fix(cli): format arrays of empty objects without keys as JSON (#7058)

When formatting arrays of objects that have no own enumerable keys as
Markdown, formatObjectTable derives empty headers and outputs a malformed
table with an invalid delimiter row. Require at least one item with keys in
isObjectArray and fall back to JSON serialization when headers are empty.

Fixes #7057

src/cli/cliOutputFormatter.ts

@@ -40,7 +40,8 @@ const isObjectArray = (value: unknown): value is Record<string, unknown>[] =>
   value.length > 0 &&
   value.every(
     (item) => typeof item === "object" && item !== null && !Array.isArray(item),
-  );
+  ) &&
+  value.some((item) => Object.keys(item).length > 0);
 
 const escapeTableCell = (value: string): string =>
   value
@@ -55,6 +56,7 @@ const renderTable = (
   headers: readonly string[],
   rows: readonly string[][],
 ): string => {
+  if (headers.length === 0) return "";
   const escapedHeaders = headers.map(escapeTableCell);
   const widths = escapedHeaders.map((header) => header.length);
   for (const row of rows) {
@@ -105,6 +107,7 @@ const formatObjectTable = (
         seen.add(key);
         headers.push(key);
       }
+  if (headers.length === 0) return markdownCell(items);
   return renderTable(
     headers,
     items.map((item) =>

tests/boundary/cli/cliOutput.test.ts

@@ -34,6 +34,14 @@ describe("CLI output argument boundary", () => {
     expect(JSON.parse(renderCliDocument(value, "json", false))).toEqual(value);
   });
 
+  it("formats arrays of empty objects without column keys as JSON in Markdown", () => {
+    expect(renderCliDocument([{}], "md", false)).toBe("[{}]\n");
+    expect(renderCliDocument([{}, {}], "md", false)).toBe("[{},{}]\n");
+    expect(renderCliDocument({ empty: [{}] }, "md", false)).toBe(
+      "## empty\n\n[{}]\n",
+    );
+  });
+
   it("rejects token windows that would corrupt structured output", () => {
     for (const format of ["json", "jsonl", "yaml"] as const) {
       const validation = parseCliOutputArguments([

tests/boundary/cli/filteredArrayOutput.test.ts

@@ -91,6 +91,15 @@ it("renders only each record's own fields in heterogeneous Markdown tables", asy
   ]);
 });
 
+it("formats arrays of empty objects without columns as JSON in Markdown", async () => {
+  const value: unknown = [{}, {}];
+  freezeRecursively(value);
+  const json = await runFilteredCli(value, "json");
+  expect(JSON.parse(json)).toEqual(value);
+  const markdown = await runFilteredCli(value, "md");
+  expect(markdown).toBe("[{},{}]\n");
+});
+
 it.each(
   (["json", "jsonl"] as const).flatMap((format) =>
     [false, true].map((full) => ({ format, full })),