Set-if-undefined — set a parameter only if the request didn't

setParams and setParamsByID normally force their values, overriding whatever the client sent. Appending ? to a parameter key makes it set-if-undefined: the value is applied only when the request does not already carry that parameter.

models:
  qwen:
    filters:
      setParamsByID:
        "${MODEL_ID}:extraction":
          temperature: 0.2      # forced — the client cannot change it
          max_tokens?: 32768    # default — a request with max_tokens wins

A request to qwen:extraction without max_tokens gets 32768. A request that sets max_tokens: 4096 keeps 4096. temperature stays 0.2 either way.

This fills the gap between "the server has no opinion" (parameter absent from the config) and "the client has no say" (plain setParams): the server supplies a tuned value, the client can still deviate for a single request without anyone adding a new alias or copying the value into every client.

It works the same everywhere setParams works: model filters, per-alias setParamsByID entries, and peer filters.

Semantics

Filters apply like a pipe — stripParams | setParams | setParamsByID — and a ? key only fills what is undefined at its own stage:

• Any value present in the body counts as defined — including 0, false, "" and null. A ? key only fills a genuinely missing key.
• Stripped parameters count as undefined. stripParams runs first, so a stripped key can be refilled by a later ? key.
• A value set by an earlier stage counts as defined. A ? key in setParamsByID is a no-op when setParams already set that key — only a hard setParamsByID key overrides setParams.
• A hard spelling wins over a ? one. If the same block contains both max_tokens and max_tokens?, the hard value is used and the ? one is ignored.
• Keys are paths, as everywhere in filters. A dotted key like chat_template_kwargs.enable_thinking? checks and sets the nested value; the presence check targets exactly the location the write would modify, same as a hard setParams key.
• Protected params (model) cannot be set, with or without ?.

When to use which

Related

• guides/api-integration/filters-and-request-rewriting — the full filter pipeline and order of operations