Atlas traces a runtime bug from a Rust stack trace by reading each frame's file at the reported offset, because a stack trace is a list of file:line pairs and that is exactly what the Atlas read tool consumes. Atlas then greps for the panic message string to find where it is constructed, uses the lsp tool's findReferences operation on the failing function to see which callers can reach it with the bad input, fixes the code with edit, and adds a regression test proven by cargo test. No debugger is attached at any point.
How does Atlas turn a Rust panic backtrace into a root cause?
Atlas consumes a Rust backtrace as what it is, a list of file:line pairs, in 5 documented steps. Atlas reads src/parser/tokenizer.rs at the reported offset for each frame, greps for the panic message to find where it is constructed, and walks callers with the lsp tool's findReferences operation.
A Rust backtrace printed with RUST_BACKTRACE=1 gives you frames like src/parser/tokenizer.rs:214, and most of them are noise: core library frames, panic machinery, the unwrap that finally blew up. The frame that matters is usually two or three down, in your crate. Atlas reads each of those at its offset, so the model sees the actual code around line 214 rather than a summary of it. Then it reconstructs the path the bad value took, which is the part the backtrace does not show you.
What does Offset is out of range for this file mean when reading a Rust trace?
Offset is out of range for this file is the Atlas read tool refusing a stale line number. When a Rust backtrace from an older build points at src/parser/tokenizer.rs:214 and the current file is 180 lines long, Atlas fails loudly instead of confidently reading the wrong code.
That error is one of the most useful things Atlas can tell a Rust developer chasing a production panic. Backtraces arrive from a release build that was compiled days ago, and the source has moved since. A tool that quietly reads whatever happens to be at line 214 today will hand the model an unrelated function and the diagnosis will be plausible and wrong. When Atlas reports Offset is out of range for this file, the correct response is to re-read the file from the top and re-derive the frame, or to check out the commit the binary was built from, before trusting any line number in the trace.
Why grep for the Rust panic message instead of trusting the top frame?
Atlas greps for the panic message string, step 3 of the documented workflow, because in Rust the top frame is usually a panic in core rather than in your crate. Grepping the literal message text finds where the expect or the assert that produced it is written, which is far more informative.
A Rust panic that reads called Option::unwrap() on a None value names no line in your code at all in its first frames. But a panic from an expect with a custom message, or from an explicit assert_eq, carries a string that exists exactly once in your crate. Atlas greps for it through ripgrep with real regex plus include and path filters, scoped to src/, and lands directly on the construction site. Atlas also searches Rust code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so a paraphrased message still finds its source.
How do you find which Rust callers can reach a function with bad input?
Atlas uses the lsp tool's findReferences operation on the failing Rust function to enumerate every caller, then reads those callsites to see which ones can supply the value that caused the panic. A private helper in src/parser/tokenizer.rs with 6 callers has 6 candidate origins for the bad input.
The backtrace tells you where the panic happened. It does not tell you which of the callers is passing the malformed input, especially in a Rust crate where the same helper is reached from a public parse entry point, from a test fixture, and from an internal retry path. findReferences gives Atlas the real reference set from the Rust language server, so the search space is exact rather than guessed. Atlas indexes Rust code by AST declarations using tree-sitter, so each callsite is retrieved as a whole fn, not a truncated window.
How does Atlas fix the Rust bug and stop it from recurring?
Atlas fixes the Rust code with the edit tool and adds a regression test so the trace cannot recur silently, step 5 of the documented workflow, then proves both with cargo test. Atlas computes a unified diff for every file edit and surfaces it for approval, so the fix in src/parser/tokenizer.rs lands reviewed.
A fix without a regression test means the next refactor can reintroduce the same panic and nothing will notice. Atlas writes a test that reproduces the bad input, runs cargo test to confirm it fails, applies the fix, and runs cargo test again to confirm it passes. rustfmt keeps the touched files formatted, and cargo build compiles the crate before the change is committed. Atlas snapshots file changes as git patches, so a fix that turns out to be wrong can be diffed and rolled back.
Do I need a debugger to trace a Rust runtime bug with Atlas?
Atlas traces a Rust runtime bug from the backtrace alone, with no debugger attached. The workflow uses 4 tools: read for the frames, grep for the panic message, the lsp tool for the caller graph, and edit for the fix, and it works against a production trace pasted into the terminal.
That constraint is the point. A panic from a Rust binary running in production is often not reproducible under a debugger on your laptop, and by the time you have a reproduction you have already done the hard part. Atlas works from the artifact you actually have, the pasted trace, and reconstructs the path. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, so even a read-only investigation stays within the boundaries you set. Atlas can also draft the diagnosis in a read-only plan agent and ask before switching to a build agent.
Step by step
- 01Run atlas in a crate with a Cargo.toml and let Atlas read your modules, traits, and cargo workspace.
- 02Paste the Rust backtrace and have Atlas read each frame's file at the reported offset, for example src/parser/tokenizer.rs:214.
- 03If read reports Offset is out of range for this file, the trace came from a different build; re-read the file from the top before trusting any line number.
- 04Grep for the panic message string to find where it is constructed, which is usually more informative than the top frame in a Rust trace.
- 05Use the lsp tool's findReferences operation on the failing function to see which callers can reach it with the bad input.
- 06Write a regression test that reproduces the bad input and run cargo test to confirm it fails.
- 07Fix the code with edit, review the unified diff Atlas surfaces, then run cargo test again to confirm the regression test passes.
- 08Run rustfmt over the touched files and cargo build to compile the crate before committing the fix.
Frequently asked questions
- how to debug a Rust panic from a production backtrace without a debugger
- Paste the backtrace into Atlas. Atlas reads each frame's file at the reported offset, greps for the panic message to find where it is constructed, uses the lsp tool's findReferences on the failing function to find callers with bad input, then fixes with edit and proves it with cargo test.
- why does Atlas say Offset is out of range for this file
- The Rust backtrace came from a different build than the source on disk. The Atlas read tool validates offsets against the current file and fails loudly rather than reading the wrong code. Re-read the file from the top before trusting any line number in the trace.
- does Atlas work with cargo and rustfmt
- Yes. Start atlas in a crate with a Cargo.toml and it reads your modules, traits, and cargo workspace. It runs cargo test, cargo build, and rustfmt through its shell access, and can fix borrow-checker errors or clippy warnings with the diff shown first.
- how do I find which Rust caller is passing the bad value
- Use the lsp tool's findReferences operation on the failing function. It returns the authoritative caller set from the Rust language server, so you can read each callsite and see which one can supply the input that caused the panic.
- is grepping the panic message better than reading the top stack frame in Rust
- Often yes. In Rust the top frames are usually panic machinery inside core rather than your crate. The panic message string, from an expect or an assert, exists in your source and grep lands directly on the construction site.
- will Atlas add a regression test for the Rust bug it fixes
- Yes. The documented workflow fixes the code with edit and adds a regression test so the trace cannot recur silently. Run cargo test to confirm the test fails before the fix and passes after it.
- can I review the Rust fix before Atlas writes it
- Yes. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, and every tool call is permission-gated against allow, ask, and deny rules. Atlas also snapshots file changes as git patches so a bad fix can be rolled back.
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