huggingface · Prawal-Sharma · Aug 22, 2025
diff --git a/src/transformers/utils/chat_template_utils.py b/src/transformers/utils/chat_template_utils.py
@@ -450,13 +450,25 @@ def tojson(x, ensure_ascii=False, indent=None, separators=None, sort_keys=False)
         # We also expose some options like custom indents and separators
         return json.dumps(x, ensure_ascii=ensure_ascii, indent=indent, separators=separators, sort_keys=sort_keys)
 
+    def fromjson(x):
+        # Parse a JSON string into a Python object
+        # This is useful for parsing tool call arguments from JSON strings
+        if isinstance(x, str):
+            try:
+                return json.loads(x)
+            except (json.JSONDecodeError, TypeError):
+                # If parsing fails, return the original string
+                return x
+        return x
+
     def strftime_now(format):
         return datetime.now().strftime(format)
 
     jinja_env = ImmutableSandboxedEnvironment(
         trim_blocks=True, lstrip_blocks=True, extensions=[AssistantTracker, jinja2.ext.loopcontrols]
     )
     jinja_env.filters["tojson"] = tojson
+    jinja_env.filters["fromjson"] = fromjson
     jinja_env.globals["raise_exception"] = raise_exception
     jinja_env.globals["strftime_now"] = strftime_now
     return jinja_env.from_string(chat_template)

diff --git a/tests/test_tokenization_common.py b/tests/test_tokenization_common.py
@@ -1303,6 +1303,48 @@ def test_jinja_strftime(self):
                 self.assertEqual(len(strftime_output), 10)
                 self.assertEqual(len(strftime_output.split("-")), 3)
 
+    @require_jinja
+    def test_jinja_fromjson(self):
+        # Test fromjson filter for parsing JSON strings in chat templates
+        fromjson_template = (
+            """{% set args = '{"name": "test_func", "value": 42}' | fromjson %}{{ args.name }}: {{ args.value }}"""
+        )
+
+        # Test with tool calls that have JSON string arguments
+        tool_call_template = """{% for message in messages %}{% if message.tool_calls %}{% for tc in message.tool_calls %}{% set args = tc.function.arguments | fromjson %}Function: {{ tc.function.name }}, Args: {% for k, v in args.items() %}{{ k }}={{ v }}{% if not loop.last %}, {% endif %}{% endfor %}{% endfor %}{% endif %}{% endfor %}"""
+
+        dummy_conversation = [{"role": "user", "content": "test"}]
+
+        tool_conversation = [
+            {
+                "role": "assistant",
+                "content": "I'll help with that.",
+                "tool_calls": [{"function": {"name": "search", "arguments": '{"query": "hello world", "limit": 10}'}}],
+            }
+        ]
+
+        tokenizers = self.get_tokenizers()
+        for tokenizer in tokenizers:
+            with self.subTest(f"{tokenizer.__class__.__name__}"):
+                # Test basic fromjson usage
+                fromjson_output = tokenizer.apply_chat_template(
+                    dummy_conversation, chat_template=fromjson_template, tokenize=False
+                )
+                self.assertEqual(fromjson_output, "test_func: 42")
+
+                # Test fromjson with tool calls
+                tool_output = tokenizer.apply_chat_template(
+                    tool_conversation, chat_template=tool_call_template, tokenize=False
+                )
+                self.assertEqual(tool_output, "Function: search, Args: query=hello world, limit=10")
+
+                # Test that fromjson handles non-string inputs gracefully
+                graceful_template = """{{ 123 | fromjson }}"""
+                graceful_output = tokenizer.apply_chat_template(
+                    dummy_conversation, chat_template=graceful_template, tokenize=False
+                )
+                self.assertEqual(graceful_output, "123")
+
     @require_torch
     @require_jinja
     def test_chat_template_return_assistant_tokens_mask(self):