fix(test): use POST /v1/messages to validate anthropic-compatible connections

GET /models is not part of the Anthropic API spec and many compatible
proxies do not implement it, causing the connection test to always fail
even with a valid API key.

Switch to POST /v1/messages with max_tokens=1 — the same approach used
for the built-in anthropic provider — and treat any non-401/403 response
as valid, since 400/529 still confirms the key was accepted.

Use node.defaultModel / connection.defaultModel when set so the test
respects the configured model rather than always falling back to
claude-3-haiku-20240307.

Co-authored-by: Rehan Choirul <rehanchrl@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/api/providers/[id]/test/testUtils.js

@@ -355,10 +355,25 @@ async function testApiKeyConnection(connection, effectiveProxy = null) {
     try {
       modelsBase = modelsBase.replace(/\/$/, "");
       if (modelsBase.endsWith("/messages")) modelsBase = modelsBase.slice(0, -9);
-      const res = await fetchWithConnectionProxy(`${modelsBase}/models`, {
+      const messagesUrl = `${modelsBase}/v1/messages`;
-        headers: { "x-api-key": connection.apiKey, "anthropic-version": "2023-06-01", "Authorization": `Bearer ${connection.apiKey}` },
+      const model = connection.defaultModel || "claude-3-haiku-20240307";
+      const res = await fetchWithConnectionProxy(messagesUrl, {
+        method: "POST",
+        headers: {
+          "x-api-key": connection.apiKey,
+          "anthropic-version": "2023-06-01",
+          "content-type": "application/json",
+          "Authorization": `Bearer ${connection.apiKey}`,
+        },
+        body: JSON.stringify({
+          model,
+          max_tokens: 1,
+          messages: [{ role: "user", content: "test" }],
+        }),
       }, effectiveProxy);
-      return { valid: res.ok, error: res.ok ? null : "Invalid API key or base URL" };
+      // 400/529 still confirms key accepted; only 401/403 = bad key
+      const valid = res.status !== 401 && res.status !== 403;
+      return { valid, error: valid ? null : "Invalid API key or base URL" };
     } catch (err) {
       return { valid: false, error: err.message };
     }

src/app/api/providers/validate/route.js

@@ -155,17 +155,26 @@ export async function POST(request) {
           normalizedBase = normalizedBase.slice(0, -9); // remove /messages
         }
 
-        const modelsUrl = `${normalizedBase}/models`;
+        const messagesUrl = `${normalizedBase}/v1/messages`;
+        const model = node.defaultModel || "claude-3-haiku-20240307";
 
-        const res = await fetch(modelsUrl, {
+        const res = await fetch(messagesUrl, {
+          method: "POST",
           headers: {
             "x-api-key": apiKey,
             "anthropic-version": "2023-06-01",
-            "Authorization": `Bearer ${apiKey}`
+            "content-type": "application/json",
+            "Authorization": `Bearer ${apiKey}`,
           },
+          body: JSON.stringify({
+            model,
+            max_tokens: 1,
+            messages: [{ role: "user", content: "test" }],
+          }),
         });
 
-        isValid = res.ok;
+        // 400/529 still confirms key accepted; only 401/403 = bad key
+        isValid = res.status !== 401 && res.status !== 403;
         return NextResponse.json({
           valid: isValid,
           error: isValid ? null : "Invalid API key",