diff --git a/src/openai/lib/__init__.py b/src/openai/lib/__init__.py
index 5c6cb782c0..7ed2893aaa 100644
--- a/src/openai/lib/__init__.py
+++ b/src/openai/lib/__init__.py
@@ -1,2 +1,4 @@
 from ._tools import pydantic_function_tool as pydantic_function_tool
 from ._parsing import ResponseFormatT as ResponseFormatT
+from ._reasoning import get_default_reasoning as get_default_reasoning
+from ._reasoning import get_reasoning_effort_from_env as get_reasoning_effort_from_env
diff --git a/src/openai/lib/_reasoning.py b/src/openai/lib/_reasoning.py
new file mode 100644
index 0000000000..c4107ce159
--- /dev/null
+++ b/src/openai/lib/_reasoning.py
@@ -0,0 +1,145 @@
+"""Utilities for reasoning configuration.
+
+This module provides utilities for configuring reasoning behavior,
+including support for the OPENAI_REASONING_EFFORT environment variable.
+
+Example usage:
+    from openai import OpenAI
+    from openai.lib import get_default_reasoning
+
+    client = OpenAI()
+
+    # Uses OPENAI_REASONING_EFFORT env var if set, otherwise returns None
+    reasoning = get_default_reasoning()
+
+    response = client.responses.create(
+        model="gpt-5",
+        input="Hello",
+        reasoning=reasoning,
+    )
+"""
+
+from __future__ import annotations
+
+import os
+import warnings
+from typing import Optional, Literal
+
+from ..types.shared_params.reasoning import Reasoning
+from ..types.shared.reasoning_effort import ReasoningEffort
+
+
+__all__ = [
+    "get_default_reasoning",
+    "get_reasoning_effort_from_env",
+]
+
+# Valid reasoning effort values
+VALID_REASONING_EFFORTS: tuple[str, ...] = (
+    "none",
+    "minimal",
+    "low",
+    "medium",
+    "high",
+    "xhigh",
+)
+
+# Environment variable name
+REASONING_EFFORT_ENV_VAR = "OPENAI_REASONING_EFFORT"
+
+
+def get_reasoning_effort_from_env() -> Optional[ReasoningEffort]:
+    """Get reasoning effort from the OPENAI_REASONING_EFFORT environment variable.
+
+    Returns:
+        The reasoning effort value if set and valid, None otherwise.
+
+    Valid values are: none, minimal, low, medium, high, xhigh
+
+    If an invalid value is set, a warning is emitted and None is returned.
+
+    Example:
+        >>> import os
+        >>> os.environ["OPENAI_REASONING_EFFORT"] = "low"
+        >>> get_reasoning_effort_from_env()
+        'low'
+    """
+    value = os.environ.get(REASONING_EFFORT_ENV_VAR)
+    if value is None:
+        return None
+
+    # Normalize to lowercase
+    value_lower = value.lower().strip()
+
+    if value_lower not in VALID_REASONING_EFFORTS:
+        warnings.warn(
+            f"Invalid {REASONING_EFFORT_ENV_VAR} value: '{value}'. "
+            f"Valid values are: {', '.join(VALID_REASONING_EFFORTS)}. "
+            "Ignoring environment variable.",
+            UserWarning,
+            stacklevel=2,
+        )
+        return None
+
+    return value_lower  # type: ignore[return-value]
+
+
+def get_default_reasoning(
+    effort: Optional[ReasoningEffort] = None,
+    summary: Optional[Literal["auto", "concise", "detailed"]] = None,
+) -> Optional[Reasoning]:
+    """Get a Reasoning configuration, using environment variable as default.
+
+    This function allows you to easily configure reasoning with support for
+    the OPENAI_REASONING_EFFORT environment variable.
+
+    Args:
+        effort: Override the reasoning effort. If None, uses OPENAI_REASONING_EFFORT
+                environment variable if set.
+        summary: Optional summary configuration for reasoning output.
+
+    Returns:
+        A Reasoning TypedDict if effort is configured (either explicitly or via
+        environment variable), None otherwise.
+
+    Precedence:
+        1. Explicit `effort` parameter (if provided)
+        2. OPENAI_REASONING_EFFORT environment variable (if set)
+        3. None (SDK default behavior)
+
+    Example:
+        >>> # With environment variable set:
+        >>> import os
+        >>> os.environ["OPENAI_REASONING_EFFORT"] = "low"
+        >>> get_default_reasoning()
+        {'effort': 'low'}
+
+        >>> # With explicit override:
+        >>> get_default_reasoning(effort="high")
+        {'effort': 'high'}
+
+        >>> # With no configuration:
+        >>> del os.environ["OPENAI_REASONING_EFFORT"]
+        >>> get_default_reasoning()
+        None
+    """
+    # Determine the effort value
+    final_effort: Optional[ReasoningEffort] = None
+
+    if effort is not None:
+        final_effort = effort
+    else:
+        final_effort = get_reasoning_effort_from_env()
+
+    # If no effort is configured, return None
+    if final_effort is None and summary is None:
+        return None
+
+    # Build the Reasoning config
+    result: Reasoning = {}
+    if final_effort is not None:
+        result["effort"] = final_effort
+    if summary is not None:
+        result["summary"] = summary
+
+    return result
diff --git a/tests/lib/test_reasoning.py b/tests/lib/test_reasoning.py
new file mode 100644
index 0000000000..4f7aab9bdc
--- /dev/null
+++ b/tests/lib/test_reasoning.py
@@ -0,0 +1,114 @@
+"""Tests for reasoning utilities.
+
+Tests the OPENAI_REASONING_EFFORT environment variable support.
+Relates to issue #2686: Allow setting reasoning effort via environment variable
+"""
+
+from __future__ import annotations
+
+import os
+import warnings
+
+import pytest
+
+from openai.lib import get_default_reasoning, get_reasoning_effort_from_env
+
+
+class TestGetReasoningEffortFromEnv:
+    """Tests for get_reasoning_effort_from_env function."""
+
+    def setup_method(self) -> None:
+        """Clean up env var before each test."""
+        if "OPENAI_REASONING_EFFORT" in os.environ:
+            del os.environ["OPENAI_REASONING_EFFORT"]
+
+    def teardown_method(self) -> None:
+        """Clean up env var after each test."""
+        if "OPENAI_REASONING_EFFORT" in os.environ:
+            del os.environ["OPENAI_REASONING_EFFORT"]
+
+    def test_returns_none_when_not_set(self) -> None:
+        """Returns None when env var is not set."""
+        result = get_reasoning_effort_from_env()
+        assert result is None
+
+    @pytest.mark.parametrize(
+        "value",
+        ["none", "minimal", "low", "medium", "high", "xhigh"],
+    )
+    def test_returns_valid_values(self, value: str) -> None:
+        """Returns the value when it's valid."""
+        os.environ["OPENAI_REASONING_EFFORT"] = value
+        result = get_reasoning_effort_from_env()
+        assert result == value
+
+    def test_case_insensitive(self) -> None:
+        """Accepts case-insensitive values."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "HIGH"
+        result = get_reasoning_effort_from_env()
+        assert result == "high"
+
+    def test_strips_whitespace(self) -> None:
+        """Strips leading/trailing whitespace."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "  low  "
+        result = get_reasoning_effort_from_env()
+        assert result == "low"
+
+    def test_warns_on_invalid_value(self) -> None:
+        """Warns and returns None for invalid values."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "invalid"
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            result = get_reasoning_effort_from_env()
+            assert result is None
+            assert len(w) == 1
+            assert "Invalid" in str(w[0].message)
+            assert "OPENAI_REASONING_EFFORT" in str(w[0].message)
+
+
+class TestGetDefaultReasoning:
+    """Tests for get_default_reasoning function."""
+
+    def setup_method(self) -> None:
+        """Clean up env var before each test."""
+        if "OPENAI_REASONING_EFFORT" in os.environ:
+            del os.environ["OPENAI_REASONING_EFFORT"]
+
+    def teardown_method(self) -> None:
+        """Clean up env var after each test."""
+        if "OPENAI_REASONING_EFFORT" in os.environ:
+            del os.environ["OPENAI_REASONING_EFFORT"]
+
+    def test_returns_none_when_no_config(self) -> None:
+        """Returns None when no effort configured."""
+        result = get_default_reasoning()
+        assert result is None
+
+    def test_uses_env_var(self) -> None:
+        """Uses env var when no explicit effort provided."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "low"
+        result = get_default_reasoning()
+        assert result == {"effort": "low"}
+
+    def test_explicit_effort_overrides_env(self) -> None:
+        """Explicit effort parameter overrides env var."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "low"
+        result = get_default_reasoning(effort="high")
+        assert result == {"effort": "high"}
+
+    def test_with_summary(self) -> None:
+        """Can configure summary alongside effort."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "medium"
+        result = get_default_reasoning(summary="concise")
+        assert result == {"effort": "medium", "summary": "concise"}
+
+    def test_summary_only(self) -> None:
+        """Can configure summary without effort."""
+        result = get_default_reasoning(summary="detailed")
+        assert result == {"summary": "detailed"}
+
+    def test_explicit_none_effort_uses_env(self) -> None:
+        """Explicit None for effort still uses env var."""
+        os.environ["OPENAI_REASONING_EFFORT"] = "high"
+        result = get_default_reasoning(effort=None)
+        assert result == {"effort": "high"}