fix(cli): emit valid JSON for filtered keyframes

docs/developers/cli.mdx

@@ -42,6 +42,10 @@ remains authoritative: run `npx hyperframes <command> --help`.
 
 `hyperframes timeline` also accepts agent editing verbs. Use `move`, `trim`, `split`, or `delete` with a row ref; add `--plan` to inspect the before/after rows and diff without writing, or `--json` for a machine-readable receipt. Time values accept seconds, relative offsets, frames, and anchors such as `after:#intro`.
 
+For scripts, `npx hyperframes keyframes --runtime css --json` writes one JSON
+result to stdout. Use `--runtime anime` for Anime.js. The static authoring
+notice appears only in human-readable output.
+
 ## Rendering and automation
 
 The CLI also renders in batches and from variables, writes image sequences and

packages/cli/src/commands/keyframes.test.ts

@@ -9,7 +9,8 @@ import {
 } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { beforeAll, describe, expect, it, onTestFinished } from "vitest";
+import { beforeAll, describe, expect, it, onTestFinished, vi } from "vitest";
+import { runCommand } from "citty";
 import type { ArgsDef } from "citty";
 import { ensureDOMParser } from "../utils/dom.js";
 import keyframesCommand from "./keyframes.js";
@@ -38,6 +39,27 @@ beforeAll(() => ensureDOMParser());
 const wrap = (script: string) =>
   `<!doctype html><html><body><div id="root" data-composition-id="main" data-duration="4"><div id="dot" class="clip"></div></div><script>${script}</script></body></html>`;
 
+describe("keyframes JSON output", () => {
+  it.each(["css", "anime"])("emits parseable JSON with --runtime %s", async (runtime) => {
+    const dir = mkdtempSync(join(tmpdir(), "hf-keyframes-json-"));
+    onTestFinished(() => rmSync(dir, { recursive: true, force: true }));
+    const source = join(dir, "index.html");
+    writeFileSync(source, wrap(""));
+    const log = vi.spyOn(console, "log").mockImplementation(() => {});
+    onTestFinished(() => log.mockRestore());
+
+    await runCommand(keyframesCommand, { rawArgs: [source, "--runtime", runtime, "--json"] });
+
+    const output = log.mock.calls.map((args) => args.join(" ")).join("\n");
+    const result: unknown = JSON.parse(output);
+    expect(result).toMatchObject({
+      project: "index.html",
+      runtime,
+      compositions: [{ composition: "index.html", tweens: [], cssKeyframes: [], anime: [] }],
+    });
+  });
+});
+
 describe("keyframes direct composition scope", () => {
   it("keeps the project root and passes the nested HTML entry to --shot", () => {
     const projectDir = mkdtempSync(join(tmpdir(), "hf-keyframes-target-"));

packages/cli/src/commands/keyframes.ts

@@ -1085,14 +1085,6 @@ function createKeyframesCommand(options: Partial<KeyframesCommandOptions> = {})
     async run({ args }) {
       ensureDOMParser();
       const runtime = normalizeRuntime(args.runtime, commandOptions.defaultRuntime);
-      if (runtime === "css" || runtime === "anime") {
-        console.log(
-          c.dim(
-            `${commandOptions.name}: ${runtime} output is a static authoring surface; use validate/render/snapshot to verify runtime adapter seekability.`,
-          ),
-        );
-        console.log();
-      }
       const { comps: rawComps, allComps, projectName, projectDir, entryFile } = resolveScope(args);
       const comps = filterCompositionsByRuntime(rawComps, runtime);
 
@@ -1107,6 +1099,15 @@ function createKeyframesCommand(options: Partial<KeyframesCommandOptions> = {})
         return;
       }
 
+      if (runtime === "css" || runtime === "anime") {
+        console.log(
+          c.dim(
+            `${commandOptions.name}: ${runtime} output is a static authoring surface; use validate/render/snapshot to verify runtime adapter seekability.`,
+          ),
+        );
+        console.log();
+      }
+
       const total = comps.reduce(
         (n, cmp) => n + cmp.tweens.length + cmp.cssKeyframes.length + cmp.anime.length,
         0,