Mcpp

An MCP server that gives AI agents call graphs, cross-references, and impact analysis for C++ codebases.

With mcpp, your agents don’t have to grep through source files. No more guessing call relationships, missing template references, or wondering "what breaks if I change this?". mcpp gives precise, Clang-parsed semantic data: every caller, every call chain, every override.

What your agent can do with mcpp:

• "Who calls HashGrid::query?" → precise callers, not grep matches
• "Show the call path from main to Renderer::render" → full chain
• "What's the blast radius if I change Scene::update?" → transitive impact
• "Is Aabb::intersects dead code?" → zero callers = dead

Quick start

mcpp index path/to/compile_commands.json

This builds xref.db and prints a config snippet — paste it into your MCP client and you're done.

Usage

index — Build cross-reference database

Parses every translation unit in a compile_commands.json, extracts symbols, references, call edges, and inheritance relations into a SQLite database.

mcpp index path/to/compile_commands.json          # output: xref.db next to compdb
mcpp index path/to/build/ -o /tmp/xref.db         # explicit output path
mcpp index compile_commands.json -j 4             # limit threads (default: all cores)

After indexing, mcpp prints a .mcp.json snippet to stdout for easy copy-paste. Drop it into your MCP client's config:

• Claude/Copilot CLI: //.mcp.json (repo-root), ~/.copilot/mcp-config.json or /mcp add
• VS Code Copilot: //.github/copilot/mcp.json in your repo
• Claude Desktop: claude_desktop_config.json

The snippet uses the mcpServers format supported by most MCP clients:

{
    "mcpServers": {
        "mcpp": {
            "type": "stdio",
            "command": "/path/to/mcpp",
            "args": ["serve", "/path/to/xref.db"]
        }
    }
}

MCP tools (11)

Tool	Description
search_cpp_symbols	Search symbols by name substring
get_cpp_definition	Find where a symbol is defined
find_cpp_references	Find all references to a symbol
find_callers	Direct callers of a function (1-hop in)
find_callees	Direct callees of a function (1-hop out)
get_class_members	List members of a class/struct
get_symbol_stats	Per-member fan-in counts, dead code detection
find_virtual_overrides	Find classes overriding a virtual method
find_call_path	Find call chains between two functions
find_call_path_via	Call path A→B constrained through waypoint C
get_impact_radius	Transitive blast radius of a change (N hops)

Building

Prerequisites

• .NET 8 SDK
• Visual Studio 2022 (v143 toolset, for ClangXref.dll)
• LLVM 21.1.1 static libs (auto-downloaded on first build)

Build steps

# 1. Build native DLL (VS 2022 Developer Command Prompt)
#    Auto-downloads ~540 MB of LLVM libs on first run.
msbuild src\ClangXref\ClangXref.vcxproj /p:Configuration=Release /p:Platform=x64

# 2. Build C# server
dotnet build -c Release src\Mcpp

Both outputs go to bin/.

Generating compile_commands.json

For CMake projects (most common):

cmake -S . -B build -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja

For MSBuild projects, use bt to extract from .binlog, or other tools that generate Clang-compatible compdbs.

Architecture

C# (.NET 8)                           C++ (LLVM 21.1.1)
┌────────────────┐                    ┌──────────────────────┐
│ mcpp.exe       │                    │ ClangXref.dll        │
│ ┌────────────┐ │  P/Invoke (Cdecl)  │ ┌──────────────────┐ │
│ │ CppIndexer │◄├───────────────────►│ │IndexDataConsumer │ │
│ └────────────┘ │  7 callbacks       │ │PPCallbacks       │ │
│ ┌────────────┐ │                    │ └──────────────────┘ │
│ │ XrefDB     │ │                    └──────────────────────┘
│ │ (SQLite)   │ │
│ └────────────┘ │
└────────────────┘