diff --git a/src/features/boulder-state/storage.test.ts b/src/features/boulder-state/storage.test.ts
index 9d685e798..967c090cf 100644
--- a/src/features/boulder-state/storage.test.ts
+++ b/src/features/boulder-state/storage.test.ts
@@ -43,6 +43,78 @@ describe("boulder-state", () => {
       expect(result).toBeNull()
     })
 
+    test("should return null for JSON null value", () => {
+      //#given - boulder.json containing null
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, "null")
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).toBeNull()
+    })
+
+    test("should return null for JSON primitive value", () => {
+      //#given - boulder.json containing a string
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, '"just a string"')
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).toBeNull()
+    })
+
+    test("should default session_ids to [] when missing from JSON", () => {
+      //#given - boulder.json without session_ids field
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/path/to/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
+    test("should default session_ids to [] when not an array", () => {
+      //#given - boulder.json with session_ids as a string
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/path/to/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        session_ids: "not-an-array",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
+    test("should default session_ids to [] for empty object", () => {
+      //#given - boulder.json with empty object
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({}))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
     test("should read valid boulder state", () => {
       // given - valid boulder.json
       const state: BoulderState = {
@@ -129,6 +201,23 @@ describe("boulder-state", () => {
       // then
       expect(result).toBeNull()
     })
+
+    test("should not crash when boulder.json has no session_ids field", () => {
+      //#given - boulder.json without session_ids
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = appendSessionId(TEST_DIR, "ses-new")
+
+      //#then - should not crash and should contain the new session
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toContain("ses-new")
+    })
   })
 
   describe("clearBoulderState", () => {
diff --git a/src/features/boulder-state/storage.ts b/src/features/boulder-state/storage.ts
index c42fc8812..2b0d1bdec 100644
--- a/src/features/boulder-state/storage.ts
+++ b/src/features/boulder-state/storage.ts
@@ -22,7 +22,14 @@ export function readBoulderState(directory: string): BoulderState | null {
 
   try {
     const content = readFileSync(filePath, "utf-8")
-    return JSON.parse(content) as BoulderState
+    const parsed = JSON.parse(content)
+    if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
+      return null
+    }
+    if (!Array.isArray(parsed.session_ids)) {
+      parsed.session_ids = []
+    }
+    return parsed as BoulderState
   } catch {
     return null
   }
@@ -48,7 +55,10 @@ export function appendSessionId(directory: string, sessionId: string): BoulderSt
   const state = readBoulderState(directory)
   if (!state) return null
 
-  if (!state.session_ids.includes(sessionId)) {
+  if (!state.session_ids?.includes(sessionId)) {
+    if (!Array.isArray(state.session_ids)) {
+      state.session_ids = []
+    }
     state.session_ids.push(sessionId)
     if (writeBoulderState(directory, state)) {
       return state
diff --git a/src/hooks/atlas/event-handler.ts b/src/hooks/atlas/event-handler.ts
index 4d33b7dab..0857c3db8 100644
--- a/src/hooks/atlas/event-handler.ts
+++ b/src/hooks/atlas/event-handler.ts
@@ -41,7 +41,7 @@ export function createAtlasEventHandler(input: {
 
       // Read boulder state FIRST to check if this session is part of an active boulder
       const boulderState = readBoulderState(ctx.directory)
-      const isBoulderSession = boulderState?.session_ids.includes(sessionID) ?? false
+      const isBoulderSession = boulderState?.session_ids?.includes(sessionID) ?? false
 
       const isBackgroundTaskSession = subagentSessions.has(sessionID)
 
diff --git a/src/hooks/atlas/tool-execute-after.ts b/src/hooks/atlas/tool-execute-after.ts
index a922e422e..f82f3e494 100644
--- a/src/hooks/atlas/tool-execute-after.ts
+++ b/src/hooks/atlas/tool-execute-after.ts
@@ -65,7 +65,7 @@ export function createToolExecuteAfterHandler(input: {
       if (boulderState) {
         const progress = getPlanProgress(boulderState.active_plan)
 
-        if (toolInput.sessionID && !boulderState.session_ids.includes(toolInput.sessionID)) {
+        if (toolInput.sessionID && !boulderState.session_ids?.includes(toolInput.sessionID)) {
           appendSessionId(ctx.directory, toolInput.sessionID)
           log(`[${HOOK_NAME}] Appended session to boulder`, {
             sessionID: toolInput.sessionID,
diff --git a/src/hooks/prometheus-md-only/agent-resolution.ts b/src/hooks/prometheus-md-only/agent-resolution.ts
index 6035209b1..b59c5a3a3 100644
--- a/src/hooks/prometheus-md-only/agent-resolution.ts
+++ b/src/hooks/prometheus-md-only/agent-resolution.ts
@@ -43,7 +43,7 @@ export function getAgentFromSession(sessionID: string, directory: string): strin
 
   // Check boulder state (persisted across restarts) - fixes #927
   const boulderState = readBoulderState(directory)
-  if (boulderState?.session_ids.includes(sessionID) && boulderState.agent) {
+  if (boulderState?.session_ids?.includes(sessionID) && boulderState.agent) {
     return boulderState.agent
   }