fix(cursor): accept BOM-prefixed hook JSON (#12652)

* fix(cursor): accept BOM-prefixed hook JSON

* test(cursor): pin the hook BOM allowance to one leading U+FEFF

Document why the BOM strip exists and cover the narrowness the fix
claims: a double BOM, a whitespace-then-BOM prefix, and a BOM inside
the JSON body are all still rejected.

---------

Co-authored-by: Brennan Benson <79079362+brennanb2025@users.noreply.github.com>

src/shared/agent-hook-listener.test.ts

@@ -152,6 +152,45 @@ describe('shared agent-hook-listener', () => {
     expect(event!.payload.agentType).toBe('claude')
   })
 
+  it('normalizes a BOM-prefixed Cursor hook payload to a working state', () => {
+    const event = normalizeHookPayload(
+      state,
+      'cursor',
+      {
+        paneKey: PANE_KEY,
+        payload: '\uFEFF{"hook_event_name":"beforeSubmitPrompt","prompt":"Synthetic Cursor prompt"}'
+      },
+      'production'
+    )
+
+    expect(event?.payload).toMatchObject({
+      agentType: 'cursor',
+      state: 'working',
+      prompt: 'Synthetic Cursor prompt'
+    })
+    expect(event?.hookEventName).toBe('beforeSubmitPrompt')
+  })
+
+  // Why: pins the allowance to exactly one leading U+FEFF, so nobody widens it into a trim.
+  it('still rejects a hook payload that is malformed once the BOM is removed', () => {
+    const bom = '\uFEFF'
+    const body = '{"hook_event_name":"beforeSubmitPrompt"}'
+    for (const payload of [
+      `${bom}${bom}${body}`,
+      `${bom}not json`,
+      ` ${bom}${body}`,
+      `{"hook_event_name"${bom}:"beforeSubmitPrompt"}`
+    ]) {
+      const event = normalizeHookPayload(
+        state,
+        'cursor',
+        { paneKey: PANE_KEY, payload },
+        'production'
+      )
+      expect(event).toBeNull()
+    }
+  })
+
   it('normalizes Gemini BeforeTool to working with tool fields', () => {
     const event = normalizeHookPayload(
       state,

src/shared/agent-hook-listener.ts

@@ -85,8 +85,12 @@ const AGENT_HOOK_JSON_STRUCTURE_LIMITS = {
 } as const
 
 function parseAgentHookJson(content: string): unknown {
-  assertJsonTextStructureWithinLimits(content, AGENT_HOOK_JSON_STRUCTURE_LIMITS)
-  return JSON.parse(content) as unknown
+  // Why: Cursor on Windows writes UTF-8-with-BOM to the hook's stdin and `JSON.parse` rejects U+FEFF,
+  // so the whole event was dropped. Strip exactly one leading BOM — not a trim — to keep every other
+  // malformed payload rejected as before.
+  const normalizedContent = content.charCodeAt(0) === 0xfeff ? content.slice(1) : content
+  assertJsonTextStructureWithinLimits(normalizedContent, AGENT_HOOK_JSON_STRUCTURE_LIMITS)
+  return JSON.parse(normalizedContent) as unknown
 }
 
 /** Bound the warn-once Sets so a client varying `version`/`env` per request can't grow them unbounded. */