---

JSON Diff Tool & API Response Inspector

Compare two JSON documents side by side and find exactly what changed. FastTL’s JSON Diff Tool helps you inspect API responses, compare nested JSON objects, detect missing fields, and identify value or data type changes—all in one place.

Whether you’re debugging an API, checking a configuration update, or reviewing JSON data from different environments, you can compare the two versions without manually scanning every line.

What you can do: Compare JSON · Validate syntax · Find changed fields · Inspect nested objects · Export comparison reports

---

Compare JSON Online

Use the JSON Diff Tool above to compare your original JSON with an updated version. Paste your data into the two panels, validate the input, and run the comparison to review the differences.

You can use it to investigate API response changes, review JSON configuration updates, or check whether two structured data objects contain the same information.

What Does This JSON Diff Tool Check?

Added Fields

Find keys that appear in the updated JSON but were not present in the original version. This is useful when an API introduces a new property or a configuration file gains a new setting.

Removed Fields

Identify keys that existed in the original JSON but are missing from the updated version. This can help you spot unexpected changes in API responses or data structures.

Changed Values

See which fields contain different values between the two JSON documents. Instead of reading both documents line by line, you can focus on the fields that actually changed.

Data Type Changes

A field can keep the same name while its data type changes. For example, an API may return a number in one response and a string in another. Detecting these changes can help identify compatibility problems.

Nested JSON Differences

JSON often contains objects inside objects and arrays inside other structures. The comparison tool can inspect nested fields and show differences at specific paths, making complex responses easier to review.

How to Compare Two JSON Responses

1. Add the original JSON: Paste or import the first JSON document into the original panel.
2. Add the updated JSON: Paste or import the second document into the updated panel.
3. Validate the input: Check that both documents contain valid JSON syntax.
4. Run the comparison: Review the detected differences, including added, removed, changed, and type-changed fields.
5. Inspect the results: Use the available search and filters to focus on the fields you need to investigate.
6. Export if needed: Download a JSON or CSV report for debugging, documentation, or further review.

Example: Comparing Two API Responses

Suppose your application originally receives this response:

Original JSON

{
  "id": 101,
  "name": "Product A",
  "price": 25,
  "available": true
}

After an API update, the response becomes:

Updated JSON

{
  "id": 101,
  "name": "Product A",
  "price": "29",
  "available": true,
  "category": "Accessories"
}

The comparison should identify these differences:

• price changed from the number 25 to the string "29". This is a value change and a data type change.
• category was added to the updated response.
• id, name, and available remain unchanged.

This kind of comparison is useful when an API change might affect calculations, frontend rendering, form validation, or application logic.

More Than a Basic JSON Formatter

A JSON formatter makes structured data easier to read. A JSON validator checks whether the syntax is valid. A JSON diff tool answers a different question: what changed between these two versions?

FastTL combines useful JSON inspection features in one workflow, so you can validate, format, compare, and export results without switching between several separate tools.

Who Can Use This Tool?

• Developers: Compare API responses while debugging applications.
• QA testers: Check whether an endpoint returns unexpected data after a change.
• Backend engineers: Review response structures between development, staging, and production.
• Frontend developers: Find missing properties or changed types that may affect the user interface.
• Students and learners: Understand how JSON objects differ and practice working with structured data.
• Technical teams: Review JSON configuration files and keep track of changes during development.

Privacy and Browser-Based Fixture Storage

FastTL’s comparison workflow does not require a paid API or an external AI API. Saved comparison fixtures are stored in your browser’s local storage rather than being synced through a fixture-storage server.

Local storage is specific to the browser and device you use. Clearing browser data or switching devices may make saved fixtures unavailable. Avoid saving passwords, access tokens, personal information, or other confidential data in fixtures, and review your site’s privacy policy before using sensitive information.

Frequently Asked Questions

What is a JSON Diff Tool?

A JSON Diff Tool compares two JSON documents and identifies differences such as added fields, removed keys, changed values, and data type changes.

Can I compare API responses online?

Yes. Paste the original and updated API response JSON into the comparison panels to inspect the differences. The tool compares the JSON you provide; it does not automatically send requests to an API endpoint.

Can it compare nested JSON objects?

Yes. It can compare nested objects and report differences at individual field paths, helping you locate changes in complex data structures.

Does it detect changes between numbers and strings?

Yes. For example, the number 42 and the string "42" are different JSON values and have different data types.

Can I export JSON comparison results?

The tool provides JSON and CSV export options for reviewing or sharing comparison reports.

Does this tool require an API key?

No paid API key or external AI API is required for the local JSON comparison workflow.

Are saved fixtures available on every device?

No. Fixtures are stored in the current browser’s local storage, so they are not automatically synced between browsers or devices.

Does the tool automatically fetch a live API response?

No. You provide the JSON documents you want to compare. The tool inspects those documents rather than fetching live endpoints automatically.

---

Compare Your JSON Data with Confidence

Unexpected JSON changes can be difficult to spot when two responses contain many nested fields. Use FastTL’s JSON Diff Tool to compare versions, identify important changes, and make your debugging workflow more manageable.

Start with your original JSON, add the updated version, and inspect the differences.