diff --git a/src/hooks/prometheus-md-only/index.test.ts b/src/hooks/prometheus-md-only/index.test.ts
index d5644240..07dff9bf 100644
--- a/src/hooks/prometheus-md-only/index.test.ts
+++ b/src/hooks/prometheus-md-only/index.test.ts
@@ -352,6 +352,121 @@ describe("prometheus-md-only", () => {
     })
   })
 
+  describe("boulder state priority over message files (fixes #927)", () => {
+    const BOULDER_DIR = join(tmpdir(), `boulder-test-${randomUUID()}`)
+    const BOULDER_FILE = join(BOULDER_DIR, ".sisyphus", "boulder.json")
+
+    beforeEach(() => {
+      mkdirSync(join(BOULDER_DIR, ".sisyphus"), { recursive: true })
+    })
+
+    afterEach(() => {
+      rmSync(BOULDER_DIR, { recursive: true, force: true })
+    })
+
+    //#given session was started with prometheus (first message), but /start-work set boulder agent to atlas
+    //#when user types "continue" after interruption (memory cleared, falls back to message files)
+    //#then should use boulder state agent (atlas), not message file agent (prometheus)
+    test("should prioritize boulder agent over message file agent", async () => {
+      // given - prometheus in message files (from /plan)
+      setupMessageStorage(TEST_SESSION_ID, "prometheus")
+      
+      // given - atlas in boulder state (from /start-work)
+      writeFileSync(BOULDER_FILE, JSON.stringify({
+        active_plan: "/test/plan.md",
+        started_at: new Date().toISOString(),
+        session_ids: [TEST_SESSION_ID],
+        plan_name: "test-plan",
+        agent: "atlas"
+      }))
+
+      const hook = createPrometheusMdOnlyHook({
+        client: {},
+        directory: BOULDER_DIR,
+      } as never)
+
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: "/path/to/code.ts" },
+      }
+
+      // when / then - should NOT block because boulder says atlas, not prometheus
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).resolves.toBeUndefined()
+    })
+
+    test("should use prometheus from boulder state when set", async () => {
+      // given - atlas in message files (from some other agent)
+      setupMessageStorage(TEST_SESSION_ID, "atlas")
+      
+      // given - prometheus in boulder state (edge case, but should honor it)
+      writeFileSync(BOULDER_FILE, JSON.stringify({
+        active_plan: "/test/plan.md",
+        started_at: new Date().toISOString(),
+        session_ids: [TEST_SESSION_ID],
+        plan_name: "test-plan",
+        agent: "prometheus"
+      }))
+
+      const hook = createPrometheusMdOnlyHook({
+        client: {},
+        directory: BOULDER_DIR,
+      } as never)
+
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: "/path/to/code.ts" },
+      }
+
+      // when / then - should block because boulder says prometheus
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).rejects.toThrow("can only write/edit .md files")
+    })
+
+    test("should fall back to message files when session not in boulder", async () => {
+      // given - prometheus in message files
+      setupMessageStorage(TEST_SESSION_ID, "prometheus")
+      
+      // given - boulder state exists but for different session
+      writeFileSync(BOULDER_FILE, JSON.stringify({
+        active_plan: "/test/plan.md",
+        started_at: new Date().toISOString(),
+        session_ids: ["other-session-id"],
+        plan_name: "test-plan",
+        agent: "atlas"
+      }))
+
+      const hook = createPrometheusMdOnlyHook({
+        client: {},
+        directory: BOULDER_DIR,
+      } as never)
+
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: "/path/to/code.ts" },
+      }
+
+      // when / then - should block because falls back to message files (prometheus)
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).rejects.toThrow("can only write/edit .md files")
+    })
+  })
+
   describe("without message storage", () => {
     test("should handle missing session gracefully (no agent found)", async () => {
       // given
diff --git a/src/hooks/prometheus-md-only/index.ts b/src/hooks/prometheus-md-only/index.ts
index 1c311dc0..7c9131f9 100644
--- a/src/hooks/prometheus-md-only/index.ts
+++ b/src/hooks/prometheus-md-only/index.ts
@@ -4,6 +4,7 @@ import { join, resolve, relative, isAbsolute } from "node:path"
 import { HOOK_NAME, PROMETHEUS_AGENT, ALLOWED_EXTENSIONS, ALLOWED_PATH_PREFIX, BLOCKED_TOOLS, PLANNING_CONSULT_WARNING, PROMETHEUS_WORKFLOW_REMINDER } from "./constants"
 import { findNearestMessageWithFields, findFirstMessageWithAgent, MESSAGE_STORAGE } from "../../features/hook-message-injector"
 import { getSessionAgent } from "../../features/claude-code-session-state"
+import { readBoulderState } from "../../features/boulder-state"
 import { log } from "../../shared/logger"
 import { SYSTEM_DIRECTIVE_PREFIX } from "../../shared/system-directive"
 import { getAgentDisplayName } from "../../shared/agent-display-names"
@@ -70,8 +71,31 @@ function getAgentFromMessageFiles(sessionID: string): string | undefined {
   return findFirstMessageWithAgent(messageDir) ?? findNearestMessageWithFields(messageDir)?.agent
 }
 
-function getAgentFromSession(sessionID: string): string | undefined {
-  return getSessionAgent(sessionID) ?? getAgentFromMessageFiles(sessionID)
+/**
+ * Get the effective agent for the session.
+ * Priority order:
+ * 1. In-memory session agent (most recent, set by /start-work)
+ * 2. Boulder state agent (persisted across restarts, fixes #927)
+ * 3. Message files (fallback for sessions without boulder state)
+ *
+ * This fixes issue #927 where after interruption:
+ * - In-memory map is cleared (process restart)
+ * - Message files return "prometheus" (oldest message from /plan)
+ * - But boulder.json has agent: "atlas" (set by /start-work)
+ */
+function getAgentFromSession(sessionID: string, directory: string): string | undefined {
+  // Check in-memory first (current session)
+  const memoryAgent = getSessionAgent(sessionID)
+  if (memoryAgent) return memoryAgent
+
+  // Check boulder state (persisted across restarts) - fixes #927
+  const boulderState = readBoulderState(directory)
+  if (boulderState?.session_ids.includes(sessionID) && boulderState.agent) {
+    return boulderState.agent
+  }
+
+  // Fallback to message files
+  return getAgentFromMessageFiles(sessionID)
 }
 
 export function createPrometheusMdOnlyHook(ctx: PluginInput) {
@@ -80,7 +104,7 @@ export function createPrometheusMdOnlyHook(ctx: PluginInput) {
       input: { tool: string; sessionID: string; callID: string },
       output: { args: Record<string, unknown>; message?: string }
     ): Promise<void> => {
-      const agentName = getAgentFromSession(input.sessionID)
+      const agentName = getAgentFromSession(input.sessionID, ctx.directory)
 
       if (agentName !== PROMETHEUS_AGENT) {
         return