Providing IDE & tooling info

Libraries can contribute extra, domain-specific information about types that is not part of the core language — for example, @typespec/http surfaces the effective HTTP route of an operation. This information is shown in IDE hover tooltips and can also be queried programmatically by tooling (such as AI agents).

Caution

This is an experimental feature. It must be enabled with the type-info-hook compiler feature flag in your tspconfig.yaml:

features:
  - type-info-hook

The $onInfo hook

A library provides this information by exporting an $onInfo function from its main entry point. Use the defineInfoHook helper for typing. The hook receives an InfoContext (with the current program and the target type) and returns a single TypeInfo object, or undefined when it has nothing to contribute.

import { defineInfoHook } from "@typespec/compiler";
import { getHttpOperation } from "./operations.js";

export const $onInfo = defineInfoHook(({ program, target }) => {
  if (target.kind !== "Operation") {
    return undefined;
  }
  const [operation] = getHttpOperation(program, target);
  if (!operation) {
    return undefined;
  }
  return {
    content: `\`HTTP Route\`: \`${operation.verb.toUpperCase()} ${operation.uriTemplate}\``,
  };
});

A TypeInfo currently has a single content field: the markdown content to show for this piece of information.

Important constraints

Unlike $onValidate, the $onInfo hook:

• is never run during compilation. It is invoked lazily and on demand.
• must not mutate the type graph. It should only read the program and answer questions about it.

Because it does not change the type graph, there are no ordering or race concerns between libraries — the content from every library is simply concatenated.

Querying info programmatically

Tooling can query all registered providers for a given type with program.getTypeInfo, which merges the contributions from every library into a single TypeInfo (or undefined when nothing is contributed or the feature is disabled):

const info = program.getTypeInfo(type);
// { content: "`HTTP Route`: `GET /pets/{id}`\n\n`Responses`: `204`" }

The language server uses this same API to enrich hover documentation.