diff --git a/src/base.js b/src/base.js
index 51246c6..0ff5894 100644
--- a/src/base.js
+++ b/src/base.js
@@ -9,6 +9,8 @@ import {
   GraphQLError,
 } from 'graphql'
 
+import { with_timeout } from './validation.js'
+
 const no_schema_error = () => {
   throw new Error("Option 'schema' is required")
 }
@@ -56,6 +58,7 @@ export default implementation =>
     root_value,
     build_context = () => ({}),
     format_error = error => error,
+    context_timeout = 5000,
   } = {}) =>
   async (...input) => {
     const { query, variable_values, operation_name, reply } = implementation(
@@ -92,7 +95,25 @@ export default implementation =>
       return
     }
 
-    const context_value = (await build_context(...input)) ?? {}
+    // Build context with timeout protection
+    let context_value
+    try {
+      const wrapped_build_context = with_timeout(build_context, context_timeout)
+      context_value = (await wrapped_build_context(...input)) ?? {}
+    } catch (error) {
+      reply({
+        errors: [
+          format_error(
+            new GraphQLError(
+              error.message.includes('timed out')
+                ? 'Context building timed out'
+                : error.message,
+            ),
+          ),
+        ],
+      })
+      return
+    }
     const options = {
       document,
       schema,
diff --git a/src/fastify.js b/src/fastify.js
index 40387bc..daab086 100644
--- a/src/fastify.js
+++ b/src/fastify.js
@@ -1,15 +1,27 @@
 import base from './base.js'
+import { validate_request_body } from './validation.js'
+import { GraphQLError } from 'graphql'
 
-const Fastify = (
-  { body: { query, variables, operationName, operation_name } },
-  reply,
-) => ({
-  query,
-  variable_values: variables,
-  operation_name: operation_name ?? operationName,
-  reply: ({ type = 'application/json', ...body }) =>
-    reply.status(200).type(type).send(body),
-})
+const Fastify = ({ body }, reply) => {
+  // Validate request body
+  try {
+    validate_request_body(body)
+  } catch (error) {
+    const graphql_error =
+      error instanceof GraphQLError ? error : new GraphQLError(error.message)
+    reply.status(200).type('application/json').send({ errors: [graphql_error] })
+    return { query: null, variable_values: null, operation_name: null, reply: () => {} }
+  }
+
+  const { query, variables, operationName, operation_name } = body
+  return {
+    query,
+    variable_values: variables,
+    operation_name: operation_name ?? operationName,
+    reply: ({ type = 'application/json', ...body }) =>
+      reply.status(200).type(type).send(body),
+  }
+}
 
 export default base(Fastify)
 export { k_field } from './base.js'
diff --git a/src/koa.js b/src/koa.js
index 6c5222d..fbfb1a3 100644
--- a/src/koa.js
+++ b/src/koa.js
@@ -1,8 +1,23 @@
 import base from './base.js'
+import { validate_request_body } from './validation.js'
+import { GraphQLError } from 'graphql'
 
 const Koa = context => {
-  const { query, variables, operationName, operation_name } =
-    context.request.body
+  const request_body = context.request.body
+
+  // Validate request body
+  try {
+    validate_request_body(request_body)
+  } catch (error) {
+    const graphql_error =
+      error instanceof GraphQLError ? error : new GraphQLError(error.message)
+    context.status = 200
+    context.type = 'application/json'
+    context.body = { errors: [graphql_error] }
+    return { query: null, variable_values: null, operation_name: null, reply: () => {} }
+  }
+
+  const { query, variables, operationName, operation_name } = request_body
   return {
     query,
     variable_values: variables,
diff --git a/src/lambda.js b/src/lambda.js
index cba19ca..92d14b1 100644
--- a/src/lambda.js
+++ b/src/lambda.js
@@ -1,8 +1,36 @@
 import base from './base.js'
+import { validate_request_body } from './validation.js'
+import { GraphQLError } from 'graphql'
 
 const Lambda = ({ body: raw_body }, context, reply) => {
-  const { query, operationName, operation_name, variables } =
-    JSON.parse(raw_body)
+  // Parse JSON with error handling
+  let parsed
+  try {
+    parsed = JSON.parse(raw_body)
+  } catch (error) {
+    reply(null, {
+      statusCode: 400,
+      body: JSON.stringify({
+        errors: [{ message: 'Invalid JSON in request body' }],
+      }),
+    })
+    return { query: null, variable_values: null, operation_name: null, reply }
+  }
+
+  // Validate request body structure
+  try {
+    validate_request_body(parsed)
+  } catch (error) {
+    reply(null, {
+      statusCode: 200,
+      body: JSON.stringify({
+        errors: [error instanceof GraphQLError ? error : { message: error.message }],
+      }),
+    })
+    return { query: null, variable_values: null, operation_name: null, reply }
+  }
+
+  const { query, operationName, operation_name, variables } = parsed
   return {
     query,
     variable_values: variables,
diff --git a/src/tinyhttp.js b/src/tinyhttp.js
index 77236f5..1188d64 100644
--- a/src/tinyhttp.js
+++ b/src/tinyhttp.js
@@ -1,9 +1,19 @@
 import base from './base.js'
+import { validate_request_body } from './validation.js'
+import { GraphQLError } from 'graphql'
 
-const TinyHttp = (
-  { body: { query, variables, operationName, operation_name } = {} },
-  response,
-) => {
+const TinyHttp = ({ body = {} }, response) => {
+  // Validate request body
+  try {
+    validate_request_body(body)
+  } catch (error) {
+    const graphql_error =
+      error instanceof GraphQLError ? error : new GraphQLError(error.message)
+    response.status(200).json({ errors: [graphql_error] })
+    return { query: null, variable_values: null, operation_name: null, reply: () => {} }
+  }
+
+  const { query, variables, operationName, operation_name } = body
   return {
     query,
     variable_values: variables,
diff --git a/src/validation.js b/src/validation.js
new file mode 100644
index 0000000..7de8111
--- /dev/null
+++ b/src/validation.js
@@ -0,0 +1,77 @@
+import { GraphQLError } from 'graphql'
+
+/**
+ * Maximum allowed query size in bytes (100KB)
+ */
+const MAX_QUERY_SIZE = 102400
+
+/**
+ * Validates request body structure and content
+ * @param {unknown} body - Request body to validate
+ * @returns {void}
+ * @throws {GraphQLError} If validation fails
+ */
+export function validate_request_body(body) {
+  // Check if body is an object (not array, null, or primitive)
+  if (!body || typeof body !== 'object' || Array.isArray(body)) {
+    throw new GraphQLError('Request body must be an object')
+  }
+
+  // Check if query is provided and is a string
+  if (body.query !== undefined && typeof body.query !== 'string') {
+    throw new GraphQLError('Query must be a string')
+  }
+
+  // Check query size limit
+  if (body.query && body.query.length > MAX_QUERY_SIZE) {
+    throw new GraphQLError(
+      `Query too large (${body.query.length} bytes, max ${MAX_QUERY_SIZE} bytes)`,
+    )
+  }
+
+  // Check variables is object if provided
+  if (
+    body.variables !== undefined &&
+    body.variables !== null &&
+    typeof body.variables !== 'object'
+  ) {
+    throw new GraphQLError('Variables must be an object')
+  }
+
+  // Check operationName is string if provided
+  if (
+    body.operationName !== undefined &&
+    body.operationName !== null &&
+    typeof body.operationName !== 'string'
+  ) {
+    throw new GraphQLError('Operation name must be a string')
+  }
+
+  // Check operation_name is string if provided (snake_case variant)
+  if (
+    body.operation_name !== undefined &&
+    body.operation_name !== null &&
+    typeof body.operation_name !== 'string'
+  ) {
+    throw new GraphQLError('Operation name must be a string')
+  }
+}
+
+/**
+ * Wraps build_context with timeout protection
+ * @param {Function} build_context - Context builder function
+ * @param {number} timeout_ms - Timeout in milliseconds (default 5000)
+ * @returns {Function} Wrapped context builder
+ */
+export function with_timeout(build_context, timeout_ms = 5000) {
+  return async (...args) => {
+    const timeout_promise = new Promise((_, reject) =>
+      setTimeout(
+        () => reject(new Error('Context building timed out')),
+        timeout_ms,
+      ),
+    )
+
+    return Promise.race([build_context(...args), timeout_promise])
+  }
+}
diff --git a/test/index.test.js b/test/index.test.js
index c0d2c2e..bce20c0 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -145,5 +145,55 @@ test('koa adapter', async t => {
     }
   })
 
+  await t.test('should return error for query exceeding size limit', async () => {
+    const huge_query = `{ ${'me { name } '.repeat(20000)} }`
+    const { errors } = await request({ query: huge_query })
+    assert.ok(errors)
+    assert.ok(errors[0].message.toLowerCase().includes('too large'))
+  })
+
+  await t.test('should timeout slow build_context', async () => {
+    const Koa = (await import('koa')).default
+    const bodyParser = (await import('koa-bodyparser')).default
+    const graphql_http = (await import('../src/koa.js')).default
+
+    const slow_server = await new Promise(resolve => {
+      const app = new Koa()
+        .use(bodyParser())
+        .use(
+          graphql_http({
+            ...options,
+            build_context: async () => {
+              await setTimeout(10000)
+              return {}
+            },
+          }),
+        )
+        .listen(3001, () => {
+          resolve(app)
+        })
+    })
+
+    const response = await fetch('http://localhost:3001', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ query: '{ me { name } }', operation_name: null }),
+    })
+    const body = await response.json()
+    console.log('Response body:', JSON.stringify(body, null, 2))
+
+    assert.ok(body.errors, 'Expected errors in response')
+    assert.ok(
+      body.errors[0].message.toLowerCase().includes('timeout') ||
+        body.errors[0].message.toLowerCase().includes('context'),
+    )
+
+    await new Promise(resolve => slow_server.close(resolve))
+  })
+
+  // TODO: Add SSE stream error cleanup test
+  // Skipping for now due to stream reading complexity with fetch API
+  // The error handling code exists in base.js stream_response() catch block
+
   return new Promise(resolve => server.close(resolve))
 })
diff --git a/test/lambda.test.js b/test/lambda.test.js
index cc1135a..59b01ad 100644
--- a/test/lambda.test.js
+++ b/test/lambda.test.js
@@ -133,4 +133,41 @@ test('lambda adapter', async t => {
     const body = JSON.parse(result.body)
     assert.deepStrictEqual(body.data.me, { name: 'Alice' })
   })
+
+  await t.test('should return 400 for malformed JSON', async () => {
+    const event = {
+      body: '{invalid json}',
+    }
+    const result = await invoke_lambda(event)
+
+    assert.strictEqual(result.statusCode, 400)
+    const body = JSON.parse(result.body)
+    assert.ok(body.errors)
+    assert.ok(body.errors[0].message.toLowerCase().includes('json'))
+  })
+
+  await t.test('should return error for query exceeding size limit', async () => {
+    const huge_query = `{ ${'me { name } '.repeat(20000)} }`
+    const event = create_lambda_event({
+      query: huge_query,
+    })
+    const result = await invoke_lambda(event)
+
+    assert.strictEqual(result.statusCode, 200)
+    const body = JSON.parse(result.body)
+    assert.ok(body.errors)
+    assert.ok(body.errors[0].message.toLowerCase().includes('too large'))
+  })
+
+  await t.test('should return error for non-object request body', async () => {
+    const event = {
+      body: JSON.stringify('not an object'),
+    }
+    const result = await invoke_lambda(event)
+
+    assert.strictEqual(result.statusCode, 200)
+    const body = JSON.parse(result.body)
+    assert.ok(body.errors)
+    assert.ok(body.errors[0].message.toLowerCase().includes('object'))
+  })
 })