fix(vercel): honor author-supplied runtime instead of forcing the eve image

.changeset/vercel-honor-runtime.md

@@ -0,0 +1,5 @@
+---
+"eve": patch
+---
+
+The Vercel sandbox backend now honors an author-supplied `runtime` (e.g. `vercel({ runtime: "python3.13" })`) instead of always forcing the `vercel/eve:latest` image. The default eve image is still used when no runtime is requested.

packages/eve/src/execution/sandbox/bindings/vercel-create-sdk.ts

@@ -24,11 +24,17 @@ export async function createVercelEveImageSandbox(input: {
   readonly createOptions: VercelSandboxCreateParams;
   readonly sandboxModule: VercelModule;
 }): Promise<VercelSandbox> {
-  const createOptions: VercelSandboxCreateParams = {
-    ...input.createOptions,
-    __image: VERCEL_EVE_SANDBOX_IMAGE,
-  };
+  // The eve base image is only a default. `__image` takes precedence over
+  // `runtime` in the SDK, so forcing it unconditionally silently drops an
+  // author-supplied runtime — apply it only when no runtime was requested.
+  const createOptions: VercelSandboxCreateParams = hasAuthorRuntime(input.createOptions)
+    ? input.createOptions
+    : { ...input.createOptions, __image: VERCEL_EVE_SANDBOX_IMAGE };
   return await input.sandboxModule.Sandbox.create(createOptions);
 }
 
+function hasAuthorRuntime(createOptions: VercelSandboxCreateParams): boolean {
+  return (createOptions as VercelSandboxCreateParams & { runtime?: unknown }).runtime !== undefined;
+}
+
 const VERCEL_EVE_SANDBOX_IMAGE = "vercel/eve:latest";

packages/eve/src/execution/sandbox/bindings/vercel.test.ts

@@ -198,6 +198,31 @@ describe("createVercelSandbox", () => {
     );
   });
 
+  it("honors an author-supplied runtime instead of forcing the eve image", async () => {
+    const templateSandbox = createMockSandbox({ name: "template-key" });
+    const sandboxModule = {
+      Sandbox: {
+        create: vi.fn().mockResolvedValueOnce(templateSandbox),
+        get: vi.fn().mockResolvedValueOnce(null),
+      },
+    };
+
+    const backend = createVercelSandbox({
+      createOptions: { runtime: "python3.13" } as never,
+      loadSandboxModule: async () => sandboxModule as never,
+    });
+
+    await backend.prewarm({
+      runtimeContext: { appRoot: "/tmp/test-app-root" },
+      seedFiles: [],
+      templateKey: "template-key",
+    });
+
+    const createArgs = sandboxModule.Sandbox.create.mock.calls[0]?.[0];
+    expect(createArgs).toMatchObject({ runtime: "python3.13" });
+    expect(createArgs).not.toHaveProperty("__image");
+  });
+
   it("passes resolved credentials to Vercel sandbox lookups instead of inferring scope", async () => {
     const existingTemplate = createMockSandbox({
       name: "template-key",