diff --git a/src/server/plugins/engine/__stubs__/definitions.ts b/src/server/plugins/engine/__stubs__/definitions.ts
new file mode 100644
index 000000000..4762193f8
--- /dev/null
+++ b/src/server/plugins/engine/__stubs__/definitions.ts
@@ -0,0 +1,155 @@
+import {
+  ComponentType,
+  ConditionType,
+  ControllerType,
+  Engine,
+  OperatorName,
+  SchemaVersion,
+  type FormDefinition,
+  type PageQuestion
+} from '@defra/forms-model'
+
+const YES_NO_COMPONENT_ID = 'a7c0242f-2a31-45b2-8c71-ff2ac7f53288'
+const USER_TYPE_LIST_ID = '4fa26e9c-07cf-47cd-a9dd-5cec0dd3f544'
+const EXISTING_USER_ITEM_ID = '55fe0067-d011-4d33-886c-e1aa266637c3'
+
+/**
+ * A minimal, schema-valid V2 definition: one YesNo question page and a
+ * summary page. Each call returns a fresh object, so tests can mutate their
+ * copy freely without affecting each other.
+ */
+export function buildDefinition(): FormDefinition {
+  return {
+    name: 'Stub definition',
+    engine: Engine.V2,
+    schema: SchemaVersion.V2,
+    startPage: '/summary',
+    pages: [
+      {
+        id: '449c053b-9201-4312-9a75-187afc6ba48b',
+        path: '/licence',
+        title: 'Licence',
+        components: [
+          {
+            id: YES_NO_COMPONENT_ID,
+            name: 'xVrYaJ',
+            type: ComponentType.YesNoField,
+            title: 'Do you have a licence?',
+            shortDescription: 'Licence',
+            options: { required: true }
+          }
+        ],
+        next: []
+      },
+      {
+        id: '449c053b-9201-4312-9a75-187afc6ba48c',
+        path: '/summary',
+        title: 'Summary',
+        controller: ControllerType.Summary,
+        components: []
+      }
+    ],
+    lists: [],
+    sections: [],
+    conditions: []
+  }
+}
+
+/**
+ * A definition that passes schema validation but cannot be used by the
+ * engine: a ListItemRef condition points at the YesNoField (boolean), so the
+ * condition expression builder produces an unparseable expression. Distilled
+ * from a real production incident.
+ */
+export function buildBrokenConditionDefinition(): FormDefinition {
+  const definition = buildDefinition()
+
+  definition.name = 'Broken condition fixture'
+
+  const questionPage = definition.pages[0] as PageQuestion
+  // A YesNoField cannot legitimately carry a custom list — that is the
+  // corruption this fixture models — so the TS component types (rightly)
+  // have no `list` property here and a cast is required.
+  ;(questionPage.components[0] as { list?: string }).list = USER_TYPE_LIST_ID
+  definition.lists = [
+    {
+      id: USER_TYPE_LIST_ID,
+      name: 'XtfRYR',
+      title: 'User type list',
+      type: 'string',
+      items: [
+        {
+          id: EXISTING_USER_ITEM_ID,
+          text: 'existing user',
+          value: 'existing user'
+        },
+        {
+          id: '2277c7e5-7fef-46c6-993b-d294116d6d6b',
+          text: 'new user',
+          value: 'new user'
+        }
+      ]
+    }
+  ]
+  definition.conditions = [
+    {
+      id: '3f9d3a35-6dee-4706-806c-3f776129f631',
+      displayName: 'Existing user',
+      items: [
+        {
+          id: '7d7f58ee-c860-4d24-8a13-de5cb9af53d8',
+          componentId: YES_NO_COMPONENT_ID,
+          operator: OperatorName.Is,
+          type: ConditionType.ListItemRef,
+          value: {
+            listId: USER_TYPE_LIST_ID,
+            itemId: [EXISTING_USER_ITEM_ID]
+          }
+        }
+      ]
+    }
+  ]
+
+  return definition
+}
+
+/**
+ * A schema-valid definition whose first page names a controller that is
+ * neither built in nor registered by the host application.
+ */
+export function buildUnknownControllerDefinition(): FormDefinition {
+  const definition = buildDefinition()
+
+  definition.name = 'Unknown controller fixture'
+  definition.pages[0].controller =
+    'NoSuchPageController' as unknown as ControllerType
+
+  return definition
+}
+
+/**
+ * Schema-valid (component types are free strings in the schema), but the
+ * question uses a type the engine has no component class for.
+ */
+export function buildUnknownComponentDefinition(): FormDefinition {
+  const definition = buildDefinition()
+
+  definition.name = 'Unknown component fixture'
+  const questionPage = definition.pages[0] as PageQuestion
+  questionPage.components[0].type = 'MyUnknownField' as unknown as ComponentType
+
+  return definition
+}
+
+/**
+ * A definition that fails schema validation: the question page appears
+ * twice, violating the pages uniqueness rule.
+ */
+export function buildSchemaInvalidDefinition(): FormDefinition {
+  const definition = buildDefinition()
+
+  definition.name = 'Schema invalid fixture'
+  definition.pages.push(buildDefinition().pages[0])
+
+  return definition
+}
diff --git a/src/server/plugins/engine/components/helpers/components.ts b/src/server/plugins/engine/components/helpers/components.ts
index ff10294dc..728fc8616 100644
--- a/src/server/plugins/engine/components/helpers/components.ts
+++ b/src/server/plugins/engine/components/helpers/components.ts
@@ -6,6 +6,7 @@ import { ListFormComponent } from '~/src/server/plugins/engine/components/ListFo
 import { escapeMarkdown } from '~/src/server/plugins/engine/components/helpers/index.js'
 import * as Components from '~/src/server/plugins/engine/components/index.js'
 import { markdown } from '~/src/server/plugins/engine/components/markdownParser.js'
+import { UnknownComponentTypeError } from '~/src/server/plugins/engine/errors.js'
 import { type Translator } from '~/src/server/plugins/engine/i18n/types.js'
 import { type FormState } from '~/src/server/plugins/engine/types.js'
 
@@ -210,7 +211,7 @@ export function createComponent(
   }
 
   if (typeof component === 'undefined') {
-    throw new Error(`Component type ${def.type} does not exist`)
+    throw new UnknownComponentTypeError(def.type)
   }
 
   return component
diff --git a/src/server/plugins/engine/errors.test.ts b/src/server/plugins/engine/errors.test.ts
new file mode 100644
index 000000000..e4a72f073
--- /dev/null
+++ b/src/server/plugins/engine/errors.test.ts
@@ -0,0 +1,130 @@
+import {
+  buildBrokenConditionDefinition,
+  buildSchemaInvalidDefinition,
+  buildUnknownComponentDefinition,
+  buildUnknownControllerDefinition
+} from '~/src/server/plugins/engine/__stubs__/definitions.js'
+import {
+  ConditionBuildError,
+  InvalidFormDefinitionError,
+  SchemaValidationError,
+  UnknownComponentTypeError,
+  UnknownPageControllerError
+} from '~/src/server/plugins/engine/errors.js'
+import { FormModel } from '~/src/server/plugins/engine/models/FormModel.js'
+
+describe('InvalidFormDefinitionError hierarchy', () => {
+  it('ConditionBuildError carries the condition name and cause', () => {
+    const cause = new Error('parse error [1:24]: Expected EOF')
+    const err = new ConditionBuildError('Existing user', { cause })
+
+    expect(err).toBeInstanceOf(InvalidFormDefinitionError)
+    expect(err.name).toBe('ConditionBuildError')
+    expect(err.conditionName).toBe('Existing user')
+    expect(err.message).toBe("Failed to build condition 'Existing user'")
+    expect(err.cause).toBe(cause)
+  })
+
+  it('UnknownPageControllerError carries the controller name', () => {
+    const err = new UnknownPageControllerError('NoSuchPageController')
+
+    expect(err).toBeInstanceOf(InvalidFormDefinitionError)
+    expect(err.name).toBe('UnknownPageControllerError')
+    expect(err.controllerName).toBe('NoSuchPageController')
+    expect(err.message).toBe(
+      'Page controller NoSuchPageController does not exist'
+    )
+  })
+
+  it('UnknownComponentTypeError carries the component type', () => {
+    const err = new UnknownComponentTypeError('NopeField')
+
+    expect(err).toBeInstanceOf(InvalidFormDefinitionError)
+    expect(err.name).toBe('UnknownComponentTypeError')
+    expect(err.componentType).toBe('NopeField')
+    expect(err.message).toBe('Component type NopeField does not exist')
+  })
+})
+
+describe('typed errors thrown from real failure sites', () => {
+  it('FormModel throws ConditionBuildError for an uncompilable condition', () => {
+    const build = () =>
+      new FormModel(buildBrokenConditionDefinition(), { basePath: 'test' })
+
+    expect(build).toThrow(ConditionBuildError)
+
+    let thrown: unknown
+    try {
+      build()
+    } catch (err) {
+      thrown = err
+    }
+
+    const conditionErr = thrown as ConditionBuildError
+    expect(conditionErr.conditionName).toBe('Existing user')
+    expect(conditionErr.cause).toBeInstanceOf(Error)
+    expect((conditionErr.cause as Error).message).toContain('parse error')
+  })
+
+  it('FormModel throws UnknownPageControllerError for an unregistered controller', () => {
+    const build = () =>
+      new FormModel(buildUnknownControllerDefinition(), { basePath: 'test' })
+
+    expect(build).toThrow(UnknownPageControllerError)
+
+    let thrown: unknown
+    try {
+      build()
+    } catch (err) {
+      thrown = err
+    }
+
+    expect((thrown as UnknownPageControllerError).controllerName).toBe(
+      'NoSuchPageController'
+    )
+  })
+})
+
+describe('typed errors thrown from real failure sites (components)', () => {
+  it('FormModel throws UnknownComponentTypeError for an unregistered component type', () => {
+    const build = () =>
+      new FormModel(buildUnknownComponentDefinition(), { basePath: 'test' })
+
+    expect(build).toThrow(UnknownComponentTypeError)
+
+    let thrown: unknown
+    try {
+      build()
+    } catch (err) {
+      thrown = err
+    }
+
+    expect((thrown as UnknownComponentTypeError).componentType).toBe(
+      'MyUnknownField'
+    )
+  })
+})
+
+describe('SchemaValidationError', () => {
+  it('is thrown by FormModel for a schema-invalid definition, wrapping the raw Joi error', () => {
+    const build = () =>
+      new FormModel(buildSchemaInvalidDefinition(), { basePath: 'test' })
+
+    let thrown: unknown
+    try {
+      build()
+    } catch (err) {
+      thrown = err
+    }
+
+    expect(thrown).toBeInstanceOf(SchemaValidationError)
+    expect(thrown).toBeInstanceOf(InvalidFormDefinitionError)
+
+    const schemaError = thrown as SchemaValidationError
+    expect(schemaError.name).toBe('SchemaValidationError')
+    expect(schemaError.message).toContain('Invalid form definition:')
+
+    expect(schemaError.cause.isJoi).toBe(true)
+    expect(Array.isArray(schemaError.cause.details)).toBe(true)
+  })
+})
diff --git a/src/server/plugins/engine/errors.ts b/src/server/plugins/engine/errors.ts
new file mode 100644
index 000000000..493af107c
--- /dev/null
+++ b/src/server/plugins/engine/errors.ts
@@ -0,0 +1,78 @@
+import { type ValidationError } from 'joi'
+
+/**
+ * Base class for errors caused by an invalid form definition — whether it
+ * fails schema validation or passes the schema but cannot be used by the
+ * engine. Never thrown directly — throw a subclass. Consumers (e.g.
+ * forms-runner error pages) detect the family with
+ * `instanceof InvalidFormDefinitionError`.
+ *
+ * When subclassing, remember that the message and the messages of any
+ * `cause` chain are shown to form authors on preview error pages. Only
+ * describe the problem with the form itself — never include sensitive
+ * details such as file paths, configuration values or secrets.
+ */
+export class InvalidFormDefinitionError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options)
+    this.name = 'InvalidFormDefinitionError'
+  }
+}
+
+/**
+ * Thrown when a condition in the definition cannot be compiled into an
+ * evaluatable expression (e.g. it references the wrong component or list).
+ */
+export class ConditionBuildError extends InvalidFormDefinitionError {
+  public readonly conditionName: string
+
+  constructor(conditionName: string, options?: ErrorOptions) {
+    super(`Failed to build condition '${conditionName}'`, options)
+    this.name = 'ConditionBuildError'
+    this.conditionName = conditionName
+  }
+}
+
+/**
+ * Thrown when a page names a controller that is neither built in nor
+ * registered by the host application.
+ */
+export class UnknownPageControllerError extends InvalidFormDefinitionError {
+  public readonly controllerName: string
+
+  constructor(controllerName: string) {
+    super(`Page controller ${controllerName} does not exist`)
+    this.name = 'UnknownPageControllerError'
+    this.controllerName = controllerName
+  }
+}
+
+/**
+ * Thrown when a component declares a type with no registered implementation.
+ */
+export class UnknownComponentTypeError extends InvalidFormDefinitionError {
+  public readonly componentType: string
+
+  constructor(componentType: string) {
+    super(`Component type ${componentType} does not exist`)
+    this.name = 'UnknownComponentTypeError'
+    this.componentType = componentType
+  }
+}
+
+/**
+ * Thrown when a form definition fails Joi schema validation. The raw
+ * ValidationError is preserved as `cause` so consumers can reach the
+ * per-field details; the message carries Joi's own summary so log lines
+ * remain diagnostic for consumers that only read `error.message`.
+ */
+export class SchemaValidationError extends InvalidFormDefinitionError {
+  // Error types `cause` as unknown; this constructor only accepts a Joi
+  // ValidationError, so narrow the declaration for consumers (type-only).
+  declare cause: ValidationError
+
+  constructor(cause: ValidationError) {
+    super(`Invalid form definition: ${cause.message}`, { cause })
+    this.name = 'SchemaValidationError'
+  }
+}
diff --git a/src/server/plugins/engine/models/FormModel.test.ts b/src/server/plugins/engine/models/FormModel.test.ts
index 76574e1cc..d3084e2b7 100644
--- a/src/server/plugins/engine/models/FormModel.test.ts
+++ b/src/server/plugins/engine/models/FormModel.test.ts
@@ -9,6 +9,7 @@ import {
 } from '@defra/forms-model'
 
 import { todayAsDateOnly } from '~/src/server/plugins/engine/date-helper.js'
+import { SchemaValidationError } from '~/src/server/plugins/engine/errors.js'
 import { FormModel } from '~/src/server/plugins/engine/models/FormModel.js'
 import { buildFormContextRequest } from '~/src/server/plugins/engine/pageControllers/__stubs__/request.js'
 import { type FormContextRequest } from '~/src/server/plugins/engine/types.js'
@@ -107,13 +108,22 @@ describe('FormModel', () => {
       ).toBeDefined()
     })
 
-    it('throws an error if schema validation fails', () => {
+    it('throws a SchemaValidationError if schema validation fails', () => {
+      const validationError = new Error('Validation error')
       formDefinitionV2Schema.validate = jest.fn().mockReturnValueOnce({
-        error: 'Validation error'
+        error: validationError
       })
 
       expect(() => new FormModel(definitionV2, { basePath: 'test' })).toThrow(
-        'Validation error'
+        SchemaValidationError
+      )
+
+      formDefinitionV2Schema.validate = jest.fn().mockReturnValueOnce({
+        error: validationError
+      })
+
+      expect(() => new FormModel(definitionV2, { basePath: 'test' })).toThrow(
+        'Invalid form definition: Validation error'
       )
     })
 
diff --git a/src/server/plugins/engine/models/FormModel.ts b/src/server/plugins/engine/models/FormModel.ts
index 5bbf07aa5..0309e8826 100644
--- a/src/server/plugins/engine/models/FormModel.ts
+++ b/src/server/plugins/engine/models/FormModel.ts
@@ -39,6 +39,10 @@ import {
   type Component
 } from '~/src/server/plugins/engine/components/helpers/components.js'
 import { todayAsDateOnly } from '~/src/server/plugins/engine/date-helper.js'
+import {
+  ConditionBuildError,
+  SchemaValidationError
+} from '~/src/server/plugins/engine/errors.js'
 import {
   findPage,
   getError,
@@ -124,7 +128,7 @@ export class FormModel {
     const result = schema.validate(def, { abortEarly: false })
 
     if (result.error) {
-      throw result.error
+      throw new SchemaValidationError(result.error)
     }
 
     // Make a clone of the shallow copy returned
@@ -292,7 +296,13 @@ export class FormModel {
     })
 
     const { name, displayName, value } = condition
-    const expr = this.toConditionExpression(value, parser)
+
+    let expr
+    try {
+      expr = this.toConditionExpression(value, parser)
+    } catch (cause) {
+      throw new ConditionBuildError(displayName, { cause })
+    }
 
     const fn = (evaluationState: FormState) => {
       const ctx = this.toConditionContext(evaluationState, this.conditions)
diff --git a/src/server/plugins/engine/pageControllers/helpers/pages.ts b/src/server/plugins/engine/pageControllers/helpers/pages.ts
index c87c9b3dd..e30b4fe8d 100644
--- a/src/server/plugins/engine/pageControllers/helpers/pages.ts
+++ b/src/server/plugins/engine/pageControllers/helpers/pages.ts
@@ -4,6 +4,7 @@ import {
   type Page
 } from '@defra/forms-model'
 
+import { UnknownPageControllerError } from '~/src/server/plugins/engine/errors.js'
 import { type FormModel } from '~/src/server/plugins/engine/models/index.js'
 import * as PageControllers from '~/src/server/plugins/engine/pageControllers/index.js'
 
@@ -66,7 +67,7 @@ export function createPage(model: FormModel, pageDef: Page) {
   }
 
   if (typeof controller === 'undefined') {
-    throw new Error(`Page controller ${pageDef.controller} does not exist`)
+    throw new UnknownPageControllerError(pageDef.controller)
   }
 
   return controller