[BUG] Claude Desktop v1.1.8629 shows MCP tool results (Write/Bash) as "failed" despite successful execution

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Desktop v1.1.8629 displays "Tool execution failed" for MCP tool calls (Write, Bash) that actually succeed. The Read tool displays correctly.

All MCP tool calls were verified via server-side logs and file system checks:

| Operation | MCP Execution | File on Disk | Claude Desktop Display |
|-----------|--------------|--------------|----------------------|
| Read | ✅ Success | - | ✅ Shows success |
| Write (id:4) | ✅ Success | ✅ Written | ❌ Shows failed |
| Bash (id:6) | ✅ Success | stdout correct | ❌ Shows failed |
| Write (id:9) | ✅ Success | ✅ Written | ❌ Shows failed |
| Bash (id:10) | ✅ Success | - | ❌ Shows failed |

The AI layer reacts to these false failures with unnecessary error recovery behavior and retries.

Suspected cause: The isError field handling or response format parsing may have regressed in v1.1.8629.

Related issue: #36043 (similar symptoms on WSL, this report is for Windows native)

What Should Happen?

Claude Desktop should correctly display MCP tool results as successful when the MCP server returns a success response. Write and Bash results should be shown the same way Read results are shown.

Error Messages/Logs

MCP server log shows successful execution:
id:4 Write → test file written successfully
id:5 Bash → echo redirect success
id:6 Bash → echo hello success (stdout: "hello")
id:9 Write → test file written successfully
id:10 Bash → echo redirect success

Claude Desktop UI displays: "Tool execution failed" for all Write and Bash operations above.

Steps to Reproduce

1. Open Claude Desktop v1.1.8629 with claude-code MCP server connected
2. Confirm MCP server is running and tools are loaded
3. Ask Claude to execute any Write command (e.g., write "test" to a file)
4. Observe: UI shows "Tool execution failed"
5. Check the file system — the file was actually written successfully
6. Ask Claude to execute any Bash command (e.g., echo hello)
7. Observe: UI shows "Tool execution failed"
8. Check MCP server logs — stdout contains expected output ("hello")
9. Ask Claude to execute a Read command
10. Observe: Read displays correctly (no false failure)

Key finding: Only Write and Bash show false failures. Read works correctly.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.83 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

• Claude Desktop version: v1.1.8629
• Environment: Windows native (not WSL)
• The bug is a display-only issue — all MCP tool operations succeed on the backend
• Workaround: Use Read tool after Write/Bash operations to verify results
• Related issue: #36043 reports similar symptoms on WSL
• This was also reported to Anthropic support via email (Conversation ID: 215473625157740)