diff --git a/changelog.d/fixes/9102-fix.plan.md b/changelog.d/fixes/9102-fix.plan.md
new file mode 100644
index 0000000000..bfbfed5c22
--- /dev/null
+++ b/changelog.d/fixes/9102-fix.plan.md
@@ -0,0 +1 @@
+- fix(providers): modal.com validation returns clear error when Base URL is missing, instead of leaking "Invalid outbound URL" (#9102)
\ No newline at end of file
diff --git a/src/lib/providers/validation.ts b/src/lib/providers/validation.ts
index e9b2cef768..8312ea58e2 100644
--- a/src/lib/providers/validation.ts
+++ b/src/lib/providers/validation.ts
@@ -211,15 +211,30 @@ export async function validateProviderApiKey({ provider, apiKey, providerSpecifi
     oci: validateOciProvider,
     sap: validateSapProvider,
     bedrock: validateBedrockProvider,
-    modal: ({ apiKey, providerSpecificData }: any) =>
-      validateOpenAILikeProvider({
+    modal: ({ apiKey, providerSpecificData }: any) => {
+      // Modal is bring-your-own-deploy — it requires a Base URL pointing to the user's
+      // OpenAI-compatible Modal app. Without it, validateOpenAILikeProvider would build an
+      // empty probe URL and trip parseOutboundUrl with a raw guard error ("Invalid outbound
+      // URL: "). Surface an actionable message instead. See #9102.
+      const baseUrl = (providerSpecificData?.baseUrl || "").trim();
+      if (!baseUrl) {
+        return {
+          valid: false,
+          error:
+            "Modal requires a Base URL pointing to your OpenAI-compatible Modal app " +
+            "(e.g. https://<workspace>--<app>.modal.run/v1). " +
+            "Fill in the \"Base URL override\" field.",
+        };
+      }
+      return validateOpenAILikeProvider({
         provider: "modal",
         apiKey,
         providerSpecificData,
-        baseUrl: normalizeBaseUrl(providerSpecificData?.baseUrl || ""),
+        baseUrl: normalizeBaseUrl(baseUrl),
         modelId: MODAL_DEFAULT_VALIDATION_MODEL_ID,
         isLocal,
-      }),
+      });
+    },
     "nous-research": validateNousResearchProvider,
     poe: validatePoeProvider,
     clarifai: validateClarifaiProvider,
diff --git a/tests/unit/probe-9102-modal-nobaseurl.test.ts b/tests/unit/probe-9102-modal-nobaseurl.test.ts
new file mode 100644
index 0000000000..b99f7c24c2
--- /dev/null
+++ b/tests/unit/probe-9102-modal-nobaseurl.test.ts
@@ -0,0 +1,30 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { validateProviderApiKey } = await import("../../src/lib/providers/validation.ts");
+
+test("modal validation without baseUrl returns clear actionable error (not Invalid outbound URL)", async () => {
+  // Ensure no actual fetch ever happens — the bug is a pre-fetch URL parse failure
+  globalThis.fetch = async (_url: RequestInfo | URL, _init?: RequestInit) => {
+    throw new Error("unexpected fetch: validation should fail before any network request");
+  };
+
+  const result = await validateProviderApiKey({
+    provider: "modal",
+    apiKey: "ak-test:as-test",
+    providerSpecificData: {},
+  });
+
+  // The bug: when baseUrl is empty, validateOpenAILikeProvider gets an empty URL,
+  // parseOutboundUrl throws "Invalid outbound URL: " — a raw guard message.
+  // The fix must return a clear actionable message mentioning Base URL.
+  const errorMsg = result.error || "";
+  assert.ok(
+    !errorMsg.includes("Invalid outbound URL"),
+    `bug: leaked raw guard message -> ${JSON.stringify(errorMsg)}`
+  );
+  assert.ok(
+    errorMsg.toLowerCase().includes("base url") || errorMsg.toLowerCase().includes("base"),
+    `expected error to mention Base URL, got: ${JSON.stringify(errorMsg)}`
+  );
+});