apicmp

Did this API quietly change? Find out in one command.

APIs change silently. A field gets renamed. A type flips from string to integer. A key disappears. A number drifts 15% without anyone noticing.

apicmp compares two JSON responses — from URLs, files, or stdin — and tells you exactly what changed: fields added, fields removed, type changes, value changes, and numeric drift beyond a threshold.

No auth setup. No config files. No enterprise pricing. Just point it at two things and see the diff.

Install

pip install apicmp

Or with pipx:

pipx install apicmp

Usage

# Two live URLs
apicmp https://api.example.com/v1/user https://api.example.com/v2/user

# Two local files
apicmp before.json after.json

# File vs live URL
apicmp snapshot.json https://api.example.com/endpoint

# Pipe first response from stdin
curl https://api.example.com/v1 | apicmp - https://api.example.com/v2

# Flag numeric drift over 10% (default is 5%)
apicmp a.json b.json --threshold 0.1

# Add auth or custom headers
apicmp a.json b.json -H "Authorization: Bearer TOKEN"
apicmp a.json b.json -H "X-Api-Key: abc123"

# Export diff as Markdown
apicmp a.json b.json --output diff.md

# JSON output — pipe to jq
apicmp a.json b.json --format json | jq '.added'

What it compares

apicmp flattens nested JSON to dot-notation paths and compares them structurally:

Nested objects are flattened — user.address.city is a single comparable path. Arrays are compared by index up to 20 elements, and array lengths are tracked separately.

Output formats

--format table (default) — colour-coded rich terminal tables, one per change category.

--format markdown — GitHub-flavored Markdown, paste into PRs, wikis, or incident reports.

--format json — machine-readable, pipe to jq or use in scripts.

All options

Arguments:
  SOURCE_A      First JSON source (URL, file, or '-' for stdin).
  SOURCE_B      Second JSON source (URL or file).

Options:
  -t, --threshold   Flag numeric values that differ by more than this fraction
                    (default: 0.05 = 5%).
  -H, --header      Extra HTTP headers (format: 'Name: Value'). Repeatable.
  --timeout         HTTP request timeout in seconds (default: 15).
  -o, --output      Write Markdown diff to this file.
  -f, --format      Output format: table | markdown | json (default: table).
  --help            Show this message and exit.

Examples

Snapshot an API before a deploy, diff after:

curl https://api.example.com/user/1 > before.json
# deploy...
apicmp before.json https://api.example.com/user/1

Compare staging vs production:

apicmp https://staging.api.example.com/config \
         https://api.example.com/config \
         -H "Authorization: Bearer $TOKEN"

Export diff for a PR comment:

apicmp before.json after.json --output diff.md
cat diff.md

Use in CI — fail if APIs diverge:

apicmp snapshot.json https://api.example.com/endpoint \
  --format json | python -c "
import json, sys
d = json.load(sys.stdin)
if d['added'] or d['removed'] or d['type_changed']:
    sys.exit(1)
"

Development

git clone https://github.com/gitwingo/apicmp
cd apicmp
pip install -e .

Support

If apicmp has been useful to you, consider supporting its development:

Connect

• GitHub: @gitwingo
• Reddit: u/gitwingo
• X / Twitter: @gitwingo

License

MIT © gitwingo