AE-YPSPX8EQ
MCP filesystem server head/tail reads corrupt CJK characters that straddle a 1024-byte chunk boundary
MULTILINGUAL FAILUREseverity: LOWcause: LIKELYoutcome: RESOLVED UNVERIFIEDconfidence: MEDIUM
The filesystem MCP server's head and tail file operations decoded each fixed 1024-byte chunk to UTF-8 independently, so a multi-byte character (for example a 3-byte CJK character) split across chunks became replacement characters in the text returned to the agent. A full read of the same file was not affected. A merged fix decodes across chunk boundaries and adds regression tests.
- Framework / agent
- @modelcontextprotocol/server-filesystem · MCP client agent (Claude Desktop) using the filesystem server
- Remediation attempts
- TESTED
- Recurrence
- not documented
- Source languages
- en
- Updated
- 2026-09-29
Sources
- GITHUB ISSUE headFile/tailFile in filesystem server corrupts multi-byte UTF-8 characters at 1024-byte chunk boundaries — github.com/modelcontextprotocol/servers, retrieved 2026-09-29
- GITHUB PULL REQUEST fix(filesystem): preserve UTF-8 across head and tail chunks — github.com/modelcontextprotocol/servers, retrieved 2026-09-29
Symptoms
- Text returned by head/tail contains corrupted characters (U+FFFD) near a 1024-byte boundary in Japanese/CJK files
- A full read of the same file shows no corruption
The full record — root-cause evidence, every remediation attempt with its status and verification, failed attempts, patch references, verbatim quotes and recurrence — is a paid lookup (0.018 USDC via x402). Agents:
GET /api/v1/cases/AE-YPSPX8EQ. PricingSimilarity to your system is not implied. A remediation that worked in the documented context may not work in yours.