diff --git a/tests/tool_parsers/test_internlm2_tool_parser.py b/tests/tool_parsers/test_internlm2_tool_parser.py
index 2e5069dbed94..62be0b300493 100644
--- a/tests/tool_parsers/test_internlm2_tool_parser.py
+++ b/tests/tool_parsers/test_internlm2_tool_parser.py
@@ -10,6 +10,7 @@
     ToolParserTests,
 )
 from vllm.tokenizers import TokenizerLike
+from vllm.tool_parsers.internlm2_tool_parser import Internlm2ToolParser
 
 
 class TestInternLM2ToolParser(ToolParserTests):
@@ -120,3 +121,42 @@ def test_config(self) -> ToolParserTestConfig:
                 ),
             },
         )
+
+
+def test_streaming_first_arguments_delta_survives_compact_json_formatting():
+    """The first streamed arguments delta must not depend on delta_text
+    appearing verbatim inside json.dumps(cur_arguments): a model emitting
+    compact JSON (no space after ':') never matches Python's default
+    json.dumps formatting, so cur_arguments_json.index(delta_text) raises
+    ValueError, which the caller swallows into a lost delta.
+    """
+    parser = Internlm2ToolParser(tokenizer=None, tools=[])
+
+    first = parser.extract_tool_calls_streaming(
+        previous_text="",
+        current_text='<|action_start|><|plugin|>{"name":"get_weather"',
+        delta_text='<|action_start|><|plugin|>{"name":"get_weather"',
+        previous_token_ids=[],
+        current_token_ids=[],
+        delta_token_ids=[],
+        request=None,
+    )
+    assert first is not None
+    assert first.tool_calls[0].function.name == "get_weather"
+
+    second = parser.extract_tool_calls_streaming(
+        previous_text='<|action_start|><|plugin|>{"name":"get_weather"',
+        current_text=(
+            '<|action_start|><|plugin|>{"name":"get_weather",'
+            '"parameters":{"city":"SF"}}'
+        ),
+        delta_text=',"parameters":{"city":"SF"}}',
+        previous_token_ids=[],
+        current_token_ids=[],
+        delta_token_ids=[],
+        request=None,
+    )
+
+    assert second is not None
+    assert second.tool_calls[0].function.arguments is not None
+    assert parser.streamed_args_for_tool[0] != ""
diff --git a/vllm/tool_parsers/internlm2_tool_parser.py b/vllm/tool_parsers/internlm2_tool_parser.py
index f4aaeef71a0f..60f937d7625b 100644
--- a/vllm/tool_parsers/internlm2_tool_parser.py
+++ b/vllm/tool_parsers/internlm2_tool_parser.py
@@ -146,9 +146,11 @@ def extract_tool_calls_streaming(
                 elif cur_arguments and not prev_arguments:
                     cur_arguments_json = json.dumps(cur_arguments, ensure_ascii=False)
 
-                    arguments_delta = cur_arguments_json[
-                        : cur_arguments_json.index(delta_text) + len(delta_text)
-                    ]
+                    # delta_text may not appear verbatim in cur_arguments_json
+                    # (e.g. compact model output vs. json.dumps spacing), so
+                    # diff against an empty previous string instead of
+                    # locating delta_text as a substring.
+                    arguments_delta = extract_intermediate_diff(cur_arguments_json, "")
                     delta = DeltaMessage(
                         tool_calls=[
                             DeltaToolCall(