diff --git a/pyproject.toml b/pyproject.toml
index f8e0353..1750cf4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,6 +58,12 @@ allowed-confusables = ["σ"]
 "src/pf1e_simulator/runner.py" = ["PLR0913"]
 # client.py: LLMClient/FakeLLMClient need the full safeguard knobs (cache, budget, pricing, retry, breaker)
 "src/pf1e_simulator/client.py" = ["PLR0913"]
+# provider.py: OpenAILLMClient.__init__ needs the full knob set (model, key,
+# base url, timeout, transport, schema, cache, budget, pricing, retry, breaker)
+# and the chat-completions URL is built from an env-gated base URL defaulting
+# to HTTPS (section 15), so the S310 audit and the 14-param constructor are
+# deliberate
+"src/pf1e_simulator/provider.py" = ["PLR0913", "S310"]
 # robustness.py: the cartography API takes the full knob set (scenario, specs,
 # side, runs, seed, fractions, target, precision, budget) as keyword parameters
 "src/pf1e_simulator/robustness.py" = ["PLR0913"]
diff --git a/src/pf1e_simulator/config.py b/src/pf1e_simulator/config.py
index 03e8d63..50e1348 100644
--- a/src/pf1e_simulator/config.py
+++ b/src/pf1e_simulator/config.py
@@ -1,4 +1,4 @@
-"""Environment-driven configuration for the phase-2 LLM layer (P1).
+"""Environment-driven configuration for the phase-2 LLM layer (P1/P5.1).
 
 Architecture doc section 15: the phase-2 code never hard-codes data or
 configuration paths -- every externally supplied value is resolved here from
@@ -14,9 +14,19 @@ from pathlib import Path
 
 # Environment variable names (documented in the architecture doc, sections
 # 14-15). ``PF1E_PROMPT_PACK_DIR`` points at the external, versioned prompt
-# pack; ``PF1E_LLM_API_KEY`` feeds the remote provider client (P5.1).
+# pack; ``PF1E_LLM_API_KEY`` feeds the remote provider client (P5.1);
+# ``PF1E_LLM_BASE_URL`` and ``PF1E_LLM_MODEL`` select the provider endpoint
+# and model, so a self-hosted OpenAI-compatible gateway (vLLM, llama.cpp...)
+# is configured without touching the code.
 ENV_PROMPT_PACK_DIR = "PF1E_PROMPT_PACK_DIR"
 ENV_LLM_API_KEY = "PF1E_LLM_API_KEY"
+ENV_LLM_BASE_URL = "PF1E_LLM_BASE_URL"
+ENV_LLM_MODEL = "PF1E_LLM_MODEL"
+
+# Sensible offline defaults: the OpenAI-compatible chat endpoint and a small,
+# cheap model. Nothing here is a secret -- the API key stays environment-only.
+DEFAULT_LLM_BASE_URL = "https://api.openai.com/v1"
+DEFAULT_LLM_MODEL = "gpt-4o-mini"
 
 
 def env_str(name: str, *, default: str | None = None) -> str | None:
@@ -34,3 +44,19 @@ def llm_api_key() -> str | None:
     """The provider API key, or None when the caller runs offline."""
     return env_str(ENV_LLM_API_KEY)
 
+
+def llm_base_url() -> str:
+    """The OpenAI-compatible base URL (``PF1E_LLM_BASE_URL``, else default).
+
+    The returned value has no trailing slash; ``OpenAILLMClient`` appends
+    ``/chat/completions`` to it (P5.1).
+    """
+    raw = env_str(ENV_LLM_BASE_URL)
+    return raw.rstrip("/") if raw else DEFAULT_LLM_BASE_URL
+
+
+def llm_model() -> str:
+    """The provider model name (``PF1E_LLM_MODEL``, else default)."""
+    raw = env_str(ENV_LLM_MODEL)
+    return raw or DEFAULT_LLM_MODEL
+
diff --git a/src/pf1e_simulator/provider.py b/src/pf1e_simulator/provider.py
new file mode 100644
index 0000000..e16c0b1
--- /dev/null
+++ b/src/pf1e_simulator/provider.py
@@ -0,0 +1,227 @@
+"""Real OpenAI-compatible chat client for the phase-2 LLM layer (P5.1).
+
+``OpenAILLMClient`` speaks the OpenAI chat-completions wire format over HTTPS
+using only the standard library (``urllib.request``), so the repository ships
+no HTTP dependency. The base URL, model and API key are resolved from the
+environment (:mod:`pf1e_simulator.config`, section 15); the key is never
+embedded in code.
+
+Error taxonomy (shared with :mod:`pf1e_simulator.client`):
+
+- ``ProviderError`` for transient failures (transport errors, HTTP 429, any
+  5xx) -- retried by ``LLMClient``'s backoff loop and counted by the breaker;
+- ``ProviderRejectedError`` for permanent 4xx rejections (bad key, unknown
+  model, quota) -- raised immediately, never retried;
+- ``ProviderResponseError`` for a 2xx body that is not a valid
+  chat-completions payload -- raised immediately, never retried.
+"""
+
+from __future__ import annotations
+
+import json
+import time
+import urllib.request
+from typing import TYPE_CHECKING, Protocol, Self, TypeGuard
+from urllib.error import HTTPError
+from urllib.request import Request
+
+from pf1e_simulator.client import (
+    CircuitBreaker,
+    ClientError,
+    ClientParams,
+    Completion,
+    CostBudget,
+    LLMClient,
+    Pricing,
+    ProviderError,
+    ResponseCache,
+    TokenUsage,
+    estimate_tokens,
+)
+from pf1e_simulator.combat import OBSERVATION_SCHEMA_VERSION
+from pf1e_simulator.config import (
+    ENV_LLM_API_KEY,
+    llm_api_key,
+    llm_base_url,
+    llm_model,
+)
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+# Status-code buckets of the chat-completions wire (RFC 9110): 2xx is a
+# success, 429 and every 5xx mean "try again later" (the only statuses safe
+# to retry), and any other 4xx is a permanent rejection.
+_OK_MIN = 200
+_OK_EXCLUSIVE = 300
+_RATE_LIMITED = 429
+_SERVER_ERROR_MIN = 500
+
+
+class ProviderRejectedError(ClientError):
+    """A permanent 4xx provider rejection - never retried."""
+
+
+class ProviderResponseError(ClientError):
+    """A 2xx body that is not a valid chat-completions payload."""
+
+
+class HttpResponse(Protocol):
+    """The minimal ``urllib.response`` surface the wire call uses."""
+
+    status: int
+
+    def read(self) -> bytes: ...
+
+    def __enter__(self) -> Self: ...
+
+    def __exit__(self, _exc_type: object, _exc_value: object, _traceback: object) -> None: ...
+
+
+def _is_object_dict(value: object) -> TypeGuard[dict[str, object]]:
+    return isinstance(value, dict)
+
+
+def _is_object_list(value: object) -> TypeGuard[list[object]]:
+    return isinstance(value, list)
+
+
+def _parse_completion(payload: dict[str, object]) -> str:
+    """Extract the completion text from a chat-completions payload."""
+    choices_raw = payload.get("choices")
+    if not _is_object_list(choices_raw) or not choices_raw:
+        msg = "provider response has no choices"
+        raise ProviderResponseError(msg)
+    choice = choices_raw[0]
+    if not _is_object_dict(choice):
+        msg = "provider response choice is not a JSON object"
+        raise ProviderResponseError(msg)
+    message_raw = choice.get("message")
+    if not _is_object_dict(message_raw):
+        msg = "provider response message is not a JSON object"
+        raise ProviderResponseError(msg)
+    content = message_raw.get("content")
+    if not isinstance(content, str):
+        msg = "provider response content is not a string"
+        raise ProviderResponseError(msg)
+    return content
+
+
+def _parse_usage(payload: dict[str, object], prompt: str, text: str) -> TokenUsage:
+    """Read the usage block, or estimate tokens when the provider omits it."""
+    usage_raw = payload.get("usage")
+    if not _is_object_dict(usage_raw):
+        return TokenUsage(
+            prompt_tokens=estimate_tokens(prompt),
+            completion_tokens=estimate_tokens(text),
+        )
+    prompt_tokens = usage_raw.get("prompt_tokens")
+    completion_tokens = usage_raw.get("completion_tokens")
+    if not isinstance(prompt_tokens, int) or not isinstance(completion_tokens, int):
+        msg = "provider usage block is malformed"
+        raise ProviderResponseError(msg)
+    return TokenUsage(prompt_tokens=prompt_tokens, completion_tokens=completion_tokens)
+
+
+class OpenAILLMClient(LLMClient):
+    """Real chat-completions client (OpenAI or any compatible gateway).
+
+    Endpoint, model and key default to the environment (section 15); all may
+    be overridden explicitly. ``urlopen`` and ``sleep`` are injectable so
+    tests exercise the full retry/breaker machinery offline.
+    """
+
+    def __init__(
+        self,
+        *,
+        prompt_version: str,
+        model: str | None = None,
+        api_key: str | None = None,
+        base_url: str | None = None,
+        timeout_s: float = 30.0,
+        urlopen: Callable[..., HttpResponse] = urllib.request.urlopen,
+        schema_version: int = OBSERVATION_SCHEMA_VERSION,
+        cache: ResponseCache | None = None,
+        budget: CostBudget | None = None,
+        pricing: Pricing | None = None,
+        retries: int = 2,
+        backoff_base_s: float = 0.25,
+        breaker: CircuitBreaker | None = None,
+        sleep: Callable[[float], None] | None = None,
+    ) -> None:
+        key = api_key if api_key is not None else llm_api_key()
+        if key is None or key == "":
+            msg = (
+                f"no LLM API key: set the {ENV_LLM_API_KEY} environment "
+                "variable or pass an explicit api_key"
+            )
+            raise ValueError(msg)
+        if timeout_s <= 0:
+            msg = f"timeout_s must be > 0, got {timeout_s}"
+            raise ValueError(msg)
+        super().__init__(
+            model=model if model is not None else llm_model(),
+            prompt_version=prompt_version,
+            schema_version=schema_version,
+            cache=cache,
+            budget=budget,
+            pricing=pricing,
+            retries=retries,
+            backoff_base_s=backoff_base_s,
+            sleep=sleep if sleep is not None else time.sleep,
+            breaker=breaker,
+        )
+        resolved_base_url = base_url if base_url is not None else llm_base_url()
+        self._api_key = key
+        self._base_url = resolved_base_url.rstrip("/")
+        self._timeout_s = timeout_s
+        self._urlopen = urlopen
+
+    def _call_once(self, prompt: str, *, params: ClientParams) -> Completion:
+        """One chat-completions attempt; raise ``ProviderError`` when retryable."""
+        body = {
+            "model": self.model,
+            "messages": [{"role": "user", "content": prompt}],
+            "temperature": params.temperature,
+            "max_tokens": params.max_tokens,
+            "top_p": params.top_p,
+            "response_format": {"type": "json_object"},
+        }
+        request = Request(
+            f"{self._base_url}/chat/completions",
+            data=json.dumps(body).encode("utf-8"),
+            headers={
+                "Authorization": f"Bearer {self._api_key}",
+                "Content-Type": "application/json",
+            },
+            method="POST",
+        )
+        try:
+            with self._urlopen(request, timeout=self._timeout_s) as response:
+                status = response.status
+                data = response.read()
+        except HTTPError as exc:
+            status = exc.code
+            data = exc.read()
+        except OSError as exc:
+            msg = f"provider unreachable: {exc}"
+            raise ProviderError(msg) from exc
+
+        if status == _RATE_LIMITED or status >= _SERVER_ERROR_MIN:
+            msg = f"provider transient failure: HTTP {status}"
+            raise ProviderError(msg)
+        if not _OK_MIN <= status < _OK_EXCLUSIVE:
+            msg = f"provider rejected request: HTTP {status}"
+            raise ProviderRejectedError(msg)
+
+        try:
+            payload = json.loads(data.decode("utf-8"))
+        except (UnicodeDecodeError, json.JSONDecodeError) as exc:
+            msg = f"provider response is not valid JSON: {exc}"
+            raise ProviderResponseError(msg) from exc
+        if not _is_object_dict(payload):
+            msg = "provider payload is not a JSON object"
+            raise ProviderResponseError(msg)
+        text = _parse_completion(payload)
+        usage = _parse_usage(payload, prompt, text)
+        return Completion(text=text, usage=usage)
diff --git a/tests/test_phase2_provider.py b/tests/test_phase2_provider.py
new file mode 100644
index 0000000..d5ef59d
--- /dev/null
+++ b/tests/test_phase2_provider.py
@@ -0,0 +1,336 @@
+"""Phase-2 real provider client (docs/phase-2 P5.1, sections 15/16).
+
+``OpenAILLMClient`` (``provider.py``) speaks the OpenAI chat-completions
+wire format over ``urllib.request`` with zero HTTP dependency. These tests
+pin the wire format, the error taxonomy (transient 429/5xx retried, 4xx
+rejected, invalid payload rejected), the env-key resolution, the token
+ledger and the cache behaviour - all through an injectable fake transport,
+never the network. Every key used below is a fake test credential.
+"""
+
+from __future__ import annotations
+
+import io
+import json
+from typing import TYPE_CHECKING, Self
+from urllib.error import HTTPError
+
+import pytest
+
+from pf1e_simulator.client import (
+    RetryExhaustedError,
+    estimate_tokens,
+)
+from pf1e_simulator.config import (
+    ENV_LLM_API_KEY,
+    ENV_LLM_BASE_URL,
+    ENV_LLM_MODEL,
+)
+from pf1e_simulator.provider import (
+    OpenAILLMClient,
+    ProviderRejectedError,
+    ProviderResponseError,
+)
+
+if TYPE_CHECKING:
+    from collections.abc import Sequence
+    from urllib.request import Request
+
+_KEY = "sk-test"
+_ENV_KEY = "sk-env-key"
+_PROMPT_VERSION = "fixture:provider:0"
+_BASE_URL = "https://api.openai.com/v1"
+_MODEL = "gpt-4o-mini"
+
+
+class _FakeResponse:
+    """Minimal ``urllib.response`` double: status + body, context manager."""
+
+    def __init__(self, status: int, body: bytes) -> None:
+        self.status = status
+        self._body = body
+
+    def read(self) -> bytes:
+        return self._body
+
+    def __enter__(self) -> Self:
+        return self
+
+    def __exit__(self, _exc_type: object, _exc_value: object, _traceback: object) -> None:
+        return None
+
+
+class _FakeTransport:
+    """Injectable ``urlopen``: replays a script, keeping the last entry.
+
+    An ``Exception`` entry is raised; provider code sees it exactly as it
+    would see a real transport failure.
+    """
+
+    def __init__(self, script: Sequence[_FakeResponse | Exception]) -> None:
+        self._script = tuple(script)
+        self._calls: list[tuple[Request, float]] = []
+
+    def __call__(self, request: Request, *, timeout: float) -> _FakeResponse:
+        self._calls.append((request, timeout))
+        index = min(len(self._calls) - 1, len(self._script) - 1)
+        entry = self._script[index]
+        if isinstance(entry, Exception):
+            raise entry
+        return entry
+
+    @property
+    def calls(self) -> tuple[tuple[Request, float], ...]:
+        return tuple(self._calls)
+
+
+def _http_error(status: int) -> HTTPError:
+    return HTTPError(
+        f"{_BASE_URL}/chat/completions",
+        status,
+        "simulated HTTP error",
+        hdrs={},
+        fp=io.BytesIO(b"{}"),
+    )
+
+
+def _ok_response(*, content: str = "action json", include_usage: bool = True) -> _FakeResponse:
+    body: dict[str, object] = {
+        "choices": [{"message": {"content": content}}],
+    }
+    if include_usage:
+        body["usage"] = {"prompt_tokens": 12, "completion_tokens": 7}
+    return _FakeResponse(200, json.dumps(body).encode("utf-8"))
+
+
+def _fake_transport(script: Sequence[_FakeResponse | Exception]) -> _FakeTransport:
+    return _FakeTransport(script)
+
+
+def _client(
+    transport: _FakeTransport,
+    *,
+    api_key: str = _KEY,
+    retries: int = 2,
+    timeout_s: float = 30.0,
+    base_url: str = _BASE_URL,
+    model: str = _MODEL,
+) -> OpenAILLMClient:
+    return OpenAILLMClient(
+        prompt_version=_PROMPT_VERSION,
+        model=model,
+        api_key=api_key,
+        base_url=base_url,
+        timeout_s=timeout_s,
+        urlopen=transport,
+        retries=retries,
+        sleep=lambda _: None,
+    )
+
+
+# ── wire format ─────────────────────────────────────────────────────────────
+
+
+def test_wire_format_matches_openai_chat_completions() -> None:
+    """The request is a POST to /v1/chat/completions with the raw body."""
+    transport = _fake_transport((_ok_response(),))
+    client = _client(transport)
+    assert client.complete("hello") == "action json"
+    assert len(transport.calls) == 1
+    request, timeout = transport.calls[0]
+    assert timeout == 30.0
+    assert request.full_url == f"{_BASE_URL}/chat/completions"
+    assert request.method == "POST"
+    assert request.get_header("Authorization") == f"Bearer {_KEY}"
+    # urllib capitalizes header keys via str.capitalize(): "Content-Type" is
+    # stored as "Content-type".
+    assert request.get_header("Content-type") == "application/json"
+    assert request.data is not None
+    body = json.loads(request.data)
+    assert body["model"] == _MODEL
+    assert body["messages"] == [{"role": "user", "content": "hello"}]
+    assert body["temperature"] == 0.0
+    assert body["max_tokens"] == 1024
+    assert body["top_p"] == 1.0
+    assert body["response_format"] == {"type": "json_object"}
+
+
+def test_ledger_records_provider_usage() -> None:
+    """Provider-reported token counts land in the client ledger."""
+    transport = _fake_transport((_ok_response(),))
+    client = _client(transport)
+    client.complete("hello")
+    assert client.stats.calls == 1
+    assert client.stats.prompt_tokens == 12
+    assert client.stats.completion_tokens == 7
+    assert client.stats.cost_usd == 0.0
+    assert client.stats.cache_hits == 0
+    assert client.stats.cache_misses == 1
+
+
+def test_usage_estimated_when_provider_omits_it() -> None:
+    """Without a usage block, tokens are estimated from character counts."""
+    transport = _fake_transport((_ok_response(include_usage=False),))
+    client = _client(transport)
+    client.complete("hello")
+    assert client.stats.prompt_tokens == estimate_tokens("hello")
+    assert client.stats.completion_tokens == estimate_tokens("action json")
+
+
+# ── key resolution ──────────────────────────────────────────────────────────
+
+
+def test_missing_or_empty_api_key_raises(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A missing or empty key is rejected with the env-var name in the error."""
+    monkeypatch.delenv(ENV_LLM_API_KEY, raising=False)
+    with pytest.raises(ValueError, match="PF1E_LLM_API_KEY"):
+        OpenAILLMClient(
+            prompt_version=_PROMPT_VERSION,
+            api_key=None,
+            base_url=_BASE_URL,
+            urlopen=_fake_transport((_ok_response(),)),
+        )
+    with pytest.raises(ValueError, match="PF1E_LLM_API_KEY"):
+        OpenAILLMClient(
+            prompt_version=_PROMPT_VERSION,
+            api_key="",
+            base_url=_BASE_URL,
+            urlopen=_fake_transport((_ok_response(),)),
+        )
+
+
+def test_api_key_read_from_environment(monkeypatch: pytest.MonkeyPatch) -> None:
+    """With no explicit key, PF1E_LLM_API_KEY is used for the bearer token."""
+    monkeypatch.setenv(ENV_LLM_API_KEY, _ENV_KEY)
+    transport = _fake_transport((_ok_response(),))
+    client = OpenAILLMClient(
+        prompt_version=_PROMPT_VERSION,
+        api_key=None,
+        base_url=_BASE_URL,
+        urlopen=transport,
+        sleep=lambda _: None,
+    )
+    client.complete("hello")
+    request, _timeout = transport.calls[0]
+    assert request.get_header("Authorization") == f"Bearer {_ENV_KEY}"
+
+
+# ── error taxonomy and retries ──────────────────────────────────────────────
+
+
+def test_rate_limit_retries_then_succeeds() -> None:
+    """HTTP 429 is transient: with retries=2 the third attempt succeeds."""
+    transport = _fake_transport(
+        (_http_error(429), _http_error(429), _ok_response()),
+    )
+    client = _client(transport, retries=2)
+    assert client.complete("hello") == "action json"
+    assert client.calls == 3
+
+
+def test_transient_oserror_retries_then_exhausts() -> None:
+    """A transport error is retried, then RetryExhaustedError names it."""
+    transport = _fake_transport((TimeoutError("simulated timeout"),))
+    client = _client(transport, retries=1)
+    with pytest.raises(RetryExhaustedError, match="unreachable"):
+        client.complete("hello")
+    assert client.calls == 2
+
+
+def test_permanent_4xx_is_not_retried() -> None:
+    """HTTP 401 is a permanent rejection: one attempt, then raise."""
+    transport = _fake_transport((_http_error(401),))
+    client = _client(transport, retries=3)
+    with pytest.raises(ProviderRejectedError, match="HTTP 401"):
+        client.complete("hello")
+    assert client.calls == 1
+
+
+def test_server_error_retries_then_exhausts() -> None:
+    """HTTP 500 is transient: retries exhausted raises with the status."""
+    transport = _fake_transport((_http_error(500),))
+    client = _client(transport, retries=1)
+    with pytest.raises(RetryExhaustedError, match="HTTP 500"):
+        client.complete("hello")
+    assert client.calls == 2
+
+
+# ── payload validation ──────────────────────────────────────────────────────
+
+
+def test_invalid_json_body_raises_provider_response_error() -> None:
+    """A 2xx body that is not JSON is a permanent response error."""
+    transport = _fake_transport((_FakeResponse(200, b"<html>oops</html>"),))
+    client = _client(transport)
+    with pytest.raises(ProviderResponseError, match="not valid JSON"):
+        client.complete("hello")
+
+
+@pytest.mark.parametrize(
+    ("payload", "message"),
+    [
+        ({"choices": []}, "no choices"),
+        ({"choices": [{}]}, "message"),
+        ({"choices": [{"message": {}}]}, "content"),
+        ([], "not a JSON object"),
+    ],
+)
+def test_malformed_payload_raises_provider_response_error(
+    payload: object,
+    message: str,
+) -> None:
+    """Non-chat-completions payloads are rejected with a stable message."""
+    transport = _fake_transport(
+        (_FakeResponse(200, json.dumps(payload).encode("utf-8")),),
+    )
+    client = _client(transport)
+    with pytest.raises(ProviderResponseError, match=message):
+        client.complete("hello")
+
+
+# ── caching and environment defaults ────────────────────────────────────────
+
+
+def test_cache_serves_second_call_without_provider() -> None:
+    """An identical second call is served from the cache, no wire traffic."""
+    transport = _fake_transport((_ok_response(),))
+    client = _client(transport)
+    assert client.complete("hello") == "action json"
+    assert client.complete("hello") == "action json"
+    assert len(transport.calls) == 1
+    assert client.calls == 1
+    assert client.cache_hits == 1
+    assert client.cache_misses == 1
+
+
+def test_base_url_and_model_read_from_environment(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """PF1E_LLM_BASE_URL/PF1E_LLM_MODEL drive the endpoint and the body."""
+    monkeypatch.setenv(ENV_LLM_BASE_URL, "http://localhost:8080/v1/")
+    monkeypatch.setenv(ENV_LLM_MODEL, "local-model")
+    transport = _fake_transport((_ok_response(),))
+    client = OpenAILLMClient(
+        prompt_version=_PROMPT_VERSION,
+        api_key=_KEY,
+        base_url=None,
+        model=None,
+        urlopen=transport,
+        sleep=lambda _: None,
+    )
+    client.complete("hello")
+    request, _timeout = transport.calls[0]
+    assert request.full_url == "http://localhost:8080/v1/chat/completions"
+    assert request.data is not None
+    assert json.loads(request.data)["model"] == "local-model"
+
+
+def test_invalid_timeout_rejected() -> None:
+    """A non-positive timeout_s is a construction error, not a wire call."""
+    with pytest.raises(ValueError, match="timeout_s"):
+        OpenAILLMClient(
+            prompt_version=_PROMPT_VERSION,
+            api_key=_KEY,
+            base_url=_BASE_URL,
+            timeout_s=0,
+        )