diff --git a/crates/graphicle/src/api.rs b/crates/graphicle/src/api.rs
index 7487132..e13fdf5 100644
--- a/crates/graphicle/src/api.rs
+++ b/crates/graphicle/src/api.rs
@@ -2001,16 +2001,26 @@ fn mcp_in(
                     }
                 })
             }
-            _ => Err(Error::bad("Unsupported MCP method")),
+            _ => Err(Error::bad(UNSUPPORTED_METHOD)),
         }
     })();
     Some(match result {
         Ok(value) => json!({"jsonrpc":"2.0","id":request["id"],"result":value}),
         Err(error) => {
-            json!({"jsonrpc":"2.0","id":request["id"],"error":{"code":-32602,"message":error.message}})
+            // JSON-RPC's "Method not found" for a method this connection
+            // does not serve, so that a client can tell it from a refused
+            // call.
+            let code = if error.message == UNSUPPORTED_METHOD {
+                -32601
+            } else {
+                -32602
+            };
+            json!({"jsonrpc":"2.0","id":request["id"],"error":{"code":code,"message":error.message}})
         }
     })
 }
+/// The error message of a method a connection does not serve.
+const UNSUPPORTED_METHOD: &str = "Unsupported MCP method";
 async fn mcp_http(
     State(app): State<App>,
     Path((workspace, repository)): Path<(String, String)>,
diff --git a/crates/graphicle/tests/it/workspace_scope.rs b/crates/graphicle/tests/it/workspace_scope.rs
index 916c057..686023d 100644
--- a/crates/graphicle/tests/it/workspace_scope.rs
+++ b/crates/graphicle/tests/it/workspace_scope.rs
@@ -1256,6 +1256,11 @@ fn a_workspace_connection_serves_the_graph_tools_of_the_project_repo_names() {
         "Unsupported MCP method",
         "{refused}"
     );
+    // JSON-RPC's "Method not found", not "Invalid params".
+    assert_eq!(refused["error"]["code"], -32601, "{refused}");
+    // A refused call of a served method keeps "Invalid params".
+    let unknown_tool = rpc(&app, "tools/call", json!({"name":"no_such_tool"}));
+    assert_eq!(unknown_tool["error"]["code"], -32602, "{unknown_tool}");
     // A navigation tool names its project, by id or key.
     let unnamed = call(&app, "get_node", json!({"label":"quantize_cents"}));
     assert!(