Zen gateway (/go/v1) streaming: tool_calls continuation deltas send "id":null / "function":{"name":null} instead of omitting absent fields #43328

Description

Streaming chat completions from the Zen gateway (https://opencode.ai/zen/go/v1/chat/completions) emit tool_calls continuation deltas with explicit null values for id and function.name instead of omitting the absent fields, as the OpenAI streaming format specifies.

The first tool_calls delta of a call carries the real values, e.g.:

"delta":{"role":null,"content":"","tool_calls":[{"index":0,"id":"chatcmpl-tool-b4d3cf2ed9145904","type":"function","function":{"name":"calculator","arguments":""}}]}

but every continuation delta for the same call sends:

"delta":{"role":null,"content":"","tool_calls":[{"index":0,"id":null,"type":null,"function":{"name":null,"arguments":""}}]}

This breaks strict OpenAI-compatible clients that accumulate deltas and treat any present field as an update. Concrete case: DeepSeek Harness (dsh) accumulates if (call.function?.name !== undefined) block.name = call.function.name — the null overwrites the previously accumulated name, and every tool call ends up dispatched as unknown tool "". The same request non-streamed returns a correct message.tool_calls[] with id and function.name intact.

Observed model: deepseek-v4-flash (stream mode). The behavior is intermittent across time windows — client-side session logs show tool calls failing on 2026-08-17 ~04:34–12:08 UTC and 2026-08-19 ~02:37–02:50 UTC, while requests outside those windows (e.g., all of 2026-08-18) received spec-compliant chunks with the fields omitted. That pattern suggests different upstream backends behind the gateway, some of which emit null-filled deltas.

Plugins

n/a

OpenCode version

n/a — Zen gateway (opencode.ai/zen/go/v1), captured 2026-08-19 02:56 UTC

Steps to reproduce

curl -N https://opencode.ai/zen/go/v1/chat/completions \
  -H "Authorization: Bearer $KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "deepseek-v4-flash",
    "messages": [{"role":"user","content":"Use the calculator tool to compute 2+2"}],
    "tools": [{"type":"function","function":{"name":"calculator","description":"compute an arithmetic expression","parameters":{"type":"object","properties":{"expr":{"type":"string"}},"required":["expr"]}}}],
    "tool_choice": "auto",
    "max_tokens": 200,
    "stream": true
  }'

The streamed SSE shows the chunk sequence quoted above: id/name present on the first tool_calls delta, explicit null on the continuation deltas.

Expected: continuation deltas omit id / type / function.name when they carry no value (per the OpenAI streaming format), so field-presence-based delta accumulators keep the values from the first delta.

Screenshot and/or share link

n/a (raw SSE text)

Operating System

macOS 25.5 (arm64), curl — issue is server-side