# Upgrade an OCaml Dependency and Fix Breakage with Atlas in 2026 > OCaml developers in 2026 leverage Atlas to upgrade `opam` dependencies, automatically fixing `dune` compile and `dune runtest` failures by reading real compiler output and applying `ocamlformat` for In 2026, OCaml developers facing a major dependency upgrade can rely on Atlas to automate the complex process of fixing compile and test failures. Atlas integrates directly with the OCaml toolchain, driving `opam` for package management, interpreting `dune` build errors, and applying `ocamlformat` for style consistency. It reads your `dune-project` configuration and `.mli` interface files to understand your codebase, then uses its AI capabilities to identify and repair breaking changes, ensuring your OCaml application remains functional and well-formatted after a significant library bump. ## Key takeaways - Atlas drives `opam` and `dune` directly for OCaml dependency upgrades. - Real compiler output from `dune build` guides Atlas's OCaml fixes. - `webfetch` provides critical release notes for OCaml breaking changes. - Atlas uses `lsp` `goToDefinition` to inspect new OCaml package signatures. - All OCaml code changes are formatted with `ocamlformat` before approval. - Unified diffs and permission-gated tools ensure safe OCaml migrations. ## How does Atlas upgrade OCaml dependencies and fix `dune` build errors? Atlas streamlines OCaml dependency upgrades in 2026 by directly interacting with `opam` and `dune`. It initiates the upgrade via `bash`, captures the full output, and then systematically addresses every compile and test failure reported by the OCaml compiler, ensuring a robust migration. Atlas begins the dependency upgrade process by executing `opam` commands through its `bash` tool. For instance, to upgrade a package like `lwt` to a new major version, Atlas might run `opam upgrade lwt --ask-on-version-change`. The complete output from `opam` is captured, even if it exceeds typical terminal limits, and saved to a file for Atlas to `read` and analyze. Once the package is updated, Atlas doesn't guess at potential breakage. Instead, it invokes `dune build` or `dune runtest` via `bash` to let the OCaml compiler and test runner enumerate all compile-time and runtime failures. This direct approach ensures that every reported error, from type mismatches in `.mli` files to runtime exceptions, is addressed based on concrete compiler feedback, rather than speculative fixes. Atlas's ability to read and interpret this output is crucial for an accurate and efficient repair process. ## What OCaml commands and files does Atlas interact with during an upgrade? Atlas interacts with core OCaml tools and files, including `opam`, `dune`, `ocamlformat`, and `.mli` interface files, to perform dependency upgrades. In 2026, it reads your `dune-project` configuration and `opam` switch pins to understand the project's structure and dependencies. During an OCaml dependency upgrade, Atlas uses specific, real-world commands and file interactions. It drives `opam` for package management, executing commands like `opam update` and `opam upgrade ` through its `bash` tool. For building and testing, Atlas repeatedly calls `dune build` and `dune runtest`, capturing their output to identify and track errors. When fixing code, Atlas leverages the `lsp` tool's `goToDefinition` operation to inspect new signatures in upgraded packages, ensuring fixes align with the new API. After all functional changes are made, Atlas invokes `ocamlformat` to ensure the codebase adheres to the project's style guidelines, preventing style-related noise in the final diff. Atlas also reads critical OCaml project files, such as the `dune-project` file at the root of your workspace, individual `dune` stanzas, and `.mli` interface files, to build a comprehensive understanding of the codebase's structure and type declarations. This deep integration allows Atlas to make precise, context-aware modifications. ## How does Atlas use release notes to fix OCaml breaking changes? Atlas ensures accurate OCaml fixes by fetching and analyzing library release notes with `webfetch` in 2026. This provides crucial context for breaking changes, allowing Atlas to understand the rationale behind API modifications and apply fixes that align with the library's new design. When an OCaml library undergoes a major version bump, understanding the breaking changes is paramount for correct migration. Atlas addresses this by using its `webfetch` tool to retrieve the library's release notes or changelog. For example, if `ppx_deriving` is upgraded, Atlas might fetch its GitHub release page to understand specific syntax changes or new attribute requirements. This information is then incorporated into Atlas's planning and execution phases. By having the actual breaking changes in context, Atlas can formulate fixes that are not merely reactive to compiler errors but are aligned with the library's intended new API. This prevents "guessing" at solutions and ensures that the modifications made to OCaml source files, such as `.ml` and `.mli` files, are robust and forward-compatible. This capability is especially valuable for complex migrations where a simple type change might mask deeper architectural shifts. ## How does Atlas ensure safety and iterative repair for OCaml upgrades? Atlas ensures safety and iterative repair for OCaml upgrades through a multi-stage process involving read-only planning, permission-gated tool calls, and unified diffs. In 2026, every proposed change to your OCaml codebase, from `dune` stanzas to `.ml` files, requires explicit user approval. Atlas prioritizes safety and user control throughout the OCaml dependency upgrade process. It first drafts a plan in a read-only plan agent, outlining the steps it intends to take, before switching to a build agent. Every Atlas tool call, whether it's `bash` to run `dune build`, `edit` to modify an `.ml` file, or `webfetch` to get release notes, is permission-gated against `allow`, `ask`, and `deny` rules, giving the developer granular control. Atlas employs an iterative repair loop: it fixes one or more errors with its `edit` tool, then re-runs `dune build` and `dune runtest` via `bash` to confirm the fixes and identify remaining issues. This cycle continues until the OCaml project compiles cleanly and all tests pass. Crucially, for every file edit, Atlas computes a unified diff and surfaces it for approval before writing any changes to disk. This allows OCaml developers to review every modification to their `.ml`, `.mli`, and `dune` files, ensuring transparency and preventing unintended consequences. Atlas also snapshots file changes as git patches, enabling easy rollback if needed. ## How does Atlas understand OCaml code for precise fixes? Atlas understands OCaml code with precision by indexing AST declarations using tree-sitter, not blind line windows. In 2026, it builds its code index with local Ollama embeddings, keeping sensitive OCaml project code off third-party servers and ensuring privacy. Atlas's ability to make precise fixes in OCaml code stems from its sophisticated understanding of the language's structure. Unlike tools that rely on generic keyword searches or blind line windows, Atlas indexes code by Abstract Syntax Tree (AST) declarations using tree-sitter. This means it understands the semantic relationships between OCaml modules, functions, types, and variants, as defined in `.ml` and `.mli` files. For example, when asked to make a variant exhaustive or add an `Alcotest` case to a `test/ dune stanza`, Atlas can work through the AST to identify the exact location and context for the modification. Furthermore, Atlas can build its code index with local Ollama embeddings, ensuring that your proprietary OCaml codebase remains on your local machine, never exposed to third-party servers. This commitment to privacy, combined with its deep semantic understanding of OCaml, allows Atlas to generate highly accurate and contextually appropriate code modifications during complex dependency upgrades. ## Steps 1. Initiate OCaml Upgrade: Ask Atlas to upgrade your target `opam` package, for example, "Atlas, please upgrade `core` to its latest major version." Atlas will use `bash` to run `opam upgrade core --ask-on-version-change` and capture the full output. 2. Fetch Release Notes: Atlas will then use `webfetch` to retrieve the release notes or changelog for the upgraded OCaml library, providing context for breaking changes before any code modifications begin. 3. Build and Identify Breakage: Atlas runs `dune build` via `bash` to compile your OCaml project. It reads the compiler output to enumerate all compile-time errors, rather than predicting them, ensuring every actual breakage is identified. 4. Fix Compile Errors: Atlas uses its `edit` tool to fix each identified OCaml compile error. It leverages the `lsp` tool's `goToDefinition` operation to inspect new signatures in the upgraded `opam` package, ensuring fixes are accurate. 5. Run Tests and Fix Failures: After compile errors are resolved, Atlas runs `dune runtest` via `bash`. It reads the test runner's output and uses `edit` to address any runtime failures, potentially adding an `Alcotest` case to a `test/ dune stanza` if needed. 6. Iterate Until Clean: Atlas repeatedly runs `dune build` and `dune runtest` through `bash`, fixing errors with `edit` in an iterative loop until the OCaml project compiles cleanly and all tests pass. 7. Format OCaml Code: Once functional, Atlas automatically runs `ocamlformat` to ensure all modified `.ml` and `.mli` files adhere to your project's style guidelines, producing a clean, style-consistent diff. 8. Review and Commit: Atlas presents a unified diff of all changes for your approval. You review the modifications to your OCaml code and `dune` files, then Atlas can stage and create a commit on your behalf. ## FAQ ### How does Atlas handle OCaml type errors after a major dependency upgrade? Atlas addresses OCaml type errors by running `dune build` and reading the precise compiler output. It then uses its `edit` tool to modify `.ml` and `.mli` files, often leveraging the `lsp` tool's `goToDefinition` to inspect the new type signatures in the upgraded `opam` package and apply correct fixes. ### Can Atlas upgrade specific OCaml packages, like `lwt` or `core`? Yes, Atlas can upgrade specific OCaml packages. You instruct it to upgrade a particular `opam` package, and Atlas uses its `bash` tool to execute the appropriate `opam upgrade ` command, then proceeds to fix any resulting breakage. ### How does Atlas ensure my OCaml code style is maintained after fixes? Atlas integrates `ocamlformat` into its workflow. After all functional fixes are applied and the OCaml project compiles and tests cleanly, Atlas automatically runs `ocamlformat` on the modified `.ml` and `.mli` files to ensure the diff is style-clean and adheres to your project's formatting standards. ### What if an OCaml dependency upgrade introduces new test failures? Atlas runs `dune runtest` after fixing compile errors. If new test failures emerge, Atlas reads the test output and uses its `edit` tool to address them. This might involve modifying existing tests or adding new `Alcotest` cases to a `test/ dune stanza` to cover new API behavior. ### Is my OCaml code safe when Atlas is making changes? Yes, Atlas prioritizes safety. It operates with permission-gated tool calls, meaning every action (like `edit`ing an `.ml` file or running `dune`) requires approval. Furthermore, Atlas presents a unified diff for every file edit for your review before writing changes, and it snapshots file changes as git patches for easy rollback. ### How does Atlas know about my OCaml project's structure and modules? Atlas builds a deep understanding of your OCaml project by reading your `dune-project` file, individual `dune` stanzas, and `.mli` interface files. It also indexes code by AST declarations using tree-sitter, providing semantic context for precise modifications. ### Can Atlas help with OCaml variant exhaustiveness after a type change? Yes, Atlas can assist with OCaml variant exhaustiveness. By understanding the AST, Atlas can identify non-exhaustive pattern matches resulting from an upgraded type and suggest or apply modifications to make the variant exhaustive, ensuring type safety. --- Canonical HTML: https://runatlas.sh/resources/stacks/upgrade-a-dependency-and-fix-the-breakage-in-ocaml Source of truth: aeo_pages row `/resources/stacks/upgrade-a-dependency-and-fix-the-breakage-in-ocaml` (segment: Stacks) (this file is generated from it, never hand-edited). Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.