If you want to use a relative path to something in a workspace folder for the url property of a JSON schema setting item, you need to define that setting in the workspace settings (<workspace-folder-path>/.vscode/settings.json) instead of in the user settings. Related docs are here.

If you insist to define it in the user settings.json, I think you need to use an absolute path.

I think this is a sensible design. User settings apply to all workspaces, so if if relative paths were supported there, it would be an assumption that every single folder you open as a workspace would have a schema file that you'd want to use at such a path (presumably relative to the workspace folder, which I think would be a rather poor assumption, or lead to surprising users (in a bad way)).

Also note that if you want to specify schema files on a per-JSON-file-basis, you can use the $schema property in the JSON file you want to be validated to specify the schema to use.

Found an answer by digging through vscode source code. Searched all files, that contains package.json and dependencies (regex search (package.json[\s\S\n]*dependencies)|(dependencies[\s\S\n]*package.json)) and found a class PackageJSONContribution and this was exactly what I was looking for.

Then I made a simple extension using yo code (as from the example of Your first extension).

Some interfaces and classes I was unable to import from vscode, I just made a local copies for my extension. Made my own class FooJSONContribution with the only difference - it's getDocumentSelector() function changed to

    public getDocumentSelector(): vscode.DocumentSelector {
        return [{ language: 'json', scheme: '*', pattern: '**/foo.json' }];
    }

Adjusted extensions package.json activationEvents and added "onLanguage:json" (this could be optimised).

Adjusted activate event of the extension by adding

    const contribution = new FooJSONContribution(xhr, true);
    const disposableCompletionItemProvider = vscode.languages.registerCompletionItemProvider(
        contribution.getDocumentSelector(),
        new JSONCompletionItemProvider(contribution),
        '"',
        ':'
    );
    context.subscriptions.push(disposableCompletionItemProvider);

And that's it. All of the foo.json files now work like package.json files -

Things I need to do now - adjust the code of FooJSONContribution.ts to work for my own needs, for my own API's, for my own files.

I Uploaded this example here https://github.com/leqwasd/VsCodeAsyncJsonAutocomplete

VSCode will give intellisense for JSON whenever it has a valid schema to use, hosted at schemastore.org or anywhere else. It just needs to know what schema to use for which files.

As you noted, VSCode just "knows" what schema to use for certain well-known files, like tsconfig.json.

For other files, you can specify the schema directly in the JSON, which is what VSCode is prompting you to do in your screenshot:

If you don't want to specify the schema in the JSON file itself, you can map file patterns to schemas in settings.json:

  "json.schemas": [
    {
      "fileMatch": ["stryker.conf.json"],
      "url": "https://raw.githubusercontent.com/stryker-mutator/stryker/master/packages/api/schema/stryker-core.json"
    }
  ]

The association of JSON schemas to files is done in the settings (File, Preferences, User Settings or Workspace Settings), under the property 'json.schemas'.

This is an example how the JSON schema for bower is associated to the bower schema.

"json.schemas": [
    {
        "fileMatch": [
            "/bower.json",
            "/.bower.json"
        ],
        "url": "http://json.schemastore.org/bower"
    },
    ...

You can also use schemas located in your workspace or define a schema right in the settings itself. Check https://code.visualstudio.com/docs/languages/json for examples.