Move oauthClientSecret out of mcp.json into the OS keychain

[cliffhall]

Summary

InspectorServerSettings.oauthClientSecret currently persists in plaintext inside ~/.mcp-inspector/mcp.json. The file is 0o600, which is appropriate for a local dev tool, but the secret is now mixed into a file users may:

• Symlink to / from Claude Desktop's claude_desktop_config.json (the spec calls this out as an interop benefit; the file is meant to be tool-shareable).
• cat for debugging, paste into bug reports, sync via dotfile managers, or commit by accident.

Goal: lift oauthClientSecret out of the on-disk JSON and store it in the OS keychain (macOS Keychain Services, Windows Credential Manager, Linux libsecret) keyed by (servers map key, "oauth-client-secret"). The mcp.json settings.oauthClientSecret field becomes a reference (or is absent) once a value has been promoted to the keychain.

Files

• core/mcp/types.ts — InspectorServerSettings shape; may need a discriminated oauthClientSecret: { stored: "keychain" } or similar.
• core/mcp/remote/node/server.ts — /api/servers POST/PUT/DELETE need to read/write the keychain alongside the on-disk JSON.
• core/mcp/serverList.ts — converter that hydrates the secret from the keychain into ServerEntry.settings.oauthClientSecret for in-memory use (browser never sees the raw value; it only sees a flag "secret configured").
• clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx — the Client Secret field becomes write-only (show "Configured" placeholder, "Clear" button) rather than echoing the value.
• specification/v2_servers_file.md — document the keychain integration; explain that other tools reading the file won't see the secret.

Proposal

Recommended library: keytar (mature, cross-platform). Adds a native dep.

• New helper core/auth/secret-store.ts with getSecret(serverId, field) / setSecret(serverId, field, value) / deleteSecret(serverId, field) backed by keytar.
• /api/servers POST/PUT: when body contains settings.oauthClientSecret, write to keychain and drop the field from disk.
• /api/servers GET: rehydrate from keychain on read so useServers sees the secret on the active session.
• /api/servers DELETE: tombstone keychain entries for that server id.
• Migration: on first read of a mcp.json that contains a oauthClientSecret value, lift to keychain and overwrite the file with the field removed.

Out of scope / open questions

• Linux without libsecret installed: keytar falls back to in-memory. Document.
• Tests in CI: keytar requires platform native bindings; integration tests need a mock backend (keytar provides setPassword/getPassword interfaces that are mockable).
• Whether to extend the same treatment to stdio env values (which are also secrets in many setups). Argue separately.

Acceptance criteria

• A oauthClientSecret entered in ServerSettingsForm is persisted to the OS keychain, not to mcp.json.
• cat ~/.mcp-inspector/mcp.json does not show the secret.
• Re-opening the settings modal shows "Configured" rather than the raw value.
• Removing the server (DELETE) also removes the keychain entry.
• Existing mcp.json files with oauthClientSecret in plaintext are migrated to the keychain on first read.