diff --git a/EVAL_WRITEUP.md b/EVAL_WRITEUP.md
index 1a97dc9..3bbbb04 100644
--- a/EVAL_WRITEUP.md
+++ b/EVAL_WRITEUP.md
@@ -105,18 +105,25 @@ them at all is the obvious next change.
 
 ### Reproducing it
 
+Add to `backend/.env`:
+
+```
+PROVIDER=groq
+GROQ_API_KEY=gsk_...
+GROQ_MODEL=openai/gpt-oss-120b
+```
+
+Then:
+
 ```bash
-printf 'PROVIDER=openai
-OPENAI_API_KEY=gsk_...
-OPENAI_MODEL=openai/gpt-oss-120b
-OPENAI_BASE_URL=https://api.groq.com/openai/v1
-' >> backend/.env
 backend/.venv/Scripts/python -m evals.runner
 ```
 
-`OPENAI_BASE_URL` points the tested OpenAI adapter at any compatible endpoint
-(Groq, Together, OpenRouter, a local server) without touching code. Leave it unset
-for OpenAI itself.
+Groq is a first-class provider reusing the tested OpenAI adapter over its
+OpenAI-compatible endpoint, with its own key and model rather than borrowing
+`OPENAI_API_KEY`. For anything else that speaks the same wire format (Together,
+OpenRouter, a local server), `OPENAI_BASE_URL` points that adapter wherever you
+like without touching code.
 
 ## Honest limitations
 
diff --git a/backend/.env.example b/backend/.env.example
index 3bf6847..4ad79b8 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -2,12 +2,18 @@
 # Copy to `.env` and fill in. Leave the selected provider's key unset to build
 # and test with the offline demo model (no network, no credentials required).
 
-# Model provider: "openai", "anthropic", or "google".
+# Model provider: "openai", "anthropic", "google", or "groq".
 PROVIDER=openai
 
 # OpenAI (used when PROVIDER=openai). The SDK also reads OPENAI_API_KEY directly.
 OPENAI_API_KEY=
 OPENAI_MODEL=gpt-4o
+# Optional: point the OpenAI adapter at any other compatible endpoint.
+OPENAI_BASE_URL=
+
+# Groq (used when PROVIDER=groq; OpenAI-compatible, so it reuses that adapter).
+GROQ_API_KEY=
+GROQ_MODEL=openai/gpt-oss-120b
 
 # Anthropic (used when PROVIDER=anthropic).
 ANTHROPIC_API_KEY=
diff --git a/backend/app/agent/llm.py b/backend/app/agent/llm.py
index edcc0a2..9ccd63d 100644
--- a/backend/app/agent/llm.py
+++ b/backend/app/agent/llm.py
@@ -185,6 +185,16 @@ def build_llm(settings) -> LLMClient:
                 base_url=settings.openai_base_url,
             )
         return DemoLLMClient()
+    if settings.provider == "groq":
+        if settings.groq_api_key:
+            from app.agent.openai_llm import OpenAILLMClient
+
+            return OpenAILLMClient(
+                api_key=settings.groq_api_key,
+                model=settings.groq_model,
+                base_url=settings.groq_base_url,
+            )
+        return DemoLLMClient()
     if settings.provider == "google":
         if settings.google_api_key:
             from app.agent.google_llm import GoogleLLMClient
diff --git a/backend/app/config.py b/backend/app/config.py
index a9431e6..120c8e5 100644
--- a/backend/app/config.py
+++ b/backend/app/config.py
@@ -28,7 +28,7 @@ class Settings(BaseSettings):
     # The agent core is written against an LLMClient seam, so the provider is a
     # config switch. Without the selected provider's key, the app falls back to
     # the offline DemoLLMClient (deterministic, no network).
-    provider: str = "openai"  # "openai" | "anthropic" | "google"
+    provider: str = "openai"  # "openai" | "anthropic" | "google" | "groq"
 
     openai_api_key: str | None = None
     openai_model: str = "gpt-4o"
@@ -37,6 +37,14 @@ class Settings(BaseSettings):
     # that lets a run reach one without editing code.
     openai_base_url: str | None = None
 
+    # Groq speaks the OpenAI wire format, so it reuses the same tested adapter.
+    # It gets its own key and model rather than borrowing OPENAI_API_KEY: a
+    # gsk_ key living under a name that says "openai" is a trap for whoever
+    # reads the file next.
+    groq_api_key: str | None = None
+    groq_model: str = "openai/gpt-oss-120b"
+    groq_base_url: str = "https://api.groq.com/openai/v1"
+
     anthropic_api_key: str | None = None
     anthropic_model: str = "claude-opus-5"
 
@@ -57,6 +65,7 @@ def active_model(self) -> str:
             "openai": self.openai_model,
             "anthropic": self.anthropic_model,
             "google": self.google_model,
+            "groq": self.groq_model,
         }.get(self.provider, self.openai_model)
 
     @property
@@ -66,6 +75,7 @@ def has_credentials(self) -> bool:
                 "openai": self.openai_api_key,
                 "anthropic": self.anthropic_api_key,
                 "google": self.google_api_key,
+                "groq": self.groq_api_key,
             }.get(self.provider)
         )
 
diff --git a/backend/tests/test_agent.py b/backend/tests/test_agent.py
new file mode 100644
index 0000000..e92de2d
--- /dev/null
+++ b/backend/tests/test_agent.py
@@ -0,0 +1,29 @@
+"""Which client the provider switch actually builds."""
+from __future__ import annotations
+
+from app.agent.llm import build_llm
+from app.config import Settings
+
+
+class TestGroqIsItsOwnProvider:
+    """A gsk_ key under a name that says "openai" is a trap for the next reader."""
+
+    def test_groq_builds_the_openai_adapter_against_groqs_endpoint(self):
+        settings = Settings(provider="groq", groq_api_key="gsk_test", _env_file=None)
+        client = build_llm(settings)
+
+        assert type(client).__name__ == "OpenAILLMClient"
+        assert client.model == "openai/gpt-oss-120b"
+
+    def test_without_its_own_key_it_falls_back_to_the_demo_model(self):
+        settings = Settings(
+            provider="groq", groq_api_key=None, openai_api_key="sk-not-this-one",
+            _env_file=None,
+        )
+
+        assert type(build_llm(settings)).__name__ == "DemoLLMClient"
+
+    def test_the_active_model_reports_groqs_model(self):
+        settings = Settings(provider="groq", groq_model="llama-3.3-70b", _env_file=None)
+
+        assert settings.active_model == "llama-3.3-70b"