fix(mcp-server): get_selection lists every selected id, with detail for the first 100

packages/mcp-server/src/selection.ts

@@ -69,14 +69,15 @@ export function parseSelectionAnswer(value: unknown): SelectionAnswer | null {
     return { pageId: text(given.pageId), pageName: text(given.pageName), cells }
 }
 
-/** describeSelection lists at most this many cells; the rest are counted */
+/** describeSelection details at most this many cells; the rest by id only */
 export const MAX_LISTED_CELLS = 100
 
 /**
  * The get_selection result text. `externalDrawio` names the draw.io origin
  * when the preview loads it from another origin, where the page cannot
- * read the editor. A selection of a whole large diagram is counted, not
- * listed in full: the model needs the ids of a few hundred cells at most.
+ * read the editor. A selection of a whole large diagram would fill the
+ * model's context with labels and geometry: the first cells get them, the
+ * rest are listed by id alone (an edit on the selection needs every id).
  */
 export function describeSelection(
     answer: SelectionAnswer,
@@ -110,10 +111,10 @@ export function describeSelection(
         const parent = cell.parent ? ` inside "${cell.parent}"` : ""
         return `- id="${cell.id}" shape ${label}${where}${parent}`
     })
-    const more = answer.cells.length - listed.length
-    if (more > 0) {
+    const rest = answer.cells.slice(MAX_LISTED_CELLS)
+    if (rest.length > 0) {
         lines.push(
-            `- and ${more} more cells not listed here; call get_diagram for the rest`,
+            `- and ${rest.length} more cells, listed by id only: ${rest.map((c) => `"${c.id}"`).join(", ")}`,
         )
     }
     const count =

packages/mcp-server/tests/selection.test.ts

@@ -53,7 +53,7 @@ describe("get_selection text", () => {
         expect(text).toContain('edit_diagram (page_id="p1")')
     })
 
-    it("counts a large selection instead of listing every cell", () => {
+    it("details the first cells of a large selection and lists the rest by id", () => {
         const cells = (n: number) =>
             Array.from({ length: n }, (_, i) => ({
                 id: `c${i + 1}`,
@@ -67,8 +67,13 @@ describe("get_selection text", () => {
         )
         expect(text).toContain(`${MAX_LISTED_CELLS + 5} cells selected`)
         expect(text).toContain(`id="c${MAX_LISTED_CELLS}"`)
+        // No detail past the cap, but every id: the model acts on all of
+        // them, and get_diagram cannot tell which cells were selected
         expect(text).not.toContain(`id="c${MAX_LISTED_CELLS + 1}"`)
-        expect(text).toContain("and 5 more cells not listed here")
+        expect(text).not.toContain(`shape "Cell ${MAX_LISTED_CELLS + 1}"`)
+        expect(text).toContain(
+            `- and 5 more cells, listed by id only: "c${MAX_LISTED_CELLS + 1}", "c${MAX_LISTED_CELLS + 2}", "c${MAX_LISTED_CELLS + 3}", "c${MAX_LISTED_CELLS + 4}", "c${MAX_LISTED_CELLS + 5}"`,
+        )
         const exact = describeSelection(
             { ...page, cells: cells(MAX_LISTED_CELLS) },
             null,