diff --git a/src/agents/models-config.fills-missing-provider-apikey-from-env-var.test.ts b/src/agents/models-config.fills-missing-provider-apikey-from-env-var.test.ts
index 9b6318153d3..ef03fb3863b 100644
--- a/src/agents/models-config.fills-missing-provider-apikey-from-env-var.test.ts
+++ b/src/agents/models-config.fills-missing-provider-apikey-from-env-var.test.ts
@@ -65,7 +65,7 @@ async function runCustomProviderMergeTest(params: {
     baseUrl: string;
     apiKey: string;
     api: string;
-    models: Array<{ id: string; name: string; input: string[] }>;
+    models: Array<{ id: string; name: string; input: string[]; api?: string }>;
   };
   existingProviderKey?: string;
   configProviderKey?: string;
@@ -261,6 +261,28 @@ describe("models-config", () => {
     });
   });
 
+  it("replaces stale merged baseUrl when only model-level apis change", async () => {
+    await withTempHome(async () => {
+      const parsed = await runCustomProviderMergeTest({
+        seedProvider: {
+          baseUrl: "https://agent.example/v1",
+          apiKey: "AGENT_KEY", // pragma: allowlist secret
+          api: "",
+          models: [
+            {
+              id: "agent-model",
+              name: "Agent model",
+              input: ["text"],
+              api: "openai-completions",
+            },
+          ],
+        },
+      });
+      expect(parsed.providers.custom?.apiKey).toBe("AGENT_KEY");
+      expect(parsed.providers.custom?.baseUrl).toBe("https://config.example/v1");
+    });
+  });
+
   it("replaces stale merged apiKey when provider is SecretRef-managed in current config", async () => {
     await withTempHome(async () => {
       await writeAgentModelsJson({
diff --git a/src/agents/models-config.ts b/src/agents/models-config.ts
index 7e96e685fe9..db7e3a5f1a7 100644
--- a/src/agents/models-config.ts
+++ b/src/agents/models-config.ts
@@ -178,6 +178,33 @@ function resolveProviderApi(entry: { api?: unknown } | undefined): string | unde
   return api || undefined;
 }
 
+function resolveModelApiSurface(entry: { models?: unknown } | undefined): string | undefined {
+  if (!Array.isArray(entry?.models)) {
+    return undefined;
+  }
+
+  const apis = entry.models
+    .flatMap((model) => {
+      if (!model || typeof model !== "object") {
+        return [];
+      }
+      const api = (model as { api?: unknown }).api;
+      return typeof api === "string" && api.trim() ? [api.trim()] : [];
+    })
+    .toSorted();
+
+  if (apis.length === 0) {
+    return undefined;
+  }
+  return JSON.stringify(apis);
+}
+
+function resolveProviderApiSurface(
+  entry: ({ api?: unknown; models?: unknown } & Record<string, unknown>) | undefined,
+): string | undefined {
+  return resolveProviderApi(entry) ?? resolveModelApiSurface(entry);
+}
+
 function shouldPreserveExistingApiKey(params: {
   providerKey: string;
   existing: ExistingProviderConfig;
@@ -207,8 +234,8 @@ function shouldPreserveExistingBaseUrl(params: {
     return false;
   }
 
-  const existingApi = resolveProviderApi(existing);
-  const nextApi = resolveProviderApi(nextEntry);
+  const existingApi = resolveProviderApiSurface(existing);
+  const nextApi = resolveProviderApiSurface(nextEntry);
   return !existingApi || !nextApi || existingApi === nextApi;
 }