1 files changed, 44 insertions, 11 deletions

diff --git a/src/client.ts b/src/client.ts
index c7dac15..3e48d7a 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -19,7 +19,7 @@ import {
   RateLimited,
   ServerError,
 } from "./errors.js";
-import { Address, Health, type RawObject } from "./models.js";
+import { Address, Faixa, Health, type RawObject } from "./models.js";
 import { VERSION } from "./version.js";
 
 /** Where the public API lives. */
@@ -235,6 +235,19 @@ export class Client {
    * message: the format already passed local validation, so the server knows
    * something this client does not.
    *
+   * A CEP that does not exist throws {@link CEPNotFound}, which may carry a
+   * {@link Faixa} naming the municipality the number would belong to:
+   *
+   * ```ts
+   * try {
+   *   await client.lookup("25845000");
+   * } catch (error) {
+   *   if (error instanceof CEPNotFound && error.faixa) {
+   *     error.faixa.localidade; // "Areal" — still a 404, still no address
+   *   }
+   * }
+   * ```
+   *
    * @throws {InvalidCEP} the CEP is not 8 digits — no request is made.
    * @throws {CEPNotFound} well-formed, but not in the database.
    * @throws {RateLimited} 429 after every attempt.
@@ -247,7 +260,13 @@ export class Client {
     const reply = await this.request(`/${number}`);
 
     if (reply.status === 404) {
-      throw new CEPNotFound(`CEP ${number} does not exist in the DNE database`, number);
+      const faixa = Faixa.fromJson(errorBody(reply.body)?.faixa);
+      const belongs = faixa === undefined ? "" : ` — that band belongs to ${faixa.label}`;
+      throw new CEPNotFound(
+        `CEP ${number} does not exist in the DNE database${belongs}`,
+        number,
+        faixa,
+      );
     }
     if (reply.status === 400) {
       throw new InvalidCEP(errorMessage(reply.body, `CEP ${number} rejected`));
@@ -417,17 +436,31 @@ export class Client {
   }
 }
 
-function errorMessage(body: string, fallback: string): string {
+/**
+ * The body of an error response as an object, or `undefined` when it is not
+ * one.
+ *
+ * Never throws. An error response is already the bad news; a proxy answering
+ * with HTML in its place must not upgrade that into a second, different
+ * failure.
+ */
+function errorBody(body: string): RawObject | undefined {
+  let data: unknown;
   try {
-    const data: unknown = JSON.parse(body);
-    if (typeof data === "object" && data !== null && !Array.isArray(data)) {
-      const message = (data as RawObject).mensagem;
-      if (typeof message === "string" && message !== "") {
-        return message;
-      }
-    }
+    data = JSON.parse(body);
   } catch {
-    return fallback;
+    return undefined;
+  }
+  if (typeof data !== "object" || data === null || Array.isArray(data)) {
+    return undefined;
+  }
+  return data as RawObject;
+}
+
+function errorMessage(body: string, fallback: string): string {
+  const message = errorBody(body)?.mensagem;
+  if (typeof message === "string" && message !== "") {
+    return message;
   }
   return fallback;
 }