Mock sample for your project: CodeScan API

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The Entity Search API lets you send a search query to Bing and get back search results that include entities and places. Place results include restaurants, hotel, or other local businesses. For places, the query can specify the name of the local business or it can ask for a list (for example, restaurants near me). Entity results include persons, places, or things. Place in this context is tourist attractions, states, countries, etc.

This API provides a similarity key used to match with other similar full name data when data fields are parsed into first name and last name components, including for purposes of deduplication, fuzzy matching, or merging of datasets. A much higher match rate will be achieved by matching on the similarity key rather than the data itself. This API is for full name data where first and last name are in separate fields. Use the Full Name Similarity Key API for first and last name data that are in the same field.

Wikipedia for Web APIs. Repository of API specs in OpenAPI 3.0 format.
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Client sample: [Demo] [Repo]

Airbyte Configuration API
https://airbyte.io.
This API is a collection of HTTP RPC-style methods. While it is not a REST API, those familiar with REST should find the conventions of this API recognizable.
Here are some conventions that this API follows:
All endpoints are http POST methods.
All endpoints accept data via application/json request bodies. The API does not accept any data via query params.
The naming convention for endpoints is: localhost:8000/{VERSION}/{METHODFAMILY}/{METHODNAME} e.g. localhost:8000/v1/connections/create.
For all update methods, the whole object must be passed in, even the fields that did not change.
Change Management:
The major version of the API endpoint can be determined / specified in the URL localhost:8080/v1/connections/create
Minor version bumps will be invisible to the end user. The user cannot specify minor versions in requests.
All backwards incompatible changes will happen in major version bumps. We will not make backwards incompatible changes in minor version bumps. Examples of non-breaking changes (includes but not limited to...):
Adding fields to request or response bodies.
Adding new HTTP endpoints.

vRealize Network Insight API Reference

User Recommendation Engine and Chat Network

The Bing Custom Image Search API lets you send an image search query to Bing and get back image search results customized to meet your custom search definition.

Converter and validator for Swagger 2.0 to OpenAPI 3.0.x definitions