diff --git a/tests/unit/backends/engine-contract.test.ts b/tests/unit/backends/engine-contract.test.ts
new file mode 100644
index 00000000..c30c1a72
--- /dev/null
+++ b/tests/unit/backends/engine-contract.test.ts
@@ -0,0 +1,153 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { registerBuiltInEngines } from '../../../src/backends/bootstrap.js';
+import {
+	getEngine,
+	getEngineCatalog,
+	getRegisteredEngines,
+} from '../../../src/backends/registry.js';
+
+const EXPECTED_ENGINE_IDS = ['llmist', 'claude-code', 'codex', 'opencode'] as const;
+const KNOWN_AGENT_TYPES = ['implementation', 'review', 'splitting'] as const;
+
+beforeAll(() => {
+	registerBuiltInEngines();
+});
+
+describe('registerBuiltInEngines', () => {
+	it('registers all 4 built-in engines', () => {
+		const registeredIds = getRegisteredEngines();
+		for (const id of EXPECTED_ENGINE_IDS) {
+			expect(registeredIds, `Expected engine "${id}" to be registered`).toContain(id);
+		}
+	});
+
+	it('registers exactly the expected engines', () => {
+		const registeredIds = getRegisteredEngines();
+		for (const id of EXPECTED_ENGINE_IDS) {
+			expect(registeredIds).toContain(id);
+		}
+		expect(registeredIds).toHaveLength(EXPECTED_ENGINE_IDS.length);
+	});
+});
+
+describe.each(EXPECTED_ENGINE_IDS)('engine: %s', (engineId) => {
+	it('is retrievable from the registry', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+	});
+
+	it('has a definition with required fields', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		const { definition } = engine;
+
+		expect(typeof definition.id).toBe('string');
+		expect(definition.id.length).toBeGreaterThan(0);
+
+		expect(typeof definition.label).toBe('string');
+		expect(definition.label.length).toBeGreaterThan(0);
+
+		expect(typeof definition.description).toBe('string');
+		expect(definition.description.length).toBeGreaterThan(0);
+
+		expect(Array.isArray(definition.capabilities)).toBe(true);
+
+		expect(definition.modelSelection).toBeDefined();
+		expect(['free-text', 'select']).toContain(definition.modelSelection.type);
+
+		expect(typeof definition.logLabel).toBe('string');
+		expect(definition.logLabel.length).toBeGreaterThan(0);
+	});
+
+	it("definition.id matches the engine's registry key", () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		expect(engine.definition.id).toBe(engineId);
+	});
+
+	it('has execute as a function', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		expect(typeof engine.execute).toBe('function');
+	});
+
+	it('has supportsAgentType as a function', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		expect(typeof engine.supportsAgentType).toBe('function');
+	});
+
+	it.each(KNOWN_AGENT_TYPES)('supportsAgentType("%s") returns a boolean', (agentType) => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		const result = engine.supportsAgentType(agentType);
+		expect(typeof result).toBe('boolean');
+	});
+
+	it('optional resolveModel is a function if present', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		if (engine.resolveModel !== undefined) {
+			expect(typeof engine.resolveModel).toBe('function');
+		}
+	});
+
+	it('optional beforeExecute is a function if present', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		if (engine.beforeExecute !== undefined) {
+			expect(typeof engine.beforeExecute).toBe('function');
+		}
+	});
+
+	it('optional afterExecute is a function if present', () => {
+		const engine = getEngine(engineId);
+		expect(engine).toBeDefined();
+		if (!engine) return;
+		if (engine.afterExecute !== undefined) {
+			expect(typeof engine.afterExecute).toBe('function');
+		}
+	});
+});
+
+describe('getEngineCatalog', () => {
+	it('returns definitions for all registered engines', () => {
+		const catalog = getEngineCatalog();
+		const catalogIds = catalog.map((def) => def.id);
+
+		for (const id of EXPECTED_ENGINE_IDS) {
+			expect(catalogIds, `Expected catalog to include engine "${id}"`).toContain(id);
+		}
+	});
+
+	it('returns the same definition objects as the registry', () => {
+		const catalog = getEngineCatalog();
+
+		for (const def of catalog) {
+			const engine = getEngine(def.id);
+			expect(engine).toBeDefined();
+			if (!engine) continue;
+			expect(engine.definition).toBe(def);
+		}
+	});
+
+	it('each catalog entry has the required fields', () => {
+		const catalog = getEngineCatalog();
+
+		for (const def of catalog) {
+			expect(typeof def.id).toBe('string');
+			expect(typeof def.label).toBe('string');
+			expect(typeof def.description).toBe('string');
+			expect(Array.isArray(def.capabilities)).toBe(true);
+			expect(def.modelSelection).toBeDefined();
+			expect(typeof def.logLabel).toBe('string');
+		}
+	});
+});