diff --git a/README.md b/README.md
index bfd15b35..c01ba993 100644
--- a/README.md
+++ b/README.md
@@ -381,6 +381,12 @@ A client-requested `ttl` must be a non-negative integer number of milliseconds;
 `ttl` is clamped between 1 second and `taskMaxTtlMs`, and the effective value is returned in the
 `CreateTaskResult`.
 
+`taskDefaultTtlMs`, `taskMaxTtlMs` and `taskLeaseMs` must be positive integer numbers of
+milliseconds and `taskMaxRetainedPerPrincipal` a positive integer. Registration throws a
+`TypeError` naming the option otherwise, so a typo or a value read from an environment variable
+as a string fails at startup rather than failing every task. A `taskDefaultTtlMs` above
+`taskMaxTtlMs` is capped to it.
+
 Tasks are stored in memory by default and in Redis when a `redis` option is given, so any
 instance can serve a poll for a task created on another.
 
diff --git a/src/index.ts b/src/index.ts
index a57f9b31..f255cb26 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -102,6 +102,14 @@ const mcpPlugin = fp(async function (app: FastifyInstance, opts: MCPPluginOption
     // Checked before any Redis connection is opened, so a bad value cannot leak one
     validateTaskLimit('taskMaxRetainedPerPrincipal', opts.taskMaxRetainedPerPrincipal)
   }
+  // A zero lease lapses at once and a NaN or negative ttl expires every task, so
+  // reject them (and strings read from env variables) here rather than at runtime
+  for (const name of ['taskLeaseMs', 'taskDefaultTtlMs', 'taskMaxTtlMs'] as const) {
+    const value: unknown = opts[name]
+    if (value !== undefined && !(Number.isSafeInteger(value) && (value as number) > 0)) {
+      throw new TypeError(`${name} must be a positive integer number of milliseconds, got ${String(value)}`)
+    }
+  }
   const tools = new Map<string, MCPTool>()
   const resources = new Map<string, MCPResource>()
   const prompts = new Map<string, MCPPrompt>()
diff --git a/test/task-options-validation.test.ts b/test/task-options-validation.test.ts
new file mode 100644
index 00000000..2330872c
--- /dev/null
+++ b/test/task-options-validation.test.ts
@@ -0,0 +1,83 @@
+import { test, describe } from 'node:test'
+import type { TestContext } from 'node:test'
+import Fastify from 'fastify'
+import mcpPlugin from '../src/index.ts'
+import type { MCPPluginOptions } from '../src/types.ts'
+
+async function assertRegistrationRejects (t: TestContext, opts: Partial<MCPPluginOptions>, pattern: RegExp): Promise<void> {
+  const app = Fastify()
+  try {
+    await t.assert.rejects(
+      async () => { await app.register(mcpPlugin, { enableTasks: true, ...opts } as MCPPluginOptions) },
+      (err: Error) => {
+        t.assert.ok(err instanceof TypeError, `expected a TypeError, got ${err}`)
+        t.assert.match(err.message, pattern)
+        return true
+      }
+    )
+  } finally {
+    await app.close()
+  }
+}
+
+async function assertRegistrationAccepts (t: TestContext, opts: Partial<MCPPluginOptions>): Promise<void> {
+  const app = Fastify()
+  try {
+    await app.register(mcpPlugin, { enableTasks: true, ...opts } as MCPPluginOptions)
+    await app.ready()
+  } finally {
+    await app.close()
+  }
+  t.assert.ok(true)
+}
+
+// Values that would otherwise surface only at runtime: a lease that lapses at
+// once, a ttl that is NaN or negative, or a string read from an env variable
+const invalidValues: unknown[] = [0, -5, NaN, Infinity, -Infinity, 1.5, '30000', null, Number.MAX_SAFE_INTEGER + 1]
+
+describe('task options validation (#302)', () => {
+  for (const option of ['taskLeaseMs', 'taskDefaultTtlMs', 'taskMaxTtlMs'] as const) {
+    test(`registration throws a TypeError naming ${option} for invalid values`, async (t: TestContext) => {
+      for (const value of invalidValues) {
+        await assertRegistrationRejects(t, { [option]: value as any }, new RegExp(`^${option} must be a positive integer`))
+      }
+    })
+
+    test(`registration throws for an invalid ${option} even when tasks are disabled`, async (t: TestContext) => {
+      await assertRegistrationRejects(t, { enableTasks: false, [option]: 0 }, new RegExp(`^${option} must be`))
+    })
+
+    test(`registration accepts a positive integer or undefined ${option}`, async (t: TestContext) => {
+      await assertRegistrationAccepts(t, { [option]: undefined })
+      await assertRegistrationAccepts(t, { [option]: 1000 })
+    })
+  }
+
+  test('registration throws for an invalid taskMaxRetainedPerPrincipal', async (t: TestContext) => {
+    for (const value of [0, -1, NaN, 1.5, '20', null]) {
+      await assertRegistrationRejects(t, { taskMaxRetainedPerPrincipal: value as any }, /^taskMaxRetainedPerPrincipal must be a positive integer/)
+    }
+    await assertRegistrationAccepts(t, { taskMaxRetainedPerPrincipal: undefined })
+    await assertRegistrationAccepts(t, { taskMaxRetainedPerPrincipal: 20 })
+  })
+
+  test('registration accepts a taskDefaultTtlMs above taskMaxTtlMs, which is capped', async (t: TestContext) => {
+    await assertRegistrationAccepts(t, { taskDefaultTtlMs: 120_000, taskMaxTtlMs: 60_000 })
+    await assertRegistrationAccepts(t, { taskDefaultTtlMs: 2 * 3600_000 })
+  })
+
+  test('registration accepts taskDefaultTtlMs equal to taskMaxTtlMs', async (t: TestContext) => {
+    await assertRegistrationAccepts(t, { taskDefaultTtlMs: 60_000, taskMaxTtlMs: 60_000 })
+    await assertRegistrationAccepts(t, { taskDefaultTtlMs: 15 * 60_000, taskMaxTtlMs: 60 * 60_000 })
+  })
+
+  test('registration accepts a taskMaxTtlMs below the default taskDefaultTtlMs', async (t: TestContext) => {
+    // An unset taskDefaultTtlMs is capped by the ceiling, as before
+    await assertRegistrationAccepts(t, { taskMaxTtlMs: 30_000 })
+  })
+
+  test('registration fails before connecting to Redis', async (t: TestContext) => {
+    // An unreachable port: had the plugin connected first, registration would hang or leak
+    await assertRegistrationRejects(t, { redis: { host: '127.0.0.1', port: 1, lazyConnect: true }, taskLeaseMs: 0 } as any, /^taskLeaseMs must be/)
+  })
+})