diff --git a/litellm/litellm_core_utils/get_supported_openai_params.py b/litellm/litellm_core_utils/get_supported_openai_params.py
index b8cdc8210fc..7c4f9941523 100644
--- a/litellm/litellm_core_utils/get_supported_openai_params.py
+++ b/litellm/litellm_core_utils/get_supported_openai_params.py
@@ -22,9 +22,11 @@ def get_supported_openai_params(  # noqa: PLR0915
     ```
 
     Args:
-        base_model: For Azure, the true underlying model (e.g. ``"azure/gpt-5.2"``)
-            when the deployment name differs. Used for model-type detection so that
-            non-standard deployment names route to the correct config.
+        base_model: An optional capability hint for deployments whose ``model``
+            label isn't recognized on its own (e.g. an Azure deployment name, or a
+            friendly Bedrock alias). It is additive: the result is the union of the
+            params supported by ``model`` and by ``base_model``, so a hint can only
+            add capabilities, never strip ones the real model already supports.
 
     Returns:
     - List if custom_llm_provider is mapped
@@ -52,7 +54,15 @@ def get_supported_openai_params(  # noqa: PLR0915
         provider_config = None
 
     if provider_config and request_type == "chat_completion":
-        return provider_config.get_supported_openai_params(model=base_model or model)
+        supported_params = provider_config.get_supported_openai_params(model=model)
+        if base_model and base_model != model:
+            base_model_params = provider_config.get_supported_openai_params(
+                model=base_model
+            )
+            supported_params = list(
+                dict.fromkeys([*supported_params, *base_model_params])
+            )
+        return supported_params
 
     if custom_llm_provider == "bedrock":
         return litellm.AmazonConverseConfig().get_supported_openai_params(model=model)
diff --git a/litellm/main.py b/litellm/main.py
index da8624d11b8..62b64716e96 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -1319,7 +1319,9 @@ def completion(  # type: ignore # noqa: PLR0915
     preset_cache_key = kwargs.get("preset_cache_key", None)
     hf_model_name = kwargs.get("hf_model_name", None)
     supports_system_message = kwargs.get("supports_system_message", None)
-    base_model = kwargs.get("base_model", None)
+    base_model = kwargs.get("base_model", None) or (
+        model_info.get("base_model") if isinstance(model_info, dict) else None
+    )
     ### DISABLE FLAGS ###
     disable_add_transform_inline_image_block = kwargs.get(
         "disable_add_transform_inline_image_block", None
@@ -1531,11 +1533,7 @@ def completion(  # type: ignore # noqa: PLR0915
             "logit_bias": logit_bias,
             "user": user,
             # params to identify the model
-            "model": (
-                model_info.get("base_model")
-                if isinstance(model_info, dict) and model_info.get("base_model")
-                else model
-            ),
+            "model": model,
             "custom_llm_provider": custom_llm_provider,
             "response_format": response_format,
             "seed": seed,
diff --git a/tests/test_litellm/litellm_core_utils/test_get_supported_openai_params.py b/tests/test_litellm/litellm_core_utils/test_get_supported_openai_params.py
new file mode 100644
index 00000000000..3c280c6ba92
--- /dev/null
+++ b/tests/test_litellm/litellm_core_utils/test_get_supported_openai_params.py
@@ -0,0 +1,134 @@
+import os
+import sys
+
+import pytest
+
+sys.path.insert(0, os.path.abspath("../../.."))
+
+from litellm.litellm_core_utils.get_supported_openai_params import (
+    get_supported_openai_params,
+)
+
+BEDROCK_REAL_MODEL = "eu.anthropic.claude-haiku-4-5-20251001-v1:0"
+BEDROCK_LABEL = "claude-haiku-4-5"
+
+
+def test_base_model_label_does_not_strip_bedrock_tools():
+    """Regression for #29618.
+
+    A Bedrock deployment whose ``model_info.base_model`` is a friendly label
+    (``claude-haiku-4-5``) must still advertise ``tools``/``tool_choice``. The label
+    on its own resolves to no tool support, so before the fix it stripped the
+    capability the real model id exposes, silently dropping function calling under
+    ``drop_params``."""
+    params = get_supported_openai_params(
+        model=BEDROCK_REAL_MODEL,
+        custom_llm_provider="bedrock",
+        base_model=BEDROCK_LABEL,
+    )
+
+    assert params is not None
+    assert "tools" in params
+    assert "tool_choice" in params
+
+
+def test_base_model_label_alone_lacks_bedrock_tools():
+    """The label by itself does not advertise tools; this is what made the union
+    necessary. Guards against the discrepancy disappearing (and the regression test
+    above silently passing for the wrong reason)."""
+    params = get_supported_openai_params(
+        model=BEDROCK_LABEL, custom_llm_provider="bedrock"
+    )
+
+    assert params is not None
+    assert "tools" not in params
+
+
+def test_base_model_is_additive_not_replacement():
+    """``base_model`` may only add capabilities, never remove ones the real model has.
+
+    Bedrock: real id supports ``tools`` but not the label's reasoning hint; the union
+    must contain the real model's ``tools`` regardless of the label being a subset."""
+    real_only = set(
+        get_supported_openai_params(
+            model=BEDROCK_REAL_MODEL, custom_llm_provider="bedrock"
+        )
+    )
+    label_only = set(
+        get_supported_openai_params(model=BEDROCK_LABEL, custom_llm_provider="bedrock")
+    )
+    combined = set(
+        get_supported_openai_params(
+            model=BEDROCK_REAL_MODEL,
+            custom_llm_provider="bedrock",
+            base_model=BEDROCK_LABEL,
+        )
+    )
+
+    assert combined == real_only | label_only
+    assert real_only - label_only  # the label really is a strict subset here
+    assert real_only <= combined
+
+
+def test_base_model_adds_capabilities_the_real_model_lacks():
+    """Regression for #27717 (the behavior the union must preserve).
+
+    ``gemini-3.1-pro`` isn't in the cost map so it advertises no reasoning support,
+    but the registered ``gemini-3.1-pro-preview`` base_model does. The hint must add
+    ``reasoning_effort``/``thinking`` without the call erroring."""
+    real_only = set(
+        get_supported_openai_params(
+            model="gemini-3.1-pro", custom_llm_provider="gemini"
+        )
+    )
+    assert "reasoning_effort" not in real_only
+
+    combined = set(
+        get_supported_openai_params(
+            model="gemini-3.1-pro",
+            custom_llm_provider="gemini",
+            base_model="gemini-3.1-pro-preview",
+        )
+    )
+    assert "reasoning_effort" in combined
+    assert "thinking" in combined
+
+
+def test_no_base_model_is_unchanged():
+    """Omitting ``base_model`` must resolve purely from ``model``."""
+    with_none = get_supported_openai_params(
+        model=BEDROCK_REAL_MODEL, custom_llm_provider="bedrock", base_model=None
+    )
+    plain = get_supported_openai_params(
+        model=BEDROCK_REAL_MODEL, custom_llm_provider="bedrock"
+    )
+
+    assert with_none == plain
+
+
+def test_base_model_equal_to_model_is_unchanged():
+    """A ``base_model`` identical to ``model`` must not double-resolve or reorder."""
+    plain = get_supported_openai_params(
+        model=BEDROCK_REAL_MODEL, custom_llm_provider="bedrock"
+    )
+    same = get_supported_openai_params(
+        model=BEDROCK_REAL_MODEL,
+        custom_llm_provider="bedrock",
+        base_model=BEDROCK_REAL_MODEL,
+    )
+
+    assert same == plain
+
+
+def test_azure_base_model_detection_preserved():
+    """Azure relies on ``base_model`` for model-type detection when the deployment
+    name is opaque; the union must keep advertising the gpt-5 capabilities."""
+    params = get_supported_openai_params(
+        model="my-opaque-deployment",
+        custom_llm_provider="azure",
+        base_model="azure/gpt-5.2",
+    )
+
+    assert params is not None
+    assert "reasoning_effort" in params
+    assert "tools" in params
diff --git a/tests/test_litellm/test_main.py b/tests/test_litellm/test_main.py
index b03579c2dbd..113e1bc0df8 100644
--- a/tests/test_litellm/test_main.py
+++ b/tests/test_litellm/test_main.py
@@ -849,12 +849,13 @@ def test_gpt_5_4_responses_bridge_preserves_reasoning_summary_dict(
 
 
 @pytest.mark.parametrize(
-    "model, model_info, expected_model_param",
+    "model, model_info, expected_model_param, expected_base_model_param",
     [
-        ("gemini/gemini-3.1-pro", None, "gemini-3.1-pro"),
+        ("gemini/gemini-3.1-pro", None, "gemini-3.1-pro", None),
         (
             "gemini/gemini-3.1-pro",
             {"base_model": "gemini-3.1-pro-preview"},
+            "gemini-3.1-pro",
             "gemini-3.1-pro-preview",
         ),
     ],
@@ -863,7 +864,13 @@ def test_completion_optional_params_base_model(
     model: str,
     model_info: dict | None,
     expected_model_param: str,
+    expected_base_model_param: str | None,
 ):
+    """``model_info.base_model`` must reach ``get_optional_params`` as ``base_model``
+    (an additive capability hint), without overwriting ``model`` with the label.
+
+    Regression for #29618: overwriting ``model`` with a friendly ``base_model``
+    label made Bedrock drop ``tools``/``tool_choice`` under ``drop_params``."""
     with patch("litellm.main.get_optional_params") as mock_get_optional_params:
         mock_get_optional_params.return_value = MagicMock()
 
@@ -881,10 +888,9 @@ def test_completion_optional_params_base_model(
         litellm.completion(**kwargs)
 
         assert mock_get_optional_params.called is True
-        get_optional_params_model_param = mock_get_optional_params.call_args.kwargs[
-            "model"
-        ]
-        assert get_optional_params_model_param == expected_model_param
+        call_kwargs = mock_get_optional_params.call_args.kwargs
+        assert call_kwargs["model"] == expected_model_param
+        assert call_kwargs["base_model"] == expected_base_model_param
 
 
 @patch("litellm.completion_extras.responses_api_bridge.completion")
diff --git a/tests/test_litellm/test_utils.py b/tests/test_litellm/test_utils.py
index 2d75671f1cb..f2b9bef9230 100644
--- a/tests/test_litellm/test_utils.py
+++ b/tests/test_litellm/test_utils.py
@@ -4144,3 +4144,51 @@ def test_original_dict_not_mutated(self):
         validate_and_fix_thinking_param(thinking=thinking)
         assert "budgetTokens" in thinking
         assert "budget_tokens" not in thinking
+
+
+class TestBedrockBaseModelLabelKeepsTools:
+    """Regression for #29618: a Bedrock deployment whose ``base_model`` is a friendly
+    label must not silently drop ``tools``/``tool_choice`` under ``drop_params``."""
+
+    TOOLS = [
+        {
+            "type": "function",
+            "function": {
+                "name": "get_weather",
+                "parameters": {
+                    "type": "object",
+                    "properties": {"city": {"type": "string"}},
+                },
+            },
+        }
+    ]
+
+    def test_base_model_label_keeps_tools_with_drop_params(self):
+        from litellm.utils import get_optional_params
+
+        result = get_optional_params(
+            model="eu.anthropic.claude-haiku-4-5-20251001-v1:0",
+            custom_llm_provider="bedrock",
+            base_model="claude-haiku-4-5",
+            tools=self.TOOLS,
+            tool_choice="auto",
+            drop_params=True,
+        )
+
+        assert "tools" in result
+        assert "tool_choice" in result
+
+    def test_base_model_label_alone_drops_tools(self):
+        """Without the real model id the label resolves to no tool support, so passing
+        the label as ``model`` is exactly what dropped tools before the fix."""
+        from litellm.utils import get_optional_params
+
+        result = get_optional_params(
+            model="claude-haiku-4-5",
+            custom_llm_provider="bedrock",
+            tools=self.TOOLS,
+            tool_choice="auto",
+            drop_params=True,
+        )
+
+        assert "tools" not in result