feat(mcp): resolve room-scoped servers for Copilot connections

docs/acp.md

@@ -160,8 +160,9 @@ assert slow.retention.transition_timeout_s == 120
   than twice on the same one. Only a bootstrap resume candidate is closed when its configuration
   is refused.
 - **Closure is confirmed, not assumed.** The process handle stays on the room until
-  `__aexit__` succeeds and the agent's stdout reaches EOF, so a descendant that
-  inherited it keeps the room closing until it exits, with a warning naming the
+  `__aexit__` succeeds, its child is reaped and the agent's stdout reaches EOF.
+  A child left running by a successful context exit is killed and waited for;
+  a descendant that inherited stdout keeps the room closing until it exits, with a warning naming the
   agent's pid. A failed or cancelled exit leaves the room owned, and a retry kills
   and waits for the same process. `cleanup_all()` and its alias `stop()` share one
   `transition_timeout_s` budget across the rooms and the shared Band MCP server,
@@ -176,6 +177,39 @@ assert slow.retention.transition_timeout_s == 120
   permission or extension requests carry no prompt id, so a call arriving after its
   turn is served with the room's current tools.
 
+## Per-room MCP servers
+
+`resolve_mcp_servers(workspace)`, a keyword-only constructor argument of
+`ACPClientAdapter`, `CopilotACPAdapter` and `CursorACPAdapter`, returns native
+`acp.schema` descriptors for that room's workspace.
+It runs once per session setup after the agent initializes. A reused live session
+keeps its servers; a restored session and its fresh-session fallback share one
+deep-copied snapshot. Recovery and a deliberate wake resolve anew on the new
+connection, as does a new session replacing a stale Band MCP endpoint.
+
+The result replaces `config.mcp_servers`, including when empty, except Band routing:
+a configured `band` entry and the injected Band server remain after the resolved
+external servers. Callback failure never falls back to configured external servers.
+
+- `McpServerStdio` is always accepted. `HttpMcpServer` and `SseMcpServer` require the
+  agent to explicitly advertise that transport in `initialize`. `AcpMcpServer` is
+  refused. Names must be unique and may not be `band`.
+- A refused descriptor fails the turn with `ACPMcpServerResolutionError`, naming
+  only its server and transport, never its URL, headers, command or environment.
+  Resolution failures recover the room's process through its retained owner; the
+  next turn resolves again on a fresh connection.
+- Queued turns share the room's initializer. One cancelled waiter leaves it for
+  another; the last waiter cancels it once without waiting for callback settlement.
+  The initializer's retained lease holds the process and workspace until it settles.
+  A callback that swallows cancellation or requests teardown cannot publish a session
+  or pass its late result to the agent.
+- A callback requesting `on_cleanup`, `cleanup_all` or `stop` gets
+  `CleanupIncompleteError` rather than waiting on itself. The retained owner continues
+  teardown after the callback unwinds, preserving the process, workspace and Band
+  MCP server until closure is confirmed.
+- There is no resolver timeout: a callback that never returns holds that room's
+  setup, while other rooms continue independently.
+
 ## Session config
 
 `resolve_session_config` runs after each new or restored session and before its first

src/band/adapters/copilot_acp.py

@@ -34,6 +34,7 @@
 from band.integrations.acp.client_adapter import (
     ACPClientAdapter,
     ACPClientAdapterConfig,
+    McpServerResolver,
     PermissionResolver,
 )
 from band.integrations.acp.session_config import SessionConfigResolver
@@ -80,6 +81,7 @@ def __init__(
         workspace_for_room: WorkspaceResolver | None = None,
         resolve_session_config: SessionConfigResolver | None = None,
         resolve_permission: PermissionResolver | None = None,
+        resolve_mcp_servers: McpServerResolver | None = None,
         **features: Unpack[FeatureKwargs],
     ) -> None:
         """Bridge Band rooms to ``copilot --acp``.
@@ -92,6 +94,8 @@ def __init__(
             resolve_session_config: Picks session config options; exclusive
                 with ``config.model`` and ``config.reasoning_effort``.
             resolve_permission: Chooses a permission option per tool call.
+            resolve_mcp_servers: Returns native MCP server descriptors once per
+                room session setup; see :class:`ACPClientAdapter`.
         """
         config = config or CopilotACPAdapterConfig()
         super().__init__(
@@ -100,6 +104,7 @@ def __init__(
             workspace_for_room=workspace_for_room,
             resolve_session_config=resolve_session_config,
             resolve_permission=resolve_permission,
+            resolve_mcp_servers=resolve_mcp_servers,
             **features,
         )
 

src/band/adapters/cursor_acp.py

@@ -28,6 +28,7 @@
     ACPClientAdapter,
     ACPClientAdapterConfig,
     ACPPermissionRequest,
+    McpServerResolver,
 )
 from band.integrations.acp.client_profiles import (
     CURSOR_ASK_QUESTION_METHOD,
@@ -207,6 +208,7 @@ def __init__(
         additional_tools: list[CustomToolDef] | None = None,
         workspace_for_room: WorkspaceResolver | None = None,
         resolve_session_config: SessionConfigResolver | None = None,
+        resolve_mcp_servers: McpServerResolver | None = None,
         **features: Unpack[FeatureKwargs],
     ) -> None:
         """Bridge Band rooms to Cursor's ``agent acp``.
@@ -217,6 +219,8 @@ def __init__(
             workspace_for_room: Maps a room id to its absolute workspace;
                 exclusive with ``config.cwd``.
             resolve_session_config: Picks session config options.
+            resolve_mcp_servers: Returns native MCP server descriptors once per
+                room session setup; see :class:`ACPClientAdapter`.
         """
         config = config or CursorACPAdapterConfig()
         # Each room's one accepted turn, from acceptance until it actually
@@ -231,6 +235,7 @@ def __init__(
             additional_tools=additional_tools,
             workspace_for_room=workspace_for_room,
             resolve_session_config=resolve_session_config,
+            resolve_mcp_servers=resolve_mcp_servers,
             resolve_permission=self._resolve_cursor_permission,
             **features,
         )