diff --git a/crates/model/src/preserved.rs b/crates/model/src/preserved.rs
index 6d0b35d..eb8e2bb 100644
--- a/crates/model/src/preserved.rs
+++ b/crates/model/src/preserved.rs
@@ -33,10 +33,11 @@ pub struct PreservedPart {
 }
 
 /// A raw XML element of an owning part that refers to kept parts, re-emitted on save with its
-/// relationship ids rewritten.
+/// relationship ids rewritten, or that Excel needs back unchanged (the workbook's `fileVersion`).
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct PreservedXml {
-    /// Which slot of the owner it belongs in (`externalReferences`, `oleObjects`, `controls`).
+    /// Which slot of the owner it belongs in (`externalReferences`, `fileVersion`, `ext`,
+    /// `oleObjects`, `controls`).
     pub slot: String,
     /// The element, namespace declarations it needs included.
     pub xml: String,
diff --git a/crates/xlsx/src/passthrough.rs b/crates/xlsx/src/passthrough.rs
index f121977..7db21fd 100644
--- a/crates/xlsx/src/passthrough.rs
+++ b/crates/xlsx/src/passthrough.rs
@@ -6,7 +6,8 @@
 //! their `<externalReferences>`, data connections) and each sheet (query tables, form controls,
 //! ActiveX controls, OLE objects, the sheet background and printer settings, with the sheet's
 //! `<picture>`, `<oleObjects>` and `<controls>` XML, the `r:id` of `<pageSetup>` and the legacy
-//! VML shapes those controls need).
+//! VML shapes those controls need). The workbook also keeps its `<fileVersion>` and the
+//! calcFeatures extension, which Excel needs to trust the saved values.
 //!
 //! On write, kept parts get fresh part names where needed and new relationship ids; the `r:id`s in
 //! kept XML are rewritten to match. Kept content is conservative:
@@ -83,6 +84,11 @@ const WORKBOOK_ROOTS: &[&str] = &["customXml", "connections"];
 const SHEET_ROOTS: &[&str] = &["queryTable"];
 /// Sheet relationship kinds that mean the sheet XML may hold elements worth keeping.
 const SHEET_ELEMENT_KINDS: &[&str] = &["oleObject", "ctrlProp", "control", "image", "package", "printerSettings"];
+/// Attributes of `<fileVersion>` (CT_FileVersion).
+const FILE_VERSION_ATTRS: &[&str] = &["appName", "lastEdited", "lowestEdited", "rupBuild", "codeName"];
+/// The workbook extension listing the calculation features the saved values were computed with.
+const CALC_FEATURES_URI: &str = "{B58B0392-4F1F-4190-BB64-5DF3571DCE5F}";
+const NS_CALC_FEATURES: &str = "http://schemas.microsoft.com/office/spreadsheetml/2018/calcfeatures";
 /// Kept sheet elements, in schema order (all after `<legacyDrawing>`, before `<tableParts>`).
 const SHEET_SLOTS: &[&str] = &["picture", "oleObjects", "controls"];
 
@@ -369,6 +375,30 @@ impl KeptReader {
                 Err(why) => cx.warn(format!("links to external workbooks were dropped: {why}")),
             }
         }
+        // Excel trusts the saved cell values only when both are present; without them it
+        // recalculates everything on open, and links into closed workbooks come back as errors.
+        if let Some(v) = wb_xml.child("fileVersion") {
+            let mut x = String::from("<fileVersion");
+            for k in FILE_VERSION_ATTRS {
+                if let Some(a) = v.attr(k) {
+                    let _ = write!(x, " {k}=\"{}\"", esc_attr(a));
+                }
+            }
+            x.push_str("/>");
+            p.elements.push(PreservedXml { slot: "fileVersion".into(), xml: x });
+        }
+        let calc_features = wb_xml
+            .child("extLst")
+            .and_then(|l| l.kids("ext").find(|e| e.attr("uri").is_some_and(|u| u.eq_ignore_ascii_case(CALC_FEATURES_URI))))
+            .and_then(|e| e.child("calcFeatures"));
+        if let Some(c) = calc_features {
+            let mut x = format!("<ext uri=\"{CALC_FEATURES_URI}\" xmlns:xcalcf=\"{NS_CALC_FEATURES}\"><xcalcf:calcFeatures>");
+            for name in c.kids("feature").filter_map(|f| f.attr("name")) {
+                let _ = write!(x, "<xcalcf:feature name=\"{}\"/>", esc_attr(name));
+            }
+            x.push_str("</xcalcf:calcFeatures></ext>");
+            p.elements.push(PreservedXml { slot: "ext".into(), xml: x });
+        }
         (!p.is_empty()).then(|| Arc::new(p))
     }
 
@@ -853,6 +883,12 @@ pub fn write_workbook(wb: &Workbook, wb_rels: &mut Rels, out: &mut Out) -> Strin
     p.elements.iter().filter(|e| e.slot == "externalReferences").filter_map(|e| remap(&e.xml, &ids)).collect()
 }
 
+/// The workbook's kept elements of `slot` that refer to no part (`fileVersion`, `ext`).
+pub fn workbook_elements(wb: &Workbook, slot: &str) -> String {
+    let Some(p) = wb.preserved.as_deref() else { return String::new() };
+    p.elements.iter().filter(|e| e.slot == slot).map(|e| e.xml.as_str()).collect()
+}
+
 /// Adds the kept package-root relationships (custom properties…) to `_rels/.rels`.
 pub fn write_package_rels(root: &mut Rels, out: &mut Out) {
     for (ty, target, external) in std::mem::take(&mut out.kept.root) {
diff --git a/crates/xlsx/src/tests/passthrough_tests.rs b/crates/xlsx/src/tests/passthrough_tests.rs
index 4cde9d6..aac9b6b 100644
--- a/crates/xlsx/src/tests/passthrough_tests.rs
+++ b/crates/xlsx/src/tests/passthrough_tests.rs
@@ -483,3 +483,41 @@ fn kept_vml_drawings_are_charged_to_the_kept_total() {
     let (wb, _) = read_xlsx(&rich_package_with(CONTENT_TYPES, true)).unwrap();
     assert!(wb.sheets.iter().all(|s| s.preserved.as_ref().is_some_and(|p| p.vml.is_some())));
 }
+
+#[test]
+fn file_version_and_calc_features_survive_a_save() {
+    // Excel trusts the saved cell values only when workbook.xml carries both <fileVersion> and the
+    // calcFeatures extension; without them it recalculates everything on open (#799).
+    let calc_features = r#"<ext uri="{B58B0392-4F1F-4190-BB64-5DF3571DCE5F}" xmlns:xcalcf="http://schemas.microsoft.com/office/spreadsheetml/2018/calcfeatures"><xcalcf:calcFeatures><xcalcf:feature name="microsoft.com:RD"/><xcalcf:feature name="microsoft.com:Single"/><xcalcf:feature name="microsoft.com:FV"/><xcalcf:feature name="microsoft.com:CNMTM"/><xcalcf:feature name="microsoft.com:LET_WF"/><xcalcf:feature name="microsoft.com:ARRAYTEXT_WF"/></xcalcf:calcFeatures></ext>"#;
+    let workbook = format!(
+        r#"<?xml version="1.0"?><workbook xmlns="{NS_MAIN}" xmlns:r="{NS_R}"><fileVersion appName="xl" lastEdited="7" lowestEdited="7" rupBuild="11008"/><workbookPr/><sheets><sheet name="A" sheetId="1" r:id="rId1"/></sheets><calcPr calcId="191029"/><extLst><ext uri="{{140A7094-0E35-4892-8432-C4D2E57EDEB5}}" xmlns:x15="http://schemas.microsoft.com/office/spreadsheetml/2010/11/main"><x15:workbookPr chartTrackingRefBase="1"/></ext>{calc_features}</extLst></workbook>"#
+    );
+    let sheet = format!(
+        r#"<?xml version="1.0"?><worksheet xmlns="{NS_MAIN}"><sheetData><row r="1"><c r="A1"><v>1</v></c><c r="B1"><f>A1*2</f><v>2</v></c></row></sheetData></worksheet>"#
+    );
+    let bytes = make_zip(&[
+        ("[Content_Types].xml", super::CT.as_bytes()),
+        ("_rels/.rels", super::ROOT_RELS.as_bytes()),
+        ("xl/workbook.xml", workbook.as_bytes()),
+        ("xl/_rels/workbook.xml.rels", rels(&[("rId1", "worksheet", "worksheets/sheet1.xml")]).as_bytes()),
+        ("xl/worksheets/sheet1.xml", sheet.as_bytes()),
+    ]);
+    let (wb, _) = read_xlsx(&bytes).unwrap();
+    // Twice: what was kept on the first save is read back and kept again.
+    let (again, _) = read_xlsx(&write_xlsx(&wb).unwrap()).unwrap();
+    for saved in [write_xlsx(&wb).unwrap(), write_xlsx(&again).unwrap()] {
+        let u = Unpacked::new(&saved);
+        u.assert_valid();
+        let text = u.text("xl/workbook.xml");
+        let root = xml::parse(text.as_bytes()).unwrap();
+        // Schema order: fileVersion is the first child of <workbook>, extLst the last.
+        assert_eq!(root.children.first().map(|e| e.name.as_str()), Some("fileVersion"), "{text}");
+        assert_eq!(root.children.last().map(|e| e.name.as_str()), Some("extLst"), "{text}");
+        assert!(text.contains(r#"<fileVersion appName="xl" lastEdited="7" lowestEdited="7" rupBuild="11008"/>"#), "{text}");
+        assert!(text.contains(&format!("<extLst>{calc_features}</extLst>")), "{text}");
+    }
+
+    // A workbook that had neither gets neither.
+    let plain = Unpacked::new(&write_xlsx(&gridcraft_model::Workbook::new()).unwrap()).text("xl/workbook.xml");
+    assert!(!plain.contains("fileVersion") && !plain.contains("extLst"), "{plain}");
+}
diff --git a/crates/xlsx/src/write.rs b/crates/xlsx/src/write.rs
index 5492756..dddba98 100644
--- a/crates/xlsx/src/write.rs
+++ b/crates/xlsx/src/write.rs
@@ -295,6 +295,7 @@ pub fn write_xlsx(wb: &Workbook, kind: XlsxKind) -> Result<Vec<u8>, IoError> {
     }
 
     let mut w = format!("{XML_DECL}<workbook xmlns=\"{NS_MAIN}\" xmlns:r=\"{NS_REL}\">");
+    w.push_str(&crate::passthrough::workbook_elements(wb, "fileVersion"));
     w.push_str(if wb.date_system == gridcraft_core::DateSystem::D1904 { "<workbookPr date1904=\"1\"/>" } else { "<workbookPr/>" });
     if wb.protected_structure {
         let pw = wb.structure_password.as_ref().map(|h| crate::sheet_write::password_attrs(h, "workbook")).unwrap_or_default();
@@ -331,6 +332,10 @@ pub fn write_xlsx(wb: &Workbook, kind: XlsxKind) -> Result<Vec<u8>, IoError> {
     }
     w.push_str("/>");
     w.push_str(&pivots.workbook_xml(&mut wb_rels));
+    let exts = crate::passthrough::workbook_elements(wb, "ext");
+    if !exts.is_empty() {
+        let _ = write!(w, "<extLst>{exts}</extLst>");
+    }
     w.push_str("</workbook>");
 
     // Workbook-level parts.