JSON Assertion Runner

About this tool

JSON Assertion Runner is a small contract-test runner for JSON payloads. Paste an API response, fixture, or event body, then write assertions with JSONPath selectors and simple operators. The output is a pass/fail report that names the failing rule, the matched location, and the actual value when a comparison fails.

Use it when a full test framework is too heavy but you still need repeatable checks: smoke tests for API samples, fixture validation before a demo, webhook payload checks, or QA notes that should be executable instead of prose.

Assertion syntax

The line DSL is one assertion per line:

$.status equals ok
$.user.id matches ^u-\d+$
$.items length 2
$.items[*] count 2
$.items[*].price gt 0
$.items[*].price lte 100
$.user.roles contains admin
$.user.name type string
$.optional not_exists

Blank lines and # comments are ignored. The same rules can be supplied as a JSON array of objects with path, op, and optional expected fields (plus an optional name that is echoed in the report) when you want to generate checks from another script.

Supported operators include exists, not_exists, equals, not_equals, type, gt, gte, lt, lte, range, contains, not_contains, matches, count, length, empty, and not_empty.

Worked example

Payload:

{"status":"ok","user":{"id":"u-42","roles":["admin","dev"]},"items":[{"sku":"A1","price":9.5},{"sku":"B2","price":250}]}

Assertions:

$.status equals ok
$.user.id matches ^u-\d+$
$.user.roles contains admin
$.items length 2
$.items[*].price gt 0

Report:

PASS — 5 of 5 assertions passed, 0 failed

PASS  #1  $.status equals "ok"
PASS  #2  $.user.id matches ^u-\d+$
PASS  #3  $.user.roles contains "admin"
PASS  #4  $.items length 2
PASS  #5  $.items[*].price gt 0

Swap the last rule for $.items[*].price lte 100 and the run fails with the value that broke it:

FAIL — 4 of 5 assertions passed, 1 failed

FAIL  #5  $.items[*].price lte 100
          at $['items'][1]['price']: expected <= 100, got 250

Limits and edge cases

• Up to 200 assertions per run; anything more is rejected rather than truncated.
• Nothing is fetched: the payload is whatever you paste. Everything runs locally in your browser.
• JSONPath selectors are evaluated against one JSON value at a time; newline-delimited JSON must be split before running checks.
• In the line DSL the path is the first whitespace-delimited token, so a path containing spaces ($['order id']) has to go through the JSON rules format instead.
• equals compares JSON values, so unquoted DSL tokens are parsed as JSON when possible (true, 42, null) and otherwise treated as strings.
• type integer is stricter than type number; 9.5 is a number but not an integer.
• contains works on arrays (membership), strings (substring), and objects (key). Use count for how many nodes a JSONPath selected — $.items count 2 selects one array, $.items[*] count 2 selects two items — and length for the size of a matched array, string, or object.
• A value comparison must hold for every matched node, and a path that matches nothing fails; use not_exists when absence is what you mean.
• Regexes use Rust regular expressions. They are not JavaScript regex literals, so write ^u-\d+$ rather than /^u-\d+$/.
• Turning off case sensitivity affects string equality, string/array contains checks, and regex matching.

FAQ