diff --git a/src/proxy/anthropic.ts b/src/proxy/anthropic.ts
index 7f96df5..8615fec 100644
--- a/src/proxy/anthropic.ts
+++ b/src/proxy/anthropic.ts
@@ -259,6 +259,22 @@ anthropicRouter.post(
   }),
   anthropicProxy
 );
+// This is not a valid route but clients may attempt to use it.
+anthropicRouter.post("/v1/claude-3/messages", (req, res) => {
+  sendErrorToClient({
+    req,
+    res,
+    options: {
+      title: "Proxy error (wrong endpoint)",
+      message:
+        "Your client is attempting to use the /anthropic/claude-3 compatibility endpoint, but it supports the new API format.\n\nUse the normal /anthropic endpoint instead.",
+      format: "unknown",
+      statusCode: 404,
+      reqId: req.id,
+      obj: { original_url: req.originalUrl, router_url: req.url },
+    },
+  });
+});
 
 export function handleCompatibilityRequest(
   req: Request,
@@ -268,7 +284,7 @@ export function handleCompatibilityRequest(
   const alreadyInChatFormat = Boolean(req.body.messages);
   const alreadyUsingClaude3 = req.body.model?.includes("claude-3");
   if (!alreadyInChatFormat && !alreadyUsingClaude3) {
-    next();
+    return next();
   }
 
   if (alreadyInChatFormat) {
diff --git a/src/proxy/aws.ts b/src/proxy/aws.ts
index 1fcb1d9..98f073e 100644
--- a/src/proxy/aws.ts
+++ b/src/proxy/aws.ts
@@ -20,6 +20,7 @@ import {
   handleCompatibilityRequest,
   transformAnthropicChatResponseToAnthropicText,
 } from "./anthropic";
+import { sendErrorToClient } from "./middleware/response/error-generator";
 
 const LATEST_AWS_V2_MINOR_VERSION = "1";
 const CLAUDE_3_COMPAT_MODEL = "anthropic.claude-3-sonnet-20240229-v1:0";
@@ -202,6 +203,22 @@ awsRouter.post(
   ),
   awsProxy
 );
+// This is not a valid route but clients may attempt to use it.
+awsRouter.post("/v1/claude-3/messages", (req, res) => {
+  sendErrorToClient({
+    req,
+    res,
+    options: {
+      title: "Proxy error (wrong endpoint)",
+      message:
+        "Your client is attempting to use the /anthropic/v1/claude-3 compatibility endpoint, but supports the new API format and should use the normal /anthropic/v1 endpoint instead.",
+      format: "unknown",
+      statusCode: 404,
+      reqId: req.id,
+      obj: { original_url: req.originalUrl, router_url: req.url },
+    },
+  });
+});
 // OpenAI-to-AWS Anthropic compatibility endpoint.
 awsRouter.post(
   "/v1/chat/completions",
diff --git a/src/proxy/routes.ts b/src/proxy/routes.ts
index 910237a..020e653 100644
--- a/src/proxy/routes.ts
+++ b/src/proxy/routes.ts
@@ -58,8 +58,10 @@ proxyRouter.use((req, res) => {
       reqId: req.id,
       format: "unknown",
       obj: {
-        proxy_note: "Your chat client is using the wrong endpoint. Please check your configuration.",
-        requested_url: req.url,
+        proxy_note:
+          "Your chat client is using the wrong endpoint. Please check your configuration.",
+        original_url: req.originalUrl,
+        router_url: req.url,
       },
     },
   });