plugin marketplace add reports success but registration is not persisted (browse immediately fails with 'not found')

[bbecher]

Describe the bug

copilot plugin marketplace add reports success, but the marketplace registration is never persisted anywhere on disk. A subsequent copilot plugin marketplace list (even in the same command chain / immediately after) does not show the newly added marketplace, and copilot plugin marketplace browse <name> fails with Marketplace "<name>" not found.

This reproduces both for a local directory marketplace and a GitHub-hosted marketplace, so it does not appear to be specific to local-path or git-auth handling (unlike #4103).

Environment

• GitHub Copilot CLI: 1.0.74 (latest at time of filing)
• OS: Windows 11 (PowerShell 7 / native)
• Marketplace manifest: valid .claude-plugin/marketplace.json at repo root (per docs, this location is supported)

Steps to reproduce

# Local directory marketplace with a valid .claude-plugin/marketplace.json
copilot plugin marketplace add C:\Git\Agentic
copilot plugin marketplace list
copilot plugin marketplace browse company-skills

# Also reproduces with a GitHub-hosted marketplace
copilot plugin marketplace add anthropics/claude-code
copilot plugin marketplace list

Actual result

Marketplace "company-skills" added successfully.

...but copilot plugin marketplace list afterward only shows the two built-in marketplaces:

Included with GitHub Copilot:
  - copilot-plugins (GitHub: github/copilot-plugins)
  - awesome-copilot (GitHub: github/awesome-copilot)

And:

Fetching plugins from marketplace "company-skills"...
Failed to browse marketplace: Error: Marketplace "company-skills" not found

repro-marketplace.zip

Confirmed via Compare-Object of a full recursive file listing under ~/.copilot, %LOCALAPPDATA%, and %APPDATA% before/after running add — no files are created or modified anywhere as a result of the command, despite the CLI reporting success.

Expected result

copilot plugin marketplace add should persist the marketplace registration so that a subsequent list/browse in a new process reflects it, matching the documented behavior in https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-finding-installing.

Additional context

Possibly related to #3403 (~/.copilot/config.json being overwritten/reset on session start, dropping previously-written keys like hooks) — if marketplace registrations are also expected to live in config.json, the same overwrite-on-start bug could explain why the registration never survives past the single add invocation.

--- This report was investigated and drafted by GitHub Copilot CLI (Claude Sonnet 5).

Additional notes: name of marketplace changed for to company-skills. Additional details