# Plan a Multi-File Change Before Editing in Rust with Atlas (2026)

> Atlas plans Rust refactors in a read-only plan agent, so borrow-checker and trait changes across a cargo workspace are designed before any src file is edited.

Atlas plans a multi-file Rust change inside a plan agent whose description is literally Plan mode. Disallows all edit tools. Before a single line of your Cargo.toml or src/lib.rs moves, the plan agent studies your modules, traits, and cargo workspace with codebase_search, grep, read, and the lsp tool, and writes the design into a plan markdown file. Only after plan_exit asks you to switch to the build agent does implementation start, and only then do cargo test, cargo, and rustfmt run against the change.

## Key takeaways

- The Atlas plan agent denies edit for every path except .atlas/plans/*.md, so designing a Rust trait change cannot silently edit src/lib.rs.
- codebase_search, grep, read, and the lsp tool all stay allowed in plan mode, which is enough to enumerate every impl block a signature change breaks.
- plan_exit asks before switching to the build agent; answering No raises Question.RejectedError and returns you to refining the Rust plan.
- cargo test, rustfmt, and cargo verify the implemented change, and Cargo.toml records any dependency the plan added.
- Atlas snapshots file changes as git patches, so a multi-crate refactor that fails the borrow checker can be rolled back.

## Why plan a Rust refactor across a cargo workspace before editing?

A Rust refactor that changes a trait signature does not fail in one file, it fails in every crate of the cargo workspace that implements the trait. Atlas drafts a plan in a read-only plan agent and asks before switching to a build agent, so in 2026 the blast radius is written down before cargo test ever runs.

Rust makes the cost of an unplanned change visible immediately and loudly. Widening a lifetime on a struct in src/lib.rs, changing an associated type on a trait, or moving a module behind a feature gate in Cargo.toml produces borrow-checker errors and clippy warnings across every dependent crate. The Atlas plan agent is built for exactly this. Its permission set denies edit for every path except the plan markdown, so research and design cannot accidentally turn into an edit, which means you can trace every impl block that will break before you break it. When the plan is honest about the trait bounds it will change, the first cargo test run after implementation is a verification and not a discovery.

## What can the Atlas plan agent do in a Rust crate, and what is blocked?

Inside a Rust crate the Atlas plan agent may run codebase_search, grep, read, the lsp tool, question, and plan_exit. Edit is denied for every path and allowed only under .atlas/plans/*.md. Reading Cargo.toml, src/main.rs, or an impl block in 2026 therefore cannot become a write.

The research toolkit maps cleanly onto how Rust code is actually navigated. codebase_search queries the semantic index, and because Atlas indexes code by AST declarations using tree-sitter, not blind line windows, a hit lands on the fn, the struct, or the impl rather than an arbitrary window of lines. grep runs a real regex through ripgrep when you want every occurrence of a trait name such as impl Serialize for. read opens the file. The lsp tool exposes the symbol graph, which is how you find every implementor of a trait before you change its signature. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so the semantic and literal views agree before you commit to a design.

## How does plan_exit hand a Rust plan to the build agent?

plan_exit is the handoff, and it offers exactly 2 answers. Atlas asks whether the plan at the given path is complete and whether to switch to the build agent and start implementing. Yes begins editing your Rust sources. No raises Question.RejectedError and returns you to the plan.

Refining a Rust plan is normal. A first draft usually says the change touches src/parser.rs and src/lexer.rs, and a second draft admits it also touches the two integration tests under tests/ and adds a dependency to Cargo.toml that cargo will need to resolve. Answering No to plan_exit keeps the plan agent in place, still unable to edit anything outside .atlas/plans/*.md, so iterating costs nothing. Answering Yes moves you to the build agent, where every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, and where Atlas computes a unified diff for every file edit and surfaces it for approval before writing.

## What commands verify a multi-file Rust change Atlas implemented?

Run cargo test for the suite, rustfmt for formatting, and cargo to resolve any dependency the plan added to Cargo.toml. Atlas snapshots file changes as git patches, so a 2026 refactor that trips the borrow checker in three crates can be diffed and rolled back in one move.

Verification in Rust is unusually decisive because the compiler participates. cargo test is the test runner, so a trait change that broke an implementor fails at compile time rather than at assertion time. rustfmt is the formatter, so the diff you read is a semantic diff. cargo is the package manager, so a plan that pulls in a new crate declares it in Cargo.toml and cargo resolves it. Pair Atlas with Rust to work through the borrow checker, cargo, and clippy lints: when the build agent's edit produces a borrow error, you feed the compiler output straight back and the fix arrives as another reviewable unified diff.

## How does Atlas keep a Rust plan from turning into an accidental edit?

Two mechanisms. The Atlas plan agent's permissions deny edit for every path except .atlas/plans/*.md, and separately every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs. A Rust developer in 2026 therefore has both a mode-level and a call-level guarantee.

The mode-level guarantee is the plan agent's permission set, which is why the description reads Plan mode. Disallows all edit tools. The call-level guarantee is the permission engine that every tool call passes through, including in the build agent after plan_exit. On top of both, Atlas computes a unified diff for every file edit and surfaces it for approval before writing, and Atlas snapshots file changes as git patches so edits can be diffed and rolled back. For a Rust workspace where one careless edit to Cargo.toml can break the build for every crate, having the rollback path already in place matters more than it does in a single-crate script.

## Steps

1. Run atlas in a crate with a Cargo.toml and let it read your modules, traits, and cargo workspace layout.
2. Switch to the plan agent, whose permissions deny edit for every path and allow edit only under .atlas/plans/*.md.
3. Describe the change to codebase_search, for example how tokens are turned into an AST, and let the semantic index rank the candidate Rust declarations.
4. Grep for the trait or type by name to find every impl block and call site that a signature change will break.
5. Use the lsp tool to walk the Rust symbol graph and confirm the implementor list before committing to the design.
6. Write the plan into the allowed markdown path under .atlas/plans, naming every src file, every tests/ integration test, and any Cargo.toml dependency change.
7. Call plan_exit and answer No while the plan still hand-waves lifetimes or trait bounds; answering No raises Question.RejectedError and keeps you in the plan agent.
8. Answer Yes to hand off to the build agent, approve each unified diff, then run cargo test and rustfmt and let cargo resolve any new dependency.

## FAQ

### how to plan a rust refactor across multiple crates with an ai agent

Use the Atlas plan agent. Its permissions deny edit for every path except .atlas/plans/*.md, so it can map your cargo workspace, traits, and modules with codebase_search, grep, read, and the lsp tool, and write the design into a plan file before any Rust source is edited.

### can an ai coding agent change my Cargo.toml without asking

Not in Atlas. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, and Atlas computes a unified diff for every file edit and surfaces it for approval before writing. A Cargo.toml dependency addition appears as a diff you approve.

### what does atlas plan mode block in a rust project

Atlas plan mode blocks every edit tool. The plan agent's description is literally Plan mode. Disallows all edit tools, and its permission set denies edit for every path except the plan markdown, so src/lib.rs, src/main.rs, and Cargo.toml are all off limits until you exit plan mode.

### how do i find every implementor of a trait before changing it

In Atlas, ask codebase_search for the behavior, grep for the trait name to catch every impl block literally, and use the lsp tool to walk the symbol graph. All three run inside the plan agent, so the survey happens before any edit.

### does atlas run cargo test automatically after a refactor

Atlas runs commands through permission-gated tool calls, so cargo test runs when the build agent proposes it and the permission rules allow it. After a multi-file Rust change, cargo test verifies behavior and rustfmt normalizes formatting.

### how do i undo a rust change an ai agent made

Atlas snapshots file changes as git patches so edits can be diffed and rolled back, and Atlas reads git branches, status, and diffs. A refactor that breaks the borrow checker across a cargo workspace can be reverted from the snapshot rather than reconstructed by hand.

### atlas plan agent vs build agent in rust

The Atlas plan agent researches and writes only the plan markdown. The build agent implements it. plan_exit is the boundary: it asks whether to switch to the build agent and start implementing, and until you answer Yes, no Rust file outside .atlas/plans/*.md can be written.

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