# Locate where a behavior is implemented in Fortran with Atlas in 2026

> Fortran developers in 2026 can use Atlas to pinpoint the exact file and symbol responsible for a behavior, even when only knowing what the software does.

Atlas, the terminal-native AI coding agent, empowers Fortran developers in 2026 to precisely locate where a behavior is implemented within their codebase. By integrating with the Fortran Package Manager (fpm), fprettify for formatting, and gfortran diagnostics, Atlas provides a comprehensive approach to understanding complex Fortran applications, from legacy FIXED-form numerics to modern coarrays and explicit interfaces.

## Key takeaways

- Atlas uses hybrid semantic and keyword search to find Fortran code, even without exact matches.
- `grep` provides precise regex matching for Fortran source files like `*.f90` within `fpm` projects.
- The `lsp` tool navigates Fortran module dependencies and symbol graphs, revealing call paths.
- Atlas ensures safety with permission-gated tools and unified diff approval for Fortran code edits.
- Fortran developers can leverage Atlas with `fpm`, `fprettify`, and `gfortran` for a complete workflow.
- Atlas indexes Fortran code by AST declarations, understanding modules, interfaces, and dummy arguments.

## How Atlas Locates Fortran Behavior with Semantic Search

Atlas in 2026 employs a three-pronged strategy to locate Fortran behavior, starting with semantic search. This approach leverages hybrid semantic and keyword retrieval, fused by reciprocal rank fusion, to understand the intent behind your query, even if your exact words do not appear in `src/` or `test/` files.

When you need to find a Fortran behavior but only know what it does, Atlas's `codebase_search` tool is your first step. It indexes your Fortran code by AST declarations using tree-sitter, not blind line windows, giving it a deep understanding of your modules, explicit interfaces, and intent declarations on dummy arguments. This allows Atlas to build its code index with local Ollama embeddings, keeping your proprietary Fortran code off third-party servers. You can describe the behavior in natural language, and Atlas will return candidate Fortran declarations, such as a `subroutine` in `src/physics_solver.f90` or a `function` within `src/data_processing_module.f90`, even if your exact phrasing isn't present in the source.

## Confirming Fortran Code Locations with grep and File Access

After initial semantic identification, Atlas in 2026 confirms Fortran code locations using its powerful `grep` tool. This step provides exact text matching capabilities, crucial for verifying specific variable names, subroutine calls, or `COMMON` block references across your `fpm` project structure.

Once `codebase_search` provides potential Fortran files or symbols, the `grep` tool allows you to confirm the exact text. This is vital for Fortran developers who might be looking for specific `CALL` statements, `USE` module directives, or even legacy `COMMON` block declarations. Atlas's `grep` takes a real regex plus include and path filters, running through `ripgrep` for speed. For instance, you might use `grep --include='*.f90' 'CALL my_subroutine'` to pinpoint exact usage. Following confirmation, the `read` tool lets you open the best candidate Fortran source file, such as `src/calculations/matrix_ops.f90`. A wrong guess fails loudly with 'File not found' and a 'Did you mean' list, ensuring bad paths do not go unnoticed and you always access the correct Fortran source.

## Navigating Fortran Symbol Graphs with the LSP Tool

To fully understand a Fortran behavior's implementation, Atlas in 2026 utilizes its `lsp` tool, which connects to Model Context Protocol servers. This allows developers to explore the symbol graph, revealing every callsite for a specific subroutine or function, and jumping directly to declarations within `src/` modules.

The `lsp` tool is indispensable for Fortran developers tracing complex execution flows. After identifying a key Fortran `subroutine` or `function`, you can use `lsp findReferences` to see every callsite across your entire `fpm` project. This helps you understand how different modules interact, for example, how `src/main_program.f90` calls a routine in `src/utilities_module.f90`. Conversely, `lsp workspaceSymbol` allows you to jump directly to the declaration of any Fortran symbol by name, whether it's a `TYPE`, a `MODULE`, or an `INTERFACE`. This capability is crucial for navigating modern Fortran codebases with their rich module dependencies and explicit interfaces, providing a clear path from usage to definition.

## Reviewing and Approving Fortran Code Changes with Atlas

Atlas in 2026 ensures developer control and safety throughout the code location and modification process for Fortran projects. Every Atlas tool call is permission-gated, and any proposed file edits, such as moving a `COMMON` block into a module, are presented as a unified diff for explicit approval.

Atlas prioritizes safety and transparency for Fortran developers. Every Atlas tool call, including running `fpm test (test-drive)` or applying `fprettify`, is permission-gated against allow, ask, and deny rules before it runs. Atlas drafts a plan in a read-only plan agent and asks for your approval before switching to a build agent that can modify files. For any proposed changes, such as refactoring a `subroutine` in `src/legacy_code.f90` or adding new `test-drive` unit tests under `test/`, Atlas computes a unified diff for every file edit and surfaces it for your approval before writing. Atlas also connects to your Git repository, reading branches, status, and diffs, and can stage and create commits on your behalf. It even snapshots file changes as Git patches, so edits can be easily diffed and rolled back, providing robust version control for your Fortran codebase.

## Steps

1. Initialize Atlas in your Fortran project by ensuring an `fpm.toml` file exists at the root, allowing Atlas to read your modules and explicit interfaces.
2. Describe the Fortran behavior you want to locate to Atlas using `codebase_search`, for example: "Find where the `calculate_flux` subroutine is implemented within the `physics_module`."
3. Review the candidate Fortran declarations returned by `codebase_search`, which might include `src/physics_module.f90` or `src/solver_routines.f90`.
4. Confirm the exact text or pattern with `grep`, specifying Fortran-specific filters like `grep --include='*.f90' 'CALL calculate_flux'` or `grep --include='*.f90' 'MODULE my_data_types'`.
5. Open the most promising Fortran source file, such as `src/physics_module.f90`, using the `read` tool to inspect its contents and verify the implementation.
6. Use the `lsp` tool's `findReferences` operation on a specific Fortran symbol, like `calculate_flux`, to see all its callsites across your `fpm` project, including those from other modules via `USE` statements.
7. Jump to the declaration of any related Fortran symbol, such as a module variable or an explicit interface, using `lsp workspaceSymbol 'my_data_type'` or `lsp workspaceSymbol 'my_interface_name'`.
8. Summarize the identified Fortran call path and implementation details, including concrete file and line references, back to the user for documentation or further action.

## FAQ

### How does Atlas understand modern Fortran modules and interfaces?

Atlas indexes Fortran code using AST declarations via tree-sitter, allowing it to understand modules, explicit interfaces, and intent declarations on dummy arguments, providing a deep structural awareness of your `fpm` project and its `src/*.f90` files.

### Can Atlas help me find Fortran code that uses COMMON blocks?

Yes, Atlas's `codebase_search` can semantically identify code related to `COMMON` blocks, and `grep` can precisely locate their declarations and uses across your Fortran source files. Atlas can even suggest moving a `COMMON` block into a modern Fortran module for better encapsulation.

### What Fortran tools does Atlas integrate with for testing and formatting?

Atlas integrates directly with the Fortran Package Manager (`fpm`) for running `fpm test (test-drive)` behind a permission prompt, and it can run `fprettify` over changed Fortran sources before you approve a diff, ensuring consistent code style in your `src/` directory.

### How does Atlas ensure I approve changes before they are written to my Fortran project?

Atlas operates with a read-only plan agent before switching to a build agent. It computes a unified diff for every proposed file edit in your Fortran project, such as refactoring a `subroutine` or `function`, and surfaces it for your explicit approval before writing any changes to `src/*.f90`.

### Can Atlas find Fortran code even if I don't know the exact subroutine name?

Yes, Atlas's `codebase_search` uses hybrid semantic retrieval, meaning you can describe the behavior in natural language, for instance, 'find the routine that handles atmospheric pressure calculations,' and it will return candidate Fortran declarations even if your words do not appear verbatim in the `src/` code.

### How does Atlas handle large Fortran codebases for search performance?

Atlas builds its code index with local Ollama embeddings, keeping your Fortran code off third-party servers. Its hybrid semantic and keyword retrieval, fused by reciprocal rank fusion, efficiently searches large Fortran codebases for relevant declarations, including those in legacy FIXED-form numerics and modern coarrays.

### What if I make a mistake or want to revert changes Atlas made to my Fortran files?

Atlas snapshots file changes as git patches, allowing edits to be easily diffed and rolled back. It also reads git branches, status, and diffs, and can stage and create commits on your behalf for your Fortran project, providing robust version control for `src/` and `test/` directories.

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Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.