diff --git a/tests/lib/test_aws.py b/tests/lib/test_aws.py
index a571f85cf..377fdc3aa 100644
--- a/tests/lib/test_aws.py
+++ b/tests/lib/test_aws.py
@@ -135,6 +135,13 @@ def _clear_aws_url_env(monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.delenv(var, raising=False)
 
 
+@pytest.fixture(autouse=True)
+def isolate_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep AWS settings from the developer's shell out of these tests."""
+    _clear_aws_url_env(monkeypatch)
+    monkeypatch.delenv("ANTHROPIC_AWS_API_KEY", raising=False)
+
+
 def _set_ambient_first_party_credentials(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("ANTHROPIC_API_KEY", "ambient-key")
     monkeypatch.setenv("ANTHROPIC_AUTH_TOKEN", "ambient-token")
diff --git a/tests/lib/test_aws_auth.py b/tests/lib/test_aws_auth.py
index e0e95fcf7..29a22902b 100644
--- a/tests/lib/test_aws_auth.py
+++ b/tests/lib/test_aws_auth.py
@@ -15,6 +15,22 @@
     validate_credentials,
 )
 
+
+@pytest.fixture(autouse=True)
+def isolate_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep AWS settings from the developer's shell out of these tests."""
+    for name in (
+        "AWS_REGION",
+        "AWS_DEFAULT_REGION",
+        "AWS_PROFILE",
+        "AWS_BEARER_TOKEN_BEDROCK",
+        "ANTHROPIC_AWS_API_KEY",
+        "ANTHROPIC_AWS_BASE_URL",
+        "ANTHROPIC_AWS_WORKSPACE_ID",
+    ):
+        monkeypatch.delenv(name, raising=False)
+
+
 # --- validate_credentials ---
 
 
diff --git a/tests/lib/test_bedrock_mantle.py b/tests/lib/test_bedrock_mantle.py
index 6aa1bb393..34c09a0fe 100644
--- a/tests/lib/test_bedrock_mantle.py
+++ b/tests/lib/test_bedrock_mantle.py
@@ -18,6 +18,20 @@
 MODEL = "anthropic.claude-sonnet-4-5"
 
 
+@pytest.fixture(autouse=True)
+def isolate_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep AWS settings from the developer's shell out of these tests."""
+    for name in (
+        "AWS_REGION",
+        "AWS_DEFAULT_REGION",
+        "AWS_PROFILE",
+        "AWS_BEARER_TOKEN_BEDROCK",
+        "ANTHROPIC_AWS_API_KEY",
+        "ANTHROPIC_BEDROCK_MANTLE_BASE_URL",
+    ):
+        monkeypatch.delenv(name, raising=False)
+
+
 class GetAuthHeadersRecorder:
     """Stands in for `anthropic.lib.bedrock._mantle.get_auth_headers` and records each call's kwargs."""