Reading the diff

Claude's summary and the diff are not the same document. The summary describes what Claude intended. The diff shows what Claude actually wrote. In a codebase where things ship, you need the diff.

Why the summary is not enough

Claude's summaries are accurate about broad strokes and unreliable about specifics. "I renamed the function and updated all call sites" is usually true. "I did not touch anything else" is sometimes not. The diff is the only authoritative record of what changed.

The 30-second scan

Before reading line by line, scan for three categories:

1. New imports at the top of any file. An import you did not ask for is a dependency you did not choose.
2. Deleted lines, especially deleted conditionals. A removed if block may be removing a guard you needed.
3. Changed function signatures. A renamed parameter or a new default argument can break callers you cannot see in the diff.

git diff --stat HEAD
# → src/api/user.ts    | 4 ++--
# → src/utils/auth.ts  | 2 +-
# → 2 files changed, 5 insertions(+), 3 deletions(-)

git diff HEAD
# Full line-by-line view

Use --stat first to see which files changed. If a file appears that you did not expect — a test file, a config file, a dependency manifest — stop before reading the full diff and ask Claude why it touched that file.

What a clean diff looks like

A rename task on getUser → fetchUser across three files should produce: green lines adding fetchUser, red lines removing getUser, no changes to whitespace-only lines, no changes to imports, no changes outside the function names themselves. If the diff is tidy, it is safe to commit.

What a suspicious diff looks like

• A restructured if/else block that was not mentioned in the task
• A new console.log or debug statement
• A changed default value in a function signature
• An added or removed blank line that shifts the line numbers of everything below it (not harmful but a sign Claude did something beyond the task)

When you see anything outside scope, name it explicitly before accepting:

> The diff shows a restructured conditional in src/auth.ts line 47. I didn't ask for that. Revert it.

Cross-reference

The habit you are building here is the same one l5 ("When to interrupt, when to let it run") depends on. You can only interrupt at the right moment if you know what "outside scope" looks like in a diff.

If you get stuck

• git diff shows nothing: Claude may have committed automatically. Run git log --oneline -3 and then git show HEAD to see the most recent commit.
• The diff is too large to read: this is information. Reject the change, reset with git checkout ., and make the task smaller.
• You accepted before reading: run git revert HEAD to undo the commit, then re-run the task and read the diff this time.

Deliverable

For the task you ran in l2, write one sentence describing something the diff showed that Claude's summary did not mention. If the summary was perfectly accurate, write why you trust it.