feat(cli): deprecate validate, inspect, layout in favor of check

One stderr notice per invocation and _meta.deprecated: true in JSON mode
(shared helper next to withMeta; layout owns both inspect and layout via
createInspectCommand). Help descriptions gain the pointer. No behavior
change; removal ships separately once migration telemetry says usage
has decayed.

fix(producer): route info/debug logs to stderr — the compiler's
'Localized remote media' line was landing on stdout ahead of validate's
--json payload, breaking every piped consumer. Diagnostics now share
stderr with warn/error; render progress uses its own channel.

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

@@ -0,0 +1,81 @@
+import type { CommandDef } from "citty";
+import { runCommand } from "citty";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+// See layout.test.ts for why these two dynamic-import targets are mocked:
+// resolveProject skips real filesystem resolution, and bundleToSingleHtml
+// gives a fast, deterministic failure that exercises run()'s outer catch
+// (the JSON failure envelope) without needing headless Chrome.
+const FAKE_PROJECT = {
+  dir: "/fake-project",
+  name: "fake-project",
+  indexPath: "/fake-project/index.html",
+};
+
+vi.mock("../utils/project.js", () => ({
+  resolveProject: vi.fn(() => FAKE_PROJECT),
+}));
+
+vi.mock("@hyperframes/core/compiler", () => ({
+  bundleToSingleHtml: vi.fn(async () => {
+    throw new Error("bundling failed (test double)");
+  }),
+}));
+
+import inspectCommand from "./inspect.js";
+
+function metaDescription(command: CommandDef): string {
+  const meta = command.meta;
+  if (meta && typeof meta === "object" && "description" in meta) {
+    return String(meta.description ?? "");
+  }
+  throw new Error("expected a synchronous meta object");
+}
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("inspect command deprecation (U5)", () => {
+  it("is the compatibility alias for layout, sharing its deprecated description", () => {
+    expect(metaDescription(inspectCommand)).toContain("(deprecated, use check)");
+  });
+
+  it("prints a one-line deprecation notice naming 'inspect' on stderr, never stdout", async () => {
+    const stderrWrites: string[] = [];
+    const stdoutWrites: string[] = [];
+    vi.spyOn(process.stderr, "write").mockImplementation((chunk: unknown) => {
+      stderrWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process.stdout, "write").mockImplementation((chunk: unknown) => {
+      stdoutWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    vi.spyOn(console, "log").mockImplementation(() => {});
+
+    await runCommand(inspectCommand, { rawArgs: ["--json"] });
+
+    const stderrText = stderrWrites.join("");
+    expect(stderrText).toContain("hyperframes inspect");
+    expect(stderrText).toContain("hyperframes check");
+    expect(stdoutWrites.join("")).toBe("");
+  });
+
+  it("--json output is valid JSON with _meta.deprecated === true on failure", async () => {
+    vi.spyOn(process.stderr, "write").mockImplementation(() => true);
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    const logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
+
+    await runCommand(inspectCommand, { rawArgs: ["--json"] });
+
+    const jsonCall = logSpy.mock.calls.find(
+      ([arg]) => typeof arg === "string" && arg.trim().startsWith("{"),
+    );
+    expect(jsonCall).toBeDefined();
+    const parsed = JSON.parse(String(jsonCall?.[0]));
+    expect(parsed.ok).toBe(false);
+    expect(parsed._meta.deprecated).toBe(true);
+  });
+});

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

@@ -0,0 +1,104 @@
+import type { CommandDef } from "citty";
+import { runCommand } from "citty";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+// resolveProject and bundleToSingleHtml are both reached via a dynamic
+// `await import(...)` inside layout.ts's run() / runLayoutAudit(), so
+// vi.mock intercepts them the same way it would a static import. Mocking
+// resolveProject skips real filesystem project resolution; mocking
+// bundleToSingleHtml gives a deterministic, fast failure well before any
+// real browser or network work — exercising run()'s outer catch (the JSON
+// failure envelope) without needing headless Chrome.
+const FAKE_PROJECT = {
+  dir: "/fake-project",
+  name: "fake-project",
+  indexPath: "/fake-project/index.html",
+};
+
+vi.mock("../utils/project.js", () => ({
+  resolveProject: vi.fn(() => FAKE_PROJECT),
+}));
+
+vi.mock("@hyperframes/core/compiler", () => ({
+  bundleToSingleHtml: vi.fn(async () => {
+    throw new Error("bundling failed (test double)");
+  }),
+}));
+
+import { createInspectCommand } from "./layout.js";
+
+/**
+ * citty's `meta` is `Resolvable<CommandMeta>` (object | promise | thunk).
+ * This file's commands always define it as a synchronous object literal, so
+ * narrow to that shape instead of asserting it with `as`.
+ */
+function metaDescription(command: CommandDef): string {
+  const meta = command.meta;
+  if (meta && typeof meta === "object" && "description" in meta) {
+    return String(meta.description ?? "");
+  }
+  throw new Error("expected a synchronous meta object");
+}
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("layout command deprecation (U5)", () => {
+  it("marks both the layout and inspect command names' shared description as deprecated", () => {
+    expect(metaDescription(createInspectCommand("layout"))).toContain("(deprecated, use check)");
+    expect(metaDescription(createInspectCommand("inspect"))).toContain("(deprecated, use check)");
+  });
+
+  it("prints a one-line deprecation notice to stderr and never to stdout", async () => {
+    const stderrWrites: string[] = [];
+    const stdoutWrites: string[] = [];
+    vi.spyOn(process.stderr, "write").mockImplementation((chunk: unknown) => {
+      stderrWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process.stdout, "write").mockImplementation((chunk: unknown) => {
+      stdoutWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    vi.spyOn(console, "log").mockImplementation(() => {});
+
+    await runCommand(createInspectCommand("layout"), { rawArgs: ["--json"] });
+
+    const stderrText = stderrWrites.join("");
+    expect(stderrText).toContain("hyperframes layout");
+    expect(stderrText).toContain("hyperframes check");
+    expect(stdoutWrites.join("")).toBe("");
+  });
+
+  it("--json output is valid JSON with _meta.deprecated === true on failure", async () => {
+    vi.spyOn(process.stderr, "write").mockImplementation(() => true);
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    const logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
+
+    await runCommand(createInspectCommand("layout"), { rawArgs: ["--json"] });
+
+    const jsonCall = logSpy.mock.calls.find(
+      ([arg]) => typeof arg === "string" && arg.trim().startsWith("{"),
+    );
+    expect(jsonCall).toBeDefined();
+    const parsed = JSON.parse(String(jsonCall?.[0]));
+    expect(parsed.ok).toBe(false);
+    expect(parsed._meta.deprecated).toBe(true);
+  });
+
+  it("the inspect command name produces the same _meta.deprecated === true envelope", async () => {
+    vi.spyOn(process.stderr, "write").mockImplementation(() => true);
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    const logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
+
+    await runCommand(createInspectCommand("inspect"), { rawArgs: ["--json"] });
+
+    const jsonCall = logSpy.mock.calls.find(
+      ([arg]) => typeof arg === "string" && arg.trim().startsWith("{"),
+    );
+    const parsed = JSON.parse(String(jsonCall?.[0]));
+    expect(parsed._meta.deprecated).toBe(true);
+  });
+});

packages/cli/src/commands/layout.ts

@@ -7,7 +7,7 @@ import { c } from "../ui/colors.js";
 import { resolveProject } from "../utils/project.js";
 import { normalizeErrorMessage } from "../utils/errorMessage.js";
 import { serveStaticProjectHtml } from "../utils/staticProjectServer.js";
-import { withMeta } from "../utils/updateCheck.js";
+import { printDeprecationNotice, withMeta } from "../utils/updateCheck.js";
 import {
   buildLayoutSampleTimes,
   buildTransitionSampleTimes,
@@ -388,16 +388,19 @@ function resolveMotionSpec(specPath: string, json: boolean): MotionSpec {
   if (json) {
     console.log(
       JSON.stringify(
-        withMeta({
-          schemaVersion: INSPECT_SCHEMA_VERSION,
-          ok: false,
-          error: message,
-          issues: [],
-          errorCount: 0,
-          warningCount: 0,
-          infoCount: 0,
-          issueCount: 0,
-        }),
+        withMeta(
+          {
+            schemaVersion: INSPECT_SCHEMA_VERSION,
+            ok: false,
+            error: message,
+            issues: [],
+            errorCount: 0,
+            warningCount: 0,
+            infoCount: 0,
+            issueCount: 0,
+          },
+          { deprecated: true },
+        ),
         null,
         2,
       ),
@@ -422,7 +425,7 @@ export function createInspectCommand(commandName: "inspect" | "layout") {
     meta: {
       name: commandName,
       description:
-        "Inspect rendered composition layout for text/container overflow, plus optional motion verification via a *.motion.json sidecar",
+        "Inspect rendered composition layout for text/container overflow, plus optional motion verification via a *.motion.json sidecar (deprecated, use check)",
     },
     args: {
       dir: { type: "positional", description: "Project directory", required: false },
@@ -476,6 +479,7 @@ export function createInspectCommand(commandName: "inspect" | "layout") {
     // Pre-existing command-run branching; U1 only swapped the seek internals.
     // fallow-ignore-next-line complexity
     async run({ args }) {
+      printDeprecationNotice(commandName);
       const project = resolveProject(args.dir);
       const samples = Math.max(1, parseInt(args.samples as string, 10) || 9);
       const tolerance = Math.max(0, parseFloat(args.tolerance as string) || 2);
@@ -534,25 +538,28 @@ export function createInspectCommand(commandName: "inspect" | "layout") {
         if (args.json) {
           console.log(
             JSON.stringify(
-              withMeta({
-                schemaVersion: INSPECT_SCHEMA_VERSION,
-                duration: result.duration,
-                samples: result.samples,
-                transitionSamples: atTransitions ? result.transitionSamples : undefined,
-                transitionSamplesDropped: atTransitions
-                  ? result.transitionSamplesDropped
-                  : undefined,
-                tolerance,
-                strict,
-                collapseStatic,
-                motionSpec: motionSpec ? motionSpecPath : undefined,
-                motionSamples: motionSpec ? result.motionSamples : undefined,
-                ...summary,
-                totalIssueCount: limited.totalIssueCount,
-                truncated: limited.truncated,
-                ok,
-                issues: limited.issues,
-              }),
+              withMeta(
+                {
+                  schemaVersion: INSPECT_SCHEMA_VERSION,
+                  duration: result.duration,
+                  samples: result.samples,
+                  transitionSamples: atTransitions ? result.transitionSamples : undefined,
+                  transitionSamplesDropped: atTransitions
+                    ? result.transitionSamplesDropped
+                    : undefined,
+                  tolerance,
+                  strict,
+                  collapseStatic,
+                  motionSpec: motionSpec ? motionSpecPath : undefined,
+                  motionSamples: motionSpec ? result.motionSamples : undefined,
+                  ...summary,
+                  totalIssueCount: limited.totalIssueCount,
+                  truncated: limited.truncated,
+                  ok,
+                  issues: limited.issues,
+                },
+                { deprecated: true },
+              ),
               null,
               2,
             ),
@@ -602,16 +609,19 @@ export function createInspectCommand(commandName: "inspect" | "layout") {
         if (args.json) {
           console.log(
             JSON.stringify(
-              withMeta({
-                schemaVersion: INSPECT_SCHEMA_VERSION,
-                ok: false,
-                error: message,
-                issues: [],
-                errorCount: 0,
-                warningCount: 0,
-                infoCount: 0,
-                issueCount: 0,
-              }),
+              withMeta(
+                {
+                  schemaVersion: INSPECT_SCHEMA_VERSION,
+                  ok: false,
+                  error: message,
+                  issues: [],
+                  errorCount: 0,
+                  warningCount: 0,
+                  infoCount: 0,
+                  issueCount: 0,
+                },
+                { deprecated: true },
+              ),
               null,
               2,
             ),

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

@@ -1,4 +1,6 @@
-import { describe, expect, it, vi } from "vitest";
+import type { CommandDef } from "citty";
+import { runCommand } from "citty";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   extractCompositionErrorsFromLint,
   navigationTimeoutHint,
@@ -28,6 +30,29 @@ vi.mock("../utils/producer.js", () => ({
   })),
 }));
 
+// U5 deprecation tests: resolveProject and lintProject are both reached via a
+// dynamic `await import(...)` inside validate.ts's run() / validateInBrowser(),
+// so vi.mock intercepts them the same way it would a static import. Mocking
+// resolveProject skips real filesystem project resolution; mocking lintProject
+// (the first await inside validateInBrowser) gives a fast, deterministic
+// failure well before any real browser or network work — exercising run()'s
+// outer catch (the JSON failure envelope) without needing headless Chrome.
+const FAKE_PROJECT = {
+  dir: "/fake-project",
+  name: "fake-project",
+  indexPath: "/fake-project/index.html",
+};
+
+vi.mock("../utils/project.js", () => ({
+  resolveProject: vi.fn(() => FAKE_PROJECT),
+}));
+
+vi.mock("../utils/lintProject.js", () => ({
+  lintProject: vi.fn(async () => {
+    throw new Error("lint failed (test double)");
+  }),
+}));
+
 // Regression for the validate audio-duration-probe timeout: a slow-loading
 // media element's duration was snapshotted once, at a fixed point in time,
 // and any element still mid-load was permanently misreported as unreadable.
@@ -282,3 +307,62 @@ describe("navigationTimeoutHint", () => {
     expect(navigationTimeoutHint("some string failure", 10000)).toBeNull();
   });
 });
+
+function metaDescription(command: CommandDef): string {
+  const meta = command.meta;
+  if (meta && typeof meta === "object" && "description" in meta) {
+    return String(meta.description ?? "");
+  }
+  throw new Error("expected a synchronous meta object");
+}
+
+describe("validate command deprecation (U5)", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("marks the command description as deprecated", async () => {
+    const { default: validateCommand } = await import("./validate.js");
+    expect(metaDescription(validateCommand)).toContain("(deprecated, use check)");
+  });
+
+  it("prints a one-line deprecation notice to stderr and never to stdout", async () => {
+    const stderrWrites: string[] = [];
+    const stdoutWrites: string[] = [];
+    vi.spyOn(process.stderr, "write").mockImplementation((chunk: unknown) => {
+      stderrWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process.stdout, "write").mockImplementation((chunk: unknown) => {
+      stdoutWrites.push(String(chunk));
+      return true;
+    });
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    vi.spyOn(console, "log").mockImplementation(() => {});
+
+    const { default: validateCommand } = await import("./validate.js");
+    await runCommand(validateCommand, { rawArgs: ["--json"] });
+
+    const stderrText = stderrWrites.join("");
+    expect(stderrText).toContain("hyperframes validate");
+    expect(stderrText).toContain("hyperframes check");
+    expect(stdoutWrites.join("")).toBe("");
+  });
+
+  it("--json output is valid JSON with _meta.deprecated === true on failure", async () => {
+    vi.spyOn(process.stderr, "write").mockImplementation(() => true);
+    vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    const logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
+
+    const { default: validateCommand } = await import("./validate.js");
+    await runCommand(validateCommand, { rawArgs: ["--json"] });
+
+    const jsonCall = logSpy.mock.calls.find(
+      ([arg]) => typeof arg === "string" && arg.trim().startsWith("{"),
+    );
+    expect(jsonCall).toBeDefined();
+    const parsed = JSON.parse(String(jsonCall?.[0]));
+    expect(parsed.ok).toBe(false);
+    expect(parsed._meta.deprecated).toBe(true);
+  });
+});

packages/cli/src/commands/validate.ts

@@ -13,7 +13,7 @@ import { normalizeErrorMessage } from "../utils/errorMessage.js";
 import type { ProjectLintResult } from "../utils/lintProject.js";
 import { resolveCompositionViewportFromHtml } from "../utils/compositionViewport.js";
 import { c } from "../ui/colors.js";
-import { withMeta } from "../utils/updateCheck.js";
+import { printDeprecationNotice, withMeta } from "../utils/updateCheck.js";
 import {
   resolveCliChromeGpuMode,
   seekCompositionTimeline,
@@ -514,13 +514,16 @@ function emitJsonReport(
 ): void {
   console.log(
     JSON.stringify(
-      withMeta({
-        ok: errors.length === 0,
-        errors,
-        warnings,
-        contrast,
-        contrastFailures: contrastFailures.length,
-      }),
+      withMeta(
+        {
+          ok: errors.length === 0,
+          errors,
+          warnings,
+          contrast,
+          contrastFailures: contrastFailures.length,
+        },
+        { deprecated: true },
+      ),
       null,
       2,
     ),
@@ -567,7 +570,11 @@ function emitTextReport(
 function emitFailureReport(message: string, asJson: boolean): void {
   if (asJson) {
     console.log(
-      JSON.stringify(withMeta({ ok: false, error: message, errors: [], warnings: [] }), null, 2),
+      JSON.stringify(
+        withMeta({ ok: false, error: message, errors: [], warnings: [] }, { deprecated: true }),
+        null,
+        2,
+      ),
     );
     return;
   }
@@ -577,7 +584,7 @@ function emitFailureReport(message: string, asJson: boolean): void {
 export default defineCommand({
   meta: {
     name: "validate",
-    description: `Load a composition in headless Chrome and report console errors
+    description: `Load a composition in headless Chrome and report console errors (deprecated, use check)
 
 Examples:
   hyperframes validate
@@ -602,6 +609,7 @@ Examples:
     },
   },
   async run({ args }) {
+    printDeprecationNotice("validate");
     const project = resolveProject(args.dir);
     const timeout = parseInt(args.timeout as string, 10) || 3000;
     const useContrast = args.contrast ?? true;

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

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { isSafeVersion } from "./updateCheck.js";
+import { isSafeVersion, printDeprecationNotice, withMeta } from "./updateCheck.js";
 
 describe("isSafeVersion", () => {
   it("accepts strict semver, incl. prerelease/build metadata", () => {
@@ -150,6 +150,68 @@ async function checkWith(registryVersion: unknown): Promise<{
   }
 }
 
+/**
+ * U5: validate/inspect/layout are deprecated in favor of `check`. withMeta's
+ * optional `{ deprecated: true }` is the single place that adds `_meta.deprecated`
+ * to a --json envelope; every other command (check, lint, ...) calls withMeta
+ * with no second argument and must never see the key at all — not even `false`.
+ */
+describe("withMeta — deprecated flag", () => {
+  it("omits _meta.deprecated entirely when no options are passed (check/lint et al.)", () => {
+    const wrapped = withMeta({ ok: true });
+    expect("deprecated" in wrapped._meta).toBe(false);
+  });
+
+  it("omits _meta.deprecated when options.deprecated is false", () => {
+    const wrapped = withMeta({ ok: true }, { deprecated: false });
+    expect("deprecated" in wrapped._meta).toBe(false);
+  });
+
+  it("sets _meta.deprecated === true when requested (validate/inspect/layout)", () => {
+    const wrapped = withMeta({ ok: true }, { deprecated: true });
+    expect(wrapped._meta.deprecated).toBe(true);
+  });
+
+  it("preserves the rest of the _meta envelope alongside the deprecated flag", () => {
+    const wrapped = withMeta({ ok: true }, { deprecated: true });
+    expect(wrapped._meta.version).toEqual(expect.any(String));
+    expect(typeof wrapped._meta.updateAvailable).toBe("boolean");
+  });
+});
+
+/**
+ * The stderr-only deprecation notice: printed once per invocation, never on
+ * stdout, so --json output stays pure JSON while humans still see the notice.
+ */
+describe("printDeprecationNotice", () => {
+  it("writes exactly one line to stderr, never stdout", () => {
+    const stderrWrites: string[] = [];
+    const stdoutWrites: string[] = [];
+    const origErrWrite = process.stderr.write.bind(process.stderr);
+    const origOutWrite = process.stdout.write.bind(process.stdout);
+    process.stderr.write = ((chunk: unknown) => {
+      stderrWrites.push(String(chunk));
+      return true;
+    }) as typeof process.stderr.write;
+    process.stdout.write = ((chunk: unknown) => {
+      stdoutWrites.push(String(chunk));
+      return true;
+    }) as typeof process.stdout.write;
+
+    try {
+      printDeprecationNotice("validate");
+    } finally {
+      process.stderr.write = origErrWrite;
+      process.stdout.write = origOutWrite;
+    }
+
+    expect(stdoutWrites).toEqual([]);
+    expect(stderrWrites).toHaveLength(1);
+    expect(stderrWrites[0]).toContain("hyperframes validate");
+    expect(stderrWrites[0]).toContain("hyperframes check");
+  });
+});
+
 describe("checkForUpdate — registry boundary guard", () => {
   afterEach(() => {
     vi.doUnmock("../telemetry/config.js");

packages/cli/src/utils/updateCheck.ts

@@ -40,6 +40,8 @@ export interface UpdateMeta {
   version: string;
   latestVersion?: string;
   updateAvailable: boolean;
+  /** Present (and true) only for commands superseded by `check`; absent otherwise. */
+  deprecated?: boolean;
 }
 
 /**
@@ -130,9 +132,30 @@ export function getUpdateMeta(): UpdateMeta {
 /**
  * Wrap a JSON payload with the _meta version envelope.
  * Use this in all --json command outputs for consistent agent-friendly metadata.
+ *
+ * Pass `{ deprecated: true }` from a command superseded by `check` (validate,
+ * inspect, layout) to add `_meta.deprecated: true`; every other call site is
+ * unaffected — the key is only ever added, never set to `false`.
  */
-export function withMeta<T extends object>(data: T): T & { _meta: UpdateMeta } {
-  return { ...data, _meta: getUpdateMeta() };
+export function withMeta<T extends object>(
+  data: T,
+  options?: { deprecated?: boolean },
+): T & { _meta: UpdateMeta } {
+  const meta = getUpdateMeta();
+  if (options?.deprecated) meta.deprecated = true;
+  return { ...data, _meta: meta };
+}
+
+/**
+ * One-line deprecation notice for a command superseded by `check`. Always
+ * writes to stderr (never stdout), so a --json invocation's stdout stays
+ * pure, parseable JSON. Call once per invocation, before the command's own
+ * output.
+ */
+export function printDeprecationNotice(command: string): void {
+  process.stderr.write(
+    `'hyperframes ${command}' is deprecated and will be removed in a future release. Use 'hyperframes check' instead.\n`,
+  );
 }
 
 /**

packages/producer/src/logger.test.ts

@@ -1,13 +1,13 @@
-import { afterEach, beforeEach, describe, expect, it, mock } from "bun:test";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { createConsoleLogger, defaultLogger } from "./logger.js";
 import type { LogLevel, ProducerLogger } from "./logger.js";
 
 describe("createConsoleLogger", () => {
-  // We capture calls to console.{log,warn,error} via `mock` so we can
+  // We capture calls to console.{log,warn,error} via `vi.fn` so we can
   // assert what would have been printed without polluting test output.
-  let logSpy: ReturnType<typeof mock>;
-  let warnSpy: ReturnType<typeof mock>;
-  let errorSpy: ReturnType<typeof mock>;
+  let logSpy: ReturnType<typeof vi.fn>;
+  let warnSpy: ReturnType<typeof vi.fn>;
+  let errorSpy: ReturnType<typeof vi.fn>;
   let origLog: typeof console.log;
   let origWarn: typeof console.warn;
   let origError: typeof console.error;
@@ -16,9 +16,9 @@ describe("createConsoleLogger", () => {
     origLog = console.log;
     origWarn = console.warn;
     origError = console.error;
-    logSpy = mock(() => {});
-    warnSpy = mock(() => {});
-    errorSpy = mock(() => {});
+    logSpy = vi.fn();
+    warnSpy = vi.fn();
+    errorSpy = vi.fn();
     console.log = logSpy as unknown as typeof console.log;
     console.warn = warnSpy as unknown as typeof console.warn;
     console.error = errorSpy as unknown as typeof console.error;
@@ -30,6 +30,43 @@ describe("createConsoleLogger", () => {
     console.error = origError;
   });
 
+  // All four levels are stderr-bound (console.error/console.warn); console.log
+  // (stdout) must never be touched, since producer runs inside CLI commands
+  // whose stdout is a machine-readable contract (e.g. `validate --json`).
+  describe("stdout/stderr routing", () => {
+    it("info and debug write to console.error (stderr), never console.log (stdout)", () => {
+      const log = createConsoleLogger("debug");
+      log.info("info-msg");
+      log.debug("debug-msg");
+
+      expect(logSpy).not.toHaveBeenCalled();
+      expect(errorSpy.mock.calls.map((c) => c[0])).toEqual([
+        "[INFO] info-msg",
+        "[DEBUG] debug-msg",
+      ]);
+    });
+
+    it("warn and error keep their pre-existing channels (console.warn / console.error)", () => {
+      const log = createConsoleLogger("debug");
+      log.warn("warn-msg");
+      log.error("error-msg");
+
+      expect(logSpy).not.toHaveBeenCalled();
+      expect(warnSpy.mock.calls[0]?.[0]).toBe("[WARN] warn-msg");
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[ERROR] error-msg");
+    });
+
+    it("console.log is never called at any level", () => {
+      const log = createConsoleLogger("debug");
+      log.debug("d");
+      log.info("i");
+      log.warn("w");
+      log.error("e");
+
+      expect(logSpy).not.toHaveBeenCalled();
+    });
+  });
+
   describe("level filtering", () => {
     it("level=info drops debug, keeps info/warn/error", () => {
       const log = createConsoleLogger("info");
@@ -38,12 +75,12 @@ describe("createConsoleLogger", () => {
       log.warn("warn-msg");
       log.error("error-msg");
 
-      expect(logSpy.mock.calls.length).toBe(1);
-      expect(logSpy.mock.calls[0]?.[0]).toBe("[INFO] info-msg");
+      // info + error both route to console.error now (info: stderr routing, error: always stderr).
+      expect(errorSpy.mock.calls.length).toBe(2);
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[INFO] info-msg");
+      expect(errorSpy.mock.calls[1]?.[0]).toBe("[ERROR] error-msg");
       expect(warnSpy.mock.calls.length).toBe(1);
       expect(warnSpy.mock.calls[0]?.[0]).toBe("[WARN] warn-msg");
-      expect(errorSpy.mock.calls.length).toBe(1);
-      expect(errorSpy.mock.calls[0]?.[0]).toBe("[ERROR] error-msg");
     });
 
     it("level=debug keeps all four levels", () => {
@@ -53,12 +90,12 @@ describe("createConsoleLogger", () => {
       log.warn("w");
       log.error("e");
 
-      // info + debug both go to console.log
-      expect(logSpy.mock.calls.length).toBe(2);
-      expect(logSpy.mock.calls[0]?.[0]).toBe("[DEBUG] d");
-      expect(logSpy.mock.calls[1]?.[0]).toBe("[INFO] i");
+      // debug + info + error all go to console.error
+      expect(errorSpy.mock.calls.length).toBe(3);
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[DEBUG] d");
+      expect(errorSpy.mock.calls[1]?.[0]).toBe("[INFO] i");
+      expect(errorSpy.mock.calls[2]?.[0]).toBe("[ERROR] e");
       expect(warnSpy.mock.calls.length).toBe(1);
-      expect(errorSpy.mock.calls.length).toBe(1);
     });
 
     it("level=warn drops info and debug, keeps warn/error", () => {
@@ -68,9 +105,9 @@ describe("createConsoleLogger", () => {
       log.warn("w");
       log.error("e");
 
-      expect(logSpy.mock.calls.length).toBe(0);
-      expect(warnSpy.mock.calls.length).toBe(1);
       expect(errorSpy.mock.calls.length).toBe(1);
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[ERROR] e");
+      expect(warnSpy.mock.calls.length).toBe(1);
     });
 
     it("level=error drops everything except error", () => {
@@ -80,7 +117,6 @@ describe("createConsoleLogger", () => {
       log.warn("w");
       log.error("e");
 
-      expect(logSpy.mock.calls.length).toBe(0);
       expect(warnSpy.mock.calls.length).toBe(0);
       expect(errorSpy.mock.calls.length).toBe(1);
     });
@@ -90,8 +126,8 @@ describe("createConsoleLogger", () => {
       log.debug("d");
       log.info("i");
 
-      expect(logSpy.mock.calls.length).toBe(1);
-      expect(logSpy.mock.calls[0]?.[0]).toBe("[INFO] i");
+      expect(errorSpy.mock.calls.length).toBe(1);
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[INFO] i");
     });
   });
 
@@ -100,14 +136,14 @@ describe("createConsoleLogger", () => {
       const log = createConsoleLogger("info");
       log.info("hello", { a: 1, b: "two" });
 
-      expect(logSpy.mock.calls[0]?.[0]).toBe('[INFO] hello {"a":1,"b":"two"}');
+      expect(errorSpy.mock.calls[0]?.[0]).toBe('[INFO] hello {"a":1,"b":"two"}');
     });
 
     it("emits message only when meta is omitted", () => {
       const log = createConsoleLogger("info");
       log.info("plain");
 
-      expect(logSpy.mock.calls[0]?.[0]).toBe("[INFO] plain");
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[INFO] plain");
     });
 
     it("does not invoke JSON.stringify when level is filtered out", () => {
@@ -123,7 +159,7 @@ describe("createConsoleLogger", () => {
       };
       // Should not throw — debug is below the info threshold.
       log.debug("trap", trap as unknown as Record<string, unknown>);
-      expect(logSpy.mock.calls.length).toBe(0);
+      expect(errorSpy.mock.calls.length).toBe(0);
     });
   });
 
@@ -185,7 +221,7 @@ describe("createConsoleLogger", () => {
       }
 
       expect(buildCount).toBe(0);
-      expect(logSpy.mock.calls.length).toBe(0);
+      expect(errorSpy.mock.calls.length).toBe(0);
     });
 
     it("call-site gate runs the meta builder when debug is enabled", () => {
@@ -203,7 +239,7 @@ describe("createConsoleLogger", () => {
       }
 
       expect(buildCount).toBe(5);
-      expect(logSpy.mock.calls.length).toBe(5);
+      expect(errorSpy.mock.calls.length).toBe(5);
     });
 
     it("custom logger without isLevelEnabled falls back to running the meta builder (`?? true`)", () => {
@@ -242,8 +278,8 @@ describe("createConsoleLogger", () => {
       defaultLogger.info("default-info");
       defaultLogger.debug("default-debug");
 
-      expect(logSpy.mock.calls.length).toBe(1);
-      expect(logSpy.mock.calls[0]?.[0]).toBe("[INFO] default-info");
+      expect(errorSpy.mock.calls.length).toBe(1);
+      expect(errorSpy.mock.calls[0]?.[0]).toBe("[INFO] default-info");
     });
 
     it("exposes isLevelEnabled gating debug at info threshold", () => {

packages/producer/src/logger.ts

@@ -4,6 +4,12 @@
  * Lightweight pluggable logger with zero dependencies.
  * Default implementation writes to console with level filtering.
  *
+ * All levels write to stderr (console.error/console.warn), never stdout —
+ * producer runs inside CLI commands whose stdout is a machine-readable
+ * contract (e.g. `validate --json`, `check --json`); an info/debug line on
+ * stdout would corrupt that output. There is no diagnostic use case that
+ * needs these lines on stdout specifically, so the whole logger is stderr-only.
+ *
  * Users can provide their own logger (e.g. Winston, Pino) by
  * implementing the ProducerLogger interface.
  */
@@ -71,12 +77,12 @@ export function createConsoleLogger(level: LogLevel = "info"): ProducerLogger {
     },
     info(message, meta) {
       if (shouldLog("info")) {
-        console.log(`[INFO] ${message}${formatMeta(meta)}`);
+        console.error(`[INFO] ${message}${formatMeta(meta)}`);
       }
     },
     debug(message, meta) {
       if (shouldLog("debug")) {
-        console.log(`[DEBUG] ${message}${formatMeta(meta)}`);
+        console.error(`[DEBUG] ${message}${formatMeta(meta)}`);
       }
     },
     isLevelEnabled(msgLevel) {