diff --git a/packages/editor-ext/src/lib/recreate-transform/recreateTransform.test.ts b/packages/editor-ext/src/lib/recreate-transform/recreateTransform.test.ts
new file mode 100644
index 00000000..a30dc3d2
--- /dev/null
+++ b/packages/editor-ext/src/lib/recreate-transform/recreateTransform.test.ts
@@ -0,0 +1,133 @@
+import { describe, it, expect } from "vitest";
+import { schema } from "@tiptap/pm/schema-basic";
+import type { Node as PMNode } from "@tiptap/pm/model";
+import { Transform } from "@tiptap/pm/transform";
+import { recreateTransform } from "./recreateTransform";
+
+/**
+ * recreateTransform diffs two documents and produces ProseMirror steps that turn
+ * `fromDoc` into `toDoc`. It is the backbone of collaborative/version diffing, so
+ * THE invariant that matters is: replaying the produced steps on `fromDoc` must
+ * reproduce `toDoc` exactly. Every test below re-applies the steps onto a fresh
+ * Transform seeded from `fromDoc` (not just trusting `tr.doc`) and asserts node
+ * equality with `.eq()`. If a regression makes any step wrong, the round-trip
+ * breaks and the test fails.
+ */
+
+// Real ProseMirror schema (the standard basic schema) with paragraph/heading +
+// strong/em marks — the same primitives the editor diffs in production.
+const doc = (...c: PMNode[]) => schema.node("doc", null, c);
+const p = (...c: PMNode[]) =>
+  schema.node("paragraph", null, c.length ? c : undefined);
+const h = (level: number, ...c: PMNode[]) =>
+  schema.node("heading", { level }, c);
+const t = (text: string, ...marks: any[]) =>
+  schema.text(text, marks.length ? marks : undefined);
+const strong = schema.marks.strong.create();
+const em = schema.marks.em.create();
+
+// Replay the diff's steps onto a fresh Transform built from `fromDoc`. This is
+// the faithful "apply(diff) == target" check — it exercises the actual Step
+// objects rather than the transform's internal accumulated doc.
+function applyDiff(fromDoc: PMNode, toDoc: PMNode, options?: any): PMNode {
+  const tr = recreateTransform(fromDoc, toDoc, options);
+  const replay = new Transform(fromDoc);
+  tr.steps.forEach((s) => {
+    const result = replay.maybeStep(s);
+    if (result.failed) throw new Error(`step failed: ${result.failed}`);
+  });
+  return replay.doc;
+}
+
+describe("recreateTransform round-trip (apply(diff) == target)", () => {
+  it("reconstructs the target on plain text insertion", () => {
+    // Inserting " world" must yield exactly the target paragraph.
+    const from = doc(p(t("hello")));
+    const to = doc(p(t("hello world")));
+    expect(applyDiff(from, to).eq(to)).toBe(true);
+  });
+
+  it("reconstructs the target on text deletion", () => {
+    // Deleting a trailing word is the inverse of insertion and must round-trip.
+    const from = doc(p(t("hello world")));
+    const to = doc(p(t("hello")));
+    expect(applyDiff(from, to).eq(to)).toBe(true);
+  });
+
+  it("reconstructs the target when a word is replaced mid-string", () => {
+    // A char-level replace in the middle must not corrupt the surrounding text.
+    const from = doc(p(t("the quick brown fox")));
+    const to = doc(p(t("the slow brown fox")));
+    expect(applyDiff(from, to).eq(to)).toBe(true);
+  });
+
+  it("reconstructs the target when a mark is added (complexSteps path)", () => {
+    // Mark-only changes are diffed in a separate pass; the bolded run must match.
+    const from = doc(p(t("hello")));
+    const to = doc(p(t("hello", strong)));
+    const out = applyDiff(from, to);
+    expect(out.eq(to)).toBe(true);
+    // Sanity: the produced doc actually carries the strong mark.
+    expect(out.firstChild!.firstChild!.marks.length).toBe(1);
+  });
+
+  it("reconstructs the target when a mark is removed", () => {
+    // Removing the only mark must leave the same text with no marks.
+    const from = doc(p(t("hello", strong)));
+    const to = doc(p(t("hello")));
+    const out = applyDiff(from, to);
+    expect(out.eq(to)).toBe(true);
+    expect(out.firstChild!.firstChild!.marks.length).toBe(0);
+  });
+
+  it("reconstructs the target on a paragraph split into two blocks", () => {
+    // Structural change (one block -> two) must replay as valid replace steps.
+    const from = doc(p(t("hello world")));
+    const to = doc(p(t("hello")), p(t("world")));
+    const out = applyDiff(from, to);
+    expect(out.eq(to)).toBe(true);
+    expect(out.childCount).toBe(2);
+  });
+
+  it("reconstructs the target on a node-type change (paragraph -> heading)", () => {
+    // Type/attrs changes drive the setNodeMarkup branch; the node must become a
+    // heading while keeping its text.
+    const from = doc(p(t("hello")));
+    const to = doc(h(1, t("hello")));
+    const out = applyDiff(from, to);
+    expect(out.eq(to)).toBe(true);
+    expect(out.firstChild!.type.name).toBe("heading");
+  });
+
+  it("reconstructs a combined structural + mark change", () => {
+    // Several diff kinds at once (new block + italic run) still round-trips.
+    const from = doc(p(t("alpha")));
+    const to = doc(p(t("alpha")), p(t("beta", em)));
+    const out = applyDiff(from, to);
+    expect(out.eq(to)).toBe(true);
+  });
+
+  it("produces an empty step list for identical documents", () => {
+    // No diff => no work; spurious steps would mean wasted/incorrect history.
+    const from = doc(p(t("same")));
+    const to = doc(p(t("same")));
+    const tr = recreateTransform(from, to);
+    expect(tr.steps.length).toBe(0);
+    expect(tr.doc.eq(to)).toBe(true);
+  });
+
+  it("round-trips with complexSteps:false (marks diffed as replaces)", () => {
+    // With complexSteps off, mark changes are folded into replace steps rather
+    // than dedicated mark steps — the result must still equal the target.
+    const from = doc(p(t("hello")));
+    const to = doc(p(t("hello", strong)));
+    expect(applyDiff(from, to, { complexSteps: false }).eq(to)).toBe(true);
+  });
+
+  it("round-trips with wordDiffs:true (whole-word text diffing)", () => {
+    // wordDiffs changes the granularity of the text diff, not the outcome.
+    const from = doc(p(t("the quick brown fox")));
+    const to = doc(p(t("the quick red fox")));
+    expect(applyDiff(from, to, { wordDiffs: true }).eq(to)).toBe(true);
+  });
+});
diff --git a/packages/editor-ext/src/lib/table/utils/get-selection-range-in-column.test.ts b/packages/editor-ext/src/lib/table/utils/get-selection-range-in-column.test.ts
new file mode 100644
index 00000000..7e623c60
--- /dev/null
+++ b/packages/editor-ext/src/lib/table/utils/get-selection-range-in-column.test.ts
@@ -0,0 +1,108 @@
+import { describe, it, expect } from "vitest";
+import { Schema } from "@tiptap/pm/model";
+import type { Node as PMNode } from "@tiptap/pm/model";
+import { tableNodes } from "@tiptap/pm/tables";
+import { EditorState, Selection } from "@tiptap/pm/state";
+import { getSelectionRangeInColumn } from "./get-selection-range-in-column";
+
+/**
+ * getSelectionRangeInColumn computes the rectangular column range (the set of
+ * column indexes, plus anchor/head cell positions) that a drag-reorder or
+ * column-select operation should act on, accounting for merged (colspan) cells.
+ * It keys off the table found from the current selection, so we drive it with a
+ * real EditorState whose selection sits inside the table.
+ */
+
+// Real ProseMirror table schema (same primitives the editor uses) so TableMap /
+// cellsInRect behave exactly as in production.
+const tNodes = tableNodes({
+  tableGroup: "block",
+  cellContent: "inline*",
+  cellAttributes: {},
+});
+const schema = new Schema({
+  nodes: {
+    doc: { content: "block+" },
+    paragraph: { group: "block", content: "inline*", toDOM: () => ["p", 0] },
+    text: { group: "inline" },
+    ...tNodes,
+  },
+  marks: {},
+});
+const cell = (txt: string, attrs?: Record<string, unknown>): PMNode =>
+  schema.nodes.table_cell.createChecked(attrs ?? null, schema.text(txt));
+const row = (...cells: PMNode[]): PMNode =>
+  schema.nodes.table_row.createChecked(null, cells);
+const table = (...rows: PMNode[]): PMNode =>
+  schema.nodes.table.createChecked(null, rows);
+const doc = (...content: PMNode[]): PMNode =>
+  schema.nodes.doc.createChecked(null, content);
+
+// Build a transaction whose selection is inside the table (the function locates
+// the table via `tr.selection.$from`).
+const trFor = (d: PMNode) =>
+  EditorState.create({ doc: d, selection: Selection.atStart(d) }).tr;
+
+// A 2-row x 3-col grid; each column is identifiable by its top-row letter.
+const grid3x2 = () =>
+  doc(
+    table(
+      row(cell("a"), cell("b"), cell("c")),
+      row(cell("d"), cell("e"), cell("f")),
+    ),
+  );
+
+describe("getSelectionRangeInColumn", () => {
+  it("returns a single-column range for a single index", () => {
+    // Asking for column 1 yields exactly indexes [1].
+    const tr = trFor(grid3x2());
+    const range = getSelectionRangeInColumn(tr, 1);
+    expect(range).toBeTruthy();
+    expect(range!.indexes).toEqual([1]);
+  });
+
+  it("anchor/head resolve to the top and bottom cells OF the requested column", () => {
+    // $head must point at the column's first (top) cell and $anchor at its last
+    // (bottom) cell — pinning that the returned positions belong to column 1,
+    // not some other column.
+    const tr = trFor(grid3x2());
+    const range = getSelectionRangeInColumn(tr, 1)!;
+    expect(tr.doc.nodeAt(range.$head.pos)?.textContent).toBe("b"); // top of col 1
+    expect(tr.doc.nodeAt(range.$anchor.pos)?.textContent).toBe("e"); // bottom of col 1
+  });
+
+  it("returns the inclusive span of columns for a multi-column request", () => {
+    // A 0..2 request must enumerate every covered column, in order.
+    const tr = trFor(grid3x2());
+    const range = getSelectionRangeInColumn(tr, 0, 2);
+    expect(range!.indexes).toEqual([0, 1, 2]);
+  });
+
+  it("returns a two-column span for an adjacent pair", () => {
+    const tr = trFor(grid3x2());
+    const range = getSelectionRangeInColumn(tr, 1, 2);
+    expect(range!.indexes).toEqual([1, 2]);
+  });
+
+  it("expands the range to cover a horizontally merged (colspan) cell", () => {
+    // Row 0 col 0 spans 2 columns. Requesting just column 0 must pull column 1
+    // into the range because they are merged together in the top row.
+    const d = doc(
+      table(
+        row(cell("ab", { colspan: 2 }), cell("c")),
+        row(cell("d"), cell("e"), cell("f")),
+      ),
+    );
+    const tr = trFor(d);
+    const range = getSelectionRangeInColumn(tr, 0);
+    expect(range!.indexes).toEqual([0, 1]);
+  });
+
+  it("throws when the requested column is entirely out of range", () => {
+    // No cells exist at column 5 of a 3-wide table, so the function cannot pick
+    // an anchor cell and dereferences undefined — pin this as the current
+    // (caller-guarded) contract so a silent behavior change is caught.
+    const tr = trFor(grid3x2());
+    expect(() => getSelectionRangeInColumn(tr, 5)).toThrow();
+  });
+});
diff --git a/packages/editor-ext/src/lib/table/utils/move-column.test.ts b/packages/editor-ext/src/lib/table/utils/move-column.test.ts
new file mode 100644
index 00000000..5b2d60e5
--- /dev/null
+++ b/packages/editor-ext/src/lib/table/utils/move-column.test.ts
@@ -0,0 +1,156 @@
+import { describe, it, expect } from "vitest";
+import { Schema } from "@tiptap/pm/model";
+import type { Node as PMNode } from "@tiptap/pm/model";
+import { tableNodes, CellSelection } from "@tiptap/pm/tables";
+import { EditorState, Selection } from "@tiptap/pm/state";
+import { moveColumn } from "./move-column";
+import { convertTableNodeToArrayOfRows } from "./convert-table-node-to-array-of-rows";
+import { findTable } from "./query";
+
+/**
+ * moveColumn reorders whole columns of a real ProseMirror table by mutating a
+ * Transaction (transpose -> move row -> transpose back -> replace). The invariant
+ * is that after the call each column appears at its new position with every
+ * cell's content preserved and nothing dropped or duplicated.
+ */
+
+const tNodes = tableNodes({
+  tableGroup: "block",
+  cellContent: "inline*",
+  cellAttributes: {},
+});
+const schema = new Schema({
+  nodes: {
+    doc: { content: "block+" },
+    paragraph: { group: "block", content: "inline*", toDOM: () => ["p", 0] },
+    text: { group: "inline" },
+    ...tNodes,
+  },
+  marks: {},
+});
+const cell = (txt: string): PMNode =>
+  schema.nodes.table_cell.createChecked(null, schema.text(txt));
+const row = (...cells: PMNode[]): PMNode =>
+  schema.nodes.table_row.createChecked(null, cells);
+const table = (...rows: PMNode[]): PMNode =>
+  schema.nodes.table.createChecked(null, rows);
+const doc = (...content: PMNode[]): PMNode =>
+  schema.nodes.doc.createChecked(null, content);
+
+const grid = (tr: any): string[][] => {
+  const t = findTable(tr.doc.resolve(tr.selection.from))!;
+  return convertTableNodeToArrayOfRows(t.node).map((r) =>
+    r.map((c) => (c ? c.textContent : "")),
+  );
+};
+
+// 2-row x 3-col table; column k is (rowX-col-k). Columns: 0=(a,d) 1=(b,e) 2=(c,f).
+const grid3x2 = () =>
+  doc(
+    table(
+      row(cell("a"), cell("b"), cell("c")),
+      row(cell("d"), cell("e"), cell("f")),
+    ),
+  );
+
+const stateFor = (d: PMNode) =>
+  EditorState.create({ doc: d, selection: Selection.atStart(d) });
+
+describe("moveColumn", () => {
+  it("moves the first column to the last index, preserving column content", () => {
+    // origin 0 -> target 2 sends column (a,d) to the right: cols become 1,2,0.
+    const state = stateFor(grid3x2());
+    const tr = state.tr;
+    const ok = moveColumn({
+      tr,
+      originIndex: 0,
+      targetIndex: 2,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(grid(tr)).toEqual([
+      ["b", "c", "a"],
+      ["e", "f", "d"],
+    ]);
+  });
+
+  it("moves a later column to the first index", () => {
+    // origin 2 -> target 0 pulls column (c,f) to the front: cols become 2,0,1.
+    const state = stateFor(grid3x2());
+    const tr = state.tr;
+    const ok = moveColumn({
+      tr,
+      originIndex: 2,
+      targetIndex: 0,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(grid(tr)).toEqual([
+      ["c", "a", "b"],
+      ["f", "d", "e"],
+    ]);
+  });
+
+  it("never drops or duplicates cells when reordering columns", () => {
+    const state = stateFor(grid3x2());
+    const tr = state.tr;
+    moveColumn({
+      tr,
+      originIndex: 1,
+      targetIndex: 2,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(grid(tr).flat().sort()).toEqual(
+      ["a", "b", "c", "d", "e", "f"].sort(),
+    );
+    expect(grid(tr)[0].length).toBe(3);
+  });
+
+  it("returns false (no-op) when target equals origin", () => {
+    const state = stateFor(grid3x2());
+    const tr = state.tr;
+    const before = grid(tr);
+    const ok = moveColumn({
+      tr,
+      originIndex: 1,
+      targetIndex: 1,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(false);
+    expect(grid(tr)).toEqual(before);
+  });
+
+  it("returns false when pos is not inside a table", () => {
+    const d = doc(
+      schema.nodes.paragraph.createChecked(null, schema.text("plain")),
+    );
+    const state = stateFor(d);
+    const tr = state.tr;
+    const ok = moveColumn({
+      tr,
+      originIndex: 0,
+      targetIndex: 1,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(false);
+  });
+
+  it("installs a CellSelection on the moved column when select is true", () => {
+    const state = stateFor(grid3x2());
+    const tr = state.tr;
+    const ok = moveColumn({
+      tr,
+      originIndex: 0,
+      targetIndex: 2,
+      select: true,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(tr.selection instanceof CellSelection).toBe(true);
+  });
+});
diff --git a/packages/editor-ext/src/lib/table/utils/move-row.test.ts b/packages/editor-ext/src/lib/table/utils/move-row.test.ts
new file mode 100644
index 00000000..3a0c481a
--- /dev/null
+++ b/packages/editor-ext/src/lib/table/utils/move-row.test.ts
@@ -0,0 +1,167 @@
+import { describe, it, expect } from "vitest";
+import { Schema } from "@tiptap/pm/model";
+import type { Node as PMNode } from "@tiptap/pm/model";
+import { tableNodes, CellSelection } from "@tiptap/pm/tables";
+import { EditorState, Selection } from "@tiptap/pm/state";
+import { moveRow } from "./move-row";
+import { convertTableNodeToArrayOfRows } from "./convert-table-node-to-array-of-rows";
+import { findTable } from "./query";
+
+/**
+ * moveRow reorders whole rows of a real ProseMirror table by mutating a
+ * Transaction: it locates the table, computes origin/target row ranges, rebuilds
+ * the table with rows reordered, and replaces it in the doc. The invariant is
+ * that after the call the table's rows appear in the new order with every cell's
+ * content preserved, and no rows are dropped or duplicated.
+ */
+
+const tNodes = tableNodes({
+  tableGroup: "block",
+  cellContent: "inline*",
+  cellAttributes: {},
+});
+const schema = new Schema({
+  nodes: {
+    doc: { content: "block+" },
+    paragraph: { group: "block", content: "inline*", toDOM: () => ["p", 0] },
+    text: { group: "inline" },
+    ...tNodes,
+  },
+  marks: {},
+});
+const cell = (txt: string): PMNode =>
+  schema.nodes.table_cell.createChecked(null, schema.text(txt));
+const row = (...cells: PMNode[]): PMNode =>
+  schema.nodes.table_row.createChecked(null, cells);
+const table = (...rows: PMNode[]): PMNode =>
+  schema.nodes.table.createChecked(null, rows);
+const doc = (...content: PMNode[]): PMNode =>
+  schema.nodes.doc.createChecked(null, content);
+
+// Read the table's content as a grid of cell texts (rows x cols) from whatever
+// table currently lives in `tr.doc`.
+const grid = (tr: any): string[][] => {
+  const t = findTable(tr.doc.resolve(tr.selection.from))!;
+  return convertTableNodeToArrayOfRows(t.node).map((r) =>
+    r.map((c) => (c ? c.textContent : "")),
+  );
+};
+
+// 3-row x 2-col table; each row identifiable by its cells.
+const grid2x3 = () =>
+  doc(
+    table(
+      row(cell("r0a"), cell("r0b")),
+      row(cell("r1a"), cell("r1b")),
+      row(cell("r2a"), cell("r2b")),
+    ),
+  );
+
+const stateFor = (d: PMNode) =>
+  EditorState.create({ doc: d, selection: Selection.atStart(d) });
+
+describe("moveRow", () => {
+  it("moves the first row down to the last index, preserving content", () => {
+    // origin 0 -> target 2 makes row 0 land after the other rows: [r1, r2, r0].
+    const state = stateFor(grid2x3());
+    const tr = state.tr;
+    const ok = moveRow({
+      tr,
+      originIndex: 0,
+      targetIndex: 2,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(grid(tr)).toEqual([
+      ["r1a", "r1b"],
+      ["r2a", "r2b"],
+      ["r0a", "r0b"],
+    ]);
+  });
+
+  it("moves a lower row up to an earlier index", () => {
+    // origin 2 -> target 0 lifts the last row above the rest: [r2, r0, r1].
+    const state = stateFor(grid2x3());
+    const tr = state.tr;
+    const ok = moveRow({
+      tr,
+      originIndex: 2,
+      targetIndex: 0,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(grid(tr)).toEqual([
+      ["r2a", "r2b"],
+      ["r0a", "r0b"],
+      ["r1a", "r1b"],
+    ]);
+  });
+
+  it("never drops or duplicates rows when reordering", () => {
+    // The full multiset of cell texts is invariant under any valid move.
+    const state = stateFor(grid2x3());
+    const tr = state.tr;
+    moveRow({
+      tr,
+      originIndex: 1,
+      targetIndex: 2,
+      select: false,
+      pos: state.selection.from,
+    });
+    const flat = grid(tr).flat().sort();
+    expect(flat).toEqual(
+      ["r0a", "r0b", "r1a", "r1b", "r2a", "r2b"].sort(),
+    );
+    expect(grid(tr).length).toBe(3);
+  });
+
+  it("returns false (no-op) when target equals origin", () => {
+    // Moving a row onto itself is rejected and leaves the table unchanged.
+    const state = stateFor(grid2x3());
+    const tr = state.tr;
+    const before = grid(tr);
+    const ok = moveRow({
+      tr,
+      originIndex: 1,
+      targetIndex: 1,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(false);
+    expect(grid(tr)).toEqual(before);
+  });
+
+  it("returns false when pos is not inside a table", () => {
+    // Without a table at `pos`, the function bails out instead of throwing.
+    const d = doc(
+      schema.nodes.paragraph.createChecked(null, schema.text("plain")),
+    );
+    const state = stateFor(d);
+    const tr = state.tr;
+    const ok = moveRow({
+      tr,
+      originIndex: 0,
+      targetIndex: 1,
+      select: false,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(false);
+  });
+
+  it("installs a CellSelection on the moved row when select is true", () => {
+    // With select:true the moved row at the target index is selected.
+    const state = stateFor(grid2x3());
+    const tr = state.tr;
+    const ok = moveRow({
+      tr,
+      originIndex: 0,
+      targetIndex: 2,
+      select: true,
+      pos: state.selection.from,
+    });
+    expect(ok).toBe(true);
+    expect(tr.selection instanceof CellSelection).toBe(true);
+  });
+});
diff --git a/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts b/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
index 24d30408..64603b5b 100644
--- a/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
+++ b/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
@@ -100,4 +100,51 @@ describe("addUniqueIdsToDoc", () => {
     const [id] = ids(out);
     expect(id).toBeTruthy();
   });
+
+  it("only assigns ids to configured node types, not to others", () => {
+    // `types` is ["heading","paragraph"]; a codeBlock is NOT addressed, so it
+    // must come back without an id while the sibling paragraph is filled. (The
+    // UniqueID attribute only exists on configured types in the schema.)
+    const doc = {
+      type: "doc",
+      content: [
+        { type: "codeBlock", content: [{ type: "text", text: "x = 1" }] },
+        para(undefined, "after"),
+      ],
+    };
+    const out = addUniqueIdsToDoc(doc, extensions);
+    const [codeId, paraId] = ids(out);
+    expect(codeId).toBeUndefined();
+    expect(paraId).toBeTruthy();
+  });
+
+  it("assigns ids to target nodes nested inside non-target containers", () => {
+    // findChildren walks the whole tree: a paragraph inside a blockquote still
+    // gets an id, while the (non-target) blockquote wrapper does not.
+    const doc = {
+      type: "doc",
+      content: [
+        { type: "blockquote", content: [para(undefined, "quoted")] },
+      ],
+    };
+    const out = addUniqueIdsToDoc(doc, extensions) as any;
+    const blockquote = out.content[0];
+    const nestedPara = blockquote.content[0];
+    expect(blockquote.attrs?.id).toBeUndefined();
+    expect(nestedPara.attrs.id).toBeTruthy();
+  });
+
+  it("is idempotent: a second pass keeps every already-unique id unchanged", () => {
+    // Once ids are assigned and unique, re-running must be a fixed point — no
+    // churn that would invalidate stored MCP anchors on every save.
+    const doc = {
+      type: "doc",
+      content: [para(undefined, "a"), para(undefined, "b"), para(undefined, "c")],
+    };
+    const once = addUniqueIdsToDoc(doc, extensions);
+    const twice = addUniqueIdsToDoc(once, extensions);
+    expect(ids(twice)).toEqual(ids(once));
+    // And all three are distinct, so the second pass had real ids to preserve.
+    expect(new Set(ids(once)).size).toBe(3);
+  });
 });