Enforce required and named tool_choice by pinning the call's opening

[mmastrac]

vLLM enforces tool_choice: "required" and a named function with structured outputs, and it rejects structured outputs for diffusion models ("not yet supported for diffusion language models"). On the chat path both choices were therefore ignored: with tool_choice naming get_time, the model called get_weather. required only appeared to work because the model happened to call something.

A diffusion canvas can be seeded and pinned instead, which is what this server already does for its reads. For those two choices, the relay now pins the empty thought block and the opening of a tool call on the canvas, <|tool_call>call:NAME{, and sends tool_choice: "auto" so vLLM's tool parser reads the call. The model can only fill in the arguments.

For required, the name is the only function offered, or the one a one-step choice read picks over the offered functions' names and descriptions (pick_tool). The first version pinned only <|tool_call>call: and let the model write the name. When no offered tool fit the request, it wrote nonsense: get_, the punchline of a requested joke, or no call at all. Requests that seed their own canvas, and names not among the offered tools, are passed to vLLM unchanged.

The README notes the vLLM flags this assumes: --enable-auto-tool-choice --tool-call-parser gemma4, and --exclude-tools-when-tool-choice-none so that "none" gets a plain answer rather than a call the parser strips.

Tested live on a GB10 (nightly-e9757321, gemma4 tool parser), with a second server process on the same vLLM:

tool_choice	before	after
auto	get_weather({"city": "Paris"})	same
auto, no tool needed	"Bonjour !"	same
required	get_weather, unenforced	get_weather, pinned
required, "Tell me a joke."	the joke as a function name	get_time, an offered tool
required, "Say hello in French."	no call	get_time
none (with --exclude-tools-when-tool-choice-none)	empty content	a text answer
named get_time	get_weather	get_time({"city": "Paris"})

Also checked:

• A streamed named call arrives as get_time with its arguments.
• A full round trip works: a required call, then a tool result, then the final answer ("The current weather in Paris is 14°C with light rain.").

🤖 Generated with Claude Code

[mmastrac]

(this won't land as-is - just a stopgap for missing vLLM features)