fix(cli)!: keep a selected JSON null in --filter-output output (#2333)

createCliResultOutput rendered an explicit null selected by
--filter-output as the empty document {}, the same output it uses when
the selection has no value at all. On the streamed JSON/JSONL path,
a consumer could not distinguish a selected null from a missing field.

Write a selected null as null in streamed JSON and JSONL. This also
preserves an unfiltered top-level null. The {} fallback stays for
undefined, functions and symbols. The --full-output envelope already
carried data: null for a selected null. The --token-count path still
hands results that fit the formatted-string limit back to Incur and
prints 0 for a selected null. Other formats and Incur-owned result
paths are unchanged.

Consumers must accept JSON null for a present null and distinguish it
from a missing selection. Use --full-output if an object envelope is
required. Keeping the old shared {} representation would continue
hiding this distinction.


Claude-Session: https://claude.ai/code/session_019TTGAv6fe859ebbCLLfLHP

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/cli/cliResultOutput.ts

@@ -216,8 +216,9 @@ export const createCliResultOutput = (
           : filtered === undefined
             ? { ok: true, meta }
             : { ok: true, data: filtered, meta }
-        : filtered === undefined ||
-            filtered === null ||
+        : // A selected null is a value; only a selection with no JSON value
+          // falls back to the empty document.
+          filtered === undefined ||
             typeof filtered === "function" ||
             typeof filtered === "symbol"
           ? {}

tests/boundary/cli/jsonOutput.test.ts

@@ -561,3 +561,35 @@ describe("CLI result output surface", () => {
     });
   }, 60_000);
 });
+
+describe("streamed CLI filtered selections", () => {
+  it.each([
+    ["raw_result", "json", "null\n"],
+    ["raw_result", "jsonl", "null\n"],
+    ["nested.value", "json", '{"nested":{"value":null}}\n'],
+    ["missing", "json", "{}\n"],
+    ["count", "json", "0\n"],
+    ["enabled", "json", "false\n"],
+  ] as const)(
+    "keeps the selected value of --filter-output %s in %s",
+    async (filter, format, expected) => {
+      const chunks: Buffer[] = [];
+      const destination = new Writable({
+        write(chunk: Buffer, _encoding, callback) {
+          chunks.push(chunk);
+          callback();
+        },
+      });
+      const output = createCliResultOutput(
+        ["--format", format, "--filter-output", filter],
+        destination,
+      );
+      if (output === undefined) throw new Error("Missing JSON output surface");
+      await output.write(
+        { raw_result: null, nested: { value: null }, count: 0, enabled: false },
+        { command: "fixture", duration: "1ms", format },
+      );
+      expect(Buffer.concat(chunks).toString("utf8")).toBe(expected);
+    },
+  );
+});