diff --git a/README.md b/README.md
index f9b5072a..807b01eb 100644
--- a/README.md
+++ b/README.md
@@ -419,6 +419,9 @@ The following runtimes are supported:
 - Jest 28 or greater with the `"node"` environment (`"jsdom"` is not supported at this time).
 - Nitro v2.6 or greater.
 
+> [!WARNING]
+> Web browser runtimes aren't supported. The SDK will throw an error if used in a browser environment.
+
 Note that React Native is not supported at this time.
 
 If you are interested in other runtime environments, please open or upvote an issue on GitHub.
diff --git a/src/index.ts b/src/index.ts
index 0976766d..2df0939a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -109,6 +109,12 @@ export class Anthropic extends Core.APIClient {
       baseURL: baseURL || `https://api.anthropic.com`,
     };
 
+    if (Core.isRunningInBrowser()) {
+      throw new Errors.AnthropicError(
+        "It looks like you're running in a browser-like environment, which is disabled to protect your secret API credentials from attackers. If you have a strong business need for client-side use of this API, please open a GitHub issue with your use-case and security mitigations.",
+      );
+    }
+
     super({
       baseURL: options.baseURL!,
       timeout: options.timeout ?? 600000 /* 10 minutes */,