diff --git a/.nvmrc b/.nvmrc
new file mode 100644
index 000000000..2bd5a0a98
--- /dev/null
+++ b/.nvmrc
@@ -0,0 +1 @@
+22
diff --git a/CLAUDE.md b/CLAUDE.md
index 6bc846c03..c60e451ac 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -248,4 +248,4 @@ This repo uses [Greptile](https://greptile.com) for automated PR reviews. After
 
 ## Node Version
 
-Requires Node >= 22.12.0.
+Requires Node >= 22.12.0. CI (`.github/workflows/ci.yml`) pins Node 22 everywhere; `.nvmrc` mirrors that. If your local Node major version diverges from CI's — `npm run doctor` (also run automatically as `pretest`) warns loudly when it does — treat a full local `npm test` run as untrustworthy for cross-checking against CI: native-addon/V8 behavioral drift between major versions can produce a large, unrelated-looking wall of failures that aren't real regressions. Cross-check any suspicious local failure against CI (or an untouched `origin/main` checkout in the same local environment) before treating it as caused by your change.
diff --git a/src/infrastructure/doctor.ts b/src/infrastructure/doctor.ts
index d14ed62d4..2e35e36e7 100644
--- a/src/infrastructure/doctor.ts
+++ b/src/infrastructure/doctor.ts
@@ -41,10 +41,14 @@
  * This module only detects — it never mutates the environment. `scripts/doctor.ts`
  * is the CLI entry point that also knows how to *fix* what this module reports.
  */
-import { readdirSync } from 'node:fs';
+import { readdirSync, readFileSync } from 'node:fs';
 import { createRequire } from 'node:module';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
 import type { LanguageRegistryEntry } from '../types.js';
 
+const REPO_ROOT = join(dirname(fileURLToPath(import.meta.url)), '..', '..');
+
 const _require = createRequire(import.meta.url);
 
 /**
@@ -312,18 +316,100 @@ export function checkWasmGrammars(
   };
 }
 
+/**
+ * Parse a `.nvmrc`-style file's content into a Node major version number.
+ * Not exported — only `checkNodeVersion`'s default reader produces this
+ * input; tests exercise it indirectly via that check's injectable reader.
+ * Tolerates a leading `v`, a full semver (`22.12.0`), trailing whitespace,
+ * and a trailing comment. Returns null for a non-numeric value (e.g. an
+ * `lts/*` alias) rather than guessing — this check only ever warns, so a
+ * silently-wrong comparison would be worse than skipping it.
+ */
+function parseNvmrcMajor(content: string): number | null {
+  const firstLine = content.split('\n')[0]?.trim() ?? '';
+  const versionPart = firstLine.split('#')[0]?.trim() ?? '';
+  const match = /^v?(\d+)/.exec(versionPart);
+  return match ? Number(match[1]) : null;
+}
+
+/** Read `.nvmrc` from the repo root, if present. Returns null when absent or unreadable. */
+function readNvmrc(): string | null {
+  try {
+    return readFileSync(join(REPO_ROOT, '.nvmrc'), 'utf-8');
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Warn (never fail) when the running Node major version doesn't match
+ * `.nvmrc` (issue #2521): a full local `npm test` run on a Node version CI
+ * doesn't pin can produce a large, unrelated-looking wall of failures from
+ * native-addon/V8 behavioral drift, not actual regressions — wasting time
+ * triaging phantom failures or masking a real one in the noise. This is
+ * purely informational (a mismatch doesn't mean the environment is broken,
+ * just that local results shouldn't be trusted over CI's), so it only ever
+ * warns, matching the "missing optional grammar" precedent above — never
+ * blocks `pretest`/`npm test`.
+ *
+ * `readFile`/`currentVersion` are injectable so tests can simulate a
+ * missing `.nvmrc`, an unparseable one, a match, and a mismatch without
+ * touching the real file or `process.version`.
+ */
+export function checkNodeVersion(
+  readFile: () => string | null = readNvmrc,
+  currentVersion: string = process.version,
+): DoctorCheck {
+  const id = 'node-version';
+  const label = 'Node version vs .nvmrc';
+
+  const nvmrcContent = readFile();
+  if (nvmrcContent === null) {
+    return { id, label, status: 'ok', detail: 'no .nvmrc present — nothing to compare against' };
+  }
+
+  const expectedMajor = parseNvmrcMajor(nvmrcContent);
+  if (expectedMajor === null) {
+    return {
+      id,
+      label,
+      status: 'ok',
+      detail: `.nvmrc content is not a plain version number (${nvmrcContent.trim()}) — skipping comparison`,
+    };
+  }
+
+  const currentMajor = Number(/^v?(\d+)/.exec(currentVersion)?.[1]);
+  if (currentMajor === expectedMajor) {
+    return { id, label, status: 'ok', detail: `running Node ${currentVersion}, matches .nvmrc` };
+  }
+
+  return {
+    id,
+    label,
+    status: 'warn',
+    detail:
+      `running Node ${currentVersion}, but .nvmrc pins ${expectedMajor} — a full local test ` +
+      `run can show unrelated-looking failures from native-addon/V8 drift between major ` +
+      `versions; cross-check against CI (which pins ${expectedMajor}) before treating them as regressions`,
+  };
+}
+
 /**
  * Run every doctor check and aggregate the result. Fast and read-only — safe
- * to call frequently (e.g. from a `pretest` hook) since neither check spawns
- * a subprocess or mutates the environment.
+ * to call frequently (e.g. from a `pretest` hook) since no check spawns a
+ * subprocess or mutates the environment.
  *
- * `checks` is injectable (defaulting to the two real checks) so the
+ * `checks` is injectable (defaulting to the three real checks) so the
  * ok/'fail'-only aggregation rule — a 'warn' must never flip the overall
  * report unhealthy — can be unit tested directly with fake check results,
- * independent of the real native binary or grammars/ directory.
+ * independent of the real native binary, grammars/ directory, or Node version.
  */
 export function runDoctorChecks(
-  checks: readonly DoctorCheck[] = [checkBetterSqlite3Abi(), checkWasmGrammars()],
+  checks: readonly DoctorCheck[] = [
+    checkBetterSqlite3Abi(),
+    checkWasmGrammars(),
+    checkNodeVersion(),
+  ],
 ): DoctorReport {
   return { ok: checks.every((c) => c.status !== 'fail'), checks: [...checks] };
 }
diff --git a/tests/unit/doctor.test.ts b/tests/unit/doctor.test.ts
index 1cb19a9ed..0529381e3 100644
--- a/tests/unit/doctor.test.ts
+++ b/tests/unit/doctor.test.ts
@@ -22,6 +22,7 @@ import { afterEach, describe, expect, it } from 'vitest';
 import { LANGUAGE_REGISTRY } from '../../src/domain/parser.js';
 import {
   checkBetterSqlite3Abi,
+  checkNodeVersion,
   checkWasmGrammars,
   findMissingGrammars,
   parseAbiMismatchError,
@@ -294,17 +295,22 @@ describe('runDoctorChecks', () => {
     expect(report.ok).toBe(true);
   });
 
-  it('runs both real checks against the real environment and returns a well-formed report', () => {
+  it('runs all real checks against the real environment and returns a well-formed report', () => {
     // Uses the real defaults (real require('better-sqlite3'), real grammars/
-    // directory) — by the time this test runs, `pretest` has already gated
-    // `npm test` on a healthy environment, but this asserts shape rather than
-    // hard-coding a specific status so a single-file run on a mid-repair
-    // machine doesn't fail on an unrelated assertion.
+    // directory, real .nvmrc) — by the time this test runs, `pretest` has
+    // already gated `npm test` on a healthy environment, but this asserts
+    // shape rather than hard-coding a specific status so a single-file run
+    // on a mid-repair machine (or one whose Node version doesn't match
+    // .nvmrc) doesn't fail on an unrelated assertion.
     const report = runDoctorChecks();
 
     expect(typeof report.ok).toBe('boolean');
-    expect(report.checks).toHaveLength(2);
-    expect(report.checks.map((c) => c.id)).toEqual(['better-sqlite3-abi', 'wasm-grammars']);
+    expect(report.checks).toHaveLength(3);
+    expect(report.checks.map((c) => c.id)).toEqual([
+      'better-sqlite3-abi',
+      'wasm-grammars',
+      'node-version',
+    ]);
     for (const check of report.checks) {
       expect(['ok', 'warn', 'fail']).toContain(check.status);
       expect(typeof check.label).toBe('string');
@@ -314,3 +320,54 @@ describe('runDoctorChecks', () => {
     expect(report.ok).toBe(report.checks.every((c) => c.status !== 'fail'));
   });
 });
+
+describe('checkNodeVersion', () => {
+  it('reports ok when no .nvmrc is present', () => {
+    const result = checkNodeVersion(() => null, 'v22.12.0');
+    expect(result.status).toBe('ok');
+    expect(result.id).toBe('node-version');
+    expect(result.detail).toContain('no .nvmrc');
+  });
+
+  it('reports ok when .nvmrc content is not a plain version number', () => {
+    const result = checkNodeVersion(() => 'lts/*\n', 'v22.12.0');
+    expect(result.status).toBe('ok');
+    expect(result.detail).toContain('not a plain version number');
+  });
+
+  it('reports ok when the running major matches .nvmrc exactly', () => {
+    const result = checkNodeVersion(() => '22\n', 'v22.12.0');
+    expect(result.status).toBe('ok');
+    expect(result.detail).toContain('matches .nvmrc');
+  });
+
+  it('reports ok when .nvmrc pins a full semver whose major matches', () => {
+    const result = checkNodeVersion(() => '22.12.0\n', 'v22.4.1');
+    expect(result.status).toBe('ok');
+  });
+
+  it('tolerates a leading v and a trailing comment in .nvmrc', () => {
+    const result = checkNodeVersion(() => 'v22 # pinned to match CI\n', 'v22.12.0');
+    expect(result.status).toBe('ok');
+  });
+
+  // Regression test for issue #2521: this exact scenario (Node 26 installed
+  // locally, CI pins Node 22) produced 200+ unrelated-looking local test
+  // failures that reproduced identically on an untouched origin/main.
+  it('warns — never fails — when the running major does not match .nvmrc', () => {
+    const result = checkNodeVersion(() => '22\n', 'v26.4.0');
+    expect(result.status).toBe('warn');
+    expect(result.status).not.toBe('fail');
+    expect(result.detail).toContain('v26.4.0');
+    expect(result.detail).toContain('.nvmrc pins 22');
+    expect(result.detail).toContain('cross-check against CI');
+  });
+
+  it('a Node-version warning does not flip the overall report unhealthy', () => {
+    const report = runDoctorChecks([
+      { id: 'a', label: 'A', status: 'ok', detail: 'fine' },
+      checkNodeVersion(() => '22\n', 'v26.4.0'),
+    ]);
+    expect(report.ok).toBe(true);
+  });
+});