing background tasks and waking continuation hooks.

src/hooks/atlas/event-handler.ts

@@ -1,5 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { log } from "../../shared/logger"
+import { resolveMessageEventSessionID, resolveSessionEventID } from "../../shared/event-session-id"
 import { HOOK_NAME } from "./hook-name"
 import { isAbortError } from "./is-abort-error"
 import { handleAtlasSessionIdle } from "./idle-event"
@@ -17,7 +18,7 @@ export function createAtlasEventHandler(input: {
     const props = event.properties as Record<string, unknown> | undefined
 
     if (event.type === "session.error") {
-      const sessionID = props?.sessionID as string | undefined
+      const sessionID = resolveSessionEventID(props)
       if (!sessionID) return
 
       const state = getState(sessionID)
@@ -39,7 +40,7 @@ export function createAtlasEventHandler(input: {
     }
 
     if (event.type === "session.idle") {
-      const sessionID = props?.sessionID as string | undefined
+      const sessionID = resolveSessionEventID(props)
       if (!sessionID) return
       await handleAtlasSessionIdle({ ctx, options, getState, sessionID })
       return
@@ -47,7 +48,7 @@ export function createAtlasEventHandler(input: {
 
     if (event.type === "message.updated") {
       const info = props?.info as Record<string, unknown> | undefined
-      const sessionID = info?.sessionID as string | undefined
+      const sessionID = resolveMessageEventSessionID(props)
       const role = info?.role as string | undefined
       if (!sessionID) return
 
@@ -64,7 +65,7 @@ export function createAtlasEventHandler(input: {
 
     if (event.type === "message.part.updated") {
       const info = props?.info as Record<string, unknown> | undefined
-      const sessionID = info?.sessionID as string | undefined
+      const sessionID = resolveMessageEventSessionID(props)
       const role = info?.role as string | undefined
 
       if (sessionID && role === "assistant") {
@@ -78,7 +79,7 @@ export function createAtlasEventHandler(input: {
     }
 
     if (event.type === "tool.execute.before" || event.type === "tool.execute.after") {
-      const sessionID = props?.sessionID as string | undefined
+      const sessionID = resolveMessageEventSessionID(props)
       if (sessionID) {
         const state = sessions.get(sessionID)
         if (state) {
@@ -90,20 +91,20 @@ export function createAtlasEventHandler(input: {
     }
 
     if (event.type === "session.deleted") {
-      const sessionInfo = props?.info as { id?: string } | undefined
-      if (sessionInfo?.id) {
-        const deletedState = sessions.get(sessionInfo.id)
+      const sessionID = resolveSessionEventID(props)
+      if (sessionID) {
+        const deletedState = sessions.get(sessionID)
         if (deletedState?.pendingRetryTimer) {
           clearTimeout(deletedState.pendingRetryTimer)
         }
-        sessions.delete(sessionInfo.id)
-        log(`[${HOOK_NAME}] Session deleted: cleaned up`, { sessionID: sessionInfo.id })
+        sessions.delete(sessionID)
+        log(`[${HOOK_NAME}] Session deleted: cleaned up`, { sessionID })
       }
       return
     }
 
     if (event.type === "session.compacted") {
-      const sessionID = (props?.sessionID ?? (props?.info as { id?: string } | undefined)?.id) as string | undefined
+      const sessionID = resolveSessionEventID(props)
       if (sessionID) {
         const compactedState = sessions.get(sessionID)
         if (compactedState?.pendingRetryTimer) {

src/hooks/atlas/index.test.ts

@@ -1347,6 +1347,38 @@ session_id: ses_untrusted_999
       expect(callArgs.body.parts[0].text).toContain("2 remaining")
     })
 
+    test("should inject continuation when idle event carries session id in info", async () => {
+      // given - boulder state with incomplete plan and nested session event shape
+      const planPath = join(TEST_DIR, "test-plan-info-idle.md")
+      writeFileSync(planPath, "# Plan\n- [ ] Task 1\n- [x] Task 2\n- [ ] Task 3")
+
+      const state: BoulderState = {
+        active_plan: planPath,
+        started_at: "2026-01-02T10:00:00Z",
+        session_ids: [MAIN_SESSION_ID],
+        plan_name: "test-plan-info-idle",
+      }
+      writeBoulderState(TEST_DIR, state)
+
+      const mockInput = createMockPluginInput()
+      const hook = createTestAtlasHook(mockInput)
+
+      // when
+      await hook.handler({
+        event: {
+          type: "session.idle",
+          properties: { info: { id: MAIN_SESSION_ID } },
+        },
+      })
+
+      // then - should call prompt with continuation
+      expect(mockInput._promptMock).toHaveBeenCalled()
+      const callArgs = mockInput._promptMock.mock.calls[0][0]
+      expect(callArgs.path.id).toBe(MAIN_SESSION_ID)
+      expect(callArgs.body.parts[0].text).toContain("incomplete tasks")
+      expect(callArgs.body.parts[0].text).toContain("2 remaining")
+    })
+
     test("should settle idle before injecting boulder continuation", async () => {
       // given
       const planPath = join(TEST_DIR, "test-plan.md")