diff --git a/tests/config-schema.test.mjs b/tests/config-schema.test.mjs
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+++ b/tests/config-schema.test.mjs
@@ -0,0 +1,415 @@
+/**
+ * Config schema test — locks in the [E00-S04-T01] TypeBox/Ajv configuration
+ * schema for the workspace.
+ *
+ * Acceptance criteria covered (each test fails without the committed state):
+ *   - "configuration schema is defined with TypeBox/Ajv" → the new
+ *     `packages/config` package (`@personal-blog/config`) pins the
+ *     golden-tuple runtime deps exactly (`@sinclair/typebox@0.34.52`,
+ *     `ajv@8.20.0` — Technology-Stack §5.2/§7), its `src/schema.ts` defines
+ *     `configSchema` with TypeBox (`Type.Object`), and its `src/validate.ts`
+ *     compiles that schema with Ajv (`new Ajv({ allErrors: true })` +
+ *     `.compile(configSchema)`) and exports `validateConfig`; the package
+ *     boundary re-exports both. Mutation probes prove the assertions are
+ *     non-vacuous (dropping a field, relaxing a constraint, replacing
+ *     TypeBox/Ajv with a hand-rolled object all fail).
+ *   - "schema covers the validated config fields" → the schema defines the
+ *     four validated config fields with their constraints: `host` (string,
+ *     default `0.0.0.0`), `port` (integer 1–65535, default `3000`),
+ *     `databaseUrl` (optional non-empty string — the local non-container path
+ *     has no database), `sessionSecret` (required, ≥ 32 chars — the story's
+ *     secret field, EPPP_SESSION_SECRET, Security-and-Operations §32/§26),
+ *     and the object is closed (`additionalProperties: false`).
+ *   - the issue's test plan — "validate a full config against the
+ *     TypeBox/Ajv schema" → the deterministic probe imports the committed
+ *     `src/schema.ts` (Node type stripping, no build step) and validates a
+ *     full config through Ajv, plus the negative cases (missing required
+ *     field naming `sessionSecret`, secret too short, unknown property, port
+ *     out of range / not an integer, empty `databaseUrl`); when the package
+ *     is built (CI builds it first) the probe additionally exercises the
+ *     compiled `validateConfig` boundary exactly as the later configuration
+ *     adapter will consume it.
+ *
+ * Run: `node --test tests/config-schema.test.mjs`
+ * (node:test — built into Node >= 18; no dependencies, lockfile untouched.)
+ */
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync, existsSync, writeFileSync, rmSync } from 'node:fs';
+import { spawnSync } from 'node:child_process';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const REPO_ROOT = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+
+const read = (relPath) => readFileSync(path.join(REPO_ROOT, relPath), 'utf8');
+
+/** The committed config package under test. */
+const CONFIG_DIR = 'packages/config';
+const MANIFEST_PATH = `${CONFIG_DIR}/package.json`;
+const SCHEMA_SRC = `${CONFIG_DIR}/src/schema.ts`;
+const VALIDATE_SRC = `${CONFIG_DIR}/src/validate.ts`;
+const INDEX_SRC = `${CONFIG_DIR}/src/index.ts`;
+
+/** The root test glob (root `scripts.test`, E00-S01-T12) that runs every suite. */
+const ROOT_TEST_GLOB = 'tests/**/*.test.mjs';
+
+/** The CI job that gates the config-schema criterion on every PR. */
+const CI_JOB = 'config-schema';
+
+/** Golden-tuple pins for the configuration schema (Technology-Stack §5.2/§7). */
+const GOLDEN_TYPEBOX = '0.34.52';
+const GOLDEN_AJV = '8.20.0';
+
+// ---------------------------------------------------------------------------
+// Static assertions on the committed config package
+// ---------------------------------------------------------------------------
+
+/**
+ * Asserts the schema is defined with TypeBox and covers the validated config
+ * fields with their documented constraints. Fails fast on any deviation; the
+ * mutation probes below prove the assertions are non-vacuous.
+ */
+function assertSchemaShape(src) {
+  // Defined with TypeBox: the schema object is built by Type.Object from the
+  // @sinclair/typebox `Type` factory (never a hand-rolled plain object).
+  assert.match(
+    src,
+    /import \{ Type, type Static \} from '@sinclair\/typebox'/,
+    'the schema module must import Type (and the Static type) from @sinclair/typebox',
+  );
+  assert.match(
+    src,
+    /export const configSchema = Type\.Object\(/,
+    'the schema must be defined with TypeBox (Type.Object) — not a hand-rolled object',
+  );
+
+  // The validated config fields, each with its documented constraint.
+  assert.match(
+    src,
+    /host: Type\.Optional\(Type\.String\(\{ default: '0\.0\.0\.0' \}\)\)/,
+    'the schema must cover the host field (optional string, default 0.0.0.0 — the bind address)',
+  );
+  assert.match(
+    src,
+    /port: Type\.Optional\(Type\.Integer\(\{ minimum: 1, maximum: 65535, default: 3000 \}\)\)/,
+    'the schema must cover the port field (optional integer 1-65535, default 3000)',
+  );
+  assert.match(
+    src,
+    /databaseUrl: Type\.Optional\(Type\.String\(\{ minLength: 1 \}\)\)/,
+    'the schema must cover the databaseUrl field (optional non-empty string)',
+  );
+  assert.match(
+    src,
+    /sessionSecret: Type\.String\(\{ minLength: 32 \}\)/,
+    'the schema must cover the sessionSecret field (required string, at least 32 chars — never Type.Optional)',
+  );
+  assert.match(
+    src,
+    /additionalProperties: false/,
+    'the schema must close the object (additionalProperties: false) so unexpected settings are rejected',
+  );
+}
+
+/** Asserts the validator compiles the schema with Ajv and exports validateConfig. */
+function assertValidateShape(src) {
+  assert.match(
+    src,
+    /import \{ Ajv \} from 'ajv'/,
+    'the validator module must import Ajv (the golden-tuple validator)',
+  );
+  assert.match(
+    src,
+    /new Ajv\(\{ allErrors: true \}\)/,
+    'the validator must create the Ajv instance with allErrors (report every violation)',
+  );
+  assert.match(
+    src,
+    /\.compile\(configSchema\)/,
+    'the validator must compile the TypeBox configSchema with Ajv',
+  );
+  assert.match(
+    src,
+    /export function validateConfig/,
+    'the validator module must export the validateConfig entry point',
+  );
+}
+
+// ---------------------------------------------------------------------------
+// Criterion tests
+// ---------------------------------------------------------------------------
+
+test('the config package exists, pins the golden-tuple runtime deps exactly, and builds with tsc', () => {
+  const manifest = JSON.parse(read(MANIFEST_PATH));
+  assert.equal(manifest.name, '@personal-blog/config');
+  assert.equal(
+    manifest.dependencies?.['@sinclair/typebox'],
+    GOLDEN_TYPEBOX,
+    `@sinclair/typebox must be pinned exactly to the golden tuple (${GOLDEN_TYPEBOX})`,
+  );
+  assert.match(
+    manifest.dependencies?.['@sinclair/typebox'],
+    /^\d+\.\d+\.\d+$/,
+    'the @sinclair/typebox pin must be exact (no ^ / ~ / range)',
+  );
+  assert.equal(
+    manifest.dependencies?.ajv,
+    GOLDEN_AJV,
+    `ajv must be pinned exactly to the golden tuple (${GOLDEN_AJV})`,
+  );
+  assert.match(
+    manifest.dependencies?.ajv,
+    /^\d+\.\d+\.\d+$/,
+    'the ajv pin must be exact (no ^ / ~ / range)',
+  );
+  // The CI job and the local build path compile the package with tsc.
+  assert.equal(manifest.scripts?.build, 'tsc -p tsconfig.json');
+  assert.equal(manifest.scripts?.typecheck, 'tsc -p tsconfig.json --noEmit');
+});
+
+test('the configuration schema is defined with TypeBox and covers the validated config fields', () => {
+  assert.ok(existsSync(path.join(REPO_ROOT, SCHEMA_SRC)), `committed ${SCHEMA_SRC} must exist`);
+  assertSchemaShape(read(SCHEMA_SRC));
+});
+
+test('the schema is validated with Ajv: validate.ts compiles configSchema and exports validateConfig', () => {
+  assert.ok(existsSync(path.join(REPO_ROOT, VALIDATE_SRC)), `committed ${VALIDATE_SRC} must exist`);
+  assertValidateShape(read(VALIDATE_SRC));
+});
+
+test('the package boundary re-exports the schema and the validator', () => {
+  const src = read(INDEX_SRC);
+  assert.match(src, /export \{ configSchema \} from '\.\/schema\.js'/, 'the boundary must re-export configSchema');
+  assert.match(src, /export type \{ Config \} from '\.\/schema\.js'/, 'the boundary must re-export the Config type');
+  assert.match(src, /export \{ validateConfig \} from '\.\/validate\.js'/, 'the boundary must re-export validateConfig');
+  assert.match(src, /export type \{ ConfigValidationResult \} from '\.\/validate\.js'/, 'the boundary must re-export the ConfigValidationResult type');
+});
+
+test('the config-schema criterion is enforced in CI', () => {
+  // Picked up by the root test command (root `scripts.test` glob).
+  const scripts = JSON.parse(read('package.json')).scripts ?? {};
+  assert.equal(
+    scripts.test,
+    `node --test "${ROOT_TEST_GLOB}"`,
+    `root scripts.test must run the "${ROOT_TEST_GLOB}" glob so this suite runs with the rest`,
+  );
+  // And a dedicated CI job gates it on every PR.
+  const workflow = read('.gitea/workflows/ci.yml');
+  assert.ok(
+    workflow.includes(`node --test tests/config-schema.test.mjs`),
+    `CI must run the config-schema suite (job "${CI_JOB}") on every PR`,
+  );
+  assert.ok(
+    workflow.includes(`pnpm --filter @personal-blog/config build`),
+    'the CI job must build the config package (the probe exercises the compiled boundary)',
+  );
+});
+
+// ---------------------------------------------------------------------------
+// Mutation probes — the static assertions are non-vacuous
+// ---------------------------------------------------------------------------
+
+test('removing a validated config field from the schema fails the coverage assertion (mutation probe)', () => {
+  const mutated = read(SCHEMA_SRC).replace(
+    /sessionSecret: Type\.String\(\{ minLength: 32 \}\),\n/,
+    '',
+  );
+  assert.notEqual(mutated, read(SCHEMA_SRC), 'the mutation must actually remove the sessionSecret field');
+  assert.throws(() => assertSchemaShape(mutated), /sessionSecret/);
+});
+
+test('making the secret optional fails the coverage assertion (mutation probe)', () => {
+  const mutated = read(SCHEMA_SRC).replace(
+    'sessionSecret: Type.String({ minLength: 32 })',
+    'sessionSecret: Type.Optional(Type.String({ minLength: 32 }))',
+  );
+  assert.notEqual(mutated, read(SCHEMA_SRC), 'the mutation must actually make sessionSecret optional');
+  assert.throws(() => assertSchemaShape(mutated), /never Type\.Optional/);
+});
+
+test('relaxing the secret length fails the coverage assertion (mutation probe)', () => {
+  const mutated = read(SCHEMA_SRC).replace('minLength: 32', 'minLength: 8');
+  assert.notEqual(mutated, read(SCHEMA_SRC), 'the mutation must actually relax the minLength');
+  assert.throws(() => assertSchemaShape(mutated), /at least 32 chars/);
+});
+
+test('opening the object fails the coverage assertion (mutation probe)', () => {
+  // Replace every occurrence (the docstring mentions the keyword too) so the
+  // committed schema code itself is the mutation target.
+  const mutated = read(SCHEMA_SRC).replace(/additionalProperties: false/g, 'additionalProperties: true');
+  assert.notEqual(mutated, read(SCHEMA_SRC), 'the mutation must actually open the object');
+  assert.throws(() => assertSchemaShape(mutated), /additionalProperties: false/);
+});
+
+test('replacing TypeBox with a hand-rolled object fails the TypeBox assertion (mutation probe)', () => {
+  const mutated = read(SCHEMA_SRC)
+    .replace("import { Type, type Static } from '@sinclair/typebox';", '// no TypeBox')
+    .replace('export const configSchema = Type.Object(', 'export const configSchema = {');
+  assert.notEqual(mutated, read(SCHEMA_SRC), 'the mutation must actually drop TypeBox');
+  assert.throws(() => assertSchemaShape(mutated), /must import Type/);
+});
+
+test('dropping the Ajv compile fails the validator assertion (mutation probe)', () => {
+  const mutated = read(VALIDATE_SRC)
+    .replace("import { Ajv } from 'ajv';", '// no Ajv')
+    .replace('const validateConfigValue = ajv.compile(configSchema);', 'const validateConfigValue = () => true;');
+  assert.notEqual(mutated, read(VALIDATE_SRC), 'the mutation must actually drop the Ajv compile');
+  assert.throws(() => assertValidateShape(mutated), /Ajv|compile/);
+});
+
+// ---------------------------------------------------------------------------
+// Deterministic behavioral probe — the issue's test plan: validate a full
+// config against the TypeBox/Ajv schema
+// ---------------------------------------------------------------------------
+
+/**
+ * How the current Node executes TypeScript sources: `default` (>= 23.6, type
+ * stripping on by default), `strip-types-flag` (>= 22.6 via
+ * `--experimental-strip-types`) or `null` (cannot run .ts at all). The
+ * workspace pins engines.node to 24.x, where type stripping is stable.
+ */
+function tsExecMode() {
+  const [major, minor] = process.versions.node.split('.').map(Number);
+  if (major > 23 || (major === 23 && minor >= 6)) return 'default';
+  if (major === 22 && minor >= 6) return 'strip-types-flag';
+  return null;
+}
+
+const TS_STRIPPING = tsExecMode() !== null;
+
+/**
+ * The probe source: imports the committed `src/schema.ts` (type-stripped,
+ * no build step) and validates a full config plus the negative cases through
+ * Ajv — the issue's test plan. When the package is built (`dist/` present,
+ * as in the CI job), it additionally exercises the compiled
+ * `validateConfig` boundary exactly as the later configuration adapter will.
+ * Written to a temp file inside `packages/config/` so `@sinclair/typebox` and
+ * `ajv` resolve through the package's own dependency links, then removed.
+ */
+const PROBE_SOURCE = `
+import { configSchema } from './src/schema.ts';
+import { Ajv } from 'ajv';
+import { existsSync } from 'node:fs';
+
+const validate = new Ajv({ allErrors: true }).compile(configSchema);
+const errorsOf = (value) => {
+  validate(value);
+  return (validate.errors ?? []).map((error) => error.message ?? 'invalid');
+};
+
+const FULL = {
+  host: '0.0.0.0',
+  port: 3000,
+  databaseUrl: 'postgres://eppp:eppp@db:5432/eppp',
+  sessionSecret: 's'.repeat(32),
+};
+
+const result = {
+  // The issue's test plan: "validate a full config against the TypeBox/Ajv
+  // schema" — a full config is valid.
+  fullValid: validate(FULL),
+  // host/port/databaseUrl are optional (port and host carry defaults; the
+  // local non-container path has no database), so a config with only the
+  // required secret is valid too.
+  defaultsValid: validate({ sessionSecret: 's'.repeat(32) }),
+  // The required field is enforced and the error names it (E00-S04-T02 will
+  // format these as field-specific startup errors).
+  missingSecretErrors: errorsOf({ host: '0.0.0.0', port: 3000 }),
+  shortSecretErrors: errorsOf({ ...FULL, sessionSecret: 'short' }),
+  // The object is closed: an unexpected setting is rejected loudly.
+  unknownPropertyErrors: errorsOf({ ...FULL, extra: true }),
+  // Port is an integer in 1-65535.
+  portTooHighErrors: errorsOf({ ...FULL, port: 65536 }),
+  portNotIntegerErrors: errorsOf({ ...FULL, port: '3000' }),
+  // databaseUrl must be non-empty when present.
+  emptyDatabaseUrlErrors: errorsOf({ ...FULL, databaseUrl: '' }),
+};
+
+// Compiled boundary (built by the CI job): the committed validateConfig.
+if (existsSync('./dist/index.js')) {
+  const { validateConfig } = await import('./dist/index.js');
+  result.boundaryFullValid = validateConfig(FULL).valid;
+  result.boundaryMissingSecretErrors = validateConfig({ host: '0.0.0.0', port: 3000 }).errors;
+  result.boundaryUnknownPropertyErrors = validateConfig({ ...FULL, extra: true }).errors;
+}
+
+console.log('CONFIG_SCHEMA_PROBE_RESULT ' + JSON.stringify(result));
+`;
+
+test('a full config validates against the committed TypeBox/Ajv schema, and violations are rejected (deterministic probe)', { skip: !TS_STRIPPING }, () => {
+  // The issue's test plan: "validate a full config against the TypeBox/Ajv
+  // schema". The probe runs the committed schema.ts through Ajv (Node type
+  // stripping, no build step) from inside packages/config so the golden-tuple
+  // deps resolve through the package's own links.
+  const probeFile = path.join(REPO_ROOT, CONFIG_DIR, `.config-schema-probe-${process.pid}.mjs`);
+  const args =
+    tsExecMode() === 'strip-types-flag'
+      ? ['--experimental-strip-types', path.basename(probeFile)]
+      : [path.basename(probeFile)];
+  try {
+    writeFileSync(probeFile, PROBE_SOURCE);
+    const run = spawnSync(process.execPath, args, {
+      cwd: path.join(REPO_ROOT, CONFIG_DIR),
+      encoding: 'utf8',
+      timeout: 60_000,
+    });
+    assert.equal(
+      run.status,
+      0,
+      `the probe must exit 0 (status ${run.status}):\n${(run.stderr || run.stdout || '').trim()}`,
+    );
+    const match = run.stdout.match(/CONFIG_SCHEMA_PROBE_RESULT (\{.*\})/);
+    assert.ok(match, `the probe must print CONFIG_SCHEMA_PROBE_RESULT:\n${run.stdout.trim()}`);
+    const result = JSON.parse(match[1]);
+
+    // The issue's test plan: a full config validates.
+    assert.equal(result.fullValid, true, 'a full config must validate against the TypeBox/Ajv schema');
+    assert.equal(result.defaultsValid, true, 'host/port/databaseUrl are optional (defaults; no-database path)');
+
+    // The required field is enforced and the error names it.
+    assert.ok(
+      result.missingSecretErrors.some((msg) => /required property 'sessionSecret'/.test(msg)),
+      `missing sessionSecret must be rejected naming the field (got: ${JSON.stringify(result.missingSecretErrors)})`,
+    );
+    assert.ok(
+      result.shortSecretErrors.some((msg) => /fewer than 32/.test(msg)),
+      `a short sessionSecret must be rejected (got: ${JSON.stringify(result.shortSecretErrors)})`,
+    );
+
+    // The object is closed and the numeric/string constraints hold.
+    assert.ok(
+      result.unknownPropertyErrors.some((msg) => /additional properties/.test(msg)),
+      `an unknown property must be rejected (got: ${JSON.stringify(result.unknownPropertyErrors)})`,
+    );
+    assert.ok(
+      result.portTooHighErrors.some((msg) => /<= 65535/.test(msg)),
+      `a port above 65535 must be rejected (got: ${JSON.stringify(result.portTooHighErrors)})`,
+    );
+    assert.ok(
+      result.portNotIntegerErrors.some((msg) => /integer/.test(msg)),
+      `a non-integer port must be rejected (got: ${JSON.stringify(result.portNotIntegerErrors)})`,
+    );
+    assert.ok(
+      result.emptyDatabaseUrlErrors.some((msg) => /fewer than 1/.test(msg)),
+      `an empty databaseUrl must be rejected (got: ${JSON.stringify(result.emptyDatabaseUrlErrors)})`,
+    );
+
+    // When the package is built (the CI job builds it), the compiled
+    // validateConfig boundary behaves identically.
+    if (existsSync(path.join(REPO_ROOT, CONFIG_DIR, 'dist', 'index.js'))) {
+      assert.equal(result.boundaryFullValid, true, 'validateConfig must accept a full config');
+      assert.ok(
+        result.boundaryMissingSecretErrors.some((msg) => /required property 'sessionSecret'/.test(msg)),
+        `validateConfig must reject a missing secret naming the field (got: ${JSON.stringify(result.boundaryMissingSecretErrors)})`,
+      );
+      assert.ok(
+        result.boundaryUnknownPropertyErrors.some((msg) => /additional properties/.test(msg)),
+        `validateConfig must reject an unknown property (got: ${JSON.stringify(result.boundaryUnknownPropertyErrors)})`,
+      );
+    }
+  } finally {
+    rmSync(probeFile, { force: true });
+  }
+});