## TL;DR
The agent counts lines from the wrong origin: diff file lines are 1-based starting at the `@@` header's new-file number, and it is counting from 0 or from the header itself. Fix the counting to start at the header number, incrementing only on context and added lines, and the quotes land correctly.

## The error
```text
review agent failed to quote the right code line: off-by-one in diff position: how to fix
(comment quotes `db.connect()` but the finding is about the `db.query()` call on the next line)
```

## Steps
### 1. Read the hunk header correctly
For `@@ -10,5 +12,7 @@`, new-file lines start at 12, and line numbers are 1-based.
Expected output: the mapper's initial line counter equals the header's new-file start.

### 2. Walk the hunk lines with the right increments
Context and added lines advance the new-file counter; removed lines do not (they have no new-file number).
Expected output: every added or context line maps to its true file line.

### 3. Verify by quoting and checking
For each comment, fetch the mapped line from the file and confirm it matches the code the finding describes.
Expected output: quoted lines match the findings they annotate.

### 4. Add fixture tests with a known diff
Write a test with a hand-computed diff and assert each mapped line.
Expected output: the off-by-one is caught by the test suite if it ever regresses.

## When to use
- Inline comments consistently quote the line above or below the intended one
- Every quoted snippet is shifted by exactly one
- The shift reproduces on fresh diffs, not just stale ones

## When not to use
- Comments landing on completely unrelated lines (a mapping bug, not an off-by-one)
- Stale hunks after a force-push (re-fetch the diff instead)
- The API rejecting positions outright

## Tool compatibility
- Unified diff format from git and the GitHub compare API
- The 1-based convention is the same on GitLab
- Any agent-side position mapper, LLM or deterministic

## Variant phrasings
### quoted line shifted after a rebase
The diff changed under the agent. Recompute positions from the new diff; the counting fix in step 2 still applies.

### context lines miscounted in the quote
The mapper probably increments on removed lines too. Removed lines belong to the old file only.

### comment quotes the hunk header line itself
The counter started at the header instead of the header's line number. Step 1 fixes this variant directly.

## Why it happens
There are three plausible origins to count from: 0, the header line, or the header's stated line number. Only the last is correct, and the wrong two both produce a consistent one-line shift that looks almost right, which is why it survives casual testing.

## Edge cases
- Deleted lines have no new-file line number at all: never anchor a comment to one.
- Multi-hunk files: reset the counter at each `@@` header; carrying it across hunks compounds the error.
- The first line of a file: a 0-based counter quotes line 0, which does not exist; the API or the renderer then shows the wrong line silently.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_jlQ8sWNd-CPjS8ZRjZlixw