fix: repair malformed JSON in streaming tool call arguments

[mateo-di]

Description

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• Autolink: [sc-522840]

Fixes intermittent JSON parse errors in Responses API streaming with tool calls when using Gemini models.

Problem

The error manifests as:

Error: Unexpected non-whitespace character after JSON at position 95
Error: Unexpected non-whitespace character after JSON at position 60

Root cause: Gemini streaming can send duplicate/overlapping JSON chunks that when naively joined with "".join() produce malformed JSON like:

{"address":"School"}{"address":"School"}

Solution

Use Python's built-in JSONDecoder.raw_decode() to extract the first valid JSON object. This is the canonical, C-optimized O(n) solution for handling "extraneous data at the end" of JSON strings.

Reference: Python JSON Documentation

Changes

• Add _validate_and_repair_tool_arguments() function using raw_decode()
• Apply fix to get_combined_tool_content() and get_combined_function_call_content()
• Add 19 comprehensive unit tests covering:
  • Valid JSON (simple, nested, arrays, escaped quotes, unicode)
  • Malformed JSON (duplicates, concatenated, garbage suffix)
  • Edge cases (empty string, empty object, invalid JSON)
  • Long/complex JSON (100 keys, 10-level nesting, 1000-element array)
  • Gemini-specific patterns (geocode duplicate, braces in strings)

Type of change

• Fix

Acceptance

1. Deploy to a test environment with Gemini models
2. Test streaming + function calling with the geocoding tool
3. Verify no more "Unexpected non-whitespace character after JSON" errors
4. Run unit tests: pytest tests/test_litellm/litellm_core_utils/test_streaming_chunk_builder_utils.py -v

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