diff --git a/app/src/components/Actions/Actions.tsx b/app/src/components/Actions/Actions.tsx
index c3a205b..3348c98 100644
--- a/app/src/components/Actions/Actions.tsx
+++ b/app/src/components/Actions/Actions.tsx
@@ -819,12 +819,24 @@ function NotebookTabContent({ docUri }: { docUri: string }) {
             if (!data) {
               return;
             }
-            // Insert a markup (markdown) cell at the top of the notebook.
+            // Default to inserting a code cell when notebook is empty.
             const firstCell = cellDatas[0]?.snapshot;
             if (firstCell) {
-              data.addMarkupCellBefore(firstCell.refId);
+              data.addCodeCellBefore(firstCell.refId);
             } else {
-              data.appendMarkupCell();
+              const newCell = data.addCodeCellAfter("");
+              if (!newCell) {
+                // Fallback: create and persist a new code cell at the end.
+                const cell = create(parser_pb.CellSchema, {
+                  metadata: {},
+                  refId: `code_${crypto.randomUUID().replace(/-/g, "")}`,
+                  languageId: "bash",
+                  role: parser_pb.CellRole.USER,
+                  kind: parser_pb.CellKind.CODE,
+                  value: "",
+                });
+                data.updateCell(cell);
+              }
             }
           }}
         >
diff --git a/app/src/lib/notebookData.test.ts b/app/src/lib/notebookData.test.ts
index 684e51b..e2b4857 100644
--- a/app/src/lib/notebookData.test.ts
+++ b/app/src/lib/notebookData.test.ts
@@ -1,4 +1,3 @@
-// @vitest-environment node
 import { Subject } from "rxjs";
 import { create } from "@bufbuild/protobuf";
 import { beforeAll, describe, expect, it, vi } from "vitest";
@@ -217,113 +216,3 @@ describe("NotebookData.getActiveStream", () => {
     expect(model.getActiveStream(cell.refId)).toBeUndefined();
   });
 });
-
-describe("NotebookData markup cell methods", () => {
-  function makeModel(cells: InstanceType<typeof parser_pb.Cell>[]) {
-    const notebook = create(parser_pb.NotebookSchema, { cells });
-    return new NotebookData({
-      notebook,
-      uri: "nb://test",
-      name: "test",
-      notebookStore: null,
-      loaded: true,
-    });
-  }
-
-  function getCells(model: InstanceType<typeof NotebookData>) {
-    return model.getSnapshot().notebook.cells;
-  }
-
-  describe("appendMarkupCell", () => {
-    it("appends a markup cell to an empty notebook", () => {
-      const model = makeModel([]);
-      const cell = model.appendMarkupCell();
-      const cells = getCells(model);
-
-      expect(cell).toBeTruthy();
-      expect(cell.kind).toBe(parser_pb.CellKind.MARKUP);
-      expect(cell.languageId).toBe("markdown");
-      expect(cell.value).toBe("");
-      expect(cell.refId).toMatch(/^markup_/);
-      expect(cells.length).toBe(1);
-      expect(cells[0].refId).toBe(cell.refId);
-    });
-
-    it("appends a markup cell after existing cells", () => {
-      const existing = create(parser_pb.CellSchema, {
-        refId: "code_existing",
-        kind: parser_pb.CellKind.CODE,
-        languageId: "js",
-        value: "console.log(1)",
-        metadata: {},
-        outputs: [],
-      });
-      const model = makeModel([existing]);
-      const cell = model.appendMarkupCell();
-      const cells = getCells(model);
-
-      expect(cells.length).toBe(2);
-      expect(cells[0].refId).toBe("code_existing");
-      expect(cells[1].refId).toBe(cell.refId);
-      expect(cell.kind).toBe(parser_pb.CellKind.MARKUP);
-    });
-  });
-
-  describe("addMarkupCellBefore", () => {
-    it("inserts a markup cell before a target cell", () => {
-      const existing = create(parser_pb.CellSchema, {
-        refId: "code_first",
-        kind: parser_pb.CellKind.CODE,
-        languageId: "bash",
-        value: "echo hi",
-        metadata: {},
-        outputs: [],
-      });
-      const model = makeModel([existing]);
-      const cell = model.addMarkupCellBefore("code_first");
-      const cells = getCells(model);
-
-      expect(cell).toBeTruthy();
-      expect(cell!.kind).toBe(parser_pb.CellKind.MARKUP);
-      expect(cell!.languageId).toBe("markdown");
-      expect(cell!.refId).toMatch(/^markup_/);
-      expect(cells.length).toBe(2);
-      expect(cells[0].refId).toBe(cell!.refId);
-      expect(cells[1].refId).toBe("code_first");
-    });
-
-    it("returns null for an invalid refId", () => {
-      const model = makeModel([]);
-      const result = model.addMarkupCellBefore("nonexistent");
-      expect(result).toBeNull();
-      expect(getCells(model).length).toBe(0);
-    });
-
-    it("inserts before the correct cell in a multi-cell notebook", () => {
-      const cell1 = create(parser_pb.CellSchema, {
-        refId: "cell_1",
-        kind: parser_pb.CellKind.CODE,
-        languageId: "js",
-        value: "1",
-        metadata: {},
-        outputs: [],
-      });
-      const cell2 = create(parser_pb.CellSchema, {
-        refId: "cell_2",
-        kind: parser_pb.CellKind.CODE,
-        languageId: "js",
-        value: "2",
-        metadata: {},
-        outputs: [],
-      });
-      const model = makeModel([cell1, cell2]);
-      const inserted = model.addMarkupCellBefore("cell_2");
-      const cells = getCells(model);
-
-      expect(cells.length).toBe(3);
-      expect(cells[0].refId).toBe("cell_1");
-      expect(cells[1].refId).toBe(inserted!.refId);
-      expect(cells[2].refId).toBe("cell_2");
-    });
-  });
-});
diff --git a/app/src/lib/notebookData.ts b/app/src/lib/notebookData.ts
index b3e64a5..d60687f 100644
--- a/app/src/lib/notebookData.ts
+++ b/app/src/lib/notebookData.ts
@@ -410,30 +410,6 @@ export class NotebookData {
     return cell;
   }
 
-  addMarkupCellBefore(targetRefId: string): parser_pb.Cell | null {
-    const idx = this.refToIndex.get(targetRefId);
-    if (idx === undefined) {
-      return null;
-    }
-    const cell = this.createMarkupCell();
-    this.notebook.cells.splice(idx, 0, cell);
-    this.rebuildIndex();
-    this.snapshotCache = this.buildSnapshot();
-    this.emit();
-    void this.persist();
-    return cell;
-  }
-
-  appendMarkupCell(): parser_pb.Cell {
-    const cell = this.createMarkupCell();
-    this.notebook.cells.push(cell);
-    this.rebuildIndex();
-    this.snapshotCache = this.buildSnapshot();
-    this.emit();
-    void this.persist();
-    return cell;
-  }
-
   removeCell(refId: string): void {
     const idx = this.refToIndex.get(refId);
     if (idx === undefined) {
@@ -601,18 +577,6 @@ export class NotebookData {
     return this.createAndBindStreams({ cell, runID, sequence, runner });
   }
 
-  private createMarkupCell(): parser_pb.Cell {
-    const refID = `markup_${crypto.randomUUID().replace(/-/g, "")}`;
-    return create(parser_pb.CellSchema, {
-      metadata: {},
-      refId: refID,
-      languageId: "markdown",
-      role: parser_pb.CellRole.USER,
-      kind: parser_pb.CellKind.MARKUP,
-      value: "",
-    });
-  }
-
   private createCodeCell(languageId?: string | null): parser_pb.Cell {
     const refID = `code_${crypto.randomUUID().replace(/-/g, "")}`;
     const normalizedLanguage = languageId?.trim().toLowerCase();
@@ -822,16 +786,15 @@ export class CellData {
     this.notebook.removeCell(this.refId);
   }
 
-  run(): string {
+  run(): void {
     console.log(`Running CellData ${this.cellDataID}`);
     const cell = this.snapshot;
-    if (!cell) return "";
+    if (!cell) return;
     const runID = this.notebook.runCodeCell(cell);
     console.log("Started run for cell", { refId: this.refId, runID });
     // Update the snapshot after running to pick up any metadata changes.
-    this.cachedSnapshot = this.notebook.getCellSnapshot(this.refId);
+    this.cachedSnapshot = this.notebook.getCellSnapshot(this.refId); 
     this.emitRunIDChange(runID ?? "");
-    return runID;
   }
 
   getStreams(): StreamsLike | undefined {