fix: preserve non-ASCII characters in JSON serialization

[edwardpwtsoi]

Add ensure_ascii=False to json.dumps calls to prevent Chinese, Japanese,
and other non-ASCII characters from being escaped as \uXXXX sequences.

This fixes the display of non-ASCII text in trace list views, where
characters were showing as Unicode escape sequences instead of
readable text.

---

Important

Add ensure_ascii=False to JSON serialization in utils.py and serializer.py to preserve non-ASCII characters.

• Behavior:
  • Add ensure_ascii=False to json.dumps in span_formatter() in utils.py to prevent non-ASCII characters from being escaped.
  • Add ensure_ascii=False to super().encode() in encode() in serializer.py to handle non-ASCII characters correctly.
• Misc:
  • Fixes display of non-ASCII text in trace list views by preventing Unicode escape sequences.

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Disclaimer: Experimental PR review

Greptile Overview

Greptile Summary

This PR aims to preserve non-ASCII characters (Chinese, Japanese, etc.) in JSON serialization by adding ensure_ascii=False to prevent Unicode escape sequences.

Critical Issue Found:

• langfuse/_utils/serializer.py:176 contains a syntax error that will cause TypeError at runtime
• JSONEncoder.encode() does not accept ensure_ascii as a parameter - it must be set in the constructor instead
• The change in langfuse/_client/utils.py:62 is correct

Impact:

• All code paths using EventSerializer().encode() will fail with TypeError
• This affects request serialization (langfuse/_utils/request.py:63), score ingestion (langfuse/_task_manager/score_ingestion_consumer.py:88, 120), and attribute conversion (langfuse/_client/attributes.py:177)

Required Fix:
Set ensure_ascii=False in EventSerializer.__init__() instead of passing it to super().encode()

Confidence Score: 0/5

• This PR cannot be merged - it contains a critical syntax error that will cause runtime failures
• The change in langfuse/_utils/serializer.py:176 passes ensure_ascii=False to super().encode(), but Python's JSONEncoder.encode() method only accepts one parameter (the object to encode). This will cause a TypeError: encode() takes 2 positional arguments but 3 were given when the code runs
• langfuse/_utils/serializer.py requires immediate correction to prevent runtime errors

Important Files Changed

Filename	Overview
langfuse/_client/utils.py	Added ensure_ascii=False to json.dumps() call in span_formatter() to preserve non-ASCII characters
langfuse/_utils/serializer.py	Attempted to pass ensure_ascii=False to super().encode() but this parameter is not accepted by the method - will cause TypeError

Sequence Diagram

sequenceDiagram
    participant Client
    participant json.dumps()
    participant EventSerializer
    participant JSONEncoder
    
    Note over Client,JSONEncoder: Two usage patterns
    
    rect rgb(200, 220, 240)
    Note over Client,EventSerializer: Pattern 1: Using json.dumps with cls parameter
    Client->>json.dumps(): json.dumps(data, cls=EventSerializer)
    json.dumps()->>EventSerializer: __init__() with ensure_ascii
    EventSerializer->>JSONEncoder: super().__init__(ensure_ascii=False)
    json.dumps()->>EventSerializer: default(obj)
    EventSerializer-->>json.dumps(): processed obj
    json.dumps()-->>Client: JSON string with non-ASCII chars
    end
    
    rect rgb(240, 220, 200)
    Note over Client,EventSerializer: Pattern 2: Direct instantiation
    Client->>EventSerializer: serializer = EventSerializer()
    EventSerializer->>JSONEncoder: super().__init__(ensure_ascii=False)
    Client->>EventSerializer: serializer.encode(obj)
    EventSerializer->>EventSerializer: self.default(obj)
    EventSerializer->>JSONEncoder: super().encode(processed_obj)
    Note over EventSerializer,JSONEncoder: BUG: ensure_ascii passed here (line 176)<br/>but encode() doesn't accept parameters
    JSONEncoder-->>EventSerializer: JSON string
    EventSerializer-->>Client: JSON string
    end

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