fortls-lsp

fortls-lsp

Fortran language server plugin for Claude Code. Provides hover documentation, go-to-definition, find references, and symbol search for Fortran source files using fortls.

Supported Extensions

.f90, .f, .f03, .f08, .for, .fpp and their uppercase preprocessed variants .F90, .F, .F03, .F08, .FOR, .FPP

Prerequisites

Install fortls on each machine:

# macOS via Homebrew
brew install fortls

# via pip (any platform)
pip install fortls

# via pipx (recommended for CLI tools)
pipx install fortls

Verify: fortls --version

Installation

claude plugin marketplace add underwoo/claude-fortls-lsp
claude plugin install fortls-lsp

Then restart Claude Code, or run /reload-plugins in an existing session.

To update later:

claude plugin update fortls-lsp

Then run /reload-plugins or restart.

Verification

After installing, restart Claude Code, then open a Fortran source file and ask Claude a question about a symbol (e.g. "what does this subroutine do?"). Claude should use the LSP hover operation automatically. You can also ask Claude to run documentSymbol on the file to list all subroutines and functions.

Project Configuration

For best results, add a .fortls config file to your Fortran project root (not this plugin directory). fortls reads it automatically on startup.

{
  "source_dirs": ["src", "."],
  "include_dirs": [],
  "exclude_dirs": ["build", "CMakeFiles", ".git"],
  "lowercase_intrinsics": false,
  "max_line_length": -1,
  "max_comment_line_length": -1,
  "use_signature_help": true,
  "variable_hover": true,
  "hover_signature": true,
  "hover_language": "fortran",
  "sort_keywords": true,
  "incremental_sync": true,
  "nthreads": 4
}

Adjust source_dirs to match your project layout — this is the most important setting for cross-file symbol resolution. See the fortls configuration reference for all options.

What Claude Can Do With This Plugin

Once enabled, Claude can automatically:

• Explain what a subroutine or function does (hover)
• Find where a symbol is defined (go-to-definition)
• List all call sites for a subroutine (find references)
• Show all subroutines and modules in a file (document symbols)
• Search for a symbol across the project (workspace symbols)

More Information

• fortls on PyPI
• fortls documentation
• GitHub repository