[GPT-AGI]

optillm/plugins/proxy/example_config.yaml documents:

api_key: ${OPENAI_API_KEY_2:-${OPENAI_API_KEY}}  # Fallback to primary key

With OPENAI_API_KEY=sk-primary, this returned the literal ${OPENAI_API_KEY} when the secondary key was unset, or sk-secondary} when it was set.

The interpolation regex stopped at the inner closing brace. It now recognizes the documented nested-default form before the existing simple form, while preserving simple variables, literal defaults, unknown variables, and environment values verbatim.

Verified on Windows with Python 3.12 after installing the same requirements as CI: TestProxyEnvInterpolation passes (3 tests), and tests/test_proxy_plugin.py passes in full (12 tests).

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