diff --git a/CHANGELOG.md b/CHANGELOG.md
index af2dece18..28daf3c7d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -59,6 +59,14 @@ now read, each on its own labelled line. The URI leads and the name and descript
 a long name cannot push the pointer past the result cap; a server's `title` is shown over its `name`
 when it gives one. A part that already carried text is unchanged.
 
+### The Microsoft Agent Framework Bot answers on a plain OpenAI key
+
+Picked with an OpenAI key, the Microsoft Agent Framework Bot failed every run with "Connection
+error.". Compose writes `OPENAI_BASE_URL` empty when the choice is a plain OpenAI key, and the
+OpenAI SDK only defaults an absent URL, so it was given "" as the address. The Bot now falls back to
+`https://api.openai.com/v1` for an empty value, as its Anthropic branch already did for
+`ANTHROPIC_BASE_URL`. An OpenAI-compatible endpoint is unchanged.
+
 ### Skill selection keeps capabilities named across multiple JSON replies
 
 When a model wraps its skill choice in prose or sends a revised JSON object, OpenBot reads each
diff --git a/agent-microsoft/src/main.py b/agent-microsoft/src/main.py
index 82af5a934..53e7190e8 100644
--- a/agent-microsoft/src/main.py
+++ b/agent-microsoft/src/main.py
@@ -23,7 +23,10 @@ def _client() -> AnthropicClient | OpenAIChatClient:
         # Compose exports missing overrides as ""; the SDK only defaults an absent URL.
         base_url = (os.environ.get("ANTHROPIC_BASE_URL") or "").strip() or "https://api.anthropic.com"
         return AnthropicClient(model=model, base_url=base_url)
-    return OpenAIChatClient(model)
+    # The same "" for OPENAI_BASE_URL, which Compose writes whenever the model screen chose a plain
+    # OpenAI key. The SDK only defaults an absent URL; given "", every request fails to connect.
+    base_url = (os.environ.get("OPENAI_BASE_URL") or "").strip() or "https://api.openai.com/v1"
+    return OpenAIChatClient(model, base_url=base_url)
 
 
 agent = _client().as_agent(
diff --git a/agent-microsoft/tests/test_main.py b/agent-microsoft/tests/test_main.py
index e439493fa..bc0a03372 100644
--- a/agent-microsoft/tests/test_main.py
+++ b/agent-microsoft/tests/test_main.py
@@ -7,6 +7,7 @@
 from pathlib import Path
 
 import httpx
+import httpx2
 import pytest
 import uvicorn
 from fastapi import FastAPI, Request
@@ -217,3 +218,43 @@ async def respond(transport, request):
     assert '"RUN_FINISHED"' in response.text
     assert '"RUN_ERROR"' not in response.text
     assert "hello" in response.text
+
+
+def test_an_openai_key_uses_the_official_endpoint_when_compose_sets_a_blank_url(monkeypatch):
+    # The same "" Compose writes for ANTHROPIC_BASE_URL above, on the other provider: the model screen
+    # sets only the key for a plain OpenAI key, and the SDK treats "" as an address, not as absent.
+    seen = []
+    provider_seen = []
+    provider_app = _provider_app(provider_seen)
+
+    async def respond(transport, request):
+        seen.append(
+            (request.url.scheme, request.url.host, request.url.path, request.headers.get("authorization"))
+        )
+        async with httpx2.ASGITransport(app=provider_app) as local_provider:
+            return await local_provider.handle_async_request(request)
+
+    # Keep the real framework and OpenAI clients; replace only the network transport. The OpenAI SDK
+    # sends through httpx2, not httpx, so patching httpx's transport here would let the request out.
+    monkeypatch.setattr(httpx2.AsyncHTTPTransport, "handle_async_request", respond)
+    monkeypatch.setenv("MANAGED_AGENT_TOKEN", TOKEN)
+    monkeypatch.setenv("BOT_PROVIDER", "openai")
+    monkeypatch.setenv("BOT_MODEL", "gpt-5.5")
+    monkeypatch.setenv("OPENAI_API_KEY", "test-key")
+    monkeypatch.setenv("OPENAI_BASE_URL", "")
+    monkeypatch.setenv("ANTHROPIC_API_KEY", "")
+    monkeypatch.setenv("ANTHROPIC_BASE_URL", "")
+
+    from src import main
+
+    main = importlib.reload(main)
+    response = TestClient(main.app).post(
+        "/", json=RUN, headers={"x-openbot-agent-token": TOKEN}
+    )
+
+    assert seen == [("https", "api.openai.com", "/v1/responses", "Bearer test-key")]
+    assert provider_seen == [("openai", "gpt-5.5")]
+    assert response.status_code == 200
+    assert '"RUN_FINISHED"' in response.text
+    assert '"RUN_ERROR"' not in response.text
+    assert "hello" in response.text