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11 — Multi-File Config

Split one config across multiple YAML files — each file owns one concern.

What this shows

  • load(["base.yaml", "agents.yaml"]) — pass a list and strands-compose merges them
  • Neither file is complete on its own; together they form a runnable config
  • Agents in agents.yaml reference models defined in base.yaml — cross-file references just work
  • vars: are scoped per file — each source resolves its own variables independently

How it works

load(["base.yaml", "agents.yaml"])
          ↓               ↓
    vars + models    agents + entry
        └─────────┬─────────┘
             merged config

Merging rules:

  • Collections (agents, models, mcp_servers, etc.) are combined — each file contributes unique names
  • Singletons (entry, log_level) use last-wins — the last file to define it wins
  • Duplicate names across files raise ValueError — intentional, not a bug
File Contents Standalone?
base.yaml vars, models ✗ no agents, no entry
agents.yaml agents, entry ✗ no models defined
both together complete config ✓ runnable

Good to know

Infra / app separation. One team owns base.yaml (models, MCP servers), another owns agents.yaml (agent definitions). Swap base.yaml per environment without touching agent logic.

Variables are file-scoped. ${MODEL} in base.yaml resolves from base.yaml's vars: block (or env). It won't see variables defined in agents.yaml.

YAML anchors are file-scoped too. An anchor in base.yaml can't be referenced from agents.yaml. For shared blocks across files, use variables instead.

Prerequisites

  • AWS credentials configured (aws configure or environment variables)
  • Dependencies installed: uv sync

Run

# Linux / macOS
uv run python examples/11_multi_file_config/main.py

# Override the model
MODEL=us.anthropic.claude-sonnet-4-6-v1:0 uv run python examples/11_multi_file_config/main.py

# Windows PowerShell
uv run python examples/11_multi_file_config/main.py

# Override the model
$env:MODEL="us.anthropic.claude-sonnet-4-6-v1:0"; uv run python examples/11_multi_file_config/main.py

Try these prompts

  • What is the difference between concurrency and parallelism?
  • Explain Python's GIL in one paragraph.
  • What are the SOLID principles?

Advanced topic — suppress default callback logging

Strands agents log actions to the console through their default callback_handler. If you want cleaner example output, set the handler to null in agent_kwargs for any agent:

agents:
  my_agent:
    agent_kwargs:
      callback_handler: null # or ~