# Trace a Runtime Bug From a Stack Trace in Swift with Atlas (2026) > Atlas reads each Swift stack trace frame at its file:line offset, and reports Offset is out of range for this file when the trace came from a different build. Atlas goes from a Swift stack trace to the responsible line without a debugger attached. A stack trace is a list of file:line pairs, which is exactly what the Atlas read tool consumes: Atlas opens each frame at its reported offset in Sources/, greps for the error message string to find where it is constructed, and runs the lsp tool's findReferences operation on the failing function to see which callers can reach it with the bad input. Offsets are validated against the current file, so a trace from an older Swift Package Manager build fails loudly instead of pointing at the wrong code. ## Key takeaways - A Swift stack trace is a list of file:line pairs, which is exactly the input the Atlas read tool consumes. - Offset is out of range for this file means the trace came from a different Swift Package Manager build, so no line number in it can be trusted. - Grepping for the thrown error message finds the construction site, which explains the bug better than the top frame. - The lsp tool's findReferences names every Swift caller that can reach the failing function, not just the one in the trace. - The fix lands with an XCTest regression case verified by XCTest via swift test, so the same trace cannot recur silently. ## How does Atlas read a Swift stack trace? A Swift stack trace is a list of file and line pairs, which is exactly what the Atlas read tool consumes. Atlas opens each frame's file at the reported offset, 1 frame at a time, walking down from the crashing frame in Sources/App/OrderStore.swift through the callers the trace names. The mechanical part of Swift crash triage is opening ten files at ten line numbers, and that is the part Atlas removes. Each frame becomes a read at an offset, and the code around the failing line arrives in context together. What matters is the sequence: the top frame is where the process died, but the responsible line is usually two or three frames down, in the code that produced the bad Optional or the out-of-range Index. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so when Atlas needs the whole failing function rather than a window around line 214, it gets the declaration. ## What does Offset is out of range mean in an Atlas Swift trace? If the Atlas read tool reports Offset is out of range for this file, the Swift stack trace came from a different build. Atlas validates offsets against the current file, so a trace from a Package.swift build made 3 commits ago fails loudly rather than pointing confidently at the wrong line. This is the most valuable error message in the whole workflow. A production Swift crash report is generated from a binary that may be days or weeks old, and the file has moved since. An agent that trusts the line number blindly will read the code that now happens to sit at line 214 and construct an entirely fictional explanation for the crash. Atlas refuses. When the offset is out of range, re-read the file from the top before trusting any line number, and reconcile the trace against the commit the build came from. Atlas reads git branches, status, and diffs, so identifying what changed since that build is part of the same session. ## Why grep for the error message instead of reading the top Swift frame? Atlas greps for the error message string to find where it is constructed, which is usually more informative than the top frame of a Swift stack trace. The 1 line that throws OrderError.invalidState tells you far more about the bug than the Foundation frame that finally surfaced it. Swift error handling spreads the truth across two places. The throw site knows why the operation was invalid. The crash site only knows that it was. Grepping for the literal message, the enum case name, or the fatalError text lands Atlas at the throw site directly. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so when the message is assembled from an interpolated string and a plain grep for the full text finds nothing, the semantic search still reaches the constructing function. From there, the precondition that failed is readable in its own context. ## How does Atlas find which Swift callers can reach a failing function? Atlas runs the lsp tool's findReferences operation on the failing Swift function to see which callers can reach it with the bad input. A crash inside a method that 6 different view models call is only explicable once you know which of those callers passes an unwrapped Optional or an unvalidated index. The stack trace shows one path. findReferences shows every path. That difference is what turns a fix from a patch into a real repair: if the crashing function is reachable from three call sites and only one of them guards the input, the fix belongs at the boundary, not at the crash. Atlas asks the Swift language server through the lsp tool, so the caller set is authoritative rather than a text match against a common method name. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, so the investigation is read-only until you approve an edit. ## How do I fix and lock in a Swift crash found from a stack trace? Atlas closes a Swift crash with 2 changes: the fix itself, applied with the edit tool, and a regression test so the trace cannot recur silently. In a Swift Package Manager project that means a new XCTest case under Tests/, verified by swift test, with swift-format run on the touched files before the commit. Run atlas in a package with a Package.swift. Atlas reads your targets, protocols, and dependencies, so the regression test it writes lands in the right test target and imports the right module. The test should assert on the exact condition that crashed: the nil that was force-unwrapped, the index that exceeded the array's count, the state the enum was not supposed to be in. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so both the fix in Sources/ and the new XCTest case in Tests/ are reviewed. XCTest via swift test then proves the test fails without the fix and passes with it. ## Steps 1. Run atlas in a Swift package with a Package.swift so Atlas can read your targets, protocols, and dependencies. 2. Paste the Swift stack trace and have Atlas read each frame's file at the reported offset with the read tool. 3. If 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. 4. Grep for the error message string, the thrown enum case, or the fatalError text to find where the failure is constructed, which is usually more informative than the top frame. 5. Run the lsp tool's findReferences operation on the failing Swift function to see which callers can reach it with the bad input. 6. Fix the responsible line in Sources/ with the edit tool and approve the unified diff Atlas surfaces before it writes. 7. Add an XCTest regression case under Tests/ asserting on the exact condition that crashed, such as the force-unwrapped nil or the out-of-range index. 8. Run XCTest via swift test to confirm the fix, run swift-format on the touched files, then commit. ## FAQ ### how to debug a Swift crash from a production stack trace without a debugger Paste the trace into Atlas. A stack trace is a list of file:line pairs, which the Atlas read tool consumes directly: it opens each frame at its offset in Sources/, greps for the error message to find the throw site, and runs the lsp tool's findReferences on the failing function to find the caller with the bad input. ### what does Offset is out of range for this file mean The Atlas read tool validates offsets against the current file. Offset is out of range for this file means the Swift stack trace came from an older build and the file has since changed, so the line numbers no longer correspond. Re-read the file from the top before trusting any of them. ### why is the top frame of a Swift stack trace not the bug The top frame is where the process died, not where the invalid state was produced. Atlas greps for the error message string to find where it is constructed, which is usually more informative, then follows the callers with the lsp tool's findReferences to see who supplied the bad input. ### how do I find every Swift caller of a crashing function Run the lsp tool's findReferences operation on the failing function. The stack trace shows one path into the function; findReferences shows all of them, which is how you learn whether the fix belongs at the crash site or at an unguarded boundary in one specific view model. ### should I add a test after fixing a Swift crash Yes. Atlas fixes with edit and adds a regression test so the trace cannot recur silently. In a Swift Package Manager project that is an XCTest case under Tests/, asserting on the exact condition that crashed, verified with XCTest via swift test. ### can Atlas edit my Swift sources while investigating a crash Not without approval. 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. The read, grep, and lsp phases of the investigation change nothing. ### atlas Swift Package Manager setup Run atlas in a package with a Package.swift. Atlas reads your targets, protocols, and dependencies, and can add XCTest cases or adopt async/await, with the diff reviewed first. Tests run as XCTest via swift test and formatting through swift-format. ### how does Atlas find a Swift error message that is built from string interpolation Plain grep can miss an interpolated message. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion and indexes code by AST declarations using tree-sitter, so the semantic side of the search still reaches the function that constructs the error. --- Canonical HTML: https://runatlas.sh/resources/stacks/trace-a-runtime-bug-from-a-stack-trace-in-swift Source of truth: aeo_pages row `/resources/stacks/trace-a-runtime-bug-from-a-stack-trace-in-swift` (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.