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Agent Defaults

mas-runtime needs a handful of package-wide defaults for values a manifest is allowed to omit: the LLM model, the design pattern, and the context manager. These are data, not Python constants, following the same pattern as plugin aliases.

Discovery order

  1. Package defaults from src/mas/runtime/defaults.yaml.
  2. Workspace or user overrides from config.yaml's defaults: block.

Overrides win on a per-key basis — a workspace can override just design_pattern and still inherit the package's default model.

Configuration

# config.yaml
defaults:
  model: gpt-4.1-mini
  design_pattern: cot@v1
  context_manager: stack

This is validated against src/mas/runtime/defaults.schema.yaml (and against the root config.schema.yaml, which declares the same three keys under its own defaults: property).

How the values are used

  • model is exposed via mas.runtime.agent_defaults.default_model() / resolve_default_model(workspace=None), and is not a registry plugin type — it's a plain string consumed directly by callers that need an LLM model id (e.g. mas-ctl's resolve_model_name).
  • design_pattern and context_manager are registry spec keys: on startup, bootstrap.load_registry() reads defaults.yaml (merged with any config.yaml override) via mas.runtime.registry.defaults. load_defaults() and calls PluginRegistry.set_default(spec_key, plugin_id) for each one. agent_defaults.default_pattern_plugin_id() and default_context_manager_id() then simply read those back via PluginRegistry.default_for(spec_key) — there is exactly one source of truth for "what plugin does an omitted spec.design_pattern resolve to."

A plugin manifest (library.yaml, or a split-out *.plugins.yaml) can also declare its own defaults: block (see plugin-registry-manifests.md) to set the default for a type it introduces. That is scoped to a single manifest's own plugin types and complements this mechanism, which is the runtime-wide default for model, design_pattern, and context_manager.

Notes for contributors

  • Keep default values in package data (defaults.yaml), not in Python constants or registry bootstrap code.
  • When adding a new package-wide default key, add it to both defaults.schema.yaml's spec properties and config.schema.yaml's defaults properties, and decide whether it needs a registry set_default() call in bootstrap.load_registry() (only spec keys that are also registry plugin types do) or just a plain accessor (like model).
  • agent_defaults.py's public API is default_model(), resolve_default_model(workspace=None), default_pattern_plugin_id(), and default_context_manager_id().