Atlas traces a Python runtime bug from a stack trace by treating the traceback as what it is: a list of file and line pairs, which is exactly what the read tool consumes. Atlas reads each frame at its reported offset in your package, greps for the exception message string to find where it is constructed, walks the callers with the lsp tool's findReferences operation, and fixes with edit. Offsets are validated against the current file, so a traceback from an older deployment fails loudly instead of pointing at the wrong line of your Django or FastAPI service.
How do you debug a Python stack trace without attaching a debugger?
Atlas debugs a Python traceback with 4 tools and no debugger: read opens each frame at its reported offset, grep finds where the exception message is constructed, the lsp tool's findReferences operation walks the callers, and edit applies the fix plus a pytest regression test.
The job is to go from a production stack trace to the responsible line and a fix, without a debugger attached. A production traceback is often all you get. You cannot attach pdb to a container that already crashed, and reproducing a KeyError that only fires on one tenant's payload is its own project. A Python traceback names every frame with a file path, a line number, and the calling function, and Atlas's read tool consumes exactly that. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so once a frame names a function, Atlas retrieves the whole declaration rather than a slice of it.
How does Atlas read each frame of a Python traceback?
Atlas reads each frame of a Python traceback at its reported offset, using the file path and line number the interpreter printed. A traceback with 6 frames becomes 6 targeted reads, starting from the innermost frame where the exception was raised and walking outward through your app/ package toward the request handler.
Paste the traceback into the session and Atlas works it frame by frame. The bottom frame names the raise site, for example app/billing/invoice.py line 214 inside def apply_discount. The frames above it name the callers the interpreter actually traversed. Atlas reads each one at its offset, so the code in context is the code that ran, not a summary of what the module probably does. Python makes this workable because the traceback is unusually honest: it names the exception type, the message, and every frame in order, which is more than most runtimes hand you after a crash.
What if a Python stack trace points at the wrong line?
Atlas validates offsets against the current file, so a stale Python traceback fails loudly. If read reports Offset <n> is out of range for this file, the traceback came from a different build than the 1 in your working tree, and Atlas re-reads the file from the top before trusting any line number.
Stale line numbers are the quiet killer of traceback debugging. A traceback captured from a deployment two weeks old points at app/billing/invoice.py line 214, but line 214 in your working tree is now a blank line inside a different function, and a tool that reads it anyway will confidently explain a bug that does not exist. Atlas refuses. The read tool validates the offset against the current file and reports Offset <n> is out of range for this file when the traceback no longer matches the source. That loud failure is the signal to check out the deployed commit or to locate the code by name instead of by line.
How do you find where a Python exception message comes from?
Atlas greps for the exception message string to find where it is constructed, which is usually more informative than the top frame of the Python traceback. Searching for the literal text of a ValueError message lands you on the raise statement, and often on the 1 validation branch that actually rejected the input.
The top frame tells you where the exception surfaced. The raise site tells you why. In Python those are frequently different places: a ValueError raised inside a Pydantic validator or a helper in app/schemas.py surfaces four frames up in a FastAPI route, and the route is not the bug. Atlas's grep takes a real regex plus include and path filters and runs through ripgrep, so scoping to *.py and excluding .venv keeps the search on your code rather than on installed packages. Grepping the message string, including the f-string prefix around the interpolated part, finds the construction site directly.
How do you find which Python callers can trigger the bug?
Atlas uses the lsp tool's findReferences operation on the failing Python function to see which callers can reach it with the bad input. A traceback shows the 1 path that crashed. findReferences shows all of them, including the async task and the management command that never appeared in the trace.
One traceback is one sample. The bug is a property of the function's contract, and the contract is exercised by every caller. Atlas calls findReferences on def apply_discount and gets the full list: the FastAPI route, a Celery-style background job, a pytest fixture, a script under scripts/. Some of those pass values the failing branch never anticipated. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so callers reached through a dispatch table or a callback, which a plain text search would miss, still surface. Knowing every caller is what turns a patch into a fix.
How does Atlas fix the Python bug and prevent it from returning?
Atlas fixes the Python bug with the edit tool and adds 1 pytest regression test so the traceback cannot recur silently. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so the change to app/billing/invoice.py is reviewed before it lands, not after.
A fix without a test is a fix with a shelf life. Atlas writes a pytest case that feeds the function the exact input from the traceback and asserts the corrected behavior, then runs pytest through the bash tool to prove it. In a project managed with uv, that means running the suite in the project environment so the pinned dependencies from pyproject.toml are the ones under test. Run ruff format on the changed files so the diff carries no style noise. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, and Atlas snapshots file changes as git patches so edits can be diffed and rolled back if the fix turns out to be wrong.
Step by step
- 01Paste the Python traceback into the session and have Atlas read each frame's file at the reported offset, starting from the innermost frame where the exception was raised.
- 02If read reports Offset <n> is out of range for this file, the traceback came from a different build; re-read the file from the top before trusting any line number in it.
- 03Grep for the exception message string to find where it is constructed, which is usually more informative than the top frame; scope the grep to *.py and exclude .venv so installed packages stay out.
- 04Use the lsp tool's findReferences operation on the failing function to see which callers can reach it with the bad input, including background jobs and scripts the traceback never showed.
- 05Form the fix against the real code path, then apply it with the edit tool and review the unified diff Atlas surfaces before approving the write.
- 06Add a pytest regression test that feeds the function the exact input from the traceback so the trace cannot recur silently.
- 07Run pytest through the bash tool in the uv-managed environment so the pinned dependencies from pyproject.toml are the ones under test.
- 08Run ruff format on the changed files so the diff carries no formatting noise into review.
Frequently asked questions
- how do I debug a python stack trace from production without a debugger
- Paste the traceback and let Atlas read each frame's file at the reported offset. A Python traceback is a list of file and line pairs, which is what Atlas's read tool consumes, so the code that ran is loaded in context without attaching pdb to anything.
- why does my python stack trace point at the wrong line of code
- The traceback came from a different build than your working tree. Atlas's read tool validates offsets against the current file and reports Offset <n> is out of range for this file, which is your signal to check out the deployed commit rather than trust the line number.
- how do I find where a python exception is raised
- Grep for the exception message string, which is usually more informative than the top frame of the traceback. Atlas's grep takes a real regex plus include and path filters and runs through ripgrep, so scoping to *.py and excluding .venv keeps the search on your own code.
- how do I find every caller of a python function
- Use the lsp tool's findReferences operation, which Atlas calls directly. A traceback shows only the one path that crashed, while findReferences shows the background job, the management script, and the pytest fixture that can also reach the function with bad input.
- can an AI agent fix a runtime bug from just a traceback
- Yes. Atlas reads each frame at its offset, greps for the raise site, walks callers with the lsp tool, fixes with edit, and adds a pytest regression test so the trace cannot recur silently. Every tool call is permission-gated against allow, ask, and deny rules before it runs.
- does atlas work with fastapi and django projects
- Yes. Run atlas in a repo with a pyproject.toml or requirements.txt and let Atlas read your package layout, virtualenv, and installed dependencies. Atlas is used across Python projects from plain scripts to Django and FastAPI services.
- how do I write a regression test for a python traceback
- Write a pytest case that feeds the failing function the exact input from the traceback and asserts the corrected behavior, then run pytest through Atlas's bash tool. A fix without a test lets the same traceback return silently on the next refactor.
- how do I undo a python fix an AI agent made
- Atlas snapshots file changes as git patches so edits can be diffed and rolled back. Atlas also computes a unified diff for every file edit and surfaces it for approval before writing, so the change to your .py module is reviewable before it ever lands.
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