fix(cli): say stdin was empty when a JSON flag reads - and nothing is piped (#6852)

packages/cli/src/utils/jsonFlag.test.ts

@@ -35,6 +35,12 @@ describe("readJsonFlag", () => {
     expect(() => readJsonFlag("--grading", "C:\\clips\\grade.json", io)).toThrow(/no such file$/);
   });
 
+  it("says stdin was empty instead of a JSON parse error", () => {
+    expect(() => readJsonFlag("--grading", "-", { ...io, readStdin: () => "\n" })).toThrow(
+      /Could not read --grading: stdin: nothing was piped in/,
+    );
+  });
+
   it("names the flag and path when the file cannot be read", () => {
     expect(() => readJsonFlag("--grading", "missing.json", io)).toThrow(
       /Could not read --grading: missing\.json: ENOENT/,

packages/cli/src/utils/jsonFlag.ts

@@ -51,7 +51,10 @@ export function readJsonFlagText(value: string, partialIo: Partial<JsonFlagIO> =
   if (trimmed.startsWith("{") || trimmed.startsWith("[") || isJsonScalar(trimmed)) return value;
   const source = trimmed === "-" ? "stdin" : value;
   try {
-    return trimmed === "-" ? io.readStdin() : io.readFile(value);
+    if (trimmed !== "-") return io.readFile(value);
+    const piped = io.readStdin();
+    if (!piped.trim()) throw new Error("nothing was piped in");
+    return piped;
   } catch (error: unknown) {
     throw new JsonFlagReadError(source, errorText(error));
   }