pyslang-mcp

pyslang-mcp is a local Model Context Protocol server that gives AI agents compiler-backed, read-only context for Verilog and SystemVerilog projects.

It wraps pyslang so an MCP client can ask questions against parsed and elaborated HDL instead of plain text:

• What modules, interfaces, and packages are in this filelist?
• What diagnostics does the compiler frontend report?
• What is the instance hierarchy below this top?
• Where is this symbol declared or referenced?
• Did my include paths, defines, and nested .f files resolve as expected?

This is not a simulator, synthesizer, waveform viewer, linter replacement, or RTL refactoring tool. It is a small semantic analysis service for local HDL checkouts.

[!NOTE] The project is currently alpha and published on PyPI and the MCP Registry for local stdio use. Install with --pre while the package uses alpha versions.

Why ASIC And EDA Engineers Might Care

Most AI coding tools are good at searching text. HDL usually needs more than that.

In real projects, useful answers often depend on filelists, packages, includes, defines, generate blocks, and hierarchy. pyslang-mcp gives an agent a compact compiler-backed view of that structure, while keeping the server read-only and scoped to a project root you provide.

Good fits:

• triaging parse and semantic diagnostics before asking an agent to reason about RTL
• checking what a .f file expands to
• listing design units in a block or small IP
• finding a declaration without chasing comments and stale grep hits
• getting hierarchy and port-connection context for review/debug prompts
• giving workflow agents HDL context without handing them an EDA runtime

Quickstart

Install the alpha package:

pip install --pre pyslang-mcp

Run the local stdio server:

pyslang-mcp

For contributor setup, clone the repo and install it in editable mode:

git clone https://github.com/ariklapid/pyslang-mcp.git
cd pyslang-mcp
python -m venv .venv
./.venv/bin/pip install -e '.[dev]'

Run the checkout stdio server:

./.venv/bin/python -m pyslang_mcp

The installed console script works too:

./.venv/bin/pyslang-mcp

Run tests:

./.venv/bin/pytest

MCP Client Config

Use local stdio. The MCP client should launch the server on the same machine, VM, or dev container that can see your RTL checkout.

{
  "mcpServers": {
    "pyslang-mcp": {
      "command": "pyslang-mcp",
      "args": []
    }
  }
}

For editable checkout installs, point the command at the checkout virtual environment instead:

{
  "mcpServers": {
    "pyslang-mcp": {
      "command": "/absolute/path/to/pyslang-mcp/.venv/bin/python",
      "args": ["-m", "pyslang_mcp"]
    }
  }
}

Tool calls must provide a project_root. Source paths, filelists, and include directories may be absolute or relative, but they must stay under that root.

Minimal Tool Payloads

Analyze explicit files:

{
  "project_root": "/path/to/rtl-project",
  "files": ["rtl/pkg.sv", "rtl/top.sv"],
  "include_dirs": ["include"],
  "defines": {
    "WIDTH": "32"
  },
  "top_modules": ["top"]
}

Analyze a filelist:

{
  "project_root": "/path/to/rtl-project",
  "filelist": "compile/project.f"
}

Find a symbol:

{
  "project_root": "/path/to/rtl-project",
  "filelist": "compile/project.f",
  "query": "payload",
  "match_mode": "exact",
  "include_references": true
}

Tools

Need	Tool
Load explicit files	pyslang_parse_files
Load a .f filelist	pyslang_parse_filelist
Get parse and semantic diagnostics	pyslang_get_diagnostics
List modules, interfaces, and packages	pyslang_list_design_units
Inspect one design unit	pyslang_describe_design_unit
Walk the elaborated instance tree	pyslang_get_hierarchy
Find declarations and references	pyslang_find_symbol
Summarize syntax node shapes	pyslang_dump_syntax_tree_summary
Check preprocessing metadata and excerpts	pyslang_preprocess_files
Get a compact project overview	pyslang_get_project_summary

Typical flow:

1. Start with pyslang_parse_filelist or pyslang_parse_files.
2. Run pyslang_get_diagnostics.
3. Use pyslang_list_design_units to see what the compiler frontend found.
4. Use pyslang_describe_design_unit, pyslang_get_hierarchy, or pyslang_find_symbol for the actual review/debug question.

Filelist Support

The current parser intentionally supports a practical subset used by many RTL flows:

• source file entries
• nested filelists with -f and -F
• include directories with +incdir+..., -I dir, and -Idir
• defines with +define+..., -D NAME, and -DNAME

Unsupported filelist tokens are reported in the tool output instead of being silently ignored.

Example Agent Prompts

Use this server when compiler-backed context matters:

• "Parse compile/project.f with +define+DEBUG and group diagnostics by source file."
• "List every design unit in this project and identify the likely top modules."
• "From top, show the instance hierarchy down to depth 4."
• "Describe module axi_dma_top: ports, child instances, and declared names."
• "Find the declaration and references for payload_valid."
• "Confirm whether legacy_widget is instantiated anywhere the elaborator sees it."
• "Show the resolved files, include dirs, defines, and unsupported entries from this filelist."

For single-line questions, comments, naming searches, or partial/incomplete source sets, regular rg, editor search, or direct file reading is usually faster and clearer.

Guardrails

• Read-only MCP tools. No RTL edits, formatting, simulation, or synthesis.
• Strict project-root scoping. Paths outside project_root are rejected.
• Compact JSON responses with truncation metadata for large result sets.
• Process-local cache keyed by project config and tracked file mtimes.
• pyslang_preprocess_files is summary-oriented. It returns preprocessing metadata and source excerpts, not a guaranteed full standalone preprocessed stream.
• streamable-http exists only as an explicit experimental local transport; it is not a secure hosted deployment mode.

HDL Example Corpus

The repo includes generated HDL examples under examples/hdl:

• clean reference projects from single modules to small IP-style examples
• intentionally buggy variants labeled easy, medium, and hard
• local validation hooks for both pyslang and verilator --lint-only

Run the full corpus validator:

./.venv/bin/python scripts/validate_hdl_examples.py

Run the CI smoke subset:

./.venv/bin/python scripts/validate_hdl_examples.py --smoke-only

Project Status

Implemented:

• FastMCP stdio server
• CLI entrypoint via python -m pyslang_mcp and pyslang-mcp
• project loader with root checks and .f parsing
• pyslang-backed diagnostics, design-unit listing, hierarchy, symbol lookup, syntax summaries, and project summaries
• bounded in-memory cache
• fixture-backed tests and Ubuntu CI for Python 3.11 and 3.12
• package smoke CI from a built wheel
• manual PyPI Trusted Publishing release workflow with release-gate tests
• PyPI alpha release line: pyslang-mcp
• MCP Registry entry: io.github.ariklapid/pyslang-mcp

Not done yet:

• schema freeze for a non-alpha release
• broad platform validation beyond the current Linux-focused CI path

Development

Useful commands:

./.venv/bin/ruff check .
./.venv/bin/pyright
./.venv/bin/pytest

Architecture and contribution docs:

• docs/architecture.md
• docs/mcp-registry.md
• CONTRIBUTING.md
• pyslang-mcp-plan.md

License

Apache-2.0. See LICENSE.