diff --git a/packages/mcp-server/src/diagram-operations.ts b/packages/mcp-server/src/diagram-operations.ts
index 5315f191..64dd810a 100644
--- a/packages/mcp-server/src/diagram-operations.ts
+++ b/packages/mcp-server/src/diagram-operations.ts
@@ -216,6 +216,20 @@ export function applyDiagramOperations(
                 continue
             }
 
+            // A cut-off cell (its XML still streaming in a preview) would
+            // parse in Chrome to a cell without its geometry
+            const syntaxError = getXmlSyntaxError(
+                `<wrapper>${op.new_xml}</wrapper>`,
+            )
+            if (syntaxError) {
+                errors.push({
+                    type: "update",
+                    cellId: op.cell_id,
+                    message: `new_xml is not well-formed XML: ${syntaxError}`,
+                })
+                continue
+            }
+
             // Parse the new XML
             const newDoc = parser.parseFromString(
                 `<wrapper>${op.new_xml}</wrapper>`,
@@ -268,6 +282,20 @@ export function applyDiagramOperations(
                 continue
             }
 
+            // A cut-off cell (its XML still streaming in a preview) would
+            // parse in Chrome to a cell without its geometry
+            const syntaxError = getXmlSyntaxError(
+                `<wrapper>${op.new_xml}</wrapper>`,
+            )
+            if (syntaxError) {
+                errors.push({
+                    type: "add",
+                    cellId: op.cell_id,
+                    message: `new_xml is not well-formed XML: ${syntaxError}`,
+                })
+                continue
+            }
+
             // Parse the new XML
             const newDoc = parser.parseFromString(
                 `<wrapper>${op.new_xml}</wrapper>`,
diff --git a/packages/mcp-server/tests/diagram-operations.test.ts b/packages/mcp-server/tests/diagram-operations.test.ts
index 77d4de13..97af0dcd 100644
--- a/packages/mcp-server/tests/diagram-operations.test.ts
+++ b/packages/mcp-server/tests/diagram-operations.test.ts
@@ -149,3 +149,52 @@ describe("a wrapped mxCell with its wrapper's id", () => {
         expect(result).toContain('label="B"')
     })
 })
+
+describe("cut-off new_xml", () => {
+    // While edit_diagram streams, the last operation's XML is often
+    // incomplete. Chrome's DOMParser keeps the partial cell, so it must be
+    // refused before it reaches the diagram.
+    const CUT = `<mxCell id="b" value="B2" style="rounded=1;" vertex="1" parent="1">`
+
+    it("refuses an update whose XML is cut off", () => {
+        const { result, errors } = applyDiagramOperations(DOC, [
+            { operation: "update", cell_id: "b", new_xml: CUT },
+        ])
+        expect(errors).toHaveLength(1)
+        expect(errors[0]).toMatchObject({ type: "update", cellId: "b" })
+        expect(errors[0].message).toContain("not well-formed")
+        expect(result).toContain('value="B"')
+        expect(result).not.toContain("B2")
+    })
+
+    it("refuses an add whose XML is cut off", () => {
+        const { result, errors } = applyDiagramOperations(DOC, [
+            {
+                operation: "add",
+                cell_id: "n",
+                new_xml: `<mxCell id="n" value="N" vertex="1" parent="1"><mxGeometry x="1" y="2" width="3"`,
+            },
+        ])
+        expect(errors).toHaveLength(1)
+        expect(errors[0]).toMatchObject({ type: "add", cellId: "n" })
+        expect(result).not.toContain('id="n"')
+    })
+
+    it("applies the complete operations before a cut-off one", () => {
+        const { result, errors } = applyDiagramOperations(DOC, [
+            {
+                operation: "update",
+                cell_id: "b",
+                new_xml: `<mxCell id="b" value="B3" vertex="1" parent="1"><mxGeometry as="geometry"/></mxCell>`,
+            },
+            {
+                operation: "add",
+                cell_id: "n",
+                new_xml: `<mxCell id="n" value="N" vertex="1" parent="1">`,
+            },
+        ])
+        expect(errors).toHaveLength(1)
+        expect(result).toContain('value="B3"')
+        expect(result).not.toContain('id="n"')
+    })
+})