[bot] Document model fallback lists, CIMD OAuth, and sandbox network isolation (v1.0.83)

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Summary

Routine check of GitHub Copilot CLI changelog (v1.0.82–v1.0.84) against the Learning Hub found three undocumented features from v1.0.83 worth adding to existing pages. No new pages were warranted — all changes fit as additions to existing sections.

What was found

• Model fallback lists for custom agents: model in agent frontmatter can now list several models, tried in order until one is available, paired with model-policy: required to pin the agent to that list.
• Client ID Metadata Document (CIMD) support: Copilot CLI can sign in to MCP servers using CIMD for OAuth, an alternative to Dynamic Client Registration.
• Sandbox network isolation (breaking change): On macOS and Linux, sandboxed commands can no longer reach services on the local machine (including a server the command itself starts on 127.0.0.1). Linux sandboxing also now requires additional networking tools (slirp4netns, nsenter, iptables, etc.) on PATH, and can restrict egress to a configured proxy.

Sections updated

• building-custom-agents.md — added a note on model fallback lists and model-policy under the frontmatter fields section.
• understanding-mcp-servers.md — added a note on CIMD OAuth support under the Authentication section.
• copilot-configuration-basics.md — added a breaking-change note on sandbox network isolation and new Linux sandboxing prerequisites under the sandbox settings section.

Other items in the v1.0.81–v1.0.84 changelog (plugins dashboard rollout, MCP 2026-07-28 support, hook trace context) were already documented in the Learning Hub. UI-only changes (taskbar session indicator, sessions sidebar sorting, herdr terminal detection) were judged too minor to warrant new documentation.

Source

• https://github.com/github/copilot-cli/blob/main/changelog.md (v1.0.81–v1.0.83 entries)

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