MsgPack (LsMsgPack)

io.github.mlsomersv2026.10.10.2Updated Oct 10, 2026

Decodes MsgPack (LsMsgPack schemas, type ids) as text and reads it from variables while debugging.

VerifiedSTDIODesktop onlyDeveloper ToolsData & Analytics

Overview

AI-generated overview

Decodes MsgPack binary data into readable JSON and reads byte variables from a program paused in a VS Code or Visual Studio debugger.

What it does
The server turns MsgPack payloads into JSON with comments, resolving LsMsgPack indexed schemas so type and property ids become names. Tools decode data from hex, base64, byte values, a file, or a previously decoded document; list debug sessions and paused-frame variables; read and decode the bytes of an expression in a paused program; explain what a byte at a given offset is; search for a text or value; and read data into a class of a compiled .NET assembly to report mismatches. Decoded payloads get document ids for follow-up calls, and output can be narrowed by path, byte range, view, and offset options.
When to use it
Useful when an assistant needs to inspect MsgPack payloads that are hard to read as hex or base64, especially LsMsgPack data whose schema is referenced by a 16-byte id. Also useful for debugging .NET programs in VS Code or Visual Studio, where the assistant can read the bytes of a variable or member in a paused frame and decode them.
Requirements
Requires the .NET runtime 8 or later; installed as the global tool LsMsgPack.Mcp, command lsmsgpack-mcp, and runs locally over stdio. Debugger features need the MsgPack Explorer extension for VS Code or the MsgPack Debugger Extension for Visual Studio, which listen on 127.0.0.1 and write a port and token lock file under the user profile. No accounts, API keys, or environment variables are declared.
Before you install
Debugger reads touch a live paused process: by default only variable and member paths are read, but an IDE setting can allow any expression, and reading a non-seekable stream consumes it after the IDE asks the user. The read-as-class tool loads a .NET assembly in a separate process and runs the class's constructors and setters, so only use assemblies you trust. The saveTo option writes decoded bytes to a file.

Installation

In SourceWeft

  1. Open MsgPack (LsMsgPack) in the dashboard and add it to a workspace.
  2. Enable the server for the chats that should use its tools.

Desktop only via STDIO. STDIO servers start a local process, so they need the SourceWeft desktop host.

Other MCP clients

Follow the launch instructions in the repository.

README

LsMsgPack MCP server

MsgPack for AI agents. An MCP server and a command line tool that decode MsgPack data into text an AI (or a person in a terminal) can read, and that read the bytes of a variable of a program paused in the debugger of VS Code or Visual Studio.

Language models read text, not bytes. Hex or base64 costs several tokens per byte, and decoding it by hand is slow and error-prone. Some data can't be decoded from the bytes at all: LsMsgPack's indexed schema writes property names as indexes, and a schema reference holds only the 16-byte id of the schema. This tool decodes the data with the LsMsgPack library itself (the code of MsgPack Explorer) and writes it as JSON with comments:

text
MsgPack "doc1" 247 bytes from payload (byte[], Launch Api).Structure: indexed schema (inline, 3 types) followed by the bodyNo errors, 0 warnings or comments.Schema (inline): type ids and property ids in the body are indexes into these lists.  #0 Invoice: Id, Customer, Billing, Lines, Date, Reference, Tags  #1 Address: Street, Number, City  #2 Line: Product, Quantity, Price
Objects (JSON with // comments: a type name comes from a type id, a name with ~ is inferred from the schema; dictionary keys that are not strings are not quoted):{ // Invoice~  "Id": 1234,  "Customer": "Alice",  "Billing": { "Street": "Main street", "Number": 12, "City": "Amsterdam" }, // Address~  "Lines": [ // 2 items    { "Product": "Apples", "Quantity": 3, "Price": 1.25 }, // Line~, Price: decimal    { "Product": "Pears", "Quantity": 2, "Price": 0.99 } // Line~, Price: decimal  ],  "Date": "2026-10-05T12:30:00Z", // timestamp  "Reference": "5bad8f0fcbd99f46a16570867728950e", // bin, 16 bytes, as Guid 0f8fad5b-d9cb-469f-a165-70867728950e  "Tags": ["fruit", "fresh"] // 2 items}

It reads any MsgPack, not only data written by LsMsgPack. Errors come with their offset, and the values read after an error are marked.

Install

Requires the .NET runtime 8 or later.

bash
dotnet tool install -g LsMsgPack.Mcp      # the command is lsmsgpack-mcp

From source: dotnet publish LsMsgPackMcpServer -c Release -o <folder>, then run dotnet <folder>/LsMsgPackMcp.dll in place of lsmsgpack-mcp.

Add it to your agent

ClientHow
VS Code (Copilot agent mode)Nothing to do with the MsgPack Explorer extension: it registers the server "MsgPack (LsMsgPack)".
Claude Codeclaude mcp add lsmsgpack -- lsmsgpack-mcp
Visual Studio 2022 (17.14+)%USERPROFILE%\.mcp.json (or .mcp.json next to the solution): { "servers": { "lsmsgpack": { "type": "stdio", "command": "lsmsgpack-mcp" } } }
Claude Desktop, Cursor, Windsurf...{ "mcpServers": { "lsmsgpack": { "command": "lsmsgpack-mcp" } } }

In VS Code, MsgPack: Copy MCP Server Configuration... copies these: for the tool when it is installed, otherwise for the server that comes with the extension (its path has the extension's version in it, so copy it again after an update).

Tools

ToolWhat it does
msgpack_debug_statusThe IDEs with the MsgPack extension, their debug sessions, and where the program is paused.
msgpack_debug_localsThe variables of the paused stack frame with their types. The ones that can hold bytes are marked.
msgpack_debug_readReads the bytes of an expression in the paused program and decodes them. Reads byte[], streams (seekable ones go back to their position), Memory<byte>, List<byte>, HttpContent and base64 or hex strings. In VS Code also JavaScript Uint8Array/Buffer and Python bytes. schemaStore reads the schema from the program's SchemaStore when the payload refers to one. saveTo also writes the bytes to a file.
msgpack_decodeDecodes data (hex, base64 or byte values), a file, or a doc decoded before. schemas takes a SchemaStore.Export (or one schema) for schema references.
msgpack_read_as_classReads the data into a class of a compiled .NET assembly (assembly: the .dll of the build output, type: the class) and reports what doesn't match: properties the class doesn't have (with "did you mean"), values that don't convert, classes without a schema entry, type ids that aren't found, with the paths of the objects (reporting differences). Also shows the object that was read. The assembly is loaded in a separate process: it runs the class's constructors and setters.
msgpack_explain_offsetWhat the byte at an offset is: the items holding it, the object member, the header and content bytes, and the bytes around it.
msgpack_searchThe items holding a text or a value, with offsets and object paths.

Every decoded payload gets a document id (doc1, doc2...) for follow-up calls. The output can be narrowed and switched with these options:

  • view: objects (default), items (the MsgPack encodings with offsets) or both.
  • path (e.g. Lines[2]), from/to (a byte range), maxNodes, maxString.
  • offsets adds the offset of every value. issues takes errors, all or none.
  • endian and continueOnError (on by default).

Debugging

The MsgPack extensions of VS Code (MsgPack Explorer) and Visual Studio (MsgPack Debugger Extension) listen on 127.0.0.1. They write a lock file with their port and a random token to ~/.lsmsgpack/ide (%USERPROFILE%\.lsmsgpack\ide). The MCP server uses the IDE that started it, otherwise the one whose workspace holds its working directory (ide picks one when several run).

  • By default only variable and member paths are read (buffer, this._payload, response.Content, items[2].Data), no method calls. The IDE setting can allow any expression: lsmsgpack.mcp.allowAnyExpression in VS Code (user settings only), Tools > Options > MsgPack Explorer in Visual Studio.
  • A stream that can't seek is consumed by reading it, so the IDE asks the user first.
  • The IDE settings can turn the bridge off: lsmsgpack.mcp.enabled in VS Code, and the same options page in Visual Studio.

Command line

bash
lsmsgpack-mcp decode payload.msgpack                # the objectslsmsgpack-mcp decode payload.msgpack --view items   # the items with offsets and encodingslsmsgpack-mcp decode --data "82 a4 4e 61 6d 65 ..." # bytes as textlsmsgpack-mcp explain payload.msgpack 0x7b          # what the byte at 0x7b islsmsgpack-mcp search payload.msgpack Amsterdamlsmsgpack-mcp read-as payload.msgpack --assembly bin/Debug/net8.0/MyApp.dll --type Order   # what doesn't match the classlsmsgpack-mcp debug status | debug locals | debug read <expression>

lsmsgpack-mcp --help lists the options. Without arguments it runs the MCP server on stdin/stdout.

More: docs/Mcp.md.

Source: LsMsgPackMcpServer/README.md at commit 7be5fc8

Tools

0
Tool metadata has not been indexed yet.

Version history

1
  1. v2026.10.10.2LatestOct 10, 2026