diff --git a/CHANGELOG.md b/CHANGELOG.md
index 60c16f8..c717cc6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ the GitHub Release body, so a release with no entry here fails.
 
 Versioning follows [docs/versioning.md](docs/versioning.md).
 
+## [0.11.2] - 2026-09-25
+
+### Fixed
+
+- `OpenAIClient` and `OpenAIResponsesClient` now treat an empty `api_key` as unset and fall back to `OPENAI_API_KEY`, matching the behavior of `None`.
+
 ## [0.11.1] - 2026-09-24
 
 ### Added
diff --git a/agent_core/providers/openai_chat.py b/agent_core/providers/openai_chat.py
index 2c9af0a..62e6c3e 100644
--- a/agent_core/providers/openai_chat.py
+++ b/agent_core/providers/openai_chat.py
@@ -180,7 +180,10 @@ def __init__(
         # (see ``mirror_session_query``) — mirror a construction-time session
         # header into ``default_query`` so it rides every request's URL.
         self._client = AsyncOpenAI(
-            api_key=api_key,
+            # The SDK consults OPENAI_API_KEY only for ``None``; an empty
+            # string (a config read before the environment was populated)
+            # raises "Missing credentials" even with the variable set.
+            api_key=api_key or None,
             base_url=base_url or None,
             timeout=timeout,
             default_headers=default_headers,
diff --git a/agent_core/providers/openai_responses.py b/agent_core/providers/openai_responses.py
index 4f26a37..3e3d029 100644
--- a/agent_core/providers/openai_responses.py
+++ b/agent_core/providers/openai_responses.py
@@ -71,7 +71,10 @@ def __init__(
         self._reasoning = reasoning or None
         self._store = store
         self._client = AsyncOpenAI(
-            api_key=api_key,
+            # The SDK consults OPENAI_API_KEY only for ``None``; an empty
+            # string (a config read before the environment was populated)
+            # raises "Missing credentials" even with the variable set.
+            api_key=api_key or None,
             base_url=base_url or None,
             timeout=timeout,
             default_headers=default_headers,
diff --git a/pyproject.toml b/pyproject.toml
index 5522835..65ce5ec 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "apodex-agent-core"
-version = "0.11.1"
+version = "0.11.2"
 description = "Shared, product-neutral runtime primitives for Apodex agents"
 readme = "README.md"
 license = "Apache-2.0"
diff --git a/tests/test_openai_empty_api_key.py b/tests/test_openai_empty_api_key.py
new file mode 100644
index 0000000..198ea6a
--- /dev/null
+++ b/tests/test_openai_empty_api_key.py
@@ -0,0 +1,28 @@
+"""An empty ``api_key`` must fall back to ``OPENAI_API_KEY`` like ``None``."""
+
+from __future__ import annotations
+
+import pytest
+
+from agent_core.providers.openai_chat import OpenAIClient
+from agent_core.providers.openai_responses import OpenAIResponsesClient
+
+
+@pytest.mark.parametrize("client_cls", [OpenAIClient, OpenAIResponsesClient])
+def test_empty_api_key_uses_the_environment(
+    client_cls: type, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "from-env")
+
+    client = client_cls("gpt-test", api_key="")
+
+    assert client._client.api_key == "from-env"
+
+
+@pytest.mark.parametrize("client_cls", [OpenAIClient, OpenAIResponsesClient])
+def test_explicit_api_key_wins(client_cls: type, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "from-env")
+
+    client = client_cls("gpt-test", api_key="explicit")
+
+    assert client._client.api_key == "explicit"
diff --git a/uv.lock b/uv.lock
index cf914dd..459ea30 100644
--- a/uv.lock
+++ b/uv.lock
@@ -50,7 +50,7 @@ wheels = [
 
 [[package]]
 name = "apodex-agent-core"
-version = "0.11.1"
+version = "0.11.2"
 source = { editable = "." }
 dependencies = [
     { name = "anthropic", extra = ["bedrock"] },