# Migrate a deprecated API across every callsite in Actix Web with Atlas in 2026

> Atlas empowers Actix Web developers to migrate deprecated APIs across an entire codebase, ensuring every callsite is updated and validated with `cargo test (actix_web::test)`.

Migrating a deprecated API across an entire Actix Web codebase in 2026 is streamlined with Atlas, which leverages `cargo test (actix_web::test)` for validation, `cargo` for dependency management, and `rustfmt` for consistent code style. Atlas ensures every callsite is identified, tracked, and safely updated, preventing missed migrations and maintaining code integrity.

## Key takeaways

- Atlas uses `lsp findReferences` and `grep` for 100% enumeration of deprecated Actix Web API calls.
- Each Actix Web callsite migration is tracked with `todowrite`, ensuring no task is missed.
- `apply_patch` in Atlas applies context-anchored changes to Actix Web files, failing safely on drift.
- Atlas validates Actix Web changes immediately by running `cargo test (actix_web::test)` after each file edit.
- Code quality is maintained with automated `cargo clippy` and `rustfmt` runs on Actix Web code.
- Atlas provides permission-gated tool calls and unified diffs for full control over Actix Web migrations.

## How to find all deprecated Actix Web API calls with Atlas?

Identifying every deprecated Actix Web API callsite is the critical first step in any migration, and Atlas achieves this with 100% coverage by combining `lsp findReferences` and `grep`. This dual approach ensures that both statically linked and dynamically referenced usages are captured, providing a complete list for your migration project in 2026.

Atlas begins by leveraging the `lsp` tool's `findReferences` operation. When running within a Rust crate that depends on `actix-web`, Atlas queries the language server for all references to the deprecated symbol. This provides a highly accurate list of direct callers within your `src/` directory, including handler functions, extractors, and service registrations. To catch any edge cases, such as string-based or dynamically generated usages that the language server might miss, Atlas cross-checks this list with a `grep` search for the deprecated symbol. This comprehensive enumeration ensures that whether the deprecated API is used in an `actix_web::web::Data` setup, a custom middleware, or a simple route handler, no callsite is overlooked. The results are then used to populate a `todowrite` list, making every required change visible and trackable.

## How does Atlas track migration progress for Actix Web handlers?

Tracking the migration progress for potentially hundreds of Actix Web handler calls and service configurations is crucial for large codebases, and Atlas uses its `todowrite` tool to manage this. Each identified callsite becomes a distinct entry, allowing developers to see partial progress and ensuring no update is silently skipped during the migration process in 2026.

Once Atlas has enumerated all deprecated Actix Web API calls using `lsp findReferences` and `grep`, it creates a `todowrite` entry for each unique callsite. This granular tracking is essential for complex Actix Web applications, where deprecated functions might appear in various contexts, from `actix_web::App::new()` configurations to specific `web::get()` or `web::post()` handler implementations. Each `todowrite` entry represents a discrete unit of work, allowing you to monitor the migration status in real-time. As Atlas successfully applies a patch to a file and `cargo test (actix_web::test)` passes for that change, the corresponding `todowrite` entry is marked as complete. This transparent workflow prevents 'silent skips' and provides a clear audit trail of all modifications, ensuring that the entire codebase transitions off the deprecated API systematically.

## How does Atlas safely apply patches to Actix Web code?

Atlas applies patches to Actix Web code with robust safety mechanisms, using the `apply_patch` tool to ensure changes are context-anchored and precise. This tool will explicitly fail rather than misapplying a patch to a drifted file, providing a critical safeguard against unintended modifications in your `src/` files in 2026.

When migrating a deprecated Actix Web API, Atlas uses the `apply_patch` tool to modify each callsite. This tool is designed for high-fidelity changes: it seeks the exact hunk's context and `old_lines` within the target file. For instance, if you're updating a handler in `src/handlers/user.rs` that uses a deprecated extractor, Atlas will generate a patch that specifically targets that line and its surrounding context. If the file has drifted since the patch was generated - perhaps another developer modified the same lines - `apply_patch` will throw a `Failed to find expected lines` error. This prevents Atlas from guessing or applying a patch incorrectly, which is vital for maintaining the integrity of your Actix Web application. Before any patch is written, Atlas computes a unified diff for every file edit and surfaces it for your approval, giving you full control over every change to your `Cargo.toml`, `src/main.rs`, or other Rust source files.

## How does Atlas validate Actix Web API migrations with `cargo test`?

Validating Actix Web API migrations is integrated directly into the Atlas workflow, with `cargo test (actix_web::test)` run after each file modification. This immediate feedback loop ensures that every change to your `src/` files or `Cargo.toml` maintains the application's functionality, preventing regressions and ensuring a stable migration in 2026.

After Atlas applies a patch to an Actix Web source file, such as `src/routes.rs` or `src/models.rs`, it immediately invokes the `bash` tool to run the affected tests. For Actix Web projects, this means executing `cargo test (actix_web::test)`. This command runs your unit and integration tests, including those specifically written with `actix_web::test` for HTTP request simulation and handler validation. Atlas waits for these tests to pass before marking the corresponding `todowrite` entry as complete. This ensures that each individual migration step is validated against your existing test suite. If tests fail, Atlas will halt, allowing you to inspect the issue, review the diff, and make necessary adjustments. This iterative testing approach, file by file, significantly reduces the risk of introducing bugs and ensures that your Actix Web application remains functional throughout the migration.

## How does Atlas ensure Actix Web code quality after migration?

Maintaining high code quality in Actix Web after a migration is paramount, and Atlas integrates `rustfmt` and `cargo clippy` into its workflow to achieve this. After applying changes and passing tests, Atlas can automatically format your code and clear any new warnings, ensuring your codebase adheres to Rust's best practices in 2026.

Once an Actix Web API migration is complete and all `cargo test (actix_web::test)` runs have passed, Atlas takes an additional step to ensure code quality. It can be prompted to run `cargo clippy` to catch common lints and potential issues that might arise from the new API usage. Following this, Atlas will execute `rustfmt` across the modified files. This ensures that all changes, from updated handler signatures to new extractor implementations, conform to the standard Rust formatting guidelines. Atlas presents a unified diff for these formatting and linting changes, allowing you to review and approve them before they are written to disk. This final polish ensures that your Actix Web project not only functions correctly but also maintains a consistent and readable codebase, making future development and maintenance easier.

## What safety features does Atlas offer for Actix Web migrations?

Atlas provides multiple layers of safety for Actix Web API migrations, ensuring that every change is permission-gated and reviewable. From a read-only planning agent to unified diffs for every edit, Atlas prioritizes developer control and code integrity throughout the entire process in 2026.

Atlas is built with safety at its core, offering several features to protect your Actix Web codebase during migrations. First, Atlas drafts a plan in a read-only plan agent and explicitly asks for permission before switching to a build agent that can make changes. Every Atlas tool call, including `lsp`, `grep`, `todowrite`, `apply_patch`, and `bash`, is permission-gated against `allow`, `ask`, and `deny` rules. This means you explicitly approve each action. Before writing any changes to files like `src/main.rs` or `Cargo.toml`, Atlas computes a unified diff for every proposed edit and surfaces it for your approval. This allows you to meticulously review every line change. Furthermore, Atlas snapshots file changes as git patches, so edits can be easily diffed and rolled back if necessary. This comprehensive safety framework ensures that you maintain full control and visibility over the entire Actix Web API migration process.

## Steps

1. Run Atlas in your Actix Web crate, ensuring your `Cargo.toml` depends on `actix-web`.
2. Use Atlas's `lsp findReferences` tool on the deprecated Actix Web symbol, then cross-check with `grep` for dynamic usages across your `src/` directory.
3. Instruct Atlas to create `todowrite` entries for each identified Actix Web callsite, making every migration task visible.
4. Approve Atlas to migrate each Actix Web callsite using `apply_patch`, reviewing the unified diff for each change to files like `src/handlers/mod.rs`.
5. After each file modification, Atlas will use the `bash` tool to run `cargo test (actix_web::test)` to validate the changes; approve only if tests pass.
6. Once all `todowrite` entries are complete, use `grep` one final time for the deprecated symbol to confirm zero remaining hits in your Actix Web project.
7. Approve Atlas to delete the old deprecated Actix Web implementation and then run `cargo clippy` and `rustfmt` to ensure code quality and consistency.
8. Review the final `git diff` and allow Atlas to stage and create a commit for the completed Actix Web API migration.

## FAQ

### How does Atlas handle complex Actix Web handler signatures during migration?

Atlas's `lsp` tool, powered by tree-sitter, understands the Abstract Syntax Tree (AST) of your Rust code. This allows it to accurately identify and modify complex Actix Web handler signatures, including those with multiple extractors like `web::Path`, `web::Query`, or `web::Data`, ensuring that the replacement API is correctly integrated without breaking existing dependency injection patterns.

### Can Atlas migrate deprecated Actix Web middleware or custom extractors?

Yes, Atlas can migrate deprecated Actix Web middleware and custom extractors. By using `lsp findReferences`, Atlas can locate all instances where the deprecated middleware is registered in `actix_web::App::new()` or where a custom extractor is used in handler function arguments. The `apply_patch` tool then precisely updates these definitions and usages, ensuring the entire application conforms to the new API.

### What if `cargo test (actix_web::test)` fails after an Atlas patch?

If `cargo test (actix_web::test)` fails after Atlas applies a patch, Atlas will halt the workflow. It will surface the test output and the unified diff for your review. You can then inspect the issue, manually adjust the code if needed, or instruct Atlas to try a different approach. The `todowrite` entry for that specific callsite will remain incomplete until tests pass, ensuring no broken code is silently committed.

### How does Atlas ensure my Actix Web `Cargo.toml` dependencies are correct after a migration?

While Atlas primarily focuses on code changes, its awareness of the Rust ecosystem means it can identify if a new API requires updated or new dependencies in your `Cargo.toml`. Atlas will propose these changes as part of its plan, presenting a diff for your approval. This ensures that your Actix Web project's dependencies are always aligned with the migrated code, preventing compilation errors.

### Is it safe to let Atlas modify my Actix Web `src/main.rs` or `src/lib.rs`?

Yes, it is safe. Atlas operates with explicit permissions. It will draft a plan, show you a unified diff of all proposed changes to files like `src/main.rs` or `src/lib.rs`, and ask for your approval before writing anything. You have the final say on every modification, and Atlas's ability to snapshot changes as git patches allows for easy rollback if you change your mind.

### Can Atlas handle Actix Web applications that use `web::Data` for shared state?

Absolutely. Atlas is designed to understand Actix Web idioms, including the use of `web::Data` for injecting shared application state into handlers. If a deprecated API is related to how `web::Data` is configured or accessed, Atlas can identify these patterns and propose the correct replacement, including fixing any `Arc::clone()` calls that might follow, as part of its migration plan.

### How does Atlas prevent missing dynamic or string-based Actix Web API usages?

Atlas prevents missing dynamic or string-based usages by combining `lsp findReferences` with a `grep` search. While the language server excels at static code analysis, `grep` acts as a robust fallback, catching any instances where the deprecated symbol might appear as a string literal, in configuration files, or in dynamically generated code that the AST-based `lsp` tool might not fully parse. This dual approach ensures comprehensive coverage for your Actix Web codebase.

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