Markdown file woes

[Robert_Burdett]

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

Summary

Markdown files created or modified by AI agents (using the write tool OR shell commands executed by agents) cause Cursor to either:

1. Fail to open with error: “Assertion Failed: Argument is undefined or null”
2. Open but display as blank (content exists when viewed in other editors)

CRITICAL FINDING: Even files created via shell commands (heredoc, cat) fail to open when created from within Cursor’s agent execution context, suggesting the issue is broader than just the write tool.

Environment

• OS: macOS (darwin 25.2.0)
• Editor: Cursor
• File Type: Markdown (.md files)
• Trigger: Files created/modified by AI agent tools OR shell commands executed by agents

Detailed Symptoms

Symptom 1: Null/Undefined Error

• Error Message: “Unable to open ‘[filename].md’” followed by “Assertion Failed: Argument is undefined or null”
• Occurs: When attempting to open files created with:
  • The write tool
  • Shell commands executed by AI agents (cat, heredoc, etc.)
• Example Files:
  • test_write_tool.md (created via AI agent write tool)
  • CURSOR_BUG_REPORT.md (created via shell heredoc from within agent context)
• Status: File cannot be opened in Cursor at all

Symptom 2: Blank Display

• Symptom: File opens in Cursor but appears completely blank
• Reality: File contains valid content (verified via terminal, TextEdit, and other editors)
• Example File: untitled.md (modified via AI agent search_replace tool)
• Status: File opens but content is invisible in Cursor

Test Results

Files That Work Correctly

All created via shell commands executed OUTSIDE of Cursor’s agent context:

• test_manual.md - Created with echo command (manual)
• test_no_newline.md - Created with printf command (manual)
• test_write_tool_fixed.md - Created with shell heredoc (manual)

Files That Fail

Created/modified via AI agent tools OR shell commands executed WITHIN agent context:

• test_write_tool.md - Created with write tool → Throws null error
• CURSOR_BUG_REPORT.md - Created with shell heredoc (from agent) → Throws null error
• untitled.md - Modified with search_replace tool → Displays blank

Technical Analysis

File Integrity Verification

All failing files pass standard file integrity checks:

• Valid text encoding (ASCII or UTF-8)
• Proper file permissions (-rw-r--r--)
• Correct line endings
• Content verified via cat, hexdump, file commands
• Opens correctly in TextEdit and other editors

File Comparison

• test_write_tool.md (fails) vs test_write_tool_fixed.md (works) are byte-for-byte identical
• Same content, same encoding, same permissions
• Only difference: creation method (AI tool vs manual shell command)

Key Finding: Execution Context Matters

The critical discovery: Files created via shell commands work when created manually, but FAIL when the same shell commands are executed by AI agents from within Cursor. This suggests:

• The issue is NOT specific to the write tool
• Cursor may have problems with files created by processes spawned from its agent execution context
• Cursor’s file watcher may not properly handle files created by child processes
• There may be a race condition or metadata issue when files are created while Cursor is actively monitoring the directory

Extended Attributes

• All files have com.apple.provenance attribute (normal for macOS)
• untitled.md additionally has com.apple.lastuseddate#PS and com.apple.macl (suggesting it was opened before)

Steps to Reproduce

Reproduction Steps

1. Have an AI agent create a new markdown file using either:
   • The write tool, OR
   • Shell commands (cat, heredoc, echo, etc.)
2. Attempt to open the file in Cursor
3. Result: Either get null error or file opens blank

Note: The same shell commands work fine when executed manually outside of agent context.

Expected Behavior

Open a markdown file which was created or modified by an agent.

Operating System

MacOS

Current Cursor Version (Menu → About Cursor → Copy)

Version: 2.2.43
VSCode Version: 1.105.1
Commit: 32cfbe848b35d9eb320980195985450f244b3030
Date: 2025-12-19T06:06:44.644Z
Electron: 37.7.0
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Darwin arm64 25.2.0

Additional Information

Workarounds

1. For new files: Create files manually first (outside Cursor or before agent execution), then have AI agents edit them
2. For existing files: Use search_replace on files that already exist (though this can still cause blank display issue)
3. Recovery: Files can be recovered by opening in TextEdit or other editors, then copying content to a new file
4. For bug reports: Create markdown files manually outside of Cursor before opening

Impact

• Severity: High - Data loss risk (files appear blank, user may think content is lost)
• Frequency: Consistent - Happens every time AI agents create/modify markdown files
• User Impact: Cannot reliably use AI agents to create or modify markdown files in Cursor
• Scope: Affects both write tool AND shell commands executed by agents

Requested Fix

Please investigate:

1. How Cursor handles files created by processes spawned from its agent execution context
2. Why identical file content works when created manually but fails when created by agent-executed processes
3. Why files modified via search_replace display as blank despite containing valid content
4. Whether there’s a file metadata cache that needs to be invalidated
5. Whether Cursor’s file watcher properly handles files created by child processes
6. If there’s a race condition between file creation and Cursor’s file system monitoring

Additional Context

This issue occurs consistently across multiple projects, suggesting it’s a systematic problem with how Cursor processes files created/modified by AI agent tools or processes spawned by agents, not an isolated incident.

Meta Note: Multiple versions of this bug report were created using different methods (write tool, shell heredoc from agent) and ALL failed to open, perfectly demonstrating the bug being reported!

---

Report Generated: 2025-12-20 Test Files Available: All test files remain on Desktop for inspection Encoding Note: Some failing files are UTF-8, some are ASCII - encoding does not appear to be the root cause

Does this stop you from using Cursor

No - Cursor works, but with this issue

[deanrie]

Hey, thanks for the report. This is a known issue related to the filesystem and markdown files created by the Agent.

Related thread: Create/Update .md files -- Unable to open: Assertion Failed: Argument is ‘undefined’ or ‘null’

Your analysis matches what we’ve seen: the problem is in how Cursor handles files created by processes running in the Agent execution context.

Workarounds:

• Create the markdown files manually before running the Agent, then ask the Agent to edit them
• After the Agent creates a file, open it in another editor, then copy the contents into a new file in Cursor

The team is already tracking this. Let me know if you can share any additional details.