fix(mcp): force UTF-8 on stdio to fix -32000 on non-ASCII payloads (Windows)#1060
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On Windows, Python defaults sys.stdin/sys.stdout to the system codepage
(e.g. cp1251 on Russian locales, cp1252 on Western European), while MCP
JSON-RPC is always UTF-8. Non-ASCII payloads (Cyrillic, CJK, accented
European) get mis-decoded before reaching handlers, causing json.loads
to fail or tool handlers to receive garbled strings. Both surface to
the client as a generic MCP error -32000.
Reproduction:
1. On Windows with a non-Latin locale, call mempalace_add_drawer or
mempalace_kg_add with Cyrillic/CJK in content or KG object.
2. Server returns: MCP error -32000: Internal tool error.
3. Calling the handler directly from Python works fine -- the bug is
purely in the stdio transport.
Fix:
Reconfigure stdin/stdout to UTF-8 at the start of main(), after
_restore_stdout(). Uses errors="replace" defensively so a lone bad
byte cannot take down the server. Guarded by hasattr(reconfigure)
for exotic stream replacements.
This matches the behaviour of PYTHONUTF8=1 / python -X utf8 without
requiring users to set an env var.
4 tasks
This was referenced May 7, 2026
2 tasks
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Summary
On Windows, the MCP server misdecodes non-ASCII JSON-RPC payloads because Python defaults
sys.stdin/sys.stdoutto the system codepage (e.g.cp1251on Russian locales,cp1252on Western European) while MCP is always UTF-8. Any tool call with Cyrillic, CJK, or accented European characters in a string field returns a genericMCP error -32000: Internal tool error.Root cause
main()inmempalace/mcp_server.pyreads requests withsys.stdin.readline()and writes responses withsys.stdout.write(...)without forcing the encoding. On Windows those streams are opened with the system ANSI codepage. UTF-8 bytes for non-ASCII characters are mis-decoded beforejson.loadsruns, so the parse fails (or the handler receives garbled strings).Calling handlers like
tool_add_drawer(content='тест кириллица')directly in Python works fine — the bug is purely in the stdio transport.Fix
Reconfigure
stdin/stdoutto UTF-8 at the start ofmain(), right after_restore_stdout().errors="replace"is defensive so a single bad byte cannot take down the server. Guarded byhasattr(stream, "reconfigure")for exotic stream replacements (embedded interpreters, test harnesses).This gives the same behaviour as running under
PYTHONUTF8=1/python -X utf8without requiring users to set an env var.Repro
content→MCP error -32000.PYTHONUTF8=1set, the MCP call succeeds.Test plan
mempalace_add_drawer/mempalace_kg_addwith Cyrillic content via Claude Code → succeedsreconfigure(encoding="utf-8")is a no-op there🤖 Generated with Claude Code