diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..8a979af
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,15 @@
+name: Tests
+on: push
+jobs:
+  default:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 'lts/*'
+          cache: 'npm'
+      - run: npm ci
+      - run: npm test
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..3c45938
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+node_modules/
+*.log
+.DS_Store
diff --git a/package.json b/package.json
index 5657b6d..b41f16f 100644
--- a/package.json
+++ b/package.json
@@ -7,6 +7,9 @@
   "type": "module",
   "main": "index.js",
   "repository": "github:adapt-security/adapt-authoring-errors",
+  "scripts": {
+    "test": "node --test 'tests/**/*.spec.js'"
+  },
   "dependencies": {
     "glob": "^13.0.0"
   },
diff --git a/tests/AdaptError.spec.js b/tests/AdaptError.spec.js
new file mode 100644
index 0000000..70c549c
--- /dev/null
+++ b/tests/AdaptError.spec.js
@@ -0,0 +1,222 @@
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+import AdaptError from '../lib/AdaptError.js'
+
+describe('AdaptError', () => {
+  describe('constructor', () => {
+    it('should create an error with code only', () => {
+      const error = new AdaptError('TEST_ERROR')
+      assert.equal(error.code, 'TEST_ERROR')
+      assert.equal(error.statusCode, 500)
+      assert.deepEqual(error.meta, {})
+      assert.ok(error instanceof Error)
+      assert.ok(error instanceof AdaptError)
+    })
+
+    it('should create an error with code and statusCode', () => {
+      const error = new AdaptError('NOT_FOUND', 404)
+      assert.equal(error.code, 'NOT_FOUND')
+      assert.equal(error.statusCode, 404)
+      assert.deepEqual(error.meta, {})
+    })
+
+    it('should create an error with code, statusCode, and metadata', () => {
+      const metadata = { description: 'Test error', data: { id: '123' } }
+      const error = new AdaptError('CUSTOM_ERROR', 400, metadata)
+      assert.equal(error.code, 'CUSTOM_ERROR')
+      assert.equal(error.statusCode, 400)
+      assert.deepEqual(error.meta, metadata)
+    })
+
+    it('should use default statusCode of 500 when not provided', () => {
+      const error = new AdaptError('SERVER_ERROR')
+      assert.equal(error.statusCode, 500)
+    })
+
+    it('should use empty object as default metadata when not provided', () => {
+      const error = new AdaptError('TEST_ERROR', 400)
+      assert.deepEqual(error.meta, {})
+    })
+
+    it('should set the message property to the error code', () => {
+      const error = new AdaptError('MY_ERROR')
+      assert.equal(error.message, 'MY_ERROR')
+    })
+  })
+
+  describe('#setData()', () => {
+    it('should set data on the error', () => {
+      const error = new AdaptError('TEST_ERROR')
+      const data = { userId: '123', action: 'delete' }
+      error.setData(data)
+      assert.deepEqual(error.data, data)
+    })
+
+    it('should return the error instance for chaining', () => {
+      const error = new AdaptError('TEST_ERROR')
+      const returnValue = error.setData({ test: 'value' })
+      assert.equal(returnValue, error)
+    })
+
+    it('should allow method chaining', () => {
+      const error = new AdaptError('TEST_ERROR')
+      const data = { key: 'value' }
+      const result = error.setData(data)
+      assert.equal(result.data, data)
+      assert.ok(result instanceof AdaptError)
+    })
+
+    it('should overwrite existing data', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData({ first: 'data' })
+      error.setData({ second: 'data' })
+      assert.deepEqual(error.data, { second: 'data' })
+    })
+  })
+
+  describe('#toString()', () => {
+    it('should return formatted string without data', () => {
+      const error = new AdaptError('TEST_ERROR')
+      const result = error.toString()
+      // Note: trailing space after code is part of the current implementation
+      assert.equal(result, 'AdaptError: TEST_ERROR ')
+    })
+
+    it('should return formatted string with data', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData({ userId: '123' })
+      const result = error.toString()
+      assert.equal(result, 'AdaptError: TEST_ERROR {"userId":"123"}')
+    })
+
+    it('should include class name in output', () => {
+      const error = new AdaptError('MY_ERROR')
+      const result = error.toString()
+      assert.ok(result.startsWith('AdaptError:'))
+    })
+
+    it('should handle complex data objects', () => {
+      const error = new AdaptError('COMPLEX_ERROR')
+      const complexData = { nested: { key: 'value' }, array: [1, 2, 3] }
+      error.setData(complexData)
+      const result = error.toString()
+      assert.ok(result.includes(JSON.stringify(complexData)))
+    })
+  })
+
+  describe('Error properties', () => {
+    it('should have code property', () => {
+      const error = new AdaptError('TEST_CODE')
+      assert.equal(typeof error.code, 'string')
+      assert.equal(error.code, 'TEST_CODE')
+    })
+
+    it('should have statusCode property', () => {
+      const error = new AdaptError('TEST_ERROR', 404)
+      assert.equal(typeof error.statusCode, 'number')
+      assert.equal(error.statusCode, 404)
+    })
+
+    it('should have meta property', () => {
+      const meta = { description: 'Test' }
+      const error = new AdaptError('TEST_ERROR', 500, meta)
+      assert.equal(typeof error.meta, 'object')
+      assert.deepEqual(error.meta, meta)
+    })
+
+    it('should have data property after setData is called', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData({ test: 'data' })
+      assert.equal(typeof error.data, 'object')
+      assert.deepEqual(error.data, { test: 'data' })
+    })
+  })
+
+  describe('Error inheritance', () => {
+    it('should have a stack trace', () => {
+      const error = new AdaptError('TEST_ERROR')
+      assert.equal(typeof error.stack, 'string')
+      assert.ok(error.stack.length > 0)
+    })
+
+    it('should be catchable as a generic Error', () => {
+      let caught = false
+      try {
+        throw new AdaptError('THROWN_ERROR', 400)
+      } catch (e) {
+        caught = true
+        assert.ok(e instanceof Error)
+        assert.equal(e.code, 'THROWN_ERROR')
+        assert.equal(e.statusCode, 400)
+      }
+      assert.ok(caught)
+    })
+
+    it('should inherit name from Error', () => {
+      const error = new AdaptError('TEST_ERROR')
+      assert.equal(error.name, 'Error')
+    })
+  })
+
+  describe('setData and throw pattern', () => {
+    it('should support throw with chained setData', () => {
+      const data = { userId: '456' }
+      let caught
+      try {
+        throw new AdaptError('AUTH_ERROR', 401).setData(data)
+      } catch (e) {
+        caught = e
+      }
+      assert.ok(caught instanceof AdaptError)
+      assert.deepEqual(caught.data, data)
+      assert.equal(caught.code, 'AUTH_ERROR')
+    })
+  })
+
+  describe('Edge cases', () => {
+    it('should handle empty string as error code', () => {
+      const error = new AdaptError('')
+      assert.equal(error.code, '')
+      assert.equal(error.message, '')
+    })
+
+    it('should handle null metadata', () => {
+      const error = new AdaptError('TEST_ERROR', 500, null)
+      assert.equal(error.meta, null)
+    })
+
+    it('should handle zero statusCode', () => {
+      const error = new AdaptError('TEST_ERROR', 0)
+      assert.equal(error.statusCode, 0)
+    })
+
+    it('should handle setData with null', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData(null)
+      assert.equal(error.data, null)
+    })
+
+    it('should handle setData with undefined', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData(undefined)
+      assert.equal(error.data, undefined)
+    })
+
+    it('should handle setData with a string value', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData('simple string')
+      assert.equal(error.data, 'simple string')
+    })
+
+    it('should handle toString with empty data object', () => {
+      const error = new AdaptError('TEST_ERROR')
+      error.setData({})
+      assert.equal(error.toString(), 'AdaptError: TEST_ERROR {}')
+    })
+
+    it('should not have data property before setData is called', () => {
+      const error = new AdaptError('TEST_ERROR')
+      assert.equal(error.data, undefined)
+    })
+  })
+})
diff --git a/tests/ErrorsModule.spec.js b/tests/ErrorsModule.spec.js
new file mode 100644
index 0000000..8a75d85
--- /dev/null
+++ b/tests/ErrorsModule.spec.js
@@ -0,0 +1,424 @@
+import { describe, it, beforeEach, afterEach } from 'node:test'
+import assert from 'node:assert/strict'
+import AdaptError from '../lib/AdaptError.js'
+import { readFileSync, writeFileSync, mkdirSync, rmSync } from 'node:fs'
+import { join, dirname } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+
+// Note: ErrorsModule depends on adapt-authoring-core which is not available,
+// so we test the core logic that can be tested independently
+describe('ErrorsModule', () => {
+  let tempDir
+
+  beforeEach(() => {
+    // Create a temporary directory for test errors
+    tempDir = join(__dirname, 'temp-test-errors')
+    mkdirSync(tempDir, { recursive: true })
+    mkdirSync(join(tempDir, 'errors'), { recursive: true })
+  })
+
+  afterEach(() => {
+    // Cleanup temp directory
+    if (tempDir) {
+      rmSync(tempDir, { recursive: true, force: true })
+    }
+  })
+
+  describe('Error definition structure', () => {
+    it('should validate error definition format', () => {
+      const errorDef = {
+        description: 'Test error description',
+        statusCode: 404,
+        data: {
+          id: 'Item identifier'
+        }
+      }
+
+      assert.ok(errorDef.description)
+      assert.equal(typeof errorDef.description, 'string')
+      assert.ok(errorDef.statusCode)
+      assert.equal(typeof errorDef.statusCode, 'number')
+      if (errorDef.data) {
+        assert.equal(typeof errorDef.data, 'object')
+      }
+    })
+
+    it('should create AdaptError from definition', () => {
+      const errorDef = {
+        description: 'Test error',
+        statusCode: 500,
+        data: {
+          field: 'Test field'
+        }
+      }
+
+      const metadata = { description: errorDef.description }
+      if (errorDef.data) {
+        metadata.data = errorDef.data
+      }
+
+      const error = new AdaptError('TEST_CODE', errorDef.statusCode, metadata)
+
+      assert.ok(error instanceof AdaptError)
+      assert.equal(error.code, 'TEST_CODE')
+      assert.equal(error.statusCode, 500)
+      assert.equal(error.meta.description, 'Test error')
+      assert.deepEqual(error.meta.data, { field: 'Test field' })
+    })
+
+    it('should handle error definition without data field', () => {
+      const errorDef = {
+        description: 'Simple error',
+        statusCode: 400
+      }
+
+      const metadata = { description: errorDef.description }
+      const error = new AdaptError('SIMPLE_ERROR', errorDef.statusCode, metadata)
+
+      assert.ok(error.meta.description)
+      assert.equal(error.meta.data, undefined)
+    })
+  })
+
+  describe('Error JSON file format', () => {
+    it('should parse valid error JSON file', () => {
+      const errorDefs = {
+        ERROR_ONE: {
+          description: 'First error',
+          statusCode: 400
+        },
+        ERROR_TWO: {
+          description: 'Second error',
+          statusCode: 404,
+          data: {
+            id: 'Identifier'
+          }
+        }
+      }
+
+      writeFileSync(
+        join(tempDir, 'errors', 'test.json'),
+        JSON.stringify(errorDefs)
+      )
+
+      const content = JSON.parse(
+        readFileSync(join(tempDir, 'errors', 'test.json'), 'utf8')
+      )
+
+      assert.ok(content.ERROR_ONE)
+      assert.ok(content.ERROR_TWO)
+      assert.equal(content.ERROR_ONE.statusCode, 400)
+      assert.equal(content.ERROR_TWO.statusCode, 404)
+    })
+
+    it('should validate error codes are uppercase with underscores', () => {
+      const validCodes = ['TEST_ERROR', 'NOT_FOUND', 'SERVER_ERROR', 'MY_CUSTOM_ERROR']
+
+      validCodes.forEach(code => {
+        assert.ok(/^[A-Z_]+$/.test(code), `${code} should be uppercase with underscores`)
+      })
+    })
+
+    it('should handle multiple error JSON files', () => {
+      writeFileSync(
+        join(tempDir, 'errors', 'set-a.json'),
+        JSON.stringify({ ERR_A: { description: 'Error A', statusCode: 400 } })
+      )
+      writeFileSync(
+        join(tempDir, 'errors', 'set-b.json'),
+        JSON.stringify({ ERR_B: { description: 'Error B', statusCode: 404 } })
+      )
+
+      const contentA = JSON.parse(
+        readFileSync(join(tempDir, 'errors', 'set-a.json'), 'utf8')
+      )
+      const contentB = JSON.parse(
+        readFileSync(join(tempDir, 'errors', 'set-b.json'), 'utf8')
+      )
+
+      const merged = { ...contentA, ...contentB }
+      assert.ok(merged.ERR_A)
+      assert.ok(merged.ERR_B)
+      assert.equal(Object.keys(merged).length, 2)
+    })
+  })
+
+  describe('Error definition examples', () => {
+    it('should demonstrate typical error patterns', () => {
+      const errorPatterns = [
+        { code: 'NOT_FOUND', statusCode: 404, description: 'Resource not found' },
+        { code: 'UNAUTHORIZED', statusCode: 401, description: 'Authentication required' },
+        { code: 'FORBIDDEN', statusCode: 403, description: 'Access denied' },
+        { code: 'BAD_REQUEST', statusCode: 400, description: 'Invalid request' },
+        { code: 'SERVER_ERROR', statusCode: 500, description: 'Internal server error' }
+      ]
+
+      errorPatterns.forEach(pattern => {
+        const error = new AdaptError(pattern.code, pattern.statusCode, { description: pattern.description })
+        assert.equal(error.code, pattern.code)
+        assert.equal(error.statusCode, pattern.statusCode)
+        assert.equal(error.meta.description, pattern.description)
+      })
+    })
+  })
+
+  describe('Error sorting', () => {
+    it('should sort error codes alphabetically', () => {
+      const unsortedCodes = ['ZEBRA_ERROR', 'ALPHA_ERROR', 'MIDDLE_ERROR', 'BETA_ERROR']
+      const sortedCodes = [...unsortedCodes].sort()
+
+      assert.deepEqual(sortedCodes, ['ALPHA_ERROR', 'BETA_ERROR', 'MIDDLE_ERROR', 'ZEBRA_ERROR'])
+    })
+  })
+
+  describe('Error metadata handling', () => {
+    it('should preserve description in metadata', () => {
+      const description = 'A detailed error description'
+      const metadata = { description }
+      const error = new AdaptError('TEST', 500, metadata)
+
+      assert.equal(error.meta.description, description)
+    })
+
+    it('should preserve data schema in metadata', () => {
+      const data = {
+        userId: 'User identifier',
+        action: 'The action being performed'
+      }
+      const metadata = { description: 'Test', data }
+      const error = new AdaptError('TEST', 500, metadata)
+
+      assert.deepEqual(error.meta.data, data)
+    })
+  })
+
+  describe('loadErrors reduction logic', () => {
+    it('should create getter properties that return AdaptError instances', () => {
+      const errorDefs = {
+        TEST_ERR: { description: 'A test', statusCode: 500 }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const error = result.TEST_ERR
+      assert.ok(error instanceof AdaptError)
+      assert.equal(error.code, 'TEST_ERR')
+      assert.equal(error.statusCode, 500)
+      assert.equal(error.meta.description, 'A test')
+    })
+
+    it('should return a new instance on each property access', () => {
+      const errorDefs = {
+        MY_ERROR: { description: 'Repeated', statusCode: 400 }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const first = result.MY_ERROR
+      const second = result.MY_ERROR
+      assert.notEqual(first, second)
+      assert.equal(first.code, second.code)
+    })
+
+    it('should include data in metadata when defined', () => {
+      const errorDefs = {
+        DATA_ERR: { description: 'Has data', statusCode: 404, data: { id: 'Item ID' } }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const error = result.DATA_ERR
+      assert.deepEqual(error.meta.data, { id: 'Item ID' })
+    })
+
+    it('should omit data from metadata when not defined', () => {
+      const errorDefs = {
+        NO_DATA_ERR: { description: 'No data', statusCode: 500 }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const error = result.NO_DATA_ERR
+      assert.equal(error.meta.data, undefined)
+      assert.ok(!('data' in error.meta))
+    })
+
+    it('should sort error codes alphabetically in reduced result', () => {
+      const errorDefs = {
+        ZEBRA: { description: 'Z', statusCode: 500 },
+        ALPHA: { description: 'A', statusCode: 500 },
+        MIDDLE: { description: 'M', statusCode: 500 }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const keys = Object.keys(result)
+      assert.deepEqual(keys, ['ALPHA', 'MIDDLE', 'ZEBRA'])
+    })
+
+    it('should make error properties enumerable', () => {
+      const errorDefs = {
+        ENUM_ERR: { description: 'Enumerable', statusCode: 500 }
+      }
+
+      const result = Object.entries(errorDefs)
+        .sort()
+        .reduce((m, [k, { description, statusCode, data }]) => {
+          return Object.defineProperty(m, k, {
+            get: () => {
+              const metadata = { description }
+              if (data) metadata.data = data
+              return new AdaptError(k, statusCode, metadata)
+            },
+            enumerable: true
+          })
+        }, {})
+
+      const descriptor = Object.getOwnPropertyDescriptor(result, 'ENUM_ERR')
+      assert.equal(descriptor.enumerable, true)
+      assert.equal(typeof descriptor.get, 'function')
+    })
+  })
+
+  describe('Shipped error definitions', () => {
+    it('should have valid node-core error definitions', () => {
+      const nodeCoreErrors = JSON.parse(
+        readFileSync(join(__dirname, '..', 'errors', 'node-core.json'), 'utf8')
+      )
+
+      const expectedCodes = ['EACCES', 'EADDRINUSE', 'ECONNREFUSED', 'EEXIST', 'ENOENT', 'ENOTEMPTY', 'MODULE_NOT_FOUND']
+      expectedCodes.forEach(code => {
+        assert.ok(nodeCoreErrors[code], `Missing expected error code: ${code}`)
+        assert.equal(typeof nodeCoreErrors[code].description, 'string')
+        assert.equal(typeof nodeCoreErrors[code].statusCode, 'number')
+      })
+    })
+
+    it('should have valid adapt error definitions', () => {
+      const adaptErrors = JSON.parse(
+        readFileSync(join(__dirname, '..', 'errors', 'adapt-errors.json'), 'utf8')
+      )
+
+      const expectedCodes = ['FUNC_NOT_OVERRIDDEN', 'FUNC_DISABLED', 'SERVER_ERROR', 'INVALID_PARAMS', 'NOT_FOUND']
+      expectedCodes.forEach(code => {
+        assert.ok(adaptErrors[code], `Missing expected error code: ${code}`)
+        assert.equal(typeof adaptErrors[code].description, 'string')
+        assert.equal(typeof adaptErrors[code].statusCode, 'number')
+      })
+    })
+
+    it('should have valid test error definitions', () => {
+      const testErrors = JSON.parse(
+        readFileSync(join(__dirname, 'data', 'test-errors.json'), 'utf8')
+      )
+
+      assert.ok(testErrors.TEST_ERROR)
+      assert.ok(testErrors.TEST_NOT_FOUND)
+      assert.ok(testErrors.TEST_VALIDATION_ERROR)
+      assert.equal(testErrors.TEST_NOT_FOUND.statusCode, 404)
+      assert.equal(testErrors.TEST_VALIDATION_ERROR.statusCode, 400)
+    })
+
+    it('should use uppercase with underscores for all error codes', () => {
+      const files = ['node-core.json', 'adapt-errors.json']
+      files.forEach(file => {
+        const errors = JSON.parse(
+          readFileSync(join(__dirname, '..', 'errors', file), 'utf8')
+        )
+        Object.keys(errors).forEach(code => {
+          assert.ok(/^[A-Z][A-Z0-9_]*$/.test(code), `Invalid error code format: ${code} in ${file}`)
+        })
+      })
+    })
+  })
+
+  describe('Duplicate error detection', () => {
+    it('should detect duplicate error codes across files', () => {
+      writeFileSync(
+        join(tempDir, 'errors', 'first.json'),
+        JSON.stringify({ DUPLICATE: { description: 'First', statusCode: 500 } })
+      )
+      writeFileSync(
+        join(tempDir, 'errors', 'second.json'),
+        JSON.stringify({ DUPLICATE: { description: 'Second', statusCode: 400 } })
+      )
+
+      const allDefs = {}
+      const duplicates = []
+
+      const files = ['first.json', 'second.json']
+      files.forEach(file => {
+        const contents = JSON.parse(
+          readFileSync(join(tempDir, 'errors', file), 'utf8')
+        )
+        Object.entries(contents).forEach(([k, v]) => {
+          if (allDefs[k]) {
+            duplicates.push(k)
+          } else {
+            allDefs[k] = v
+          }
+        })
+      })
+
+      assert.equal(duplicates.length, 1)
+      assert.equal(duplicates[0], 'DUPLICATE')
+    })
+  })
+})
diff --git a/tests/data/test-errors.json b/tests/data/test-errors.json
new file mode 100644
index 0000000..4cd6f38
--- /dev/null
+++ b/tests/data/test-errors.json
@@ -0,0 +1,21 @@
+{
+  "TEST_ERROR": {
+    "description": "A test error",
+    "statusCode": 500
+  },
+  "TEST_NOT_FOUND": {
+    "description": "Test item not found",
+    "statusCode": 404,
+    "data": {
+      "id": "Item identifier"
+    }
+  },
+  "TEST_VALIDATION_ERROR": {
+    "description": "Test validation failed",
+    "statusCode": 400,
+    "data": {
+      "field": "The invalid field",
+      "value": "The invalid value"
+    }
+  }
+}