diff --git a/apps/server/Dockerfile b/apps/server/Dockerfile
index f2050e5..f8a1f85 100644
--- a/apps/server/Dockerfile
+++ b/apps/server/Dockerfile
@@ -21,14 +21,17 @@
 # T05 the runtime stage drops root privileges (runs as the image's non-root
 # `node` user).
 #
-# The app now depends on the `database-postgres` workspace package (the single
-# owner of the pg/Kysely driver, E00-S03-T02). The build stage therefore also
-# installs/builds that package — the server's `build`/`typecheck` scripts
-# build their workspace dependency first (`pnpm --filter
-# @personal-blog/database-postgres build`), and the runtime stage ships the
-# compiled `packages/database-postgres/dist` next to the copied workspace
-# node_modules links so the server's `@personal-blog/database-postgres` import
-# resolves at run time.
+# The app now depends on the `config` and `database-postgres` workspace
+# packages (the configuration service — E00-S04-T02 validates the required
+# settings at startup — and the single owner of the pg/Kysely driver,
+# E00-S03-T02). The build stage therefore also installs/builds those
+# packages — the server's `build`/`typecheck` scripts build their workspace
+# dependencies first (`pnpm --filter @personal-blog/config build` and
+# `pnpm --filter @personal-blog/database-postgres build`), and the runtime
+# stage ships the compiled `packages/config/dist` and
+# `packages/database-postgres/dist` next to the copied workspace node_modules
+# links so the server's `@personal-blog/config` and
+# `@personal-blog/database-postgres` imports resolve at run time.
 #
 # T08: the image embeds no secrets. The Dockerfile declares no secret-bearing
 # ARG/ENV instruction (the only ENV is `NODE_ENV=production`) and every COPY
@@ -68,10 +71,11 @@ COPY extensions/example/package.json extensions/example/package.json
 RUN pnpm install --frozen-lockfile
 
 # Compile the server package (tsc -p apps/server/tsconfig.json -> dist/). The
-# server's build script builds its workspace dependency first (the
-# `database-postgres` package, whose compiled dist the server imports), so a
-# single command produces both dists in the right order.
+# server's build script builds its workspace dependencies first (the `config`
+# and `database-postgres` packages, whose compiled dists the server imports),
+# so a single command produces all dists in the right order.
 COPY apps/server apps/server
+COPY packages/config packages/config
 COPY packages/database-postgres packages/database-postgres
 RUN pnpm --filter @personal-blog/server build
 
@@ -81,11 +85,13 @@ WORKDIR /app
 ENV NODE_ENV=production
 
 # The workspace install (devDependencies included — image-size pruning is a
-# later E00-S02 concern) plus the compiled server output, the compiled
-# database-postgres output the server imports, and the package manifests.
+# later E00-S02 concern) plus the compiled server output, the compiled config
+# and database-postgres outputs the server imports, and the package manifests.
 COPY --from=build /app/node_modules ./node_modules
 COPY --from=build /app/apps/server/dist ./apps/server/dist
 COPY --from=build /app/apps/server/package.json ./apps/server/package.json
+COPY --from=build /app/packages/config/dist ./packages/config/dist
+COPY --from=build /app/packages/config/package.json ./packages/config/package.json
 COPY --from=build /app/packages/database-postgres/dist ./packages/database-postgres/dist
 COPY --from=build /app/packages/database-postgres/package.json ./packages/database-postgres/package.json
 
diff --git a/apps/server/package.json b/apps/server/package.json
index 026f594..56506f2 100644
--- a/apps/server/package.json
+++ b/apps/server/package.json
@@ -3,13 +3,14 @@
   "version": "0.0.0",
   "private": true,
   "type": "module",
-  "description": "EPPP public server application. Serves the application health endpoint (E00-S02-T03) gated on the startup migration run (E00-S03-T06); the Fastify 5 application shell lands in a later story.",
+  "description": "EPPP public server application. Serves the application health endpoint (E00-S02-T03) gated on the startup migration run (E00-S03-T06), with a field-specific startup error when a required setting is missing (E00-S04-T02); the Fastify 5 application shell lands in a later story.",
   "scripts": {
-    "build": "pnpm --filter @personal-blog/database-postgres build && tsc -p tsconfig.json",
-    "typecheck": "pnpm --filter @personal-blog/database-postgres build && tsc -p tsconfig.json --noEmit",
+    "build": "pnpm --filter @personal-blog/config build && pnpm --filter @personal-blog/database-postgres build && tsc -p tsconfig.json",
+    "typecheck": "pnpm --filter @personal-blog/config build && pnpm --filter @personal-blog/database-postgres build && tsc -p tsconfig.json --noEmit",
     "start": "node dist/index.js"
   },
   "dependencies": {
+    "@personal-blog/config": "workspace:*",
     "@personal-blog/database-postgres": "workspace:*"
   },
   "devDependencies": {
diff --git a/apps/server/src/index.ts b/apps/server/src/index.ts
index c99ebaf..f258caf 100644
--- a/apps/server/src/index.ts
+++ b/apps/server/src/index.ts
@@ -18,11 +18,22 @@
  * E00-S01-T06) there are no migrations to run, so the app reports ready
  * immediately.
  *
+ * [E00-S04-T02] field-specific startup error: the required settings are
+ * validated before the server binds, so a deployment missing a required
+ * setting (the admin-session secret `EPPP_SESSION_SECRET` — the schema's
+ * required field, Security-and-Operations §32/§26) fails fast at startup
+ * with an error naming the missing field instead of booting with an invalid
+ * configuration. The parsed config keeps the committed defaults for
+ * `host`/`port`/`databaseUrl` (the `process.env` adapter that centralizes
+ * these reads is E00-S04-T04 and lands later); `assertValidConfig` throws
+ * `MissingRequiredSettingError` naming the missing field.
+ *
  * The Fastify 5 application shell (and the real HTTP API) lands in a later
  * story; this bootstrap keeps the application health-checkable until then.
  */
 
 import { createServer, type IncomingMessage, type ServerResponse } from 'node:http';
+import { assertValidConfig } from '@personal-blog/config';
 import { Pool } from '@personal-blog/database-postgres';
 import { MigrationLedger, MigrationRunner } from '@personal-blog/database-postgres';
 import type { Migration } from '@personal-blog/database-postgres';
@@ -30,6 +41,17 @@ import type { Migration } from '@personal-blog/database-postgres';
 /** Port the server listens on; `PORT` overrides the container default (3000). */
 const PORT = resolvePort(process.env.PORT);
 
+// [E00-S04-T02] field-specific startup error: validate the startup
+// configuration before anything else, so a missing required setting (e.g.
+// EPPP_SESSION_SECRET) crashes the process at startup with an error naming
+// the missing field — never boots with an invalid configuration.
+assertValidConfig({
+  host: '0.0.0.0',
+  port: PORT,
+  databaseUrl: process.env.DATABASE_URL,
+  sessionSecret: process.env.EPPP_SESSION_SECRET,
+});
+
 /** Health payload — reported once the startup migration run completes. */
 const HEALTH_PAYLOAD = JSON.stringify({ status: 'ok' });
 
diff --git a/compose.yaml b/compose.yaml
index 06aa600..856db96 100644
--- a/compose.yaml
+++ b/compose.yaml
@@ -50,6 +50,11 @@
 #
 # All values have defaults so `docker compose up -d` works from a clean clone
 # without a .env file (a committed .env.example template lands in E00-S04).
+# Since E00-S04-T02 the app validates its required settings at startup: the
+# admin-session secret `EPPP_SESSION_SECRET` (the schema's required field,
+# Security-and-Operations §32/§26) is provided here with a dev-only default —
+# override it via a `.env` file / shell environment for anything beyond local
+# development.
 
 services:
   db:
@@ -93,6 +98,10 @@ services:
         - linux/arm64
     environment:
       DATABASE_URL: postgres://eppp:eppp@db:5432/eppp
+      # E00-S04-T02: the app's required admin-session secret (the schema's
+      # required field, EPPP_SESSION_SECRET per Security-and-Operations
+      # §32/§26) — dev-only default (>= 32 chars), override via .env / shell.
+      EPPP_SESSION_SECRET: ${EPPP_SESSION_SECRET:-eppp-local-session-secret-change-me-0123456789}
     ports:
       - "${APP_PORT:-3000}:3000"
     # T02: start only once the database reports healthy (service_healthy), so
diff --git a/packages/config/package.json b/packages/config/package.json
index 700a3da..5cddb6c 100644
--- a/packages/config/package.json
+++ b/packages/config/package.json
@@ -3,7 +3,7 @@
   "version": "0.0.0",
   "private": true,
   "type": "module",
-  "description": "EPPP configuration service. Owns the TypeBox/Ajv configuration schema (E00-S04-T01); the environment adapter (E00-S04-T04), field-specific startup errors (E00-S04-T02), secret redaction (E00-S04-T03) and the .env.example template (E00-S04-T05) land in later tasks.",
+  "description": "EPPP configuration service. Owns the TypeBox/Ajv configuration schema (E00-S04-T01) and the field-specific startup error for a missing required setting (E00-S04-T02); the environment adapter (E00-S04-T04), secret redaction (E00-S04-T03) and the .env.example template (E00-S04-T05) land in later tasks.",
   "scripts": {
     "build": "tsc -p tsconfig.json",
     "typecheck": "tsc -p tsconfig.json --noEmit"
diff --git a/packages/config/src/index.ts b/packages/config/src/index.ts
index 1bc4602..b6cc631 100644
--- a/packages/config/src/index.ts
+++ b/packages/config/src/index.ts
@@ -3,12 +3,20 @@
  *
  * [E00-S04-T01] TypeBox/Ajv schema: the package boundary exposes the
  * configuration schema (`configSchema`, defined with TypeBox) and the Ajv
- * schema-validation entry point (`validateConfig`). The environment adapter
- * (E00-S04-T04), field-specific startup errors (E00-S04-T02) and secret
- * redaction (E00-S04-T03) build on this boundary in later tasks.
+ * schema-validation entry point (`validateConfig`).
+ *
+ * [E00-S04-T02] Field-specific startup error: the boundary also exposes the
+ * startup validation entry point (`assertValidConfig`) and its field-specific
+ * errors (`MissingRequiredSettingError` — names the missing required setting —
+ * and `ConfigStartupError` — names each violating field), so the application
+ * fails fast at startup when a required setting is missing.
+ *
+ * The environment adapter (E00-S04-T04) and secret redaction (E00-S04-T03)
+ * build on this boundary in later tasks.
  */
 
 export { configSchema } from './schema.js';
 export type { Config } from './schema.js';
 export { validateConfig } from './validate.js';
 export type { ConfigValidationResult } from './validate.js';
+export { assertValidConfig, ConfigStartupError, MissingRequiredSettingError } from './startup.js';
diff --git a/packages/config/src/startup.ts b/packages/config/src/startup.ts
new file mode 100644
index 0000000..f4953c2
--- /dev/null
+++ b/packages/config/src/startup.ts
@@ -0,0 +1,128 @@
+/**
+ * EPPP configuration startup validation — [E00-S04-T02] missing required
+ * setting gives a field-specific startup error.
+ *
+ * Builds on the E00-S04-T01 boundary (`configSchema` from `schema.ts`,
+ * validated with Ajv): `assertValidConfig` is the startup entry point the
+ * application calls with its parsed configuration before it binds — when a
+ * required setting is missing it throws `MissingRequiredSettingError`, whose
+ * message and `missingField` name the missing field (the issue's acceptance:
+ * "missing required setting gives a field-specific startup error", "the error
+ * names the missing field"); any other schema violation throws a
+ * `ConfigStartupError` whose message names each violating field too.
+ *
+ * This is deliberately NOT the T04 environment adapter: nothing here reads
+ * `process.env`. The adapter (E00-S04-T04) maps the environment onto the
+ * validated config shape and passes it to `assertValidConfig` at startup;
+ * secret redaction (E00-S04-T03) builds on the same boundary in a later task.
+ *
+ * Rollback note from the issue: revert the validation error handling.
+ */
+
+import { Ajv, type ErrorObject } from 'ajv';
+
+import { configSchema, type Config } from './schema.js';
+
+/** Ajv instance for the config schema — `allErrors` reports every violation. */
+const ajv = new Ajv({ allErrors: true });
+
+/** The compiled validator — TypeBox schemas are JSON Schema, so Ajv compiles them directly. */
+const validateConfigValue = ajv.compile(configSchema);
+
+/**
+ * The field-specific startup error thrown when a configuration value is
+ * invalid at startup (any schema violation). `violations` holds one
+ * field-prefixed message per violation (e.g. `"sessionSecret: must NOT have
+ * fewer than 32 characters"`), so the error names the offending field(s).
+ */
+export class ConfigStartupError extends Error {
+  /** Field-prefixed messages naming each violation (never empty). */
+  readonly violations: ReadonlyArray<string>;
+
+  constructor(message: string, violations: readonly string[]) {
+    super(message);
+    this.name = 'ConfigStartupError';
+    this.violations = violations;
+  }
+}
+
+/**
+ * The error thrown when a required setting is missing — the E00-S04-T02
+ * field-specific startup error. `missingField` and the message name the
+ * missing field (e.g. `"missing required setting: sessionSecret"`), so an
+ * operator starting the app with an incomplete configuration sees exactly
+ * which setting to provide.
+ */
+export class MissingRequiredSettingError extends ConfigStartupError {
+  /** The name of the required setting that is missing. */
+  readonly missingField: string;
+
+  constructor(missingField: string) {
+    super(`missing required setting: ${missingField}`, [`missing required setting: ${missingField}`]);
+    this.name = 'MissingRequiredSettingError';
+    this.missingField = missingField;
+  }
+}
+
+/**
+ * Formats one Ajv violation as a field-specific message: the field named by
+ * the error's `instancePath` (e.g. `/sessionSecret`) prefixes the Ajv
+ * message, so every startup error names the offending setting — never just a
+ * bare schema message. An `additionalProperties` violation points at the
+ * object (empty `instancePath`), so its offending key (Ajv
+ * `params.additionalProperty`) is used as the field instead.
+ */
+function formatViolation(error: ErrorObject): string {
+  const field = error.instancePath.replace(/^\//, '');
+  const message = error.message ?? 'invalid';
+  if (field !== '') {
+    return `${field}: ${message}`;
+  }
+  const extra = (error.params as { additionalProperty?: unknown } | undefined)?.additionalProperty;
+  return typeof extra === 'string' && extra.length > 0 ? `${extra}: ${message}` : message;
+}
+
+/**
+ * Validates a configuration value at startup and returns it as the typed
+ * `Config` — or throws a field-specific startup error:
+ *
+ * - a missing required setting throws `MissingRequiredSettingError` naming
+ *   the missing field (the issue's acceptance criteria);
+ * - any other schema violation throws `ConfigStartupError` whose message
+ *   names the violating field(s).
+ *
+ * The application calls this before it starts serving, so an invalid
+ * configuration fails fast at startup with a clear, field-specific error
+ * instead of booting with a silently-wrong setting.
+ *
+ * @param value - the parsed configuration value (the T04 adapter will hand
+ *   this the mapped environment)
+ * @returns the validated configuration
+ * @throws {MissingRequiredSettingError} when a required setting is missing
+ * @throws {ConfigStartupError} when the configuration violates the schema
+ */
+export function assertValidConfig(value: unknown): Config {
+  const valid = validateConfigValue(value);
+  if (valid) {
+    return value as Config;
+  }
+
+  const errors = validateConfigValue.errors ?? [];
+
+  // Missing required settings get the dedicated field-specific error — the
+  // Ajv `required` keyword error carries the missing property name, which is
+  // exactly the field the acceptance criteria require the error to name.
+  const missingFields = errors
+    .filter((error) => error.keyword === 'required')
+    .map((error) => {
+      const missing = (error.params as { missingProperty?: unknown } | undefined)?.missingProperty;
+      return typeof missing === 'string' ? missing : '';
+    })
+    .filter((field) => field.length > 0);
+  if (missingFields.length > 0) {
+    throw new MissingRequiredSettingError(missingFields.join(', '));
+  }
+
+  const violations = errors.map(formatViolation);
+  throw new ConfigStartupError(`invalid configuration: ${violations.join('; ')}`, violations);
+}
diff --git a/packages/config/src/validate.ts b/packages/config/src/validate.ts
index 81435d2..ef2bf52 100644
--- a/packages/config/src/validate.ts
+++ b/packages/config/src/validate.ts
@@ -8,9 +8,9 @@
  *
  * This is deliberately NOT the E00-S04-T02 field-specific startup error:
  * `validateConfig` returns the raw schema-validation outcome (valid or not,
- * with the Ajv messages) and performs no startup wiring — the adapter
- * (E00-S04-T04) and the startup error formatting (E00-S04-T02) build on it
- * in later tasks.
+ * with the Ajv messages) and performs no startup wiring — the startup error
+ * formatting (E00-S04-T02, `startup.ts`) and the environment adapter
+ * (E00-S04-T04) build on this raw outcome in their own modules.
  */
 
 import { Ajv } from 'ajv';
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 3669a69..6511571 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -14,6 +14,9 @@ importers:
 
   apps/server:
     dependencies:
+      '@personal-blog/config':
+        specifier: workspace:*
+        version: link:../../packages/config
       '@personal-blog/database-postgres':
         specifier: workspace:*
         version: link:../../packages/database-postgres