diff --git a/context/repo-map.md b/context/repo-map.md
index 059c28e..21fc761 100644
--- a/context/repo-map.md
+++ b/context/repo-map.md
@@ -93,10 +93,11 @@ pub struct Model {
 //   set_cell(&mut self, key: CellKey, value: CellValue)
 //   evaluate(&self, key: &CellKey) -> Option<CellValue>          [formulas + raw data]
 //   evaluate_aggregated(&self, key, none_cats) -> Option<CellValue>  [sums over hidden dims]
-//   add_formula(&mut self, formula: Formula)       [replaces same target+category]
+//   recompute_formulas(&mut self, none_cats)               [fixed-point formula cache]
+//   add_formula(&mut self, formula: Formula)       [replaces same target+category, adds item]
 //   remove_formula(&mut self, target, category)
 //   category_names(&self) -> Vec<&str>                    [includes virtual]
-//   regular_category_names(&self) -> Vec<&str>            [excludes _Index, _Dim]
+//   regular_category_names(&self) -> Vec<&str>            [excludes _Index, _Dim, _Measure]
 
 const MAX_CATEGORIES: usize = 12;  // virtual categories don't count
 ```
@@ -114,8 +115,10 @@ pub struct CellKey(pub Vec<(String, String)>);  // always sorted by category nam
 pub enum CellValue {
     Number(f64),
     Text(String),
+    Error(String),       // formula evaluation error (circular ref, div/0, etc.)
 }
 // CellValue::as_f64() -> Option<f64>
+// CellValue::is_error() -> bool
 
 pub struct DataStore {
     cells: HashMap<InternedKey, CellValue>,
@@ -141,7 +144,7 @@ pub struct Category {
 // Category::add_item(&mut self, name) -> ItemId  [deduplicates by name]
 // Category::ordered_item_names(&self) -> Vec<&str>  [respects group order]
 
-pub enum CategoryKind { Regular, VirtualIndex, VirtualDim, Label }
+pub enum CategoryKind { Regular, VirtualIndex, VirtualDim, VirtualMeasure, Label }
 ```
 
 ### Formula System
@@ -480,8 +483,9 @@ examples).
 1. **Commands never mutate.** They receive `&CmdContext` (read-only) and return `Vec<Box<dyn Effect>>`.
 2. **CellKey is always sorted.** Use `CellKey::new()` — never construct the inner Vec directly.
 3. **`category_mut()` for adding items.** `Model` has no `add_item` method; get the category first: `m.category_mut("Region").unwrap().add_item("East")`.
-4. **Virtual categories** `_Index` and `_Dim` always exist. `is_empty_model()` checks whether any *non-virtual* categories exist.
+4. **Virtual categories** `_Index`, `_Dim`, and `_Measure` always exist. `is_empty_model()` checks whether any *non-virtual* categories exist. `_Measure` holds numeric data fields and formula targets; `add_formula` auto-adds the target item.
 5. **Display rounding is view-only.** `format_f64` (half-away-from-zero) is only called in rendering. Formula eval uses full f64.
+5b. **Formula evaluation is fixed-point.** `recompute_formulas(none_cats)` iterates formula evaluation until values stabilize, using a cache. `evaluate_aggregated` checks the cache for formula results. Circular refs produce `CellValue::Error("circular")`.
 6. **Keybindings are per-mode.** `ModeKey::from_app_mode()` resolves the current mode, then the corresponding `Keymap` is looked up. Normal + `search_mode=true` maps to `SearchMode`.
 7. **`effect_cmd!` macro** generates a command struct that just produces effects. Use for simple commands without complex logic.
 8. **`.improv` format is markdown-like**, not JSON. See `persistence/mod.rs`. JSON is legacy only.