diff --git a/tests/entrypoints/openai/test_tool_calls_utils.py b/tests/entrypoints/openai/test_tool_calls_utils.py
new file mode 100644
index 000000000000..dabd1a15db3f
--- /dev/null
+++ b/tests/entrypoints/openai/test_tool_calls_utils.py
@@ -0,0 +1,107 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests for maybe_filter_parallel_tool_calls (issue #44948).
+
+``parallel_tool_calls`` defaults to ``true``; an explicit ``null`` means
+"unspecified" and must resolve to that default, the same as omitting the
+field. Only an explicit ``false`` trims the response to the first tool call.
+"""
+
+import pytest
+
+from vllm.entrypoints.openai.chat_completion.protocol import (
+    ChatCompletionRequest,
+    ChatCompletionResponseChoice,
+    ChatCompletionResponseStreamChoice,
+    ChatMessage,
+)
+from vllm.entrypoints.openai.engine.protocol import (
+    DeltaFunctionCall,
+    DeltaMessage,
+    DeltaToolCall,
+    FunctionCall,
+    ToolCall,
+)
+from vllm.entrypoints.serve.utils.tool_calls_utils import (
+    maybe_filter_parallel_tool_calls,
+)
+
+
+def _make_request(**kwargs) -> ChatCompletionRequest:
+    return ChatCompletionRequest(
+        messages=[{"role": "user", "content": "hi"}],
+        model="test-model",
+        **kwargs,
+    )
+
+
+def _make_choice(num_tool_calls: int) -> ChatCompletionResponseChoice:
+    tool_calls = [
+        ToolCall(function=FunctionCall(name=f"fn_{i}", arguments="{}"))
+        for i in range(num_tool_calls)
+    ]
+    return ChatCompletionResponseChoice(
+        index=0,
+        message=ChatMessage(role="assistant", tool_calls=tool_calls),
+    )
+
+
+def _make_stream_choice(num_tool_calls: int) -> ChatCompletionResponseStreamChoice:
+    tool_calls = [
+        DeltaToolCall(index=i, function=DeltaFunctionCall(name=f"fn_{i}"))
+        for i in range(num_tool_calls)
+    ]
+    return ChatCompletionResponseStreamChoice(
+        index=0,
+        delta=DeltaMessage(tool_calls=tool_calls),
+    )
+
+
+@pytest.mark.cpu_test
+def test_omitted_defaults_to_true_keeps_all_tool_calls():
+    request = _make_request()
+    assert request.parallel_tool_calls is True
+
+    choice = maybe_filter_parallel_tool_calls(_make_choice(3), request)
+    assert len(choice.message.tool_calls) == 3
+
+
+@pytest.mark.cpu_test
+def test_explicit_true_keeps_all_tool_calls():
+    request = _make_request(parallel_tool_calls=True)
+    choice = maybe_filter_parallel_tool_calls(_make_choice(3), request)
+    assert len(choice.message.tool_calls) == 3
+
+
+@pytest.mark.cpu_test
+def test_explicit_false_trims_to_first_tool_call():
+    request = _make_request(parallel_tool_calls=False)
+    choice = maybe_filter_parallel_tool_calls(_make_choice(3), request)
+    assert len(choice.message.tool_calls) == 1
+    assert choice.message.tool_calls[0].function.name == "fn_0"
+
+
+@pytest.mark.cpu_test
+def test_explicit_null_resolves_to_default_keeps_all_tool_calls():
+    """Regression test for #44948: explicit ``null`` was treated as falsy and
+    filtered like ``false``; it must behave like an omitted field instead."""
+    request = _make_request(parallel_tool_calls=None)
+    assert request.parallel_tool_calls is None
+
+    choice = maybe_filter_parallel_tool_calls(_make_choice(3), request)
+    assert len(choice.message.tool_calls) == 3
+
+
+@pytest.mark.cpu_test
+def test_streaming_explicit_false_trims_to_index_zero():
+    request = _make_request(parallel_tool_calls=False)
+    choice = maybe_filter_parallel_tool_calls(_make_stream_choice(3), request)
+    assert [tc.index for tc in choice.delta.tool_calls] == [0]
+
+
+@pytest.mark.cpu_test
+def test_streaming_explicit_null_keeps_all_tool_calls():
+    """Streaming variant of the #44948 regression."""
+    request = _make_request(parallel_tool_calls=None)
+    choice = maybe_filter_parallel_tool_calls(_make_stream_choice(3), request)
+    assert [tc.index for tc in choice.delta.tool_calls] == [0, 1, 2]
diff --git a/vllm/entrypoints/serve/utils/tool_calls_utils.py b/vllm/entrypoints/serve/utils/tool_calls_utils.py
index 648698c2a975..d6e520f7fe63 100644
--- a/vllm/entrypoints/serve/utils/tool_calls_utils.py
+++ b/vllm/entrypoints/serve/utils/tool_calls_utils.py
@@ -19,9 +19,14 @@
 def maybe_filter_parallel_tool_calls(
     choice: _ChatCompletionResponseChoiceT, request: ChatCompletionRequest
 ) -> _ChatCompletionResponseChoiceT:
-    """Filter to first tool call only when parallel_tool_calls is False."""
+    """Filter to first tool call only when parallel_tool_calls is False.
 
-    if request.parallel_tool_calls:
+    An explicit ``parallel_tool_calls: null`` means "unspecified" and resolves
+    to the documented default (``true``), same as omitting the field — only an
+    explicit ``false`` trims the response.
+    """
+
+    if request.parallel_tool_calls is not False:
         return choice
 
     if isinstance(choice, ChatCompletionResponseChoice) and choice.message.tool_calls: