feat: inject per-user MCP servers into ACP sessions

[Reese-max]

What problem does this solve?

Users can manage MCP servers via Discord slash commands such as /mcp-add, /mcp-remove, and /mcp-list, but those profile entries are never injected into ACP sessions. The profile JSON is written, but the backend session still starts without mcpServers, so /mcp-add is a dead-end UI.

This change closes the loop from Discord MCP profile management to runtime MCP activation.

At a Glance

Discord user
  |
  | /mcp-add mempalace http://...
  v
profile JSON: data/mcp-profiles/{bot}/{user_id}.json
  |
  v
message / slash command
  |
  v
discord.rs -> mcp_servers_for_user(user_id)
  |
  v
pool.get_or_create(thread, mcp_servers)
  |
  v
connection.session_new/session_load(..., mcp_servers)
  |
  v
ACP JSON-RPC session/new | session/load
  { cwd, mcpServers: [{ name, type, url, headers }] }
  |
  v
Claude / Copilot / Gemini / Codex session loads MCP tools

Prior Art

OpenClaw

OpenClaw merges MCP config from bundle defaults plus global config and injects it at session/runtime creation. For CLI backends it relies on backend-specific config/CLI arg injection such as --mcp-config, not ACP session/new.

Relevant difference:

• Scope is global, not per user.
• Injection is backend-specific.
• Good reference for runtime lifecycle and config invalidation, but not for Discord multi-user isolation.

Hermes Agent

Hermes supports mcp_servers on ACP session creation, but tools are registered into a process-wide registry rather than isolated per Discord user/session.

Relevant difference:

• Uses ACP-style injection similar to this PR.
• Registry is global, so one user's MCP setup can affect others.
• Better fit for ACP transport shape, weaker fit for Discord multi-user isolation.

Comparison

Aspect	OpenClaw	Hermes Agent	OpenAB in this PR
MCP config scope	Global	Global/process-wide	Per-user per-bot
Injection point	Config merge + CLI args	ACP session params	ACP session params
User isolation	No	No	Yes
Management surface	Chat / CLI	CLI	Discord slash commands
Activation timing	Runtime/config reload	Session/runtime	Next session / /new-session

Proposed Solution

1. Add McpServerEntry and read_mcp_profile() in src/config.rs to read {mcp_profiles_dir}/{user_id}.json.
2. Extend session_new() / session_load() in src/acp/connection.rs to accept mcp_servers and send them through ACP.
3. Extend SessionPool::get_or_create() in src/acp/pool.rs to thread mcp_servers into new or resumed sessions.
4. In src/discord.rs, resolve the calling user's MCP profile and inject it into message handling and session-creating slash commands.
5. Keep diagnostic commands on &[] so health/status flows do not depend on user MCP state.

Profile JSON stays in the existing Discord-managed format:

{
  "discord_user_id": "844236700611379200",
  "mcpServers": {
    "mempalace": {
      "type": "http",
      "url": "http://127.0.0.1:18793/mcp",
      "headers": []
    }
  },
  "enabled": true
}

Converted at runtime to ACP format:

[
  {
    "name": "mempalace",
    "type": "http",
    "url": "http://127.0.0.1:18793/mcp",
    "headers": []
  }
]

Why this approach?

• ACP-native: all target backends accept mcpServers on session creation.
• Per-user isolation: MCP config is keyed by Discord user ID, not global process state.
• Per-bot isolation: each bot keeps its own mcp_profiles_dir.
• Graceful fallback: missing or invalid profile returns an empty list.
• No backend-specific config mutation: avoids touching ~/.claude*, Copilot config files, or other local machine state.

Alternatives Considered

1. Backend-specific config file mutation.
   Rejected because it is fragile, backend-specific, and risks overwriting user-managed config.

2. Global singleton MCP registry.
   Rejected because OpenAB is a shared Discord bot process and would leak one user's MCP tools into another user's sessions.

3. In-session hot reload.
   Deferred because the current ACP/session model is simpler and the UX is already covered by “next session” plus /new-session.

Validation

Current branch validation:

• cargo check
• cargo build --release
• cargo test (42 passed, 0 failed)
• cargo fmt --check
• Branch diff excludes personal bat / local profile / runtime script files

Previously validated during the implementation session:

• Claude ACP E2E: profile -> session/new(mcpServers) -> tool discovery -> mempalace_search
• Backend format verification for Claude / Copilot / Gemini / Codex
• /mcp-add UX updated to note that changes apply on next session and /new-session can apply immediately

Notes

This PR branch also includes a small follow-up cleanup to remove dead-code warnings in the model picker path so the branch stays warning-free during review.

[github-actions]

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