Accept replaces-base option

src/start-proxy-action.ts

@@ -198,6 +198,7 @@ async function startProxy(
     .map((credential) => ({
       type: credential.type,
       url: credential.url,
+      "replaces-base": credential["replaces-base"],
     }));
   core.setOutput("proxy_urls", JSON.stringify(registry_urls));
 

src/start-proxy.ts

@@ -347,8 +347,21 @@ export function getCredentials(
       );
     }
 
+    // Construct the base credential object.
+    const baseCredential: Omit<Registry, keyof Address> = { type: e.type };
+
+    if (isDefined(e["replaces-base"])) {
+      if (typeof e["replaces-base"] === "boolean") {
+        baseCredential["replaces-base"] = e["replaces-base"];
+      } else {
+        throw new ConfigurationError(
+          "Invalid credentials - 'replaces-base' must be a boolean",
+        );
+      }
+    }
+
     out.push({
-      type: e.type,
+      ...baseCredential,
       ...authConfig,
       ...address,
     });

src/start-proxy/types.ts

@@ -246,6 +246,8 @@ export function credentialToStr(credential: Credential): string {
 export type Registry = {
   /** The type of the package registry. */
   type: string;
+  /** Whether the registry replaces the base registry for the ecosystem. */
+  "replaces-base"?: boolean;
 } & Address;
 
 // If a registry has an `url`, then that takes precedence over the `host` which may or may