diff --git a/lib/start-proxy-action.js b/lib/start-proxy-action.js
index 74ffff232..4c71f8470 100644
--- a/lib/start-proxy-action.js
+++ b/lib/start-proxy-action.js
@@ -122611,8 +122611,18 @@ function getCredentials(logger, registrySecrets, registriesCredentials, language
         `A ${e.type} private registry is configured for ${e.host || e.url} using a GitHub Personal Access Token (PAT), but no username was provided. This may not work correctly. When configuring a private registry using a PAT, select "Username and password" and enter the username of the user who generated the PAT.`
       );
     }
+    const baseCredential = { type: e.type };
+    if (isDefined2(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
     });
@@ -123158,7 +123168,8 @@ async function startProxy(binPath, config, logFilePath, logger) {
   core13.setOutput("proxy_ca_certificate", config.ca.cert);
   const registry_urls = config.all_credentials.filter((credential) => credential.url !== void 0).map((credential) => ({
     type: credential.type,
-    url: credential.url
+    url: credential.url,
+    "replaces-base": credential["replaces-base"]
   }));
   core13.setOutput("proxy_urls", JSON.stringify(registry_urls));
   return { host, port, cert: config.ca.cert, registries: registry_urls };
diff --git a/src/start-proxy-action.ts b/src/start-proxy-action.ts
index a288acc5c..091866377 100644
--- a/src/start-proxy-action.ts
+++ b/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));
 
diff --git a/src/start-proxy.ts b/src/start-proxy.ts
index 94ae303e6..868722e15 100644
--- a/src/start-proxy.ts
+++ b/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,
     });
diff --git a/src/start-proxy/types.ts b/src/start-proxy/types.ts
index 98c23662a..8e25b78af 100644
--- a/src/start-proxy/types.ts
+++ b/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