Expressions & Variables

Using built-in variables and simple expressions.

The ONR DSL is intentionally declarative, meaning there is no Turing-complete scripting language embedded. However, you can use built-in variables to dynamically construct paths, headers, and keys.

Expression Contexts

In positions where the DSL requires an <expr>, you can use:

1. String Literal: "abc"
2. Variable: $channel.key
3. Function: concat("Bearer ", $channel.key)

Note: Variables are only evaluated when used as bare expressions. If you wrap them in double quotes (e.g., "$channel.key"), they are treated as plain string literals and will not be expanded.

Built-in Variables

These variables are populated at runtime dynamically for every request.

channel.* context

• $channel.base_url: The channel base URL (string). Overrides upstream_config.base_url if set by the API caller.
• $channel.key: The downstream API key/token (string) meant to be passed to the upstream provider.
• $channel.location: The provider location or region attached to the selected upstream key, such as global or us-central1.

credential.* context

• $credential.project_id: The project id parsed from the active credential. For Google service account files, this is the JSON project_id.

request.* context

• $request.model: The original requested model name from the upstream JSON body.
• $request.model_mapped: The transformed model name, determined after any model_map operations fire.

Expression Examples

request {
  # Set a custom header based on the target mapped model
  set_header "x-upstream-model" $request.model_mapped;
}

upstream {
  # Dynamically construct the path
  set_path concat("/v1/", $request.model_mapped, "/chat/completions");
  
  # Inject query strings dynamically
  set_query key $channel.key;
}

provider "vertex" {
  match api = "gemini.generateContent" {
    upstream {
      set_path template("/v1/projects/${credential.project_id}/locations/${channel.location}/publishers/google/models/${request.model_mapped}:generateContent");
    }
  }
}