diff --git a/crates/improvise-io/src/persistence/mod.rs b/crates/improvise-io/src/persistence/mod.rs
index 505dfa2c..ee8d579a 100644
--- a/crates/improvise-io/src/persistence/mod.rs
+++ b/crates/improvise-io/src/persistence/mod.rs
@@ -1,4 +1,4 @@
-//! `.improv` document lifecycle — save, load, gzip, legacy JSON — over a
+//! `.improv` document lifecycle — save, load, gzip — over a
 //! [`Storage`] backend. The text format is in `format`, `parse` and `export`.
 
 mod export;
@@ -96,11 +96,16 @@ pub fn load_from(storage: &dyn Storage, path: &Path) -> Result<Workbook> {
     };
     let text =
         String::from_utf8(bytes).with_context(|| format!("{} is not UTF-8", path.display()))?;
+    // `.improv` text is the only native format; a JSON workbook is named, not
+    // handed to the grammar to fail on its first byte (improvise-tt22).
     if text.trim_start().starts_with('{') {
-        serde_json::from_str(&text).context("Failed to deserialize workbook")
-    } else {
-        parse_md(&text)
+        anyhow::bail!(
+            "{} is a legacy JSON workbook, which is no longer supported: \
+             only .improv text can be opened",
+            path.display()
+        );
     }
+    parse_md(&text)
 }
 
 pub fn autosave_path(path: &Path) -> std::path::PathBuf {
@@ -194,37 +199,34 @@ mod tests {
         );
     }
 
+    /// improvise-tt22: `.improv` text is the only native format, so a legacy
+    /// JSON workbook is refused by name. Evidence: `context/test-evidence/formula-and-io.md`.
     #[test]
-    fn load_dispatcher_detects_legacy_json_by_brace() {
-        // The load() function routes to JSON deserializer when text starts with '{'
-        let m = two_cat_model();
-        let json = serde_json::to_string_pretty(&m).unwrap();
-        assert!(json.trim_start().starts_with('{'), "sanity check");
-        // Deserialise via the JSON path
-        let m2: Workbook = serde_json::from_str(&json).unwrap();
-        assert_eq!(m2.model.name, "Budget");
-    }
-
-    #[test]
-    fn a_legacy_json_workbook_loads_its_numbers_bit_for_bit() {
+    fn a_legacy_json_workbook_is_refused_by_name() {
         use crate::storage::{MemoryFs, Storage};
-        const LOSSY: f64 = 0.248_157_142_857_142_85;
-        let key = coord(&[("Type", "Food"), ("Month", "Jan")]);
-        let mut wb = two_cat_model();
-        wb.model.set_cell(key.clone(), CellValue::Number(LOSSY));
-        let json = serde_json::to_string(&wb).unwrap();
-        assert!(
-            json.contains("0.24815714285714285"),
-            "the file holds the exact text"
-        );
-        let fs = MemoryFs::new();
-        let path = std::path::Path::new("legacy.improv");
-        fs.write(path, json.as_bytes()).unwrap();
-        let loaded = super::load_from(&fs, path).unwrap();
-        let Some(CellValue::Number(n)) = loaded.model.get_cell(&key) else {
-            panic!("the cell must load as a number");
-        };
-        assert_eq!(n.to_bits(), LOSSY.to_bits(), "loaded {n:?}, not {LOSSY:?}");
+        // The shape the removed loader read: `Workbook`'s serde form.
+        let legacy = r#"
+            {"model": {"name": "Budget", "categories": {}, "data": [], "formulas": []},
+             "views": {}, "active_view": "Default"}"#;
+        for name in ["legacy.improv", "legacy.improv.gz"] {
+            let fs = MemoryFs::new();
+            let path = std::path::Path::new(name);
+            let bytes = if name.ends_with(".gz") {
+                use std::io::Write;
+                let mut enc =
+                    flate2::write::GzEncoder::new(Vec::new(), flate2::Compression::default());
+                enc.write_all(legacy.as_bytes()).unwrap();
+                enc.finish().unwrap()
+            } else {
+                legacy.as_bytes().to_vec()
+            };
+            fs.write(path, &bytes).unwrap();
+            let err = format!("{:#}", super::load_from(&fs, path).unwrap_err());
+            assert!(
+                err.contains("legacy JSON") && err.contains("no longer supported"),
+                "{name}: expected the format named and refused, got {err:?}"
+            );
+        }
     }
 
     // ── save/load roundtrip via file ────────────────────────────────────