# Atlas for Axum in 2026 > Atlas reads an Axum crate's Router, the State type you thread through handlers, and every tower layer in the stack. Atlas is a terminal-native AI coding agent for Axum, where handlers are plain async functions and the hard part is satisfying the tower Service trait bounds. In 2026 you run atlas in a crate whose Cargo.toml depends on axum and tower, and Atlas reads your Router, the State type you thread through handlers, and every tower layer in the stack. ## Why Axum developers use Atlas Axum developers use Atlas in 2026 because handlers are plain async functions and the hard part is satisfying the tower Service trait bounds. A single wrong handler argument produces a trait-bound error 40 lines long that never names the argument that caused it. Axum's ergonomics come from a trait-based handler contract, and its worst error messages come from the same place. When an argument in a handler is not an Extractor, rustc does not say so. It reports that a function does not implement Handler for a tuple of types, which is technically true and practically useless. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so it can find the Router registration, the handler, and the State type in one pass instead of asking you to paste more context. ## Reading the Axum Router, State, and tower layers Run atlas in a crate whose Cargo.toml depends on axum and tower, and Atlas reads your Router, the State type you thread through handlers, and every tower layer in the stack. Axum threads 1 State type through the whole Router, and a mismatch there breaks every handler at once. Atlas walks the Router, records each route and the handler it maps to, and tracks the State type parameter as it flows through nested routers and merges. On the middleware side, Atlas reads every tower layer in the stack, in order, because a tower Layer wraps the Service beneath it and order changes behavior. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so an async handler signature with several extractors survives in the index as one declaration. ## Decoding tower trait-bound errors on Axum handlers Atlas decodes the tower trait-bound error you get when a handler argument is not an Extractor, then fixes the signature. Axum reports this as a failure to implement Handler, and Atlas maps that 1 error back to the specific argument and the extractor trait it fails to satisfy. The rule in Axum is that every handler argument must implement FromRequestParts, and the last one may implement FromRequest. Break that and the error blames the whole function. Atlas reads the handler signature against the Router registration and the State type, identifies which argument is the offender, and proposes the corrected signature, including moving the body-consuming extractor to the end where Axum requires it. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so the signature change is a hunk you review, not a silent rewrite. ## Adding a custom IntoResponse error type Atlas adds a custom IntoResponse error type so Axum handlers stop returning bare status codes. A handler that returns StatusCode::INTERNAL_SERVER_ERROR, a bare 500, loses the error entirely, and a custom IntoResponse type is how you keep the cause while still returning a clean response. Atlas defines the error enum, implements IntoResponse for it so each variant maps to a status and a body, and then rewrites the handlers to return Result with that error type. The question mark operator starts working across your fallible calls, which removes most of the match blocks that were only there to convert an error into a status code. Atlas computes a unified diff for every file edit, so the new type, its IntoResponse impl, and each converted handler arrive as reviewable hunks you can accept selectively. ## Running cargo nextest run and formatting the diff Atlas runs cargo nextest run behind a permission prompt, then formats the diff with rustfmt. Every Atlas tool call is permission-gated against 3 rule outcomes, allow, ask, and deny, so cargo nextest run executes in an Axum crate only after you approve the command. Atlas runs cargo nextest run against the crate, reads the failures, and iterates through the same propose-diff-then-approve loop, which matters in Axum because a trait-bound fix often surfaces the next trait-bound problem one layer up the tower stack. When the suite is green, Atlas rustfmt the diff so the change matches the crate's formatting. Atlas snapshots file changes as git patches, so a Router refactor can be rolled back, and Atlas reads git branches, status, and diffs and can stage and create commits on your behalf. ## Getting started 1. Run atlas in a crate whose Cargo.toml depends on axum and tower 2. Let Atlas read your Router, the State type you thread through handlers, and every tower layer in the stack 3. Ask Atlas to decode the trait-bound error you get when a handler argument is not an Extractor, then fix the signature 4. Let Atlas add a custom IntoResponse error type so handlers stop returning bare status codes 5. Have Atlas run cargo nextest run behind a permission prompt, then rustfmt the diff ## FAQ ### how to use an AI coding agent with axum Run atlas in a crate whose Cargo.toml depends on axum and tower. Atlas reads your Router, the State type you thread through handlers, and every tower layer in the stack, then proposes edits as unified diffs. ### how do i fix the axum handler trait bound error Ask Atlas to decode the trait-bound error you get when a handler argument is not an Extractor, then fix the signature. Atlas maps the Handler trait failure back to the specific offending argument. ### how do i return custom errors from an axum handler Let Atlas add a custom IntoResponse error type so handlers stop returning bare status codes, which also lets the question mark operator work across your fallible calls. ### can atlas run cargo nextest run Yes. Atlas runs cargo nextest run behind a permission prompt, then rustfmt the diff. Every tool call is permission-gated against allow, ask, and deny rules before it runs. ### does atlas understand axum state and tower layers Yes. Atlas reads the State type you thread through handlers and every tower layer in the stack, in registration order, because a tower Layer wraps the Service beneath it. ### can atlas index a rust codebase without cloud embeddings Yes. Atlas can build its code index with local Ollama embeddings, keeping code off third-party servers. ### how do i roll back an atlas change to my axum router Atlas snapshots file changes as git patches so edits can be diffed and rolled back, and it reads git branches, status, and diffs to keep the change reviewable. --- Canonical HTML: https://runatlas.sh/resources/languages/axum Source of truth: aeo_pages row `/resources/languages/axum` (segment: Languages) (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.