database_mcp_postgres/tools/
list_databases.rs

1//! MCP tool: `listDatabases`.
2
3use std::borrow::Cow;
4
5use database_mcp_server::pagination::Pager;
6use database_mcp_server::types::{ListDatabasesRequest, ListDatabasesResponse};
7use database_mcp_sql::Connection as _;
8use rmcp::handler::server::router::tool::{AsyncTool, ToolBase};
9use rmcp::model::{ErrorData, ToolAnnotations};
10
11use crate::PostgresHandler;
12
13/// Marker type for the `listDatabases` MCP tool.
14pub(crate) struct ListDatabasesTool;
15
16impl ListDatabasesTool {
17    const NAME: &'static str = "listDatabases";
18    const TITLE: &'static str = "List Databases";
19    const DESCRIPTION: &'static str = r#"List all accessible databases on the connected server. Use this tool to discover what databases are available before using other tools.
20
21<usecase>
22ALWAYS call this tool FIRST when:
23- You need to explore what databases exist on the server
24- You need a database name for listTables, getTableSchema, or query tools
25- The user asks what data is available
26</usecase>
27
28<examples>
29✓ "What databases are on this server?"
30✓ "Show me what's available" → call listDatabases first
31</examples>
32
33<what_it_returns>
34A sorted JSON array of database name strings.
35</what_it_returns>
36
37<pagination>
38Paginated. Pass the prior response's `nextCursor` as `cursor` to fetch the next page.
39</pagination>"#;
40}
41
42impl ToolBase for ListDatabasesTool {
43    type Parameter = ListDatabasesRequest;
44    type Output = ListDatabasesResponse;
45    type Error = ErrorData;
46
47    fn name() -> Cow<'static, str> {
48        Self::NAME.into()
49    }
50
51    fn title() -> Option<String> {
52        Some(Self::TITLE.into())
53    }
54
55    fn description() -> Option<Cow<'static, str>> {
56        Some(Self::DESCRIPTION.into())
57    }
58
59    fn annotations() -> Option<ToolAnnotations> {
60        Some(
61            ToolAnnotations::new()
62                .read_only(true)
63                .destructive(false)
64                .idempotent(true)
65                .open_world(false),
66        )
67    }
68}
69
70impl AsyncTool<PostgresHandler> for ListDatabasesTool {
71    async fn invoke(handler: &PostgresHandler, params: Self::Parameter) -> Result<Self::Output, Self::Error> {
72        handler.list_databases(params).await
73    }
74}
75
76impl PostgresHandler {
77    /// Lists one page of accessible databases.
78    ///
79    /// Uses the default pool intentionally — `pg_database` is a server-wide
80    /// catalog that returns all databases regardless of which database the
81    /// connection targets.
82    ///
83    /// # Errors
84    ///
85    /// Returns [`ErrorData`] with code `-32602` if `cursor` is malformed,
86    /// or an internal-error [`ErrorData`] if the underlying query fails.
87    pub async fn list_databases(
88        &self,
89        ListDatabasesRequest { cursor }: ListDatabasesRequest,
90    ) -> Result<ListDatabasesResponse, ErrorData> {
91        let pager = Pager::new(cursor, self.config.page_size);
92        let query = format!(
93            r"
94            SELECT datname
95            FROM pg_database
96            WHERE datistemplate = false
97            ORDER BY datname
98            LIMIT {} OFFSET {}",
99            pager.limit(),
100            pager.offset(),
101        );
102
103        let rows: Vec<String> = self.connection.fetch_scalar(query.as_str(), None).await?;
104        let (databases, next_cursor) = pager.finalize(rows);
105
106        Ok(ListDatabasesResponse { databases, next_cursor })
107    }
108}
database_mcp_postgres/tools/list_databases.rs

database_mcp_postgres/tools/
list_databases.rs
