# Atlas read tool: "Offset is out of range for this file ( lines)" > To fix "Offset is out of range" in Atlas, re-read the file without an offset to see its current line count, then pass an offset below that number. Atlas fails with "Offset is out of range for this file ( lines)" because the read tool validates the requested offset against the file's actual line count, and the offset you passed is at or past the end of the file as it exists right now. The fix is to re-read the file without an offset to see its current length, then re-issue the read with an offset below the reported line count. The most common trigger is a stale offset carried over from an earlier version of the file: if the file shrank since your last read, discard the old offsets entirely rather than adjusting them. ## Symptom The read tool fails with: Offset is out of range for this file ( lines). The message reports both the offset you asked for and the number of lines the file actually has. ## Cause Atlas's read tool validates the requested offset against the file's actual line count and reports both numbers, so a stale offset from an earlier version of the file is caught explicitly instead of silently returning nothing. The file is shorter than the offset you passed. ## Fix 1. Re-read the file without an offset to see its current length. The read returns the file as it exists now, not as it existed when you captured the old offset. 2. Use an offset below the reported line count. The error message already tells you the count, so the valid range is bounded by the number in parentheses. 3. If the file shrank since your last read, discard the old offsets entirely rather than subtracting from them. 4. For targeted lookups, prefer grep over paging by offset, since grep finds the line you want by content instead of by position. 5. Re-run the read with the corrected offset and confirm the file contents come back. ## Why does Atlas report "Offset is out of range for this file" Atlas reports "Offset is out of range for this file ( lines)" because the read tool validates the requested offset against the file's actual line count before returning anything. The message prints 2 numbers, the offset you asked for and the line count, so the mismatch is visible at a glance. The check is explicit by design. A read tool that silently returned an empty result for an out-of-range offset would leave the agent guessing about whether the file is empty, whether the range is wrong, or whether the read failed. Atlas instead names both figures, the offset you asked for and the line count the file actually has, which turns a confusing empty response into a single-glance diagnosis. The gap between those two numbers tells you what happened. A small gap usually means an off-by-one at the end of the file. A large gap means the file is nothing like the shape you thought it was, which almost always means it changed since you last looked at it. ## How to fix an out-of-range offset in the Atlas read tool Fix the Atlas offset error in 3 steps: re-read the file without an offset to see its current length, choose an offset below the line count the error reported, and re-run. If the file shrank since your last read, throw the old offsets away rather than adjusting them by hand. Re-reading with no offset is the cheapest possible ground-truth check. It returns the file as it exists at this moment, which resets whatever mental model went stale. Once you have the current contents, the valid offset range is obvious and you can page from a position that actually exists. Resist the temptation to guess a smaller number without looking, because a guessed offset that happens to be in range is worse than one that errors out: it returns real lines from the wrong part of the file, and nothing tells you they are the wrong lines. The error is loud. A bad guess is quiet. Prefer the loud failure and the confirmed re-read. ## Why stale offsets happen in an Atlas session Stale offsets happen in Atlas because a file changes between the moment you record a position and the moment you read from it. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so 1 approved deletion shortens the file and invalidates every offset past the new end. Long sessions make this more likely, not less. You read a file at line 800, an edit removes a block, a formatter collapses some lines, and the position you were holding no longer points where you thought. The same happens when a subagent or a background job touches the file: Atlas fans out work to subagents that can run in the foreground or in parallel background sessions, so the file under you can move while you are reasoning about a snapshot of it. The habit that avoids the whole class of problem is to treat offsets as short-lived. Capture, use, discard. An offset that survives an edit is a liability, and the read tool telling you so with a hard error is the system working correctly. ## When to use grep instead of paging by offset in Atlas Use grep instead of offsets in Atlas whenever you are looking for 1 specific thing rather than reading a file end to end. Grep finds the line by content, so it cannot go out of range the way a hardcoded offset can, and it survives edits that shift every line number in the file. Offsets are a paging mechanism. They are the right tool when you genuinely need to walk a long file in sequence and you want the next chunk after the one you just saw. They are the wrong tool when what you actually want is a definition, a call site, or a config key, because in that case you are using a position to stand in for a piece of content, and positions decay while content does not. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, which means the search path is a first-class way to find code, not a fallback. Reach for it before you reach for arithmetic on line numbers. ## How to verify the offset fix worked in Atlas Verify the Atlas offset fix by re-running read with the corrected offset. A successful call returns file contents and the message "Offset is out of range for this file ( lines)" does not appear. Then run 1 more check: confirm the lines you got back are the lines you expected. A read that succeeds is necessary but not sufficient. Because an in-range offset always returns something, the second half of verification is confirming the content matches what you were looking for. If it does not, the offset is valid but wrong, which is the failure mode the error message was protecting you from. Compare the returned lines against the section of the file you meant to reach, and if they do not line up, re-read from the top rather than nudging the offset. Atlas snapshots file changes as git patches so edits can be diffed and rolled back, so if you already acted on lines from the wrong region, the change is recoverable. Catching it at read time is still cheaper than catching it at review time. ## FAQ ### what does Offset is out of range mean in Atlas The Atlas read tool validates the requested offset against the file's actual line count. The message "Offset is out of range for this file ( lines)" means the offset you passed is at or past the end of the file as it exists now. ### how do I fix an out of range offset in Atlas read Re-read the file without an offset to see its current length, then use an offset below the line count that the error reported. The error message already gives you the upper bound. ### why did my Atlas read offset work earlier and fail now The file shrank. An edit, a formatter, or a background subagent removed lines, so an offset captured against the older version now points past the end. Discard the old offsets entirely. ### should I use grep or read offsets to find code in Atlas Use grep when you are looking for specific content. Grep finds the line by what it says rather than where it sits, so it cannot go out of range and it survives edits that shift line numbers. ### does Atlas tell me how many lines the file has Yes. The error includes the count in parentheses, as in "Offset is out of range for this file ( lines)", so the valid offset range is bounded by the number the message reports. ### is an in-range Atlas offset always the right offset No. An in-range offset always returns lines, but they may be the wrong lines if the file changed shape. Confirm the returned content is what you expected rather than assuming a successful read is a correct read. ### how do I page through a long file safely in Atlas Re-read without an offset first to establish the current line count, then page from a position below that count, and re-check the count after any edit that could change the file's length. --- Canonical HTML: https://runatlas.sh/resources/troubleshooting/read-offset-out-of-range Source of truth: aeo_pages row `/resources/troubleshooting/read-offset-out-of-range` (segment: Troubleshooting) (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.