Streaming tool_calls produce duplicate arguments chunks #361

Description

When using stream: true with tool calling, OpenCodex emits duplicate function.arguments chunks. OpenAI-compatible clients that accumulate argument deltas end up with corrupted JSON (concatenated duplicates), making tool calling unusable in streaming mode.

stream: false works correctly — each tool call has exactly one clean arguments JSON string.

Version

opencodex 2.7.35 (latest as of 2026-07-24)

Reproduction

curl -s -N http://127.0.0.1:10100/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "cursor/composer-2.5",
    "messages": [{"role": "user", "content": "Run ls /home/opc and read /home/opc/README.md. Do only tool calls, no text."}],
    "tools": [
      {"type": "function", "function": {"name": "shell", "parameters": {"type": "object", "properties": {"command": {"type": "string"}}, "required": ["command"]}}},
      {"type": "function", "function": {"name": "read_file", "parameters": {"type": "object", "properties": {"path": {"type": "string"}}, "required": ["path"]}}}
    ],
    "stream": true
  }'

Actual behavior (streaming — BROKEN)

Each tool call produces 3 SSE chunks instead of 2:

# Tool call 0 (shell):
Chunk 1: {"index":0,"id":"tool_xxx","function":{"name":"shell","arguments":""}}
Chunk 2: {"index":0,"function":{"arguments":"{\"command\":\"ls /home/opc\"}","name":"shell"}}
Chunk 3: {"index":0,"id":"tool_xxx","function":{"name":"shell","arguments":"{\"command\":\"ls /home/opc\"}"}}  ← DUPLICATE

# Tool call 1 (read_file):
Chunk 4: {"index":1,"id":"tool_yyy","function":{"name":"read_file","arguments":""}}
Chunk 5: {"index":1,"function":{"arguments":"{\"path\":\"/home/opc/README.md\"}","name":"read_file"}}
Chunk 6: {"index":1,"id":"tool_yyy","function":{"name":"read_file","arguments":"{\"path\":\"/home/opc/README.md\"}"}}  ← DUPLICATE

Chunks 3 and 6 are duplicates — they repeat the full arguments already delivered in chunks 2 and 5, but wrapped in the complete tool_call object (with id field).

Downstream clients that follow the OpenAI streaming spec (accumulate function.arguments deltas) end up with:

{"command":"ls /home/opc"}{"command":"ls /home/opc"}  ← corrupt JSON

This causes "Unrepairable tool_call arguments" errors in agents like Hermes.

Expected behavior (non-streaming — WORKS)

{
  "tool_calls": [
    {
      "id": "tool_0dc837b1-...",
      "type": "function",
      "function": {
        "name": "shell",
        "arguments": "{\"command\":\"ls /home/opc\"}"
      }
    },
    {
      "id": "tool_9bbfae2a-...",
      "type": "function",
      "function": {
        "name": "read_file",
        "arguments": "{\"path\":\"/home/opc/README.md\"}"
      }
    }
  ]
}

Each arguments appears exactly once. ✅

Impact

Severity: Medium — affects any OpenAI-compatible client that accumulates function.arguments deltas in streaming mode. The workaround is to force stream: false.

Additional context

• Cursor models affected: cursor/composer-2.5, cursor/composer-2.5-fast, cursor/auto
• Likely all Cursor models that support tool calling
• Non-streaming path is fine — the bug appears specific to the SSE streaming adapter