diff --git a/src/hooks/atlas/event-handler.ts b/src/hooks/atlas/event-handler.ts
index 0f7187fc..3930b40b 100644
--- a/src/hooks/atlas/event-handler.ts
+++ b/src/hooks/atlas/event-handler.ts
@@ -1,6 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { getPlanProgress, readBoulderState } from "../../features/boulder-state"
-import { subagentSessions } from "../../features/claude-code-session-state"
+import { getSessionAgent, subagentSessions } from "../../features/claude-code-session-state"
 import { log } from "../../shared/logger"
 import { getAgentConfigKey } from "../../shared/agent-display-names"
 import { HOOK_NAME } from "./hook-name"
@@ -97,8 +97,10 @@ export function createAtlasEventHandler(input: {
         return
       }
 
+      const sessionAgent = getSessionAgent(sessionID)
       const lastAgent = await getLastAgentFromSession(sessionID, ctx.client)
-      const lastAgentKey = getAgentConfigKey(lastAgent ?? "")
+      const effectiveAgent = sessionAgent ?? lastAgent
+      const lastAgentKey = getAgentConfigKey(effectiveAgent ?? "")
       const requiredAgent = getAgentConfigKey(boulderState.agent ?? "atlas")
       const lastAgentMatchesRequired = lastAgentKey === requiredAgent
       const boulderAgentDefaultsToAtlas = requiredAgent === "atlas"
@@ -108,7 +110,7 @@ export function createAtlasEventHandler(input: {
       if (!agentMatches) {
         log(`[${HOOK_NAME}] Skipped: last agent does not match boulder agent`, {
           sessionID,
-          lastAgent: lastAgent ?? "unknown",
+          lastAgent: effectiveAgent ?? "unknown",
           requiredAgent,
         })
         return
diff --git a/src/hooks/atlas/index.test.ts b/src/hooks/atlas/index.test.ts
index 36f30827..e7027750 100644
--- a/src/hooks/atlas/index.test.ts
+++ b/src/hooks/atlas/index.test.ts
@@ -9,7 +9,7 @@ import {
   readBoulderState,
 } from "../../features/boulder-state"
 import type { BoulderState } from "../../features/boulder-state"
-import { _resetForTesting, subagentSessions } from "../../features/claude-code-session-state"
+import { _resetForTesting, subagentSessions, updateSessionAgent } from "../../features/claude-code-session-state"
 
 const TEST_STORAGE_ROOT = join(tmpdir(), `atlas-message-storage-${randomUUID()}`)
 const TEST_MESSAGE_STORAGE = join(TEST_STORAGE_ROOT, "message")
@@ -933,7 +933,7 @@ describe("atlas hook", () => {
       expect(callArgs.body.parts[0].text).toContain("2 remaining")
     })
 
-     test("should inject when last agent is sisyphus and boulder targets atlas explicitly", async () => {
+    test("should inject when last agent is sisyphus and boulder targets atlas explicitly", async () => {
        // given - boulder explicitly set to atlas, but last agent is sisyphus (initial state after /start-work)
        const planPath = join(TEST_DIR, "test-plan.md")
        writeFileSync(planPath, "# Plan\n- [ ] Task 1\n- [ ] Task 2")
@@ -1386,5 +1386,38 @@ describe("atlas hook", () => {
       // then - should call prompt because session state was cleaned
       expect(mockInput._promptMock).toHaveBeenCalled()
     })
+
+    test("should inject when session agent was updated to atlas by start-work even if message storage agent differs", async () => {
+      // given - boulder targets atlas, but nearest stored message still says hephaestus
+      const planPath = join(TEST_DIR, "test-plan.md")
+      writeFileSync(planPath, "# Plan\n- [ ] Task 1\n- [ ] Task 2")
+
+      const state: BoulderState = {
+        active_plan: planPath,
+        started_at: "2026-01-02T10:00:00Z",
+        session_ids: [MAIN_SESSION_ID],
+        plan_name: "test-plan",
+        agent: "atlas",
+      }
+      writeBoulderState(TEST_DIR, state)
+
+      cleanupMessageStorage(MAIN_SESSION_ID)
+      setupMessageStorage(MAIN_SESSION_ID, "hephaestus")
+      updateSessionAgent(MAIN_SESSION_ID, "atlas")
+
+      const mockInput = createMockPluginInput()
+      const hook = createAtlasHook(mockInput)
+
+      // when
+      await hook.handler({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: MAIN_SESSION_ID },
+        },
+      })
+
+      // then - should continue because start-work updated session agent to atlas
+      expect(mockInput._promptMock).toHaveBeenCalled()
+    })
   })
 })