Opt-in extension inheritance for children (extension-provided model providers are unreachable)

[Sokoshy]

Problem

Child sessions are created with noExtensions: true (hardcoded in extensions/subagent.ts), so extension-provided model providers are invisible to children.

Concretely: my main model comes from a custom provider extension (registerProvider in a pi package). The parent session resolves command-code/xiaomi/mimo-v2.6-flash fine, but any agent_run child with an inherited (or explicit) command-code/* model fails before its first turn:

[scout · scout-1 · error · 0 turns]
Failed before the child started: No API key found for command-code.

The provider isn't missing credentials — it simply isn't registered in the child session.

Request

An opt-in, per-agent way to let a child load extensions, defaulting to today's behavior (none). Something in the spirit of the existing inherit_skills / system_prompt_mode frontmatter keys, for example:

---
description: Scout with corporate model access
tools: read, grep, find, ls
model: command-code/xiaomi/mimo-v2.6-flash
inherit_extensions: true
# or, preferably, an allowlist:
extensions: pi-commandcode-provider
---

Suggested semantics:

• inherit_extensions unset / false (or no extensions: list) → noExtensions: true, exactly as today. Safe default preserved.
• inherit_extensions: true → child loads the parent's extensions.
• extensions: <list> → child loads only the named extensions (mirrors the tools: allowlist philosophy: enumerate what the child may touch).

Why allowlist matters

Fully inheriting extensions would also hand the child every interactive/orchestration extension (ask-user-question, other subagent orchestrators, MCP adapters…), which is precisely the blast radius noExtensions: true was designed to close. A named allowlist keeps the scope bounded: in my case the child needs exactly one extension (the model provider), nothing else.

A cheaper moitié alternative, if full extension loading is off the table: allow inheriting providers only (inherit_providers: true / providers: [...]), since model access is the most common legitimate need and providers don't add tools to the child's surface.

Environment

• @pify/subagent 0.16.1 (npm)
• Pi 0.87.1
• Custom provider via a pi extension package (registerProvider)

Thanks for the great work on the suite — happy to test a preview build.