Let the MCP server be told which database once

`create_mcp_server` promised one database and accepted a scope covering a set,
where the write tools exist and fail on use. It refuses that now.

Resolving is the public factory's job, as it is `HaikuRAG`'s: `_covering` takes
a scope someone already resolved, so the configured name survives without a
`DatabaseScope` reaching the public signature.

The test that a scope decides the database asserted `all(...)` over a search
that could return nothing, which held whatever the server read. It reads the
listing instead, so alpha's documents being present and beta's absent both have
to be true.

docs/python.md

@@ -261,9 +261,11 @@ their documents.
 none: `search` returns no results, and `ask` and `analyze` run with no evidence
 from any database.
 
-On the constructor, `sources` is rejected alongside a database path, and `[]`
-raises `ValueError`. A selection of nothing to search is a legitimate question;
-a client over no database is not.
+On the constructor it means something else. Passing `sources` alongside a
+database path raises `AmbiguousDatabaseError` immediately, since both say which
+database to open. Passing `sources=[]` alone raises `ValueError` on entering the
+client: a selection of nothing to search is a legitimate question, a client over
+no database is not.
 
 #### Inspecting the client scope
 

haiku_rag_slim/haiku/rag/app.py

@@ -18,7 +18,7 @@ from rich.progress import (
 
 from haiku.rag.client import HaikuRAG, RebuildMode
 from haiku.rag.config import AppConfig, get_config
-from haiku.rag.mcp import create_mcp_server
+from haiku.rag.mcp import _covering as _mcp_server_covering
 from haiku.rag.store.models.chunk import SearchType
 from haiku.rag.store.models.document import Document
 
@@ -926,9 +926,7 @@ class HaikuRAGApp:
         # The resolved scope, not a derived path and configuration: a path
         # would override a configured URI, and deriving drops the name results
         # and citations carry.
-        server = create_mcp_server(
-            config=self.config, read_only=self.read_only, scope=self.scope
-        )
+        server = _mcp_server_covering(self.scope, self.config, self.read_only)
         try:
             if transport == "stdio":
                 await server.run_stdio_async()

haiku_rag_slim/haiku/rag/mcp.py

@@ -28,7 +28,6 @@ def create_mcp_server(
     db_path: Path | None = None,
     config: AppConfig | None = None,
     read_only: bool = False,
-    scope: "DatabaseScope | None" = None,
 ) -> FastMCP:
     """Create an MCP server over one database.
 
@@ -38,15 +37,30 @@ def create_mcp_server(
             must pass None rather than a local stand-in.
         config: Configuration to use.
         read_only: If True, write tools (add_document_*, delete_document) are not registered.
-        scope: The database, already resolved. Pass this rather than a derived
-            path and configuration to keep its configured name, which results
-            and citations carry as `source`.
     """
     from haiku.rag.client.scope import DatabaseScope
 
     config = config if config is not None else get_config()
-    if scope is None:
-        scope = DatabaseScope.resolve(config, database_path=db_path)
+    return _covering(
+        DatabaseScope.resolve(config, database_path=db_path), config, read_only
+    )
+
+
+def _covering(scope: "DatabaseScope", config: AppConfig, read_only: bool) -> FastMCP:
+    """An MCP server over databases someone already resolved.
+
+    Internal, as ``HaikuRAG._covering`` is: the public factory takes a path and
+    resolves it, which is its own job. A caller that resolved already passes the
+    scope, so the configured name survives, which results and citations carry as
+    ``source``.
+    """
+    from haiku.rag.store.exceptions import AmbiguousDatabaseError
+
+    if scope.covers_multiple:
+        raise AmbiguousDatabaseError(
+            "an MCP server serves one database, and this scope covers "
+            f"{', '.join(scope.names)}; name the one to serve"
+        )
     client: HaikuRAG | None = None
     stack = AsyncExitStack()
     client_lock = asyncio.Lock()

tests/multi_db/test_capabilities.py

@@ -226,7 +226,7 @@ class TestLendingANamedClient:
             run = await capability.for_run(make_context(deps))
             assert run.state is not None
 
-            # The lent client is what it reads through, whatever it was built with.
+            # A borrowed client overrides the capability's configured placement.
             assert await run._ensure_rag() is client
 
             run.state.searches["cats"] = await client.search("cats", search_type="fts")

tests/test_app.py

@@ -620,7 +620,7 @@ def test_search_result_rendering_includes_provenance(app):
 
 async def test_run_mcp_stdio(app, client, monkeypatch):
     server = AsyncMock()
-    monkeypatch.setattr("haiku.rag.app.create_mcp_server", lambda *a, **kw: server)
+    monkeypatch.setattr("haiku.rag.app._mcp_server_covering", lambda *a, **kw: server)
 
     await app.run_mcp(transport="stdio")
 
@@ -629,7 +629,7 @@ async def test_run_mcp_stdio(app, client, monkeypatch):
 
 async def test_run_mcp_http(app, client, monkeypatch):
     server = AsyncMock()
-    monkeypatch.setattr("haiku.rag.app.create_mcp_server", lambda *a, **kw: server)
+    monkeypatch.setattr("haiku.rag.app._mcp_server_covering", lambda *a, **kw: server)
 
     await app.run_mcp(host="0.0.0.0", port=9001)
 
@@ -641,7 +641,7 @@ async def test_run_mcp_http(app, client, monkeypatch):
 async def test_run_mcp_survives_interruption(app, client, monkeypatch):
     server = AsyncMock()
     server.run_stdio_async.side_effect = KeyboardInterrupt
-    monkeypatch.setattr("haiku.rag.app.create_mcp_server", lambda *a, **kw: server)
+    monkeypatch.setattr("haiku.rag.app._mcp_server_covering", lambda *a, **kw: server)
 
     await app.run_mcp(transport="stdio")
 

tests/test_mcp.py

@@ -1,6 +1,7 @@
 import pytest
 
 from haiku.rag.client import HaikuRAG
+from haiku.rag.mcp import _covering as _mcp_covering
 from haiku.rag.mcp import create_mcp_server
 from haiku.rag.store.models import Chunk, Document, SearchResult
 from haiku.rag.tools.document import DocumentInfo
@@ -624,15 +625,46 @@ class TestMCPClientLifetime:
         )
         scope = DatabaseScope.resolve(config, database_name="alpha")
 
-        mcp = create_mcp_server(config=config, read_only=True, scope=scope)
+        mcp = _mcp_covering(scope, config, read_only=True)
         async with mcp._lifespan_manager():
             search = await _get_tool(mcp, "search_documents")
             results = await search(query="artificial intelligence")
-            zebras = await search(query="zebras savannah")
+            listing = await _get_tool(mcp, "list_documents")
+            documents = await listing()
 
         assert results
         assert {r.source for r in results} == {"alpha"}
-        assert all(r.source == "alpha" for r in zebras)
+        titles = {d.title for d in documents}
+        assert "AI Overview" in titles
+        assert "Zebras" not in titles
+
+    def test_a_scope_covering_a_set_is_refused(self, tmp_path):
+        from haiku.rag.client.scope import DatabaseScope
+        from haiku.rag.config.models import AppConfig, LanceDBConfig
+        from haiku.rag.store.exceptions import AmbiguousDatabaseError
+
+        config = AppConfig(
+            lancedb=LanceDBConfig(
+                databases={"alpha": str(tmp_path / "a"), "beta": str(tmp_path / "b")}
+            )
+        )
+
+        with pytest.raises(AmbiguousDatabaseError, match="alpha, beta"):
+            _mcp_covering(DatabaseScope.resolve(config), config, read_only=True)
+
+    def test_the_public_factory_refuses_a_configured_set_too(self, tmp_path):
+        """It resolves the same scope, so it reaches the same refusal."""
+        from haiku.rag.config.models import AppConfig, LanceDBConfig
+        from haiku.rag.store.exceptions import AmbiguousDatabaseError
+
+        config = AppConfig(
+            lancedb=LanceDBConfig(
+                databases={"alpha": str(tmp_path / "a"), "beta": str(tmp_path / "b")}
+            )
+        )
+
+        with pytest.raises(AmbiguousDatabaseError, match="alpha, beta"):
+            create_mcp_server(config=config)
 
     @pytest.mark.asyncio
     async def test_the_command_hands_the_server_its_resolved_database(
@@ -653,21 +685,22 @@ class TestMCPClientLifetime:
             async def run_stdio_async(self):
                 return None
 
-        def fake_create(db_path=None, config=None, read_only=False, scope=None):
-            seen.update(db_path=db_path, scope=scope)
+        def fake_covering(scope, config, read_only):
+            seen.update(scope=scope, config=config)
             return _Server()
 
-        monkeypatch.setattr("haiku.rag.app.create_mcp_server", fake_create)
+        monkeypatch.setattr("haiku.rag.app._mcp_server_covering", fake_covering)
         app = HaikuRAGApp(
             scope=DatabaseScope.resolve(config, database_name="prod"), config=config
         )
 
         await app.run_mcp(transport="stdio")
 
-        assert seen["db_path"] is None
         [ref] = seen["scope"].databases
         assert ref.name == "prod"
         assert ref.uri == "s3://bucket/prod.lancedb"
+        # The caller's configuration, not one derived from the ref.
+        assert seen["config"].lancedb.databases == {"prod": "s3://bucket/prod.lancedb"}
 
     @pytest.mark.asyncio
     async def test_startup_fails_when_the_database_cannot_open(self, tmp_path):