diff --git a/env.example b/env.example
index a5d5c70..0b5b164 100644
--- a/env.example
+++ b/env.example
@@ -7,6 +7,8 @@
 # Republic model format: provider:model_id
 # Default in code is `openrouter:qwen/qwen3-coder-next`.
 # BUB_MODEL=openrouter:qwen/qwen3-coder-next
+# API transport: `completion`, `responses`, or `messages`.
+# BUB_API_FORMAT=completion
 # BUB_MAX_STEPS=50
 # BUB_MAX_TOKENS=1024
 # BUB_MODEL_TIMEOUT_SECONDS=300
diff --git a/src/bub/builtin/agent.py b/src/bub/builtin/agent.py
index 79c8f6d..db47942 100644
--- a/src/bub/builtin/agent.py
+++ b/src/bub/builtin/agent.py
@@ -226,6 +226,7 @@ def _build_llm(settings: AgentSettings, tape_store: AsyncTapeStore) -> LLM:
         settings.model,
         api_key=settings.api_key,
         api_base=settings.api_base,
+        api_format=settings.api_format,
         api_key_resolver=api_key_resolver,
         tape_store=tape_store,
         context=default_tape_context(),
diff --git a/src/bub/builtin/settings.py b/src/bub/builtin/settings.py
index 7accc94..12c609a 100644
--- a/src/bub/builtin/settings.py
+++ b/src/bub/builtin/settings.py
@@ -1,4 +1,5 @@
 import pathlib
+from typing import Literal
 
 from pydantic import Field
 from pydantic_settings import BaseSettings, SettingsConfigDict
@@ -18,6 +19,7 @@ class AgentSettings(BaseSettings):
     model: str = DEFAULT_MODEL
     api_key: str | None = None
     api_base: str | None = None
+    api_format: Literal["completion", "responses", "messages"] = "completion"
     max_steps: int = 50
     max_tokens: int = DEFAULT_MAX_TOKENS
     model_timeout_seconds: int | None = None
diff --git a/tests/test_builtin_settings.py b/tests/test_builtin_settings.py
new file mode 100644
index 0000000..e3f2b46
--- /dev/null
+++ b/tests/test_builtin_settings.py
@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from bub.builtin.settings import AgentSettings
+
+
+def test_agent_settings_reads_api_format_from_env(monkeypatch) -> None:
+    monkeypatch.setenv("BUB_API_FORMAT", "responses")
+
+    settings = AgentSettings()
+
+    assert settings.api_format == "responses"