Validating Response Headers | Effect Http Client

Some APIs return important metadata in response headers — pagination cursors, rate limit info, or cache tokens. You can validate these headers with HttpClientResponse.schemaHeaders().

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import {
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  FetchHttpClient,
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  HttpClient,
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  HttpClientRequest,
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  HttpClientResponse,
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} from "effect/unstable/http";
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import { Effect, Schema } from "effect";
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// Define a schema for headers we care about
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const ResponseHeaders = Schema.Struct({
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  "content-type": Schema.String,
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  "cache-control": Schema.String,
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});
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function fetchAndInspectHeaders() {
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  return Effect.gen(function* () {
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    const client = yield* HttpClient.HttpClient;
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    const request = HttpClientRequest.get("https://dummyjson.com/products/1");
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    const response = yield* client.execute(request);
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    // Validate response headers against our schema
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    const headers =
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      yield* HttpClientResponse.schemaHeaders(ResponseHeaders)(response);
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    console.log("Content-Type:", headers["content-type"]);
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    console.log("Cache-Control:", headers["cache-control"]);
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    const product = yield* response.json;
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    return product;
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  }).pipe(Effect.provide(FetchHttpClient.layer));
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}
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// Test it
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Effect.runPromise(fetchAndInspectHeaders()).then((product) => {
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  console.log("Product:", product.title);
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});

Output:

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Content-Type: application/json; charset=utf-8
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Cache-Control: no-store
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Product: Essence Mascara Lash Princess

schemaHeaders works just like schemaBodyJson, but operates on the response headers instead of the body. You define a schema for the header keys you expect, and it validates them. If a declared header is missing or doesn’t match, you get a SchemaError.

Note that header names are always lowercased when accessed through schemaHeaders, so use lowercase keys in your schema (e.g., "content-type", not "Content-Type"). This is because the HTTP specification treats header names as case-insensitive, and Effect normalizes them to lowercase for consistency.

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