diff --git a/litellm/proxy/litellm_pre_call_utils.py b/litellm/proxy/litellm_pre_call_utils.py
index 9ddc7ce2caf..6514d4e1e8c 100644
--- a/litellm/proxy/litellm_pre_call_utils.py
+++ b/litellm/proxy/litellm_pre_call_utils.py
@@ -457,12 +457,20 @@ def is_claude_code_user_agent(user_agent: str) -> bool:
     return user_agent.startswith("claude-cli/")
 
 
-def should_auto_drop_params_for_claude_code(user_agent: str, data: dict, proxy_config: ProxyConfig) -> bool:
-    """drop_params defaults to on for Claude Code so its Anthropic-specific
-    params (e.g. thinking) don't fail requests routed to non-Anthropic
-    providers. An explicit drop_params from the caller or in the operator's
-    ``litellm_settings`` always wins over this default."""
-    if not is_claude_code_user_agent(user_agent):
+def is_codex_user_agent(user_agent: str) -> bool:
+    """Codex identifies itself as ``codex_cli_rs/<version> ...`` (TUI),
+    ``codex_exec/<version> ...`` (exec mode), or ``codex_vscode/<version> ...``
+    (IDE extension); all share the ``codex_`` prefix."""
+    return user_agent.startswith("codex_")
+
+
+def should_auto_drop_params_for_agentic_cli(user_agent: str, data: dict, proxy_config: ProxyConfig) -> bool:
+    """drop_params defaults to on for agentic CLIs so their client-specific
+    params (e.g. Claude Code's thinking, Codex's service_tier) don't fail
+    requests routed to providers that reject them. An explicit drop_params
+    from the caller or in the operator's ``litellm_settings`` always wins
+    over this default."""
+    if not (is_claude_code_user_agent(user_agent) or is_codex_user_agent(user_agent)):
         return False
     if "drop_params" in data:
         return False
@@ -1687,7 +1695,7 @@ async def add_litellm_data_to_request(
         user_agent = request.headers["user-agent"]
     data[_metadata_variable_name]["user_agent"] = user_agent
 
-    if should_auto_drop_params_for_claude_code(user_agent, data, proxy_config):
+    if should_auto_drop_params_for_agentic_cli(user_agent, data, proxy_config):
         data["drop_params"] = True
 
     # Merge caller-supplied tags (x-litellm-tags header, data["tags"] root-level)
diff --git a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
index 47879ee96ad..8bee7e9f33b 100644
--- a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
+++ b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
@@ -5094,17 +5094,23 @@ def _make_request_mock(path: str, headers: dict) -> MagicMock:
         ("claude-cli/2.0.69 (external, cli)", False, None, False),
         ("claude-cli/2.0.69 (external, cli)", None, False, None),
         ("claude-cli/2.0.69 (external, cli)", None, True, None),
+        ("codex_cli_rs/0.144.5 (Mac OS 26.4.0; arm64) WezTerm", None, None, True),
+        ("codex_exec/0.144.5 (Mac OS 26.4.0; arm64) WarpTerminal (codex_exec; 0.144.5)", None, None, True),
+        ("codex_vscode/0.144.5 (Mac OS 26.4.0; arm64) vscode/1.104.1", None, None, True),
+        ("codex_exec/0.144.5 (Mac OS 26.4.0; arm64)", False, None, False),
+        ("codex_exec/0.144.5 (Mac OS 26.4.0; arm64)", None, True, None),
         ("PostmanRuntime/7.53.0", None, None, None),
         (None, None, None, None),
     ],
 )
-async def test_add_litellm_data_to_request_claude_code_drop_params(
+async def test_add_litellm_data_to_request_agentic_cli_drop_params(
     user_agent, request_drop_params, operator_drop_params, expected_drop_params
 ):
-    """Claude Code sends Anthropic-specific params that fail on non-Anthropic
-    providers, so its user agent must turn on drop_params automatically,
-    without overriding an explicit caller value, an explicit operator-level
-    litellm_settings value, or affecting other clients.
+    """Claude Code sends Anthropic-specific params and Codex sends
+    service_tier, both of which fail on providers that reject them, so those
+    user agents must turn on drop_params automatically, without overriding an
+    explicit caller value, an explicit operator-level litellm_settings value,
+    or affecting other clients.
     """
     headers = {"Content-Type": "application/json"}
     if user_agent is not None: