fix(chat): buffer streamed tool call until name arrives - #577

Summary

Fixes #560. Streaming chat completions broke tool-calling for Vercel-AI-SDK clients (e.g. opencode) with:

AI_InvalidResponseDataError: Expected 'function.name' to be a string.

Plain text generation was unaffected — only the streamed tool_call path.

Root cause

Some OpenAI-compatible upstreams (e.g. deepseek/deepseek-v4-pro, moonshotai/kimi-k2.7-code) stream a tool call whose first tool_calls delta carries only id + partial arguments, delivering function.name in a later delta:

data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_123","type":"function","function":{"arguments":""}}]}}]}
data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_123","type":"function","function":{"name":"bash","arguments":"{"}}]}}]}

ChatStreamEncoder forwarded each delta verbatim, so the first chunk a client saw had no function.name. The Vercel AI SDK (@ai-sdk/openai-compatible) rejects the first streamed chunk for a new tool-call index unless function.name is a string. (Same root cause as opencode#24137.)

PR #552 hardened the Responses / Messages / Generate Content encoders and the chat decoder against this upstream quirk, but left the chat encoder naive — that gap is this bug.

Fix

ChatStreamEncoder now holds a tool call's opening chunk back — buffering argument fragments per call — until a non-empty function.name arrives, then emits one named opening chunk and streams the remaining arguments. A call whose name never arrives is dropped, consistent with the server-tool loop's handling of nameless calls. The change is encoder-only; the decoder and the other encoders are untouched.

Test

Adds chat_encode_late_tool_name_first_chunk_carries_name, which replays the captured wire shape through the real chat decode → encode round trip and asserts the first emitted chunk names the function. It fails on main (function.name is null) and passes with the fix.

Verification

• cargo nextest run --all-features — 985 passed, 1 skipped
• cargo clippy --all-features — clean
• cargo fmt -- --check — clean

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