diff --git a/nanobot/providers/openai_compat_provider.py b/nanobot/providers/openai_compat_provider.py
index 4254a0323..37fce20d0 100644
--- a/nanobot/providers/openai_compat_provider.py
+++ b/nanobot/providers/openai_compat_provider.py
@@ -523,9 +523,6 @@ class OpenAICompatProvider(LLMProvider):
         self._proxy = proxy or None
         self._native_compaction_available = True
 
-        if api_key and spec and spec.env_key:
-            self._setup_env(api_key, api_base)
-
         effective_base = api_base or (spec.default_api_base if spec else None) or None
         self._effective_base = effective_base
         self._default_headers = {"x-session-affinity": uuid.uuid4().hex}
@@ -618,20 +615,6 @@ class OpenAICompatProvider(LLMProvider):
                 raise RuntimeError("OpenAI client initialization did not produce a client")
             return self._client
 
-    def _setup_env(self, api_key: str, api_base: str | None) -> None:
-        """Set environment variables based on provider spec."""
-        spec = self._spec
-        if not spec or not spec.env_key:
-            return
-        if spec.is_gateway:
-            os.environ[spec.env_key] = api_key
-        else:
-            os.environ.setdefault(spec.env_key, api_key)
-        effective_base = api_base or spec.default_api_base
-        for env_name, env_val in spec.env_extras:
-            resolved = env_val.replace("{api_key}", api_key).replace("{api_base}", effective_base)
-            os.environ.setdefault(env_name, resolved)
-
     @classmethod
     def _apply_cache_control(
         cls,
diff --git a/tests/providers/test_provider_env_isolation.py b/tests/providers/test_provider_env_isolation.py
new file mode 100644
index 000000000..c90f2efbb
--- /dev/null
+++ b/tests/providers/test_provider_env_isolation.py
@@ -0,0 +1,48 @@
+"""Provider credentials must not leak through process-global os.environ."""
+
+from __future__ import annotations
+
+import os
+
+from nanobot.providers.openai_compat_provider import OpenAICompatProvider
+from nanobot.providers.registry import find_by_name
+
+
+def test_provider_init_does_not_mutate_shared_env_keys(monkeypatch) -> None:
+    """Multi-provider setups must not overwrite or pin each other's keys."""
+    monkeypatch.delenv("OPENAI_API_KEY", raising=False)
+    monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
+
+    openai_spec = find_by_name("openai")
+    openrouter_spec = find_by_name("openrouter")
+    assert openai_spec is not None and openrouter_spec is not None
+
+    OpenAICompatProvider(
+        api_key="sk-openai-secret",
+        default_model="gpt-4o",
+        spec=openai_spec,
+    )
+    OpenAICompatProvider(
+        api_key="sk-or-secret",
+        default_model="openrouter/auto",
+        spec=openrouter_spec,
+        api_base="https://openrouter.ai/api/v1",
+    )
+
+    assert "OPENAI_API_KEY" not in os.environ
+    assert "OPENROUTER_API_KEY" not in os.environ
+
+
+def test_provider_init_preserves_preexisting_env_keys(monkeypatch) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "preexisting-user-key")
+
+    openai_spec = find_by_name("openai")
+    assert openai_spec is not None
+    provider = OpenAICompatProvider(
+        api_key="sk-from-config",
+        default_model="gpt-4o",
+        spec=openai_spec,
+    )
+
+    assert os.environ["OPENAI_API_KEY"] == "preexisting-user-key"
+    assert provider._api_key_for_client == "sk-from-config"