fix(start-work): fall back to sisyphus without atlas

/start-work should leave plan mode even when Atlas is unavailable. This prevents Prometheus from being persisted into boulder state and keeping resumed work sessions in md-only mode.

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

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

src/hooks/start-work/index.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, test, beforeEach, afterEach, spyOn } from "bun:test"
 import { existsSync, mkdirSync, rmSync, writeFileSync } from "node:fs"
 import { join } from "node:path"
-import { tmpdir, homedir } from "node:os"
+import { tmpdir } from "node:os"
 import { randomUUID } from "node:crypto"
 import { createStartWorkHook } from "./index"
 import {
@@ -448,6 +448,69 @@ describe("start-work hook", () => {
       expect(output.message.agent).toBe("Sisyphus (Ultraworker)")
       expect(sessionState.getSessionAgent("ses-prometheus-to-sisyphus")).toBe("sisyphus")
     })
+
+    test("should fall back to Sisyphus instead of keeping Prometheus when Atlas is unavailable", async () => {
+      // given
+      sessionState._resetForTesting()
+      sessionState.registerAgentName("prometheus")
+      sessionState.registerAgentName("sisyphus")
+      sessionState.updateSessionAgent("ses-prometheus-to-worker", "prometheus")
+
+      const plansDir = join(testDir, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+      writeFileSync(join(plansDir, "worker-plan.md"), "# Plan\n- [ ] Task 1")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        message: {} as Record<string, unknown>,
+        parts: [{ type: "text", text: "<session-context></session-context>" }],
+      }
+
+      // when
+      await hook["chat.message"](
+        { sessionID: "ses-prometheus-to-worker" },
+        output
+      )
+
+      // then
+      expect(output.message.agent).toBe("Sisyphus (Ultraworker)")
+      expect(sessionState.getSessionAgent("ses-prometheus-to-worker")).toBe("sisyphus")
+      expect(readBoulderState(testDir)?.agent).toBe("sisyphus")
+    })
+
+    test("should rewrite stale Prometheus boulder state to Sisyphus when resuming without Atlas", async () => {
+      // given
+      sessionState._resetForTesting()
+      sessionState.registerAgentName("prometheus")
+      sessionState.registerAgentName("sisyphus")
+      sessionState.updateSessionAgent("ses-prometheus-resume", "prometheus")
+
+      const planPath = join(testDir, "resume-plan.md")
+      writeFileSync(planPath, "# Plan\n- [ ] Task 1")
+      writeBoulderState(testDir, {
+        active_plan: planPath,
+        started_at: "2026-01-02T10:00:00Z",
+        session_ids: ["old-session"],
+        plan_name: "resume-plan",
+        agent: "prometheus",
+      })
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        message: {} as Record<string, unknown>,
+        parts: [{ type: "text", text: "<session-context></session-context>" }],
+      }
+
+      // when
+      await hook["chat.message"](
+        { sessionID: "ses-prometheus-resume" },
+        output
+      )
+
+      // then
+      expect(output.message.agent).toBe("Sisyphus (Ultraworker)")
+      expect(readBoulderState(testDir)?.agent).toBe("sisyphus")
+    })
   })
 
   describe("worktree support", () => {

src/hooks/start-work/start-work-hook.ts

@@ -11,7 +11,7 @@ import {
   clearBoulderState,
 } from "../../features/boulder-state"
 import { log } from "../../shared/logger"
-import { getAgentDisplayName } from "../../shared/agent-display-names"
+import { getAgentConfigKey, getAgentDisplayName } from "../../shared/agent-display-names"
 import { getSessionAgent, isAgentRegistered, updateSessionAgent } from "../../features/claude-code-session-state"
 import { detectWorktreePath } from "./worktree-detector"
 import { parseUserRequest } from "./parse-user-request"
@@ -80,9 +80,12 @@ export function createStartWorkHook(ctx: PluginInput) {
       if (!promptText.includes("<session-context>")) return
 
       log(`[${HOOK_NAME}] Processing start-work command`, { sessionID: input.sessionID })
+      const currentSessionAgent = getSessionAgent(input.sessionID)
       const activeAgent = isAgentRegistered("atlas")
         ? "atlas"
-        : getSessionAgent(input.sessionID) ?? "sisyphus"
+        : currentSessionAgent && getAgentConfigKey(currentSessionAgent) !== "prometheus"
+          ? currentSessionAgent
+          : "sisyphus"
       const activeAgentDisplayName = getAgentDisplayName(activeAgent)
       updateSessionAgent(input.sessionID, activeAgent)
       if (output.message) {
@@ -162,17 +165,20 @@ No incomplete plans available. Create a new plan with: /plan "your task"`
 
         if (!progress.isComplete) {
           const effectiveWorktree = worktreePath ?? existingState.worktree_path
+          const sessionAlreadyTracked = existingState.session_ids.includes(sessionId)
+          const updatedSessions = sessionAlreadyTracked
+            ? existingState.session_ids
+            : [...existingState.session_ids, sessionId]
+          const shouldRewriteState = existingState.agent !== activeAgent || worktreePath !== undefined
 
-          if (worktreePath !== undefined) {
-            const updatedSessions = existingState.session_ids.includes(sessionId)
-              ? existingState.session_ids
-              : [...existingState.session_ids, sessionId]
+          if (shouldRewriteState) {
             writeBoulderState(ctx.directory, {
               ...existingState,
-              worktree_path: worktreePath,
+              agent: activeAgent,
+              ...(worktreePath !== undefined ? { worktree_path: worktreePath } : {}),
               session_ids: updatedSessions,
             })
-          } else {
+          } else if (!sessionAlreadyTracked) {
             appendSessionId(ctx.directory, sessionId)
           }