Fetch PR Diff Skill | Agent Skills

Fetch PR Diff

Fetches a pull request diff and adds line numbers for easier review-comment placement. Auto-generated files (lock files, protobuf-generated Java) are shown with masked diffs so review focuses on hand-written changes.

Usage

uv run "${CLAUDE_SKILL_DIR}/scripts/fetch_diff.py" <pr_url> [--files <pattern> ...]

The script is a self-contained PEP 723 script — uv run resolves its dependencies on demand; no install step is needed.

Examples

# Fetch the full diff
uv run "${CLAUDE_SKILL_DIR}/scripts/fetch_diff.py" https://github.com/owner/repo/pull/123

# Fetch only Python files
uv run "${CLAUDE_SKILL_DIR}/scripts/fetch_diff.py" https://github.com/owner/repo/pull/123 --files '*.py'

# Fetch only frontend files
uv run "${CLAUDE_SKILL_DIR}/scripts/fetch_diff.py" https://github.com/owner/repo/pull/123 --files 'src/server/js/*'

# Multiple patterns
uv run "${CLAUDE_SKILL_DIR}/scripts/fetch_diff.py" https://github.com/owner/repo/pull/123 --files '*.py' '*.ts'

The GitHub token is auto-detected from the GH_TOKEN environment variable or gh auth token.

Inputs and outputs

scripts/fetch_diff.py takes:

pr_url (positional, required) — a GitHub PR URL of the form https://github.com/<owner>/<repo>/pull/<number>. Sub-paths like /files are not accepted; use the bare /pull/<n> form.
--files <pattern> ... (optional) — one or more fnmatch globs matched against each file's new path. Files matching any pattern are kept; the rest are dropped. No match yields an empty diff (not an error).
GH_TOKEN (env) — the token used to call the GitHub API. If unset, the script falls back to gh auth token; if that also fails it exits with an error.

It prints the PR's unified diff to stdout, reorganized per file: each file keeps its diff --git header, then either line-annotated hunks (see Line Annotation) or a one-line mask message for auto-generated and deleted files. A single diagnostic line goes to stderr when a stacked-PR incremental diff is detected. See references/output-format.md for the full schema.

Output Example

Regular file:

diff --git a/path/to/file.py b/path/to/file.py
index abc123..def456 100644
--- a/path/to/file.py
+++ b/path/to/file.py
@@ -10,7 +10,7 @@
10    10 |  import os
11    11 |  import sys
12    12 |  from typing import Optional
13       | -from old_module import OldClass
      14 | +from new_module import NewClass
14    15 |
15    16 |  def process_data(input_file: str) -> dict:

Auto-generated file (masked):

diff --git a/uv.lock b/uv.lock
index abc123..def456 100644
--- a/uv.lock
+++ b/uv.lock
[Auto-generated file - diff masked]

Deleted file (masked):

diff --git a/path/to/removed.py b/dev/null
index abc123..0000000 100644
--- a/path/to/removed.py
+++ /dev/null
[Deleted file - diff masked]

Line Annotation

Each line is annotated as old_line new_line | <marker> content:

- marker (left number only) -> deleted line; comment with side=LEFT, line=old_line
+ marker (right number only) -> added line; comment with side=RIGHT, line=new_line
No marker (both numbers) -> unchanged line; comment with side=RIGHT, line=new_line

Reference Files

references/output-format.md — consult when parsing the annotated diff: the column schema, marker meanings, how masked and deleted files render, and how stacked-PR incremental diffs are signaled.
references/troubleshooting.md — consult when the fetch fails or surprises you: auth errors, PR-not-found / private repos, large diffs, --files glob behavior and no-match output, and line-anchoring pitfalls when the result feeds add-review-comment.

Related skills

This skill is the first step of the PR-review family in agent-harness:

fetch-diff (this skill) — get the diff annotated with old/new line numbers.
add-review-comment — post an inline comment using the side/line anchors this skill produces.
pr-review — review a whole PR end to end.
fetch-unresolved-comments — pull back the review threads that still need a response.

Agent Skills: Fetch PR Diff

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