diff --git a/Auto Run Docs/02a-backend-engine.md b/Auto Run Docs/02a-backend-engine.md
index 7c11252..933a620 100644
--- a/Auto Run Docs/02a-backend-engine.md	
+++ b/Auto Run Docs/02a-backend-engine.md	
@@ -4,7 +4,7 @@ Implement the core experiment execution engine: LLM adapters, response caching,
 
 - [x] Implement backend/engine/adapters/base.py defining the BaseAdapter abstract class with methods: complete(prompt, model, params) → AdapterResponse (containing response text, tokens_in, tokens_out, latency_ms), list_models() → list of model identifiers, and test_connection() → bool. Define the AdapterResponse dataclass.
 
-- [ ] Implement backend/engine/adapters/openai_compat.py as the primary adapter. It should work with any OpenAI-compatible API (OpenWebUI, vLLM, Ollama, OpenAI, Anthropic via proxy). Use httpx for async HTTP calls. Support chat completions format with system + user messages. Parse token usage from the response. Handle errors gracefully with retries (3 attempts, exponential backoff). Support both streaming and non-streaming modes.
+- [x] Implement backend/engine/adapters/openai_compat.py as the primary adapter. It should work with any OpenAI-compatible API (OpenWebUI, vLLM, Ollama, OpenAI, Anthropic via proxy). Use httpx for async HTTP calls. Support chat completions format with system + user messages. Parse token usage from the response. Handle errors gracefully with retries (3 attempts, exponential backoff). Support both streaming and non-streaming modes.
 
 - [ ] Implement backend/engine/cache.py with the ResponseCache layer. Key function: compute_config_hash(prompt, model, params, input_data) → SHA-256 hex string. Methods: get(config_hash) → CachedResponse or None, put(config_hash, response, metadata). In SQLite mode, use the ResponseCache table directly. In Postgres mode, same table but with connection pooling. Include a cache_stats() method returning hit rate, total entries, and storage size.
 
diff --git a/backend/engine/adapters/__init__.py b/backend/engine/adapters/__init__.py
index 5b60247..5d62c03 100644
--- a/backend/engine/adapters/__init__.py
+++ b/backend/engine/adapters/__init__.py
@@ -1,5 +1,6 @@
 """LLM endpoint adapters."""
 
 from engine.adapters.base import AdapterResponse, BaseAdapter
+from engine.adapters.openai_compat import OpenAICompatAdapter
 
-__all__ = ["AdapterResponse", "BaseAdapter"]
+__all__ = ["AdapterResponse", "BaseAdapter", "OpenAICompatAdapter"]
diff --git a/backend/engine/adapters/openai_compat.py b/backend/engine/adapters/openai_compat.py
new file mode 100644
index 0000000..5c55184
--- /dev/null
+++ b/backend/engine/adapters/openai_compat.py
@@ -0,0 +1,248 @@
+"""OpenAI-compatible LLM adapter.
+
+Works with any endpoint that speaks the OpenAI chat completions API:
+OpenAI, OpenWebUI, vLLM, Ollama, Anthropic via proxy, etc.
+"""
+
+import time
+from typing import Any, AsyncIterator
+
+import httpx
+
+from engine.adapters.base import AdapterResponse, BaseAdapter
+
+
+class OpenAICompatAdapter(BaseAdapter):
+    """Adapter for OpenAI-compatible chat completion APIs.
+
+    Args:
+        base_url: API base URL (e.g. "https://api.openai.com/v1").
+        api_key: Bearer token for authentication.  Optional for local endpoints.
+        timeout: Request timeout in seconds.
+        max_retries: Number of retry attempts on transient failures.
+    """
+
+    RETRYABLE_STATUS_CODES = {429, 500, 502, 503, 504}
+
+    def __init__(
+        self,
+        base_url: str,
+        api_key: str | None = None,
+        timeout: float = 120.0,
+        max_retries: int = 3,
+    ) -> None:
+        self.base_url = base_url.rstrip("/")
+        self.api_key = api_key
+        self.timeout = timeout
+        self.max_retries = max_retries
+
+    def _headers(self) -> dict[str, str]:
+        headers: dict[str, str] = {"Content-Type": "application/json"}
+        if self.api_key:
+            headers["Authorization"] = f"Bearer {self.api_key}"
+        return headers
+
+    def _client(self) -> httpx.AsyncClient:
+        return httpx.AsyncClient(
+            timeout=httpx.Timeout(self.timeout),
+            headers=self._headers(),
+        )
+
+    async def complete(
+        self, prompt: str, model: str, params: dict[str, Any]
+    ) -> AdapterResponse:
+        body = self._build_request_body(prompt, model, params)
+        stream = params.get("stream", False)
+        body["stream"] = stream
+
+        if stream:
+            return await self._complete_stream(body, model)
+        return await self._complete_non_stream(body, model)
+
+    async def _complete_non_stream(
+        self, body: dict[str, Any], model: str
+    ) -> AdapterResponse:
+        url = f"{self.base_url}/chat/completions"
+        last_exc: Exception | None = None
+
+        for attempt in range(self.max_retries):
+            t0 = time.perf_counter()
+            try:
+                async with self._client() as client:
+                    resp = await client.post(url, json=body)
+
+                latency_ms = (time.perf_counter() - t0) * 1000
+
+                if resp.status_code == 200:
+                    data = resp.json()
+                    return self._parse_response(data, latency_ms, model)
+
+                if resp.status_code not in self.RETRYABLE_STATUS_CODES:
+                    resp.raise_for_status()
+
+                last_exc = httpx.HTTPStatusError(
+                    f"HTTP {resp.status_code}",
+                    request=resp.request,
+                    response=resp,
+                )
+            except httpx.HTTPStatusError:
+                raise
+            except httpx.HTTPError as exc:
+                last_exc = exc
+
+            if attempt < self.max_retries - 1:
+                delay = 2**attempt  # 1s, 2s, 4s …
+                import asyncio
+
+                await asyncio.sleep(delay)
+
+        raise RuntimeError(
+            f"All {self.max_retries} attempts failed for {url}"
+        ) from last_exc
+
+    async def _complete_stream(
+        self, body: dict[str, Any], model: str
+    ) -> AdapterResponse:
+        url = f"{self.base_url}/chat/completions"
+        last_exc: Exception | None = None
+
+        for attempt in range(self.max_retries):
+            t0 = time.perf_counter()
+            try:
+                async with self._client() as client:
+                    async with client.stream("POST", url, json=body) as resp:
+                        if resp.status_code != 200:
+                            await resp.aread()
+                            if resp.status_code not in self.RETRYABLE_STATUS_CODES:
+                                resp.raise_for_status()
+                            last_exc = httpx.HTTPStatusError(
+                                f"HTTP {resp.status_code}",
+                                request=resp.request,
+                                response=resp,
+                            )
+                        else:
+                            text, usage = await self._consume_stream(resp)
+                            latency_ms = (time.perf_counter() - t0) * 1000
+                            return AdapterResponse(
+                                text=text,
+                                tokens_in=usage.get("prompt_tokens", 0),
+                                tokens_out=usage.get("completion_tokens", 0),
+                                latency_ms=latency_ms,
+                                model=model,
+                                raw={"streamed": True, "usage": usage},
+                            )
+            except httpx.HTTPStatusError:
+                raise
+            except httpx.HTTPError as exc:
+                last_exc = exc
+
+            if attempt < self.max_retries - 1:
+                import asyncio
+
+                await asyncio.sleep(2**attempt)
+
+        raise RuntimeError(
+            f"All {self.max_retries} attempts failed for {url}"
+        ) from last_exc
+
+    async def _consume_stream(
+        self, resp: httpx.Response
+    ) -> tuple[str, dict[str, int]]:
+        import json as _json
+
+        chunks: list[str] = []
+        usage: dict[str, int] = {}
+
+        async for line in resp.aiter_lines():
+            if not line.startswith("data: "):
+                continue
+            payload = line[6:]
+            if payload.strip() == "[DONE]":
+                break
+            try:
+                data = _json.loads(payload)
+            except _json.JSONDecodeError:
+                continue
+
+            for choice in data.get("choices", []):
+                delta = choice.get("delta", {})
+                if "content" in delta and delta["content"]:
+                    chunks.append(delta["content"])
+
+            if "usage" in data and data["usage"]:
+                usage = data["usage"]
+
+        return "".join(chunks), usage
+
+    async def list_models(self) -> list[str]:
+        url = f"{self.base_url}/models"
+        async with self._client() as client:
+            resp = await client.get(url)
+            resp.raise_for_status()
+            data = resp.json()
+
+        models: list[str] = []
+        for entry in data.get("data", []):
+            model_id = entry.get("id")
+            if model_id:
+                models.append(model_id)
+        return sorted(models)
+
+    async def test_connection(self) -> bool:
+        try:
+            await self.list_models()
+            return True
+        except Exception:
+            return False
+
+    @staticmethod
+    def _build_request_body(
+        prompt: str, model: str, params: dict[str, Any]
+    ) -> dict[str, Any]:
+        messages: list[dict[str, str]] = []
+
+        system_message = params.get("system_message") or params.get("system")
+        if system_message:
+            messages.append({"role": "system", "content": system_message})
+
+        messages.append({"role": "user", "content": prompt})
+
+        body: dict[str, Any] = {"model": model, "messages": messages}
+
+        passthrough_keys = {
+            "temperature",
+            "max_tokens",
+            "top_p",
+            "frequency_penalty",
+            "presence_penalty",
+            "stop",
+            "seed",
+        }
+        for key in passthrough_keys:
+            if key in params:
+                body[key] = params[key]
+
+        return body
+
+    @staticmethod
+    def _parse_response(
+        data: dict[str, Any], latency_ms: float, model: str
+    ) -> AdapterResponse:
+        choices = data.get("choices", [])
+        text = ""
+        if choices:
+            message = choices[0].get("message", {})
+            text = message.get("content", "")
+
+        usage = data.get("usage", {})
+        tokens_in = usage.get("prompt_tokens", 0)
+        tokens_out = usage.get("completion_tokens", 0)
+
+        return AdapterResponse(
+            text=text,
+            tokens_in=tokens_in,
+            tokens_out=tokens_out,
+            latency_ms=latency_ms,
+            model=data.get("model", model),
+            raw=data,
+        )
diff --git a/backend/tests/test_openai_compat.py b/backend/tests/test_openai_compat.py
new file mode 100644
index 0000000..3c9b764
--- /dev/null
+++ b/backend/tests/test_openai_compat.py
@@ -0,0 +1,312 @@
+"""Tests for the OpenAI-compatible adapter."""
+
+import json
+import pytest
+import httpx
+
+from engine.adapters.base import AdapterResponse, BaseAdapter
+from engine.adapters.openai_compat import OpenAICompatAdapter
+
+
+# ---------------------------------------------------------------------------
+# Fixtures & helpers
+# ---------------------------------------------------------------------------
+
+FAKE_COMPLETION = {
+    "id": "chatcmpl-test",
+    "object": "chat.completion",
+    "model": "test-model",
+    "choices": [
+        {
+            "index": 0,
+            "message": {"role": "assistant", "content": "Hello back!"},
+            "finish_reason": "stop",
+        }
+    ],
+    "usage": {"prompt_tokens": 5, "completion_tokens": 3, "total_tokens": 8},
+}
+
+FAKE_MODELS = {
+    "object": "list",
+    "data": [
+        {"id": "model-b", "object": "model"},
+        {"id": "model-a", "object": "model"},
+    ],
+}
+
+FAKE_STREAM_LINES = [
+    'data: {"id":"c1","choices":[{"delta":{"role":"assistant"},"index":0}]}\n',
+    'data: {"id":"c1","choices":[{"delta":{"content":"Hi"},"index":0}]}\n',
+    'data: {"id":"c1","choices":[{"delta":{"content":" there"},"index":0}]}\n',
+    'data: {"id":"c1","choices":[{"delta":{},"index":0,"finish_reason":"stop"}],"usage":{"prompt_tokens":4,"completion_tokens":2}}\n',
+    "data: [DONE]\n",
+]
+
+
+@pytest.fixture
+def adapter():
+    return OpenAICompatAdapter(
+        base_url="https://fake.api/v1",
+        api_key="sk-test-key",
+        max_retries=1,
+    )
+
+
+def _mock_response(status_code=200, json_data=None, text=""):
+    """Build an httpx.Response suitable for mocking."""
+    if json_data is not None:
+        content = json.dumps(json_data).encode()
+    else:
+        content = text.encode()
+    return httpx.Response(
+        status_code=status_code,
+        content=content,
+        request=httpx.Request("POST", "https://fake.api/v1/chat/completions"),
+    )
+
+
+# ---------------------------------------------------------------------------
+# Construction & interface
+# ---------------------------------------------------------------------------
+
+
+class TestConstruction:
+    def test_is_base_adapter_subclass(self):
+        assert issubclass(OpenAICompatAdapter, BaseAdapter)
+
+    def test_base_url_trailing_slash_stripped(self):
+        a = OpenAICompatAdapter(base_url="https://api.example.com/v1/")
+        assert a.base_url == "https://api.example.com/v1"
+
+    def test_default_params(self):
+        a = OpenAICompatAdapter(base_url="https://x")
+        assert a.api_key is None
+        assert a.timeout == 120.0
+        assert a.max_retries == 3
+
+    def test_headers_with_api_key(self, adapter):
+        h = adapter._headers()
+        assert h["Authorization"] == "Bearer sk-test-key"
+        assert h["Content-Type"] == "application/json"
+
+    def test_headers_without_api_key(self):
+        a = OpenAICompatAdapter(base_url="http://localhost:11434/v1")
+        h = a._headers()
+        assert "Authorization" not in h
+
+
+# ---------------------------------------------------------------------------
+# Request body building
+# ---------------------------------------------------------------------------
+
+
+class TestBuildRequestBody:
+    def test_basic(self):
+        body = OpenAICompatAdapter._build_request_body(
+            "Hello", "gpt-4", {}
+        )
+        assert body["model"] == "gpt-4"
+        assert body["messages"] == [{"role": "user", "content": "Hello"}]
+
+    def test_with_system_message(self):
+        body = OpenAICompatAdapter._build_request_body(
+            "Hi", "m", {"system_message": "You are helpful."}
+        )
+        assert body["messages"][0] == {
+            "role": "system",
+            "content": "You are helpful.",
+        }
+        assert body["messages"][1]["role"] == "user"
+
+    def test_with_system_alias(self):
+        body = OpenAICompatAdapter._build_request_body(
+            "Hi", "m", {"system": "Be brief."}
+        )
+        assert body["messages"][0]["content"] == "Be brief."
+
+    def test_passthrough_params(self):
+        body = OpenAICompatAdapter._build_request_body(
+            "x", "m", {"temperature": 0.7, "max_tokens": 100, "top_p": 0.9}
+        )
+        assert body["temperature"] == 0.7
+        assert body["max_tokens"] == 100
+        assert body["top_p"] == 0.9
+
+    def test_unknown_params_ignored(self):
+        body = OpenAICompatAdapter._build_request_body(
+            "x", "m", {"custom_thing": 42, "stream": True}
+        )
+        assert "custom_thing" not in body
+        # stream is handled separately, not in passthrough
+        assert "stream" not in body
+
+
+# ---------------------------------------------------------------------------
+# Response parsing
+# ---------------------------------------------------------------------------
+
+
+class TestParseResponse:
+    def test_standard_response(self):
+        resp = OpenAICompatAdapter._parse_response(
+            FAKE_COMPLETION, latency_ms=55.0, model="gpt-4"
+        )
+        assert isinstance(resp, AdapterResponse)
+        assert resp.text == "Hello back!"
+        assert resp.tokens_in == 5
+        assert resp.tokens_out == 3
+        assert resp.latency_ms == 55.0
+        assert resp.model == "test-model"
+        assert resp.raw == FAKE_COMPLETION
+
+    def test_empty_choices(self):
+        data = {**FAKE_COMPLETION, "choices": []}
+        resp = OpenAICompatAdapter._parse_response(data, 10.0, "m")
+        assert resp.text == ""
+
+    def test_missing_usage(self):
+        data = dict(FAKE_COMPLETION)
+        del data["usage"]
+        resp = OpenAICompatAdapter._parse_response(data, 10.0, "m")
+        assert resp.tokens_in == 0
+        assert resp.tokens_out == 0
+
+
+# ---------------------------------------------------------------------------
+# Non-streaming complete (mocked transport)
+# ---------------------------------------------------------------------------
+
+
+class TestCompleteNonStream:
+    @pytest.mark.asyncio
+    async def test_success(self, adapter, monkeypatch):
+        async def fake_post(self_client, url, **kwargs):
+            return _mock_response(200, FAKE_COMPLETION)
+
+        monkeypatch.setattr(httpx.AsyncClient, "post", fake_post)
+
+        resp = await adapter.complete("Hi", "gpt-4", {})
+        assert resp.text == "Hello back!"
+        assert resp.tokens_in == 5
+
+    @pytest.mark.asyncio
+    async def test_non_retryable_error_raises_immediately(
+        self, adapter, monkeypatch
+    ):
+        call_count = 0
+
+        async def fake_post(self_client, url, **kwargs):
+            nonlocal call_count
+            call_count += 1
+            return _mock_response(401)
+
+        monkeypatch.setattr(httpx.AsyncClient, "post", fake_post)
+
+        with pytest.raises(httpx.HTTPStatusError):
+            await adapter.complete("Hi", "m", {})
+        assert call_count == 1
+
+    @pytest.mark.asyncio
+    async def test_retryable_error_exhausts_retries(
+        self, monkeypatch
+    ):
+        a = OpenAICompatAdapter(
+            base_url="https://fake.api/v1",
+            api_key="k",
+            max_retries=2,
+        )
+        call_count = 0
+
+        async def fake_post(self_client, url, **kwargs):
+            nonlocal call_count
+            call_count += 1
+            return _mock_response(503)
+
+        monkeypatch.setattr(httpx.AsyncClient, "post", fake_post)
+        # Patch sleep to avoid real delays
+        monkeypatch.setattr("asyncio.sleep", lambda _: __import__("asyncio").coroutine(lambda: None)())
+
+        import asyncio
+
+        async def fake_sleep(delay):
+            pass
+
+        monkeypatch.setattr(asyncio, "sleep", fake_sleep)
+
+        with pytest.raises(RuntimeError, match="All 2 attempts failed"):
+            await a.complete("Hi", "m", {})
+        assert call_count == 2
+
+    @pytest.mark.asyncio
+    async def test_retry_then_success(self, monkeypatch):
+        a = OpenAICompatAdapter(
+            base_url="https://fake.api/v1",
+            api_key="k",
+            max_retries=3,
+        )
+        call_count = 0
+
+        async def fake_post(self_client, url, **kwargs):
+            nonlocal call_count
+            call_count += 1
+            if call_count < 3:
+                return _mock_response(429)
+            return _mock_response(200, FAKE_COMPLETION)
+
+        monkeypatch.setattr(httpx.AsyncClient, "post", fake_post)
+
+        import asyncio
+
+        async def fake_sleep(delay):
+            pass
+
+        monkeypatch.setattr(asyncio, "sleep", fake_sleep)
+
+        resp = await a.complete("Hi", "m", {})
+        assert resp.text == "Hello back!"
+        assert call_count == 3
+
+
+# ---------------------------------------------------------------------------
+# list_models & test_connection
+# ---------------------------------------------------------------------------
+
+
+class TestListModels:
+    @pytest.mark.asyncio
+    async def test_success(self, adapter, monkeypatch):
+        async def fake_get(self_client, url, **kwargs):
+            return _mock_response(200, FAKE_MODELS)
+
+        monkeypatch.setattr(httpx.AsyncClient, "get", fake_get)
+
+        models = await adapter.list_models()
+        assert models == ["model-a", "model-b"]  # sorted
+
+    @pytest.mark.asyncio
+    async def test_empty_response(self, adapter, monkeypatch):
+        async def fake_get(self_client, url, **kwargs):
+            return _mock_response(200, {"data": []})
+
+        monkeypatch.setattr(httpx.AsyncClient, "get", fake_get)
+
+        models = await adapter.list_models()
+        assert models == []
+
+
+class TestTestConnection:
+    @pytest.mark.asyncio
+    async def test_returns_true_on_success(self, adapter, monkeypatch):
+        async def fake_get(self_client, url, **kwargs):
+            return _mock_response(200, FAKE_MODELS)
+
+        monkeypatch.setattr(httpx.AsyncClient, "get", fake_get)
+        assert await adapter.test_connection() is True
+
+    @pytest.mark.asyncio
+    async def test_returns_false_on_failure(self, adapter, monkeypatch):
+        async def fake_get(self_client, url, **kwargs):
+            raise httpx.ConnectError("refused")
+
+        monkeypatch.setattr(httpx.AsyncClient, "get", fake_get)
+        assert await adapter.test_connection() is False