diff --git a/docs/validator-doctor.md b/docs/validator-doctor.md
new file mode 100644
index 0000000..a3f2957
--- /dev/null
+++ b/docs/validator-doctor.md
@@ -0,0 +1,29 @@
+# Validator environment doctor
+
+Run the doctor before an audit to check whether the current Node.js environment exposes the file-opening capabilities required by the JSON validator CLIs:
+
+```sh
+node skills/security-audit/check-environment.cjs
+```
+
+The command reports the Node.js version, operating system, architecture, and whether `fs.constants.O_NOFOLLOW` and `fs.constants.O_NONBLOCK` are non-zero integers. This is the same availability rule used by the validators. The doctor reports `PASS` and exits `0` when both capabilities are available. It reports `FAIL` and exits `1` when either capability is missing, zero, or not an integer.
+
+The protected flags are required because the validators fail closed rather than read input files without no-follow and nonblocking protections. If the check fails on native Windows, run Node.js inside Linux or WSL. The doctor reports the installed Node.js version but does not impose a minimum version.
+
+For automation, request JSON output:
+
+```sh
+node skills/security-audit/check-environment.cjs --json
+```
+
+The JSON document has a versioned, stable top-level structure. `ok` is the overall result, `runtime` identifies the environment, `capabilities` describes each flag, and `missingCapabilities` lists requirements that failed. The command keeps the same `0` or `1` result exit code in JSON mode.
+
+For usage information:
+
+```sh
+node skills/security-audit/check-environment.cjs --help
+```
+
+Help exits `0`. Unknown options or extra arguments exit `2`.
+
+This command checks only the JSON validators' file-reading prerequisites. It does not verify the complete security-audit environment, an OS-enforced security sandbox, network isolation, resource limits, or safe scratch-directory configuration. It has no third-party dependencies, does not access the network, and does not install software or change system configuration.
diff --git a/skills/security-audit/check-environment.cjs b/skills/security-audit/check-environment.cjs
new file mode 100644
index 0000000..17ec9d7
--- /dev/null
+++ b/skills/security-audit/check-environment.cjs
@@ -0,0 +1,123 @@
+const fs = require("node:fs");
+
+const SCOPE = "JSON validator file-reading prerequisites only";
+const REQUIREMENT = "non-zero integer";
+const FLAG_NAMES = ["O_NOFOLLOW", "O_NONBLOCK"];
+
+function flagType(value) {
+  if (value === null) return "null";
+  if (Array.isArray(value)) return "array";
+  return typeof value;
+}
+
+function inspectFlag(value) {
+  return {
+    available: Number.isInteger(value) && value !== 0,
+    requirement: REQUIREMENT,
+    type: flagType(value),
+    value: Number.isInteger(value) ? value : null,
+  };
+}
+
+function inspectEnvironment(options = {}) {
+  const constants = options.constants || fs.constants;
+  const runtime = {
+    nodeVersion: options.nodeVersion || process.version,
+    platform: options.platform || process.platform,
+    arch: options.arch || process.arch,
+  };
+  const capabilities = {
+    O_NOFOLLOW: inspectFlag(constants.O_NOFOLLOW),
+    O_NONBLOCK: inspectFlag(constants.O_NONBLOCK),
+  };
+  const missingCapabilities = FLAG_NAMES.filter((name) => !capabilities[name].available);
+  const ok = missingCapabilities.length === 0;
+
+  return {
+    schemaVersion: 1,
+    ok,
+    scope: SCOPE,
+    runtime,
+    capabilities,
+    missingCapabilities,
+    conclusion: ok
+      ? "The JSON validator CLIs have the required protected file-opening flags."
+      : "The JSON validator CLIs cannot run because they refuse to read input without both protected file-opening flags.",
+    recommendation: ok
+      ? null
+      : "Run Node.js in Linux or WSL, where Node.js normally exposes O_NOFOLLOW and O_NONBLOCK.",
+    limitation: "This result does not verify the complete audit environment or a security sandbox.",
+  };
+}
+
+function displayValue(check) {
+  if (check.type === "undefined") return "undefined";
+  if (check.value !== null) return `${check.value} (${check.type})`;
+  return `a value of type ${check.type}`;
+}
+
+function formatText(report) {
+  const lines = [
+    "Validator environment doctor",
+    `Node.js: ${report.runtime.nodeVersion}`,
+    `OS: ${report.runtime.platform}`,
+    `Architecture: ${report.runtime.arch}`,
+  ];
+
+  for (const name of FLAG_NAMES) {
+    const check = report.capabilities[name];
+    const status = check.available ? "PASS" : "FAIL";
+    lines.push(`${name}: ${status} (expected ${check.requirement}; got ${displayValue(check)})`);
+  }
+
+  lines.push(`${report.ok ? "PASS" : "FAIL"}: ${report.conclusion}`);
+  if (report.recommendation) lines.push(`Recommendation: ${report.recommendation}`);
+  lines.push(`Scope: ${report.scope}. ${report.limitation}`);
+  return `${lines.join("\n")}\n`;
+}
+
+function formatHelp() {
+  return [
+    "Usage: node skills/security-audit/check-environment.cjs [--json | --help]",
+    "",
+    "Check whether Node.js exposes the protected file-opening flags required by the JSON validators.",
+    "",
+    "Options:",
+    "  --json  Print a stable, machine-readable JSON result.",
+    "  --help  Show this help message.",
+    "",
+  ].join("\n");
+}
+
+function main(argv = process.argv.slice(2), options = {}) {
+  const writeOut = options.writeOut || ((text) => process.stdout.write(text));
+  const writeError = options.writeError || ((text) => process.stderr.write(text));
+
+  if (argv.length === 1 && argv[0] === "--help") {
+    writeOut(formatHelp());
+    return 0;
+  }
+  if (argv.length > 1 || (argv.length === 1 && argv[0] !== "--json")) {
+    writeError("Invalid arguments. Use --help for usage.\n");
+    return 2;
+  }
+
+  const report = inspectEnvironment(options.environment);
+  if (argv[0] === "--json") {
+    writeOut(`${JSON.stringify(report, null, 2)}\n`);
+  } else {
+    writeOut(formatText(report));
+  }
+  return report.ok ? 0 : 1;
+}
+
+module.exports = {
+  FLAG_NAMES,
+  SCOPE,
+  formatHelp,
+  formatText,
+  inspectEnvironment,
+  main,
+};
+
+if (require.main === module) process.exitCode = main();
diff --git a/skills/security-audit/check-environment.test.cjs b/skills/security-audit/check-environment.test.cjs
new file mode 100644
index 0000000..c5dc4f8
--- /dev/null
+++ b/skills/security-audit/check-environment.test.cjs
@@ -0,0 +1,158 @@
+const assert = require("node:assert/strict");
+const path = require("node:path");
+const { spawnSync } = require("node:child_process");
+const test = require("node:test");
+const {
+  inspectEnvironment,
+  main,
+} = require("./check-environment.cjs");
+
+const doctorPath = path.join(__dirname, "check-environment.cjs");
+const CLI_TIMEOUT_MS = 5000;
+const TEST_RUNTIME = {
+  nodeVersion: "v-test",
+  platform: "test-os",
+  arch: "test-arch",
+};
+
+function runMain(argv, constants) {
+  let stdout = "";
+  let stderr = "";
+  const status = main(argv, {
+    environment: { ...TEST_RUNTIME, constants },
+    writeOut(text) { stdout += text; },
+    writeError(text) { stderr += text; },
+  });
+  return { status, stdout, stderr };
+}
+
+function runCli(args = []) {
+  return spawnSync(process.execPath, [doctorPath, ...args], {
+    encoding: "utf8",
+    timeout: CLI_TIMEOUT_MS,
+  });
+}
+
+function cliOutput(result) {
+  return `${result.stdout}${result.stderr}`;
+}
+
+test("reports both required capabilities as available", () => {
+  const report = inspectEnvironment({
+    ...TEST_RUNTIME,
+    constants: { O_NOFOLLOW: 131072, O_NONBLOCK: 2048 },
+  });
+
+  assert.equal(report.ok, true);
+  assert.deepEqual(report.missingCapabilities, []);
+  assert.deepEqual(report.runtime, TEST_RUNTIME);
+  assert.deepEqual(report.capabilities.O_NOFOLLOW, {
+    available: true,
+    requirement: "non-zero integer",
+    type: "number",
+    value: 131072,
+  });
+  assert.equal(report.recommendation, null);
+});
+
+const invalidCases = [
+  ["O_NOFOLLOW is missing", { O_NONBLOCK: 2048 }, "O_NOFOLLOW", "undefined"],
+  ["O_NONBLOCK is missing", { O_NOFOLLOW: 131072 }, "O_NONBLOCK", "undefined"],
+  ["O_NOFOLLOW is zero", { O_NOFOLLOW: 0, O_NONBLOCK: 2048 }, "O_NOFOLLOW", "number"],
+  ["O_NONBLOCK is zero", { O_NOFOLLOW: 131072, O_NONBLOCK: 0 }, "O_NONBLOCK", "number"],
+  ["O_NOFOLLOW has the wrong type", { O_NOFOLLOW: "131072", O_NONBLOCK: 2048 }, "O_NOFOLLOW", "string"],
+  ["O_NONBLOCK has the wrong type", { O_NOFOLLOW: 131072, O_NONBLOCK: {} }, "O_NONBLOCK", "object"],
+];
+
+for (const [name, constants, missingName, expectedType] of invalidCases) {
+  test(`fails when ${name}`, () => {
+    const report = inspectEnvironment({ ...TEST_RUNTIME, constants });
+    assert.equal(report.ok, false);
+    assert.deepEqual(report.missingCapabilities, [missingName]);
+    assert.equal(report.capabilities[missingName].available, false);
+    assert.equal(report.capabilities[missingName].type, expectedType);
+    assert.match(report.conclusion, /cannot run because they refuse to read input/);
+    assert.match(report.recommendation, /Linux or WSL/);
+  });
+}
+
+test("text output reports runtime, checks, scope, and a passing exit code", () => {
+  const result = runMain([], { O_NOFOLLOW: 1, O_NONBLOCK: 2 });
+  assert.equal(result.status, 0);
+  assert.equal(result.stderr, "");
+  assert.match(result.stdout, /Node\.js: v-test/);
+  assert.match(result.stdout, /OS: test-os/);
+  assert.match(result.stdout, /Architecture: test-arch/);
+  assert.match(result.stdout, /O_NOFOLLOW: PASS/);
+  assert.match(result.stdout, /O_NONBLOCK: PASS/);
+  assert.match(result.stdout, /does not verify the complete audit environment or a security sandbox/);
+});
+
+test("text output explains missing capabilities and exits 1", () => {
+  const result = runMain([], { O_NOFOLLOW: 1 });
+  assert.equal(result.status, 1);
+  assert.equal(result.stderr, "");
+  assert.match(result.stdout, /O_NONBLOCK: FAIL/);
+  assert.match(result.stdout, /cannot run because they refuse to read input/);
+  assert.match(result.stdout, /Run Node\.js in Linux or WSL/);
+});
+
+test("JSON output is stable, parseable, and uses the check result as its exit code", () => {
+  const first = runMain(["--json"], { O_NOFOLLOW: 1, O_NONBLOCK: 2 });
+  const second = runMain(["--json"], { O_NOFOLLOW: 1, O_NONBLOCK: 2 });
+  assert.equal(first.status, 0);
+  assert.equal(first.stdout, second.stdout);
+  assert.equal(first.stderr, "");
+  const parsed = JSON.parse(first.stdout);
+  assert.equal(parsed.schemaVersion, 1);
+  assert.equal(parsed.ok, true);
+  assert.equal(parsed.scope, "JSON validator file-reading prerequisites only");
+  assert.deepEqual(parsed.missingCapabilities, []);
+
+  const failed = runMain(["--json"], { O_NOFOLLOW: 1, O_NONBLOCK: 0 });
+  assert.equal(failed.status, 1);
+  assert.deepEqual(JSON.parse(failed.stdout).missingCapabilities, ["O_NONBLOCK"]);
+});
+
+test("help exits 0 without running the check", () => {
+  const result = runMain(["--help"], {});
+  assert.equal(result.status, 0);
+  assert.equal(result.stderr, "");
+  assert.match(result.stdout, /^Usage:/);
+  assert.match(result.stdout, /--json/);
+  assert.match(result.stdout, /--help/);
+});
+
+test("invalid arguments exit 2 and write only to stderr", () => {
+  for (const args of [["--unknown"], ["--json", "extra"], ["--help", "--json"]]) {
+    const result = runMain(args, { O_NOFOLLOW: 1, O_NONBLOCK: 2 });
+    assert.equal(result.status, 2);
+    assert.equal(result.stdout, "");
+    assert.match(result.stderr, /Invalid arguments/);
+  }
+});
+
+test("real CLI text invocation reflects the current platform", () => {
+  const expected = inspectEnvironment();
+  const result = runCli();
+  assert.equal(result.error, undefined, cliOutput(result));
+  assert.equal(result.status, expected.ok ? 0 : 1, cliOutput(result));
+  assert.match(result.stdout, new RegExp(`Node\\.js: ${process.version.replaceAll(".", "\\.")}`));
+  assert.match(result.stdout, new RegExp(`OS: ${process.platform}`));
+  assert.match(result.stdout, new RegExp(`Architecture: ${process.arch}`));
+});
+
+test("real CLI JSON, help, and invalid-argument invocations use documented exit codes", () => {
+  const expected = inspectEnvironment();
+  const jsonResult = runCli(["--json"]);
+  assert.equal(jsonResult.status, expected.ok ? 0 : 1, cliOutput(jsonResult));
+  assert.deepEqual(JSON.parse(jsonResult.stdout), expected);
+
+  const helpResult = runCli(["--help"]);
+  assert.equal(helpResult.status, 0, cliOutput(helpResult));
+  assert.match(helpResult.stdout, /^Usage:/);
+
+  const invalidResult = runCli(["--unknown"]);
+  assert.equal(invalidResult.status, 2, cliOutput(invalidResult));
+  assert.match(invalidResult.stderr, /Invalid arguments/);
+});