diff --git a/tests/code-generator.test.ts b/tests/code-generator.test.ts
new file mode 100644
index 0000000..4b3b2d4
--- /dev/null
+++ b/tests/code-generator.test.ts
@@ -0,0 +1,87 @@
+import {describe, expect, it} from 'vitest';
+import {generateCode} from '../src/generators/code-generator';
+import type {TransformedSchema} from '../src/types';
+
+describe('code generator', () => {
+  it('handles models without relationships and unknown type strings', () => {
+    const schema: TransformedSchema = {
+      enums: [],
+      models: [
+        {
+          tableName: 'Weird',
+          originalTableName: null,
+          modelName: 'Weird',
+          zeroTableName: 'weirdTable',
+          columns: {
+            id: {type: 'Custom()', isOptional: false, mappedName: null},
+          },
+          relationships: undefined as any,
+          primaryKey: ['id'],
+        },
+      ],
+    };
+
+    const output = generateCode(schema);
+
+    expect(output).toContain('export const weirdTable = table("Weird")');
+    expect(output).toContain('export const schema = createSchema({');
+    expect(output).not.toContain('relationships(');
+    expect(output).toContain(
+      'import {\n  createBuilder,\n  createSchema,\n  table,\n} from "@rocicorp/zero";',
+    );
+  });
+
+  it('omits relationships section when none exist', () => {
+    const schema: TransformedSchema = {
+      enums: [],
+      models: [],
+    };
+
+    const output = generateCode(schema);
+
+    expect(output).toContain('tables: [\n  ],');
+    expect(output).not.toContain('relationships: [');
+  });
+
+  it('only includes relationship entries for models that define them', () => {
+    const schema: TransformedSchema = {
+      enums: [],
+      models: [
+        {
+          tableName: 'WithRel',
+          originalTableName: null,
+          modelName: 'WithRel',
+          zeroTableName: 'withRelTable',
+          columns: {
+            id: {type: 'string()', isOptional: false, mappedName: null},
+          },
+          relationships: {
+            plain: {
+              type: 'one',
+              sourceField: ['id'],
+              destField: ['id'],
+              destSchema: 'plainTable',
+            },
+          },
+          primaryKey: ['id'],
+        },
+        {
+          tableName: 'Plain',
+          originalTableName: null,
+          modelName: 'Plain',
+          zeroTableName: 'plainTable',
+          columns: {
+            id: {type: 'string()', isOptional: false, mappedName: null},
+          },
+          relationships: {},
+          primaryKey: ['id'],
+        },
+      ],
+    };
+
+    const output = generateCode(schema);
+
+    expect(output).toContain('withRelTableRelationships');
+    expect(output).not.toContain('plainTableRelationships');
+  });
+});
diff --git a/tests/generator-config.test.ts b/tests/generator-config.test.ts
new file mode 100644
index 0000000..ece65dc
--- /dev/null
+++ b/tests/generator-config.test.ts
@@ -0,0 +1,169 @@
+import type {DMMF, GeneratorOptions} from '@prisma/generator-helper';
+import {beforeEach, describe, expect, it, vi} from 'vitest';
+import {createEnum, createField, createMockDMMF, createModel} from './utils';
+
+vi.mock('fs/promises', () => ({
+  writeFile: vi.fn(),
+  mkdir: vi.fn(),
+  readFile: vi.fn(),
+}));
+
+function createTestOptions(dmmf: DMMF.Document): GeneratorOptions {
+  return {
+    generator: {
+      output: {value: 'generated', fromEnvVar: null},
+      name: 'test-generator',
+      config: {},
+      provider: {value: 'test-provider', fromEnvVar: null},
+      binaryTargets: [],
+      previewFeatures: [],
+      sourceFilePath: '',
+    },
+    dmmf,
+    schemaPath: '',
+    datasources: [],
+    otherGenerators: [],
+    version: '0.0.0',
+    datamodel: '',
+  };
+}
+
+describe('Generator configuration', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    vi.clearAllMocks();
+    vi.unmock('prettier');
+    vi.unmock('@prisma/generator-helper');
+  });
+
+  it('throws when output directory is missing', async () => {
+    const {onGenerate} = await import('../src/generator');
+
+    const options = createTestOptions(createMockDMMF([]));
+    (options.generator as any).output = undefined;
+
+    await expect(onGenerate(options)).rejects.toThrow(
+      'Output directory is required',
+    );
+  });
+
+  it('formats output with prettier when enabled', async () => {
+    const format = vi.fn(
+      async (code: string, options?: unknown) =>
+        `formatted:${code}-${JSON.stringify(options)}`,
+    );
+    const resolveConfig = vi.fn(async () => ({semi: false}));
+
+    vi.doMock('prettier', () => ({
+      format,
+      resolveConfig,
+    }));
+
+    const {onGenerate} = await import('../src/generator');
+    const options = createTestOptions(
+      createMockDMMF(
+        [
+          createModel('User', [
+            createField('id', 'String', {isId: true}),
+            createField('role', 'Role', {kind: 'enum'}),
+          ]),
+        ],
+        [createEnum('Role', ['ADMIN'])],
+      ),
+    );
+    options.generator.config.prettier = 'true';
+
+    await onGenerate(options);
+
+    expect(resolveConfig).toHaveBeenCalledWith('schema.ts');
+    expect(format).toHaveBeenCalled();
+    const formatOptions = format.mock.calls[0]?.[1] as Record<string, unknown>;
+    expect(formatOptions?.parser).toBe('typescript');
+  });
+
+  it('throws a helpful error when prettier cannot be loaded', async () => {
+    vi.doMock('prettier', () => {
+      throw new Error('module not found');
+    });
+
+    const {onGenerate} = await import('../src/generator');
+    const options = createTestOptions(createMockDMMF([]));
+    options.generator.config.prettier = 'true';
+
+    await expect(onGenerate(options)).rejects.toThrow(
+      '⚠️  prisma-zero: prettier could not be found. Install it locally with\n  npm i -D prettier',
+    );
+  });
+
+  it('skips resolving prettier config when disabled', async () => {
+    const format = vi.fn(async (code: string) => code);
+    const resolveConfig = vi.fn();
+
+    vi.doMock('prettier', () => ({
+      format,
+      resolveConfig,
+    }));
+
+    const {onGenerate} = await import('../src/generator');
+    const options = createTestOptions(createMockDMMF([]));
+    options.generator.config.prettier = 'true';
+    options.generator.config.resolvePrettierConfig = 'false';
+
+    await onGenerate(options);
+
+    expect(resolveConfig).not.toHaveBeenCalled();
+    expect(format).toHaveBeenCalled();
+  });
+
+  it('validates excludeTables configuration', async () => {
+    const {onGenerate} = await import('../src/generator');
+    const options = createTestOptions(createMockDMMF([]));
+    options.generator.config.excludeTables = 'posts' as any;
+
+    await expect(onGenerate(options)).rejects.toThrow(
+      'excludeTables must be an array',
+    );
+  });
+
+  it('passes excludeTables through to schema transformation', async () => {
+    const mapperModule = await import('../src/mappers/schema-mapper');
+    const transformSpy = vi.spyOn(mapperModule, 'transformSchema');
+    const {onGenerate} = await import('../src/generator');
+
+    const options = createTestOptions(
+      createMockDMMF([
+        createModel('User', [
+          createField('id', 'String', {isId: true}),
+          createField('name', 'String'),
+        ]),
+      ]),
+    );
+    options.generator.config.excludeTables = ['User'];
+
+    await onGenerate(options);
+
+    expect(transformSpy).toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({excludeTables: ['User']}),
+    );
+  });
+
+  it('exposes manifest information through generatorHandler', async () => {
+    const handler = vi.fn();
+    vi.doMock('@prisma/generator-helper', () => ({
+      default: {generatorHandler: handler},
+      generatorHandler: handler,
+    }));
+
+    const {version} = await import('../package.json');
+    await import('../src/generator');
+
+    expect(handler).toHaveBeenCalledTimes(1);
+    const manifest = handler.mock.calls[0]?.[0]?.onManifest?.();
+    expect(manifest).toEqual({
+      version,
+      defaultOutput: 'generated/zero',
+      prettyName: 'Zero Schema',
+    });
+  });
+});
diff --git a/tests/schema-mapper.test.ts b/tests/schema-mapper.test.ts
index 08eb7ed..d4908a3 100644
--- a/tests/schema-mapper.test.ts
+++ b/tests/schema-mapper.test.ts
@@ -293,6 +293,38 @@ describe('Schema Mapper', () => {
   });
 
   describe('Relationships', () => {
+    it('uses back-references when relationFromFields is only defined on the target', () => {
+      const userModel = createModel('User', [
+        createField('id', 'String', {isId: true}),
+        createField('profile', 'Profile', {
+          relationName: 'UserProfile',
+          kind: 'object',
+        }),
+      ]);
+
+      const profileModel = createModel('Profile', [
+        createField('id', 'String', {isId: true}),
+        createField('userId', 'String'),
+        createField('user', 'User', {
+          relationName: 'UserProfile',
+          kind: 'object',
+          relationFromFields: ['userId'],
+          relationToFields: ['id'],
+        }),
+      ]);
+
+      const dmmf = createMockDMMF([userModel, profileModel]);
+      const result = transformSchema(dmmf, baseConfig);
+
+      const user = result.models.find(m => m.modelName === 'User');
+      expect(user?.relationships.profile).toEqual({
+        sourceField: ['id'],
+        destField: ['userId'],
+        destSchema: 'profileTable',
+        type: 'one',
+      });
+    });
+
     it('should correctly map one-to-many relationship with composite key on parent', () => {
       const parentModel = createModel(
         'Parent',
@@ -480,4 +512,88 @@ describe('Schema Mapper', () => {
       );
     });
   });
+
+  describe('Error handling', () => {
+    it('throws when a model has no primary key', () => {
+      const model = createModel('NoPK', [createField('name', 'String')]);
+
+      expect(() =>
+        transformSchema(createMockDMMF([model]), baseConfig),
+      ).toThrow('No primary key found for NoPK');
+    });
+
+    it('throws when a relationship target model is missing', () => {
+      const model = createModel('Post', [
+        createField('id', 'String', {isId: true}),
+        createField('author', 'User', {
+          relationName: 'Author',
+          kind: 'object',
+        }),
+      ]);
+
+      expect(() =>
+        transformSchema(createMockDMMF([model]), baseConfig),
+      ).toThrow('Target model User not found for relationship author');
+    });
+
+    it('throws when implicit many-to-many models are missing id fields', () => {
+      const postModel = createModel('Post', [
+        createField('id', 'String', {isId: true}),
+        createField('tags', 'Tag', {
+          isList: true,
+          relationName: 'PostTags',
+          kind: 'object',
+        }),
+      ]);
+
+      const tagModel = createModel(
+        'Tag',
+        [
+          createField('label', 'String'),
+          createField('posts', 'Post', {
+            isList: true,
+            relationName: 'PostTags',
+            kind: 'object',
+          }),
+        ],
+        {primaryKey: {fields: ['label'], name: null}},
+      );
+
+      expect(() =>
+        transformSchema(createMockDMMF([postModel, tagModel]), baseConfig),
+      ).toThrow('Implicit relation PostTags: Model Tag has no @id field.');
+    });
+
+    it('throws when implicit many-to-many models have primary keys without @id', () => {
+      const alphaModel = createModel(
+        'Alpha',
+        [
+          createField('key', 'String'),
+          createField('betas', 'Beta', {
+            isList: true,
+            relationName: 'AlphaBeta',
+            kind: 'object',
+          }),
+        ],
+        {primaryKey: {fields: ['key'], name: null}},
+      );
+
+      const betaModel = createModel(
+        'Beta',
+        [
+          createField('key', 'String'),
+          createField('alphas', 'Alpha', {
+            isList: true,
+            relationName: 'AlphaBeta',
+            kind: 'object',
+          }),
+        ],
+        {primaryKey: {fields: ['key'], name: null}},
+      );
+
+      expect(() =>
+        transformSchema(createMockDMMF([alphaModel, betaModel]), baseConfig),
+      ).toThrow('Implicit relation AlphaBeta: Model Alpha has no @id field.');
+    });
+  });
 });