diff --git a/src/hooks/start-work/index.test.ts b/src/hooks/start-work/index.test.ts
index d52a4edab..874c7910f 100644
--- a/src/hooks/start-work/index.test.ts
+++ b/src/hooks/start-work/index.test.ts
@@ -456,6 +456,27 @@ You are starting a Sisyphus work session.
       expect(output.message.agent).toBe(getAgentListDisplayName("atlas"))
     })
 
+    test("should switch to Atlas even when current session is Sisyphus (regression: #3155)", async () => {
+      // given: user runs /start-work while in a Sisyphus session
+      // atlas is registered, so /start-work must always hand off to atlas
+      sessionState.updateSessionAgent("ses-sisyphus-to-atlas", "sisyphus")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        message: {} as Record<string, unknown>,
+        parts: [{ type: "text", text: createStartWorkPrompt() }],
+      }
+
+      await hook["chat.message"](
+        { sessionID: "ses-sisyphus-to-atlas" },
+        output
+      )
+
+      // atlas is registered in beforeEach, so it must be selected
+      expect(output.message.agent).toBe(getAgentListDisplayName("atlas"))
+      expect(sessionState.getSessionAgent("ses-sisyphus-to-atlas")).toBe("atlas")
+    })
+
     test("should keep the current agent when Atlas is unavailable", async () => {
       // given
       sessionState._resetForTesting()
diff --git a/src/hooks/start-work/start-work-hook.ts b/src/hooks/start-work/start-work-hook.ts
index 430e96792..f916c2eae 100644
--- a/src/hooks/start-work/start-work-hook.ts
+++ b/src/hooks/start-work/start-work-hook.ts
@@ -12,12 +12,10 @@ import {
 } from "../../features/boulder-state"
 import { log } from "../../shared/logger"
 import {
-  getAgentConfigKey,
   getAgentDisplayName,
   getAgentListDisplayName,
 } from "../../shared/agent-display-names"
 import {
-  getSessionAgent,
   isAgentRegistered,
   updateSessionAgent,
 } from "../../features/claude-code-session-state"
@@ -84,18 +82,9 @@ export function createStartWorkHook(ctx: PluginInput) {
     }
 
     log(`[${HOOK_NAME}] Processing start-work command`, { sessionID: input.sessionID })
-    const currentSessionAgent = getSessionAgent(input.sessionID)
-    const currentSessionAgentKey = currentSessionAgent
-      ? getAgentConfigKey(currentSessionAgent)
-      : undefined
-    const activeAgent = currentSessionAgent
-      && currentSessionAgentKey
-      && currentSessionAgentKey !== "prometheus"
-      && currentSessionAgentKey !== "atlas"
-        ? currentSessionAgent
-        : isAgentRegistered("atlas")
-          ? "atlas"
-          : "sisyphus"
+    const activeAgent = isAgentRegistered("atlas")
+      ? "atlas"
+      : "sisyphus"
     const activeAgentDisplayName = activeAgent === "atlas"
       ? getAgentListDisplayName(activeAgent)
       : getAgentDisplayName(activeAgent)