diff --git a/.genignore b/.genignore
index 492fe6c2..a91e004c 100644
--- a/.genignore
+++ b/.genignore
@@ -1,3 +1,4 @@
 pylintrc
 docs/docs.json
 docs/overview.mdx
+src/openrouter/utils/logger.py
diff --git a/src/openrouter/utils/logger.py b/src/openrouter/utils/logger.py
index db55cd38..061ad672 100644
--- a/src/openrouter/utils/logger.py
+++ b/src/openrouter/utils/logger.py
@@ -1,4 +1,9 @@
-"""Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
+"""Debug logging for the SDK.
+
+Originally generated by Speakeasy, now hand-maintained and listed in `.genignore`
+so a regeneration cannot reinstate the `logging.basicConfig` call below — see
+`get_default_logger`.
+"""
 
 import httpx
 import logging
@@ -21,7 +26,21 @@ def get_body_content(req: httpx.Request) -> str:
 
 
 def get_default_logger() -> Logger:
+    """Return the debug logger used when `OPENROUTER_DEBUG` is set.
+
+    Configuration is scoped to the `openrouter` logger. A library must not call
+    `logging.basicConfig`: that sets the level and attaches a handler on the
+    *root* logger, turning on DEBUG output for every library in the process and
+    overriding whatever logging the host application had already set up.
+
+    A `StreamHandler` is only attached when nothing up the hierarchy would emit
+    the records already, so an application that has configured its own handlers
+    keeps them, and repeated client construction does not stack duplicates.
+    """
     if os.getenv("OPENROUTER_DEBUG"):
-        logging.basicConfig(level=logging.DEBUG)
-        return logging.getLogger("openrouter")
+        logger = logging.getLogger("openrouter")
+        logger.setLevel(logging.DEBUG)
+        if not logger.hasHandlers():
+            logger.addHandler(logging.StreamHandler())
+        return logger
     return NoOpLogger()
diff --git a/tests/test_logger.py b/tests/test_logger.py
new file mode 100644
index 00000000..3d28981a
--- /dev/null
+++ b/tests/test_logger.py
@@ -0,0 +1,71 @@
+import json
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+SRC = str(Path(__file__).resolve().parents[1] / "src")
+
+PROBE = """
+import json, logging, sys
+{setup}
+root = logging.getLogger()
+before = (root.level, len(root.handlers))
+
+from openrouter import OpenRouter
+OpenRouter(api_key="x")
+OpenRouter(api_key="x")  # twice: handlers must not stack
+
+sdk = logging.getLogger("openrouter")
+print(json.dumps({{
+    "root_before": before,
+    "root_after": (root.level, len(root.handlers)),
+    "sdk_level": sdk.level,
+    "sdk_handlers": len(sdk.handlers),
+}}))
+"""
+
+
+def _probe(setup="", **env):
+    """Run a fresh interpreter: logging config is process-global."""
+    out = subprocess.run(
+        [sys.executable, "-c", PROBE.format(setup=setup)],
+        capture_output=True,
+        text=True,
+        check=True,
+        env={**os.environ, "PYTHONPATH": SRC, **env},
+    ).stdout
+    return json.loads(out.strip().splitlines()[-1])
+
+
+def test_debug_mode_leaves_the_root_logger_alone():
+    r = _probe(OPENROUTER_DEBUG="1")
+
+    # basicConfig used to set root to DEBUG and attach a StreamHandler to it.
+    assert r["root_after"] == r["root_before"]
+    assert r["root_after"] == [30, 0]  # WARNING, no handlers
+
+    # The SDK's own logger is what gets configured.
+    assert r["sdk_level"] == 10  # DEBUG
+    assert r["sdk_handlers"] == 1  # not 2 — repeated construction must not stack
+
+
+def test_no_logging_is_touched_without_the_env_var():
+    r = _probe()
+
+    assert r["root_after"] == r["root_before"]
+    assert r["sdk_level"] == 0  # NOTSET
+    assert r["sdk_handlers"] == 0
+
+
+def test_an_application_handler_is_not_duplicated():
+    r = _probe(
+        setup="logging.basicConfig(level=logging.INFO)",
+        OPENROUTER_DEBUG="1",
+    )
+
+    # The app configured root itself; we must not add a competing handler,
+    # which would print every record twice.
+    assert r["sdk_handlers"] == 0
+    assert r["sdk_level"] == 10
+    assert r["root_after"] == r["root_before"]