diff --git a/tests/tool_parsers/test_functiongemma_tool_parser.py b/tests/tool_parsers/test_functiongemma_tool_parser.py
index d32aba3085e5..12941ae7dd90 100644
--- a/tests/tool_parsers/test_functiongemma_tool_parser.py
+++ b/tests/tool_parsers/test_functiongemma_tool_parser.py
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import json
 from unittest.mock import MagicMock
 
 import pytest
@@ -120,6 +121,61 @@ def test_argument_with_spaces(self, parser):
         assert result == {"message": "Hello World"}
 
 
+class TestExtractToolCallsStreaming:
+    def _stream(self, parser, chunks):
+        current_text = ""
+        collected_args = ""
+        for chunk in chunks:
+            previous_text = current_text
+            current_text += chunk
+            result = parser.extract_tool_calls_streaming(
+                previous_text=previous_text,
+                current_text=current_text,
+                delta_text=chunk,
+                previous_token_ids=[],
+                current_token_ids=[],
+                delta_token_ids=[],
+                request=None,
+            )
+            if result and result.tool_calls:
+                function = result.tool_calls[0].function
+                arguments = getattr(function, "arguments", None)
+                if arguments:
+                    collected_args += arguments
+        return collected_args
+
+    def test_streamed_arguments_form_valid_json_with_two_keys(self, parser):
+        # Regression test: each streaming delta previously diffed
+        # json.dumps(current_args) by raw string length against the last-sent
+        # string. Since json.dumps() closes the object with '}' on every call,
+        # the delta after the first key arrived re-sent a stale closing brace
+        # mid-object, corrupting the client-side reconstructed JSON.
+        chunks = [
+            "<start_function_call>call:get_weather{",
+            "ci",
+            "ty:<escape>NYC<escape>",
+            "unit:<escape>celsius<escape>",
+            "}<end_function_call>",
+        ]
+        reconstructed = self._stream(parser, chunks)
+        assert json.loads(reconstructed) == {"city": "NYC", "unit": "celsius"}
+
+    def test_streamed_arguments_form_valid_json_with_three_keys(self, parser):
+        chunks = [
+            "<start_function_call>call:search{",
+            "query:<escape>python<escape>",
+            "limit:<escape>10<escape>",
+            "sort:<escape>relevance<escape>",
+            "}<end_function_call>",
+        ]
+        reconstructed = self._stream(parser, chunks)
+        assert json.loads(reconstructed) == {
+            "query": "python",
+            "limit": 10,
+            "sort": "relevance",
+        }
+
+
 class TestAdjustRequest:
     def test_skip_special_tokens_disabled(self, parser, mock_request):
         mock_request.tools = [{"type": "function", "function": {"name": "test"}}]
diff --git a/vllm/tool_parsers/functiongemma_tool_parser.py b/vllm/tool_parsers/functiongemma_tool_parser.py
index 776792ea1d6d..3c145e31332b 100644
--- a/vllm/tool_parsers/functiongemma_tool_parser.py
+++ b/vllm/tool_parsers/functiongemma_tool_parser.py
@@ -22,6 +22,7 @@
 from vllm.logger import init_logger
 from vllm.tokenizers import TokenizerLike
 from vllm.tool_parsers.abstract_tool_parser import Tool, ToolParser
+from vllm.tool_parsers.utils import find_common_prefix
 
 logger = init_logger(__name__)
 
@@ -238,15 +239,23 @@ def extract_tool_calls_streaming(
                         if self.current_tool_name_sent and args_part:
                             current_args = self._parse_arguments(args_part)
                             if current_args:
+                                # Mid-stream, more keys may still arrive, so the
+                                # object isn't closed yet -- drop the trailing '}'
+                                # that json.dumps() always adds for a complete
+                                # dict. The real closing brace is only sent once,
+                                # from the "function call just ended" branch below.
                                 current_args_json = json.dumps(
                                     current_args, ensure_ascii=False
-                                )
+                                )[:-1]
                                 prev_streamed = self.streamed_args_for_tool[
                                     self.current_tool_id
                                 ]
 
-                                if len(current_args_json) > len(prev_streamed):
-                                    diff = current_args_json[len(prev_streamed) :]
+                                if current_args_json != prev_streamed:
+                                    common = find_common_prefix(
+                                        current_args_json, prev_streamed
+                                    )
+                                    diff = current_args_json[len(common) :]
                                     self.streamed_args_for_tool[
                                         self.current_tool_id
                                     ] = current_args_json
@@ -288,8 +297,9 @@ def extract_tool_calls_streaming(
                         prev_streamed = self.streamed_args_for_tool[
                             self.current_tool_id
                         ]
-                        if len(args_json) > len(prev_streamed):
-                            diff = args_json[len(prev_streamed) :]
+                        if args_json != prev_streamed:
+                            common = find_common_prefix(args_json, prev_streamed)
+                            diff = args_json[len(common) :]
                             self.streamed_args_for_tool[self.current_tool_id] = (
                                 args_json
                             )