Merge branch 'main' into ui/header-stable

benches/grid_projection.rs

@@ -3,7 +3,7 @@
 
 use std::time::{Duration, Instant};
 
-use improvise::format::parse_number_format;
+use improvise::format::FormatPlan;
 use improvise::model::cell::{CellKey, CellValue};
 use improvise::view::Axis;
 use improvise::workbook::Workbook;
@@ -94,12 +94,11 @@ fn time(iters: u32, app: &App, viewport: Viewport) -> Duration {
 fn time_widths(iters: u32, app: &App) -> Duration {
     let layout = &app.layout;
     let model = &app.model_state.workbook.model;
-    let (comma, decimals) =
-        parse_number_format(&app.model_state.workbook.active_view().number_format);
+    let plan = FormatPlan::of(model, app.model_state.workbook.active_view());
     let mut best = Duration::MAX;
     for _ in 0..iters.max(3) {
         let start = Instant::now();
-        std::hint::black_box(compute_col_widths(model, layout, comma, decimals));
+        std::hint::black_box(compute_col_widths(model, layout, &plan));
         best = best.min(start.elapsed());
     }
     best

context/code/core.md

@@ -820,3 +820,146 @@ renders a cell and asserts its digits. That gap is named rather than closed —
 those files are `improvise-command`, which was another wave's ground when this
 landed — and it is the more useful half of this entry, because it says where the
 next regression would go unseen.
+
+**That gap is now closed, and the sentence above is kept as the record of it.**
+`ui::project::cell_identity::a_formulas_format_reaches_its_own_column_and_no_other`
+renders cells *and* the totals strip and asserts their digits (improvise-e0ex);
+`ui/app.rs`'s own call is still unguarded, and now reaches the same `FormatPlan`
+rather than a second `parse_number_format`, so there is one fewer place to drift.
+
+## The decimal default and a formula's own format (improvise-e0ex)
+
+`View::new` seeds `,.2`, not `,.0`. The symptom that forced it was
+`Margin = Profit / Revenue` drawing as `1`: a 0.66 ratio under *zero* decimals,
+rounded half-away-from-zero by `format_f64`. Three things worth stating, because
+each was a candidate answer that is wrong.
+
+**It is a formatting bug and not an arithmetic one.** Exact decimal arithmetic
+(ADR 005) would not change the rendering by one character — `0.66` under `,.0`
+is still `1` — so nothing here waits on it and nothing here touches the
+evaluator. Gotcha 8 is unchanged: `format_f64` is called in rendering and
+nowhere else, and full `f64` is what eval sees.
+
+**The cost is paid on whole numbers, knowingly.** Revenue now draws
+`18,400.00`. That is what an accounting application does, and it is the cheaper
+error: a currency column with two redundant zeroes is *legible*, where a ratio
+with none is **wrong**. A format cannot be guessed from a value — 0.66 is a
+ratio and 0.66 is also two-thirds of a dollar — so the default must be the one
+whose failure mode is noise rather than a false number.
+
+**The override is spelled in the formula, not per `_Measure` item.** The user's
+ruling ("unless the format is explicit in the formula") picks the narrower of
+two nearly-coincident units: a formula *target* is a `_Measure` item (gotcha 7),
+so per-formula and per-measure differ only for a measure no formula owns — and
+such a measure is raw data, whose format is the view's job. So a formula may
+carry its own format, which wins for its target; a raw measure keeps the view's.
+
+### Where the format is resolved: `FormatPlan`
+
+`FormatPlan::of(model, view)` is the one answer, and every render path takes it
+where it used to take a `(comma, decimals)` pair: `GridLayout::display_text`,
+`ui::viewport::grid_body`/`compute_col_widths`, `ui::project::grid_in` and
+`persistence/export.rs::render_csv`. Replacing the pair rather than adding a
+second argument beside it is the `row_levels`-next-to-`row_labels` rule from
+design-principles §5 — two answers to "what format is this cell" is the disease.
+
+**It is asked of the cell's `CellKey`, never of the axes.** A cell aggregating
+over `_Measure` carries no target coordinate and so keeps the view's format,
+which falls out of the key rather than needing a rule. Deriving it from "is
+`_Measure` on a visible axis" would be a second derivation that could agree
+with itself while disagreeing with the cells.
+
+**The empty plan short-circuits**, so a document with no formula format pays one
+`is_empty` per cell and nothing else; the lookup is otherwise a scan of the
+formula list, which is short and must stay ordered for the reason below.
+
+**The owner is the formula `Model::formula_for` names, and nothing else.**
+`FormatPlan` holds every formula's `(target_category, target, Option<format>)`
+in the model's own order and takes the **first** coordinate match, which is the
+same rule and the same order evaluation uses to decide whose value the cell is.
+Two things that shape enforces, both of which a `HashMap` keyed by target got
+wrong and review caught (Codex, PR #125):
+
+- **A key may be claimed by two formulas**, because a target category need not
+  be `_Measure` — `- Tax = Revenue * 0.1 [CustomCat]` is in the format's own
+  documentation. Iterating a `HashMap` answered from whichever category hashed
+  first, so the *displayed* format was nondeterministic per process while the
+  *value* was not. That is gotcha 18 exactly: an index whose values are hashed
+  is for lookup, never for choosing between candidates.
+- **An owner that names no format still owns the cell.** Indexing only the
+  formatted formulas let a later formula's `FORMAT` reach a cell whose value it
+  does not produce. So the list holds *every* formula and a match with
+  `format: None` falls through to the view — it is a match, not a miss.
+
+The list is dropped entirely when no formula names a format, so the common
+document pays one `is_empty` per cell rather than a scan.
+
+**A column total is resolved from the column, never from a row.**
+`GridLayout::column_key(col)` is the page/filter constraints plus the column's
+own tuple — the coordinates that survive the row aggregation — and `GridBody`
+carries the resulting `col_formats` so the strip and the cells cannot disagree.
+Reading it off row 0 was the first version and is wrong twice over, also found
+by review: a formula may target a **row-axis** item (its target category is any
+category), so the total inherited a row-local override for a row it had summed
+away, **and** re-ordering the rows changed the total's spelling. The guard is
+`a_column_total_does_not_inherit_a_row_local_format`, which builds the same grid
+with the owned row first and last and requires the two strips to agree.
+
+### The grammar: `FORMAT <spec>` in the formula, not in the `.improv` line
+
+`formula.pest` gains `format_clause = ${ ^"FORMAT" ~ sp+ ~ format_spec }` after
+the optional `WHERE`. **`improv.pest` is unchanged**, and that is the design
+rather than an omission: `formula_raw` already takes the `## Formulas` line
+verbatim up to the optional `[Category]` suffix, so a format inside the formula
+*text* is carried by the existing grammar with nothing to add. Two consequences.
+The grammar-walking generator (which reads `improv.pest`, not `formula.pest`) is
+untouched, so there is no new spelling for it to violate. And the same text a
+user types into the TUI formula minibuffer is the text the file carries — one
+namespace, one spelling rule, exactly as gotcha 34 says of names.
+
+`sp+` rather than a `!bare_char` lookahead, for `WHERE`'s reason (gotcha 34):
+a predicate is invisible to a generator. `FORMAT` joins `quote::KEYWORDS`, so an
+item literally named `FORMAT` is written `|FORMAT|` — the keyword guard is
+canonical and this is one more entry in it, not a new rule.
+
+**`format_spec` is `(ASCII_ALPHANUMERIC | "," | "." | "%")+`** — a bounded token
+where the view's `format:` line is `rest_of_line`. The line can afford to be
+opaque because a newline ends it; a formula clause cannot, because something may
+follow. The set covers every spelling `parse_number_format` can read (`,.2f`,
+`.4`, `%.2f`) and nothing the tokenizer would have to disambiguate.
+
+### The trap this feature sets, and what is asserted instead
+
+`format_md` writes `Formula::raw`, so a format inside the raw text round-trips
+**whether or not the walker parsed it into a field**. That is gotcha 34's bug
+exactly — a target that kept its pipes survived every round-trip test because
+`raw` was unaffected. So the round-trip test asserts `Formula::format`, the
+parsed field, and its `raw` assertion is there to *show* that `raw` cannot be
+the witness. Injecting the fault (a walker that drops the clause) fails that one
+test and **nothing else in `improvise-io`** — 242 other tests, including the
+whole `format_md`↔`parse_md` characterization suite and the grammar-walking
+properties, pass. That is the measurement, not an argument.
+
+### What persistence does and does not do here
+
+The user's constraint — "should not affect persistence at all, except to store
+format metadata" — holds by construction and the job was to not break it.
+`format_f64` is display; `persistence/format.rs::format_number` is a different
+function in a different crate writing Rust's shortest round-trip form. **The
+regression to watch for is routing a save through the display formatter** so the
+file matches what the user sees: a model has many views (gotcha 29), so there is
+no single right answer and the last save would win. Gotcha 8 forbids feeding a
+rounded value back into *eval*; this is its persistence twin.
+
+What did change on disk is metadata only: newly created views write
+`format: ,.2` where they wrote `format: ,.0`, and a formula line may now carry
+a `FORMAT` suffix. Existing documents are unaffected — `format_md` has always
+written the `format:` line explicitly whenever `number_format` is non-empty and
+`View::new`'s value has never been empty, so every document improvise has ever
+written carries its own and no migration exists to get wrong.
+
+One consumer that is display but reads like persistence: **CSV export** goes
+through `display_text` and therefore through the view's format, as it always
+has. A document with no explicit `format:` line now exports two decimals. That
+is pre-existing behaviour meeting a new default, not a new coupling.
+

context/test-evidence/command.md

@@ -4216,3 +4216,55 @@ fixture loads **both** stacks first so a lost forward stack is visible at all.
 The neighbouring test `a_refused_switch_corrects_the_asker_and_nobody_else`
 compared `active_view` alone and stayed green through all of this: the check
 that answers a neighbouring question, in the place the doctrine warns about.
+
+---
+
+## The format a projected cell is drawn with (improvise-e0ex)
+
+### a_formulas_format_reaches_its_own_column_and_no_other
+
+`context/code/core.md` recorded, as the more useful half of improvise-711n's
+entry, that `ui/project.rs::grid_of` "has no test that renders a cell and
+asserts its digits". This is that test, and it is why the gap was worth naming:
+the per-formula format is resolved in `improvise-core` and could have been
+resolved correctly there while never reaching the grid.
+
+It asserts four things that fail independently: the formula column's cell, the
+raw column's cell beside it, and **both** totals. The totals matter separately
+because they are formatted on their own path — `GridBody::col_formats`, read off
+the top row — so a correct cell says nothing about the strip under it.
+
+Two faults injected:
+
+- `FormatPlan::at` returning the view's format for every key fails the cell
+  assertion (`left: Some("0.50") right: Some("0.5000")`).
+- Dropping only the `col_formats` write in `viewport::grid_body`, leaving the
+  cells correct, fails the totals assertion (`left: "1.50" right: "1.5000"`).
+
+The second is the one that earns the test its length: without it, the totals
+strip could round every formula column to the view's format and no assertion in
+the workspace would move.
+
+### a_column_total_does_not_inherit_a_row_local_format
+
+Found by review (ChatGPT Codex, PR #125). The first version of `grid_body` read
+each column's total format off **row 0**, and `context/code/core.md` argued that
+was safe because "where `_Measure` is on the row axis a column is not a measure
+at all". That argument is wrong: a formula's target category is *any* category,
+so a formula can own a **row-axis item**, and then row 0's format is a row-local
+override applied to a total that has summed that row away — and re-ordering the
+rows changes the total's spelling.
+
+The fixture builds the same grid twice with `12.5 FORMAT .4` owning the **first**
+row in one and the **last** row in the other, and requires the two totals strips
+to agree. Every raw cell in `raw_workbook` holds the same value per measure, so
+the two grids' totals are numerically identical and the *only* thing that can
+differ is the format — the assertion has nowhere to hide.
+
+It asserts the owned cell first (`12.5000`), so a plan that applies no format at
+all fails on the fixture rather than passing the comparison vacuously.
+
+Fault injected: `plan.at(layout.cell_key(0, ci).as_ref())` in place of
+`layout.column_key(ci)` — the rejected version, restored. Fails with
+`left: ["14.5000", "16.5000"] right: ["14.50", "16.50"]`, which is the bug in
+one line: row order decided the spelling.

context/test-evidence/core.md

@@ -1062,3 +1062,84 @@ qualification.
 It asserts `effective_item_names("_Measure")` **by value** rather than
 `!contains("Profit")`, so a fix that removed the formula and also lost
 `Revenue` or `Cost` on the way fails here rather than passing a negative.
+
+---
+
+## `format.rs` — the decimal default and a formula's own format (improvise-e0ex)
+
+### a_ratio_keeps_its_decimals_under_a_fresh_views_format
+
+The bug the ticket reports, at the smallest place it is visible.
+`View::new`'s `number_format` was `,.0` — *zero* decimals — so `2/3` reached
+`format_f64` and came back as `"1"`, half-away-from-zero. Written before the
+fix and observed failing with `left: "1" right: "0.67"`.
+
+It asks the **view's default** rather than a literal `",.2"`, so the assertion
+is about the shipped default and not about `parse_number_format`, which already
+had its own tests. Reverting `View::new` to `,.0` fails it.
+
+What it deliberately does not assert: that the default is exactly two places.
+`0.67` would also fail under `.1` or `.3`, which is the point — the test names
+"a ratio keeps its decimals", and the precise number is a product decision
+recorded in `context/code/core.md`, not a property.
+
+### a_formulas_own_format_wins_for_its_target_and_nowhere_else
+
+The override, and both halves of "nowhere else" — a raw measure beside the
+formula target, and a key with no `_Measure` coordinate at all (the aggregating
+cell). The fixture uses `.4` against a `,.2` view so the two answers differ in
+*digits*, not merely in grouping.
+
+Two faults injected, in opposite directions, because one alone is not enough:
+
+- `FormatPlan::at` returning `self.view` unconditionally (the override never
+  applies) fails with `left: "0.67" right: "0.6667"`.
+- `FormatPlan::at` returning the first entry in the plan for any key (the
+  override leaks to every cell) fails with `left: "0.6667" right: "0.67"`.
+
+A test asserting only the target's own column passes the second fault, which is
+why the two neighbours are in the same test rather than in a sibling that could
+be deleted on its own.
+
+### a_formula_naming_no_format_leaves_its_target_on_the_views
+
+The fallback, asserted where it is easiest to lose: a formula target whose
+formula carries no format must be indistinguishable from a raw measure. It
+keeps `FormatPlan::of` from indexing every target with some default and then
+"overriding" it back, which would be a second copy of the view's answer.
+
+### the_formula_that_owns_a_cell_names_its_format_even_when_it_names_none
+
+Found by review (ChatGPT Codex, PR #125), not by the tests above, which is the
+entry's point: they used one formula and so could not see an ownership question
+at all.
+
+A target category need not be `_Measure` — `- Tax = Revenue * 0.1 [CustomCat]`
+is in the format's own documentation — so a key may carry coordinates two
+formulas claim. The first `FormatPlan` indexed **only the formatted** formulas,
+so a later formula's `FORMAT` reached a cell whose value an earlier, unformatted
+formula produces: the number and its spelling came from different formulas.
+
+The fixture puts the same two formulas in both orders and asserts the owner's
+answer each time, so a plan that prefers the formatted one fails the first half
+and a plan that ignores formats entirely fails the second.
+
+Fault injected: `.filter(|t| t.format.is_some())` before the `find` — precisely
+the behaviour of the version review rejected. Fails with `left: "0.6667"
+right: "0.67"`.
+
+### the_owner_is_the_one_model_order_names_and_not_the_one_a_hash_does
+
+The other half of the same finding, and the one a single run cannot see.
+`by_target` was a `HashMap<category, HashMap<item, _>>` and `at` iterated it, so
+with two claiming categories the answer came from whichever hashed first —
+**a different answer per process**, gotcha 18's shape exactly (an index whose
+values are hashed is for lookup, never for choosing between candidates).
+
+It rebuilds the plan five times rather than asserting once, because each
+`HashMap` gets its own `RandomState` and a single construction can be right by
+luck. It is the same shape as `analyze_records`' five-iteration guard.
+
+This one cannot be made to fail by re-introducing a `HashMap` *reliably*, which
+is why the test that carries the load is its sibling above; kept because the
+repetition is what documents that hash order was the defect.

context/test-evidence/formula-and-io.md

@@ -1255,3 +1255,36 @@ well as membership — insertion order is load-bearing here (gotcha 18), and the
 neighbouring `two_sections_naming_one_category_round_trip_to_a_stable_group_order`
 already holds groups to that standard. This test was the item-list half of the
 same invariant, held to a weaker one.
+
+---
+
+## A formula's own number format on disk (improvise-e0ex)
+
+### parse_md_round_trips_the_format_a_formula_carries
+
+`format_md` writes `Formula::raw`, so a `FORMAT .4` suffix inside the formula
+text survives a save/load **whether or not the tree-walker parsed it into
+`Formula::format`**. That is gotcha 34's bug wearing new clothes: a target that
+kept its pipe delimiters round-tripped perfectly and still created a ghost
+`_Measure` item, precisely because `raw` was unaffected.
+
+So this test asserts the **parsed field**, and its `raw` assertion is there to
+show why `raw` cannot be the witness rather than to check the writer.
+
+Fault injected: `build_formula`'s `Rule::format_clause` arm discarded instead of
+recorded (`let _ = first_inner(inner);`). It fails with
+`left: None right: Some(".4")` — and **every other test in `improvise-io`
+passes**, 242 of them, including the whole `format_md`↔`parse_md`
+characterization suite and the grammar-walking generator properties. That
+measurement is the entry's point: no text-comparing test can see this field
+go missing.
+
+Note what the injected fault does *not* do: it leaves the grammar alone, so the
+line still parses and the file is still well-formed. A fault that also broke
+the parse would fail the test for the wrong reason.
+
+### parse_md_reads_a_formula_with_no_format_as_none
+
+The absent case, so the field above is something the file carried rather than a
+constant the walker invents. Without it, a walker hard-coding `Some(".4")`
+would pass the test above.

crates/improvise-command/src/command/cmd/panel.rs

@@ -406,6 +406,7 @@ mod tests {
             target_category: "Type".to_string(),
             expr: crate::formula::ast::Expr::Number(0.0),
             filter: None,
+            format: None,
         });
         let layout = make_layout(&m);
         let reg = make_registry();

crates/improvise-command/src/ui/app.rs

@@ -17,7 +17,7 @@ use std::rc::Rc;
 
 use crate::command::cmd::CmdContext;
 use crate::command::keymap::{Keymap, KeymapSet};
-use crate::format::parse_number_format;
+use crate::format::FormatPlan;
 use crate::import::wizard::ImportWizard;
 use crate::model::cell::{CellKey, CellValue, DataStore};
 use crate::persistence;
@@ -796,13 +796,8 @@ impl App {
             display_value: self.display_value_at_cursor(),
             visible_rows: (self.term_height as usize).saturating_sub(8),
             visible_cols: {
-                let (fmt_comma, fmt_decimals) = parse_number_format(&view.number_format);
-                let col_widths = compute_col_widths(
-                    &self.model_state.workbook.model,
-                    layout,
-                    fmt_comma,
-                    fmt_decimals,
-                );
+                let model = &self.model_state.workbook.model;
+                let col_widths = compute_col_widths(model, layout, &FormatPlan::of(model, view));
                 let row_header_width = compute_row_header_width(layout);
                 compute_visible_cols(
                     &col_widths,

crates/improvise-command/src/ui/host.rs

@@ -453,7 +453,7 @@ mod tests {
 
         let mut seen = vec![connect.msg];
         seen.extend(to(&frames, id));
-        assert_eq!(cell_text(&folded_grid(&seen), 0, 0), "42");
+        assert_eq!(cell_text(&folded_grid(&seen), 0, 0), "42.00");
     }
 
     /// `doctrine(check_the_checker)`: inject the fault — never send the command —
@@ -483,7 +483,7 @@ mod tests {
 
         let mut seen = vec![bootstrap.msg];
         seen.extend(to(&frames, bystander));
-        assert_eq!(cell_text(&folded_grid(&seen), 0, 0), "7");
+        assert_eq!(cell_text(&folded_grid(&seen), 0, 0), "7.00");
     }
 
     /// A view-only command reaches nobody else — the counterpart that keeps the

crates/improvise-command/src/ui/project.rs

@@ -3,7 +3,7 @@
 
 use std::collections::BTreeMap;
 
-use improvise_core::format::{format_f64, format_value, parse_number_format};
+use improvise_core::format::FormatPlan;
 use improvise_core::model::Model;
 use improvise_core::model::category::MEASURE_CATEGORY;
 use improvise_core::model::cell::CellValue;
@@ -141,12 +141,12 @@ pub fn grid_in(app: &App, view_name: &str, viewport: Viewport) -> GridCache {
     let elsewhere =
         (name != app.view_name()).then(|| GridLayout::with_frozen_records(model, view, None));
     let layout = elsewhere.as_ref().unwrap_or(&app.layout);
-    let (comma, decimals) = parse_number_format(&view.number_format);
+    let plan = FormatPlan::of(model, view);
     let rows = viewport.rows_in(layout.row_count());
     let cols = viewport.cols_in(layout.col_count());
     // ONE walk: the widths, the totals and this window's values all come off the
     // same evaluation — `context/code/command.md` (improvise-jmjz).
-    let body = grid_body(model, layout, comma, decimals, rows.clone(), cols.clone());
+    let body = grid_body(model, layout, &plan, rows.clone(), cols.clone());
 
     GridCache {
         row_frame: axis_frame(&layout.row_cats, &layout.row_items, view, rows.clone()),
@@ -161,11 +161,11 @@ pub fn grid_in(app: &App, view_name: &str, viewport: Viewport) -> GridCache {
             .map(|(r, values)| {
                 cols.clone()
                     .zip(values.iter())
-                    .map(|(c, value)| cell_display(app, layout, r, c, value, comma, decimals))
+                    .map(|(c, value)| cell_display(app, layout, r, c, value, &plan))
                     .collect()
             })
             .collect(),
-        totals: column_totals(&body, cols, comma, decimals),
+        totals: column_totals(&body, cols),
         page_coords: layout.page_coords.clone(),
         // The whole grid, not the window: this is what a client clamps its own
         // cursor against (improvise-bpq2). Off the layout, not the clipped ranges.
@@ -252,17 +252,12 @@ fn axis_frame(
 
 /// The totals strip: each visible column summed over **every** row, not just the
 /// viewport's — which is why it is projected. Empty means "no totals" (gotcha 33).
-fn column_totals(
-    body: &GridBody,
-    cols: std::ops::Range<usize>,
-    comma: bool,
-    decimals: u8,
-) -> Vec<CellDisplay> {
+fn column_totals(body: &GridBody, cols: std::ops::Range<usize>) -> Vec<CellDisplay> {
     match body.totals.is_empty() {
         true => Vec::new(),
         false => cols
-            .filter_map(|c| body.totals.get(c))
-            .map(|total| CellDisplay::number(format_f64(*total, comma, decimals)))
+            .filter_map(|c| Some((body.totals.get(c)?, body.col_formats.get(c)?)))
+            .map(|(total, fmt)| CellDisplay::number(fmt.number(*total)))
             .collect(),
     }
 }
@@ -275,8 +270,7 @@ fn cell_display(
     row: usize,
     col: usize,
     value: &Option<CellValue>,
-    comma: bool,
-    decimals: u8,
+    plan: &FormatPlan,
 ) -> CellDisplay {
     if layout.is_records_mode() {
         // The value, not its spelling: this module used to infer numeric-ness
@@ -290,16 +284,15 @@ fn cell_display(
             id: cell_id(app, layout, row, col),
         };
     }
+    let key = layout.cell_key(row, col);
     CellDisplay {
-        text: format_value(value.as_ref(), comma, decimals),
+        text: plan.at(key.as_ref()).value(value.as_ref()),
         numeric: matches!(value, Some(CellValue::Number(_))),
         error: matches!(value, Some(CellValue::Error(_))),
         id: cell_id(app, layout, row, col),
         // The *unformatted* value the TUI seeds its edit buffer from. Projecting it
         // beats a client un-formatting `text` (gotcha 47).
-        edit_text: layout
-            .cell_key(row, col)
-            .map(|key| app.display_value_in(layout, &key)),
+        edit_text: key.map(|key| app.display_value_in(layout, &key)),
     }
 }
 
@@ -1811,7 +1804,7 @@ mod tests {
             .collect();
         assert_eq!(cells.len(), 1, "expected one changed cell, got {cells:?}");
         assert_eq!(cells[0].0, (1, 1));
-        assert_eq!(cells[0].1.text, "7");
+        assert_eq!(cells[0].1.text, "7.00");
     }
 
     /// Scrolling is `ViewOnly` — no other client hears it — but the client
@@ -2733,7 +2726,8 @@ mod tests {
 
         let model = &app.model_state.workbook.model;
         assert_eq!(app.layout.row_count(), 3, "precondition: three rows");
-        let one_row = grid_body(model, &app.layout, false, 0, 0..1, 0..1);
+        let plan = improvise_core::format::FormatPlan::uniform("");
+        let one_row = grid_body(model, &app.layout, &plan, 0..1, 0..1);
         assert_eq!(
             one_row.window.len(),
             1,
@@ -2752,7 +2746,7 @@ mod tests {
         );
         assert_eq!(
             one_row.col_widths,
-            compute_col_widths(model, &app.layout, false, 0),
+            compute_col_widths(model, &app.layout, &plan),
             "the keypress path and the projection must size a column the same"
         );
     }
@@ -4396,7 +4390,7 @@ mod cell_identity {
 
         assert_eq!(
             grid_of(&app, WHOLE).cell(row, 0).map(|c| c.text.clone()),
-            Some("42".to_string())
+            Some("42.00".to_string())
         );
     }
 
@@ -4432,7 +4426,7 @@ mod cell_identity {
             vec![3],
             "exactly (1, 1) may move: {before:?} {after:?}"
         );
-        assert_eq!(after[3], "999");
+        assert_eq!(after[3], "999.00");
     }
 
     /// A surrogate that does not resolve applies *nothing*. The floor underneath the
@@ -4510,6 +4504,86 @@ mod cell_identity {
         app
     }
 
+    /// improvise-e0ex: a formula's own format reaches the drawn grid — cells and
+    /// the totals strip — and stops there. `context/test-evidence/command.md`.
+    #[test]
+    fn a_formulas_format_reaches_its_own_column_and_no_other() {
+        let mut wb = raw_workbook();
+        wb.model.add_formula(
+            improvise_formula::parse_formula("Margin = Revenue / Cost FORMAT .4", "_Measure")
+                .unwrap(),
+        );
+        let app = App::new(wb, None);
+        let grid = grid_of(&app, WHOLE);
+
+        let margin = column_named(&app, "Margin");
+        let revenue = column_named(&app, "Revenue");
+        assert_eq!(
+            grid.cell(0, margin).map(|c| c.text.clone()),
+            Some("0.5000".to_string()),
+            "the formula's own four decimals, not the view's two"
+        );
+        assert_eq!(
+            grid.cell(0, revenue).map(|c| c.text.clone()),
+            Some("1.00".to_string()),
+            "a raw measure keeps the view's format"
+        );
+        // The totals strip is formatted on its own path (`col_formats`), so it
+        // can disagree with the cells above it without any cell being wrong.
+        assert_eq!(
+            grid.totals[margin].text, "1.5000",
+            "three rows of 0.5, under the column's own format"
+        );
+        assert_eq!(grid.totals[revenue].text, "3.00");
+    }
+
+    /// A formula owning one ROW, so its format is row-local and the totals have
+    /// aggregated that row away. `context/test-evidence/command.md`.
+    fn row_targeted_app(region: &str) -> App {
+        let mut wb = raw_workbook();
+        wb.model.add_formula(
+            improvise_formula::parse_formula(&format!("{region} = 12.5 FORMAT .4"), "Region")
+                .unwrap(),
+        );
+        App::new(wb, None)
+    }
+
+    /// improvise-e0ex, found by review: reading a total's format off row 0 made
+    /// it depend on row ORDER. `context/test-evidence/command.md`.
+    #[test]
+    fn a_column_total_does_not_inherit_a_row_local_format() {
+        let first = row_targeted_app("North");
+        let last = row_targeted_app("East");
+        let (fg, lg) = (grid_of(&first, WHOLE), grid_of(&last, WHOLE));
+
+        // The row the formula owns really is drawn with the formula's format —
+        // without this the test would pass against a plan that does nothing.
+        let owned = fg.cell(0, 0).map(|c| c.text.clone());
+        assert_eq!(
+            owned,
+            Some("12.5000".to_string()),
+            "fixture: North is owned"
+        );
+
+        // North is row 0 and East is the last row, so a total read off row 0
+        // answers differently in the two grids. It must not.
+        let totals = |g: &improvise_protocol::render::GridCache| -> Vec<String> {
+            g.totals.iter().map(|t| t.text.clone()).collect()
+        };
+        assert_eq!(
+            totals(&fg),
+            totals(&lg),
+            "which row a formula owns changed the totals' spelling"
+        );
+        for t in totals(&fg) {
+            assert_eq!(
+                t.split('.').nth(1).map(str::len),
+                Some(2),
+                "a total must keep the VIEW's two decimals, got {t:?}"
+            );
+        }
+    }
+
     fn column_named(app: &App, label: &str) -> usize {
         (0..app.layout.col_count())
             .find(|&c| app.layout.col_label(c) == label)
@@ -5040,7 +5114,7 @@ mod basis_check {
                 .cells
                 .iter()
                 .flatten()
-                .any(|c| c.text == "99"),
+                .any(|c| c.text == "99.00"),
             "the accepted write never reached the document"
         );
     }
@@ -5077,7 +5151,7 @@ mod basis_check {
         assert_eq!(rejection(applied), Rejection::RecordMoved(target.record));
         assert_eq!(
             value_at(&app, 1),
-            "7",
+            "7.00",
             "a rejected batch must leave the document exactly as it was"
         );
     }
@@ -5106,7 +5180,7 @@ mod basis_check {
             matches!(applied, Applied::Diff { .. }),
             "another row's edit invalidated this one: {applied:?}"
         );
-        assert_eq!(value_at(&app, 1), "99");
+        assert_eq!(value_at(&app, 1), "99.00");
     }
 
     /// Class 2 by the other route: "gone" and "changed" must not collapse into one
@@ -5146,7 +5220,11 @@ mod basis_check {
 
         let applied = sessions.apply(&mut app, id, &write(target.clone(), basis));
         assert_eq!(rejection(applied), Rejection::RecordUnstated(target.record));
-        assert_eq!(value_at(&app, 1), "1", "the omitted claim let a write land");
+        assert_eq!(
+            value_at(&app, 1),
+            "1.00",
+            "the omitted claim let a write land"
+        );
     }
 
     // ── tier 1: the document ────────────────────────────────────────────
@@ -5273,7 +5351,7 @@ mod basis_check {
             matches!(applied, Applied::Diff { .. }),
             "a basis composed only from the published stream was refused: {applied:?}"
         );
-        assert_eq!(value_at(&app, 1), "99");
+        assert_eq!(value_at(&app, 1), "99.00");
     }
 
     /// The other side of publication: after somebody else writes, the stream carries
@@ -5331,6 +5409,6 @@ mod basis_check {
             matches!(applied, Applied::Diff { .. }),
             "a re-composed batch was refused: {applied:?}"
         );
-        assert_eq!(value_at(&app, 1), "99");
+        assert_eq!(value_at(&app, 1), "99.00");
     }
 }

crates/improvise-command/src/ui/viewport.rs

@@ -5,8 +5,8 @@ use unicode_width::UnicodeWidthStr;
 
 use improvise_protocol::header_level_widths;
 
-use crate::format::format_f64;
-use crate::model::cell::CellValue;
+use crate::format::{FormatPlan, NumberFormat};
+use crate::model::cell::{CellKey, CellValue};
 use crate::model::{Aggregation, Model};
 use crate::view::{AxisEntry, GridLayout};
 
@@ -23,6 +23,9 @@ pub use improvise_protocol::{MAX_ROW_HEADER_W, MIN_ROW_HEADER_W};
 pub struct GridBody {
     /// One per column of the WHOLE grid, clamped, never windowed.
     pub col_widths: Vec<u16>,
+    /// The format each column's TOTAL is drawn with, resolved from the
+    /// coordinates that survive the row aggregation — `context/code/core.md`.
+    pub col_formats: Vec<NumberFormat>,
     /// Column sums over every row. **Empty** in records mode or an empty grid,
     /// which is "this grid has no totals" rather than a strip of zeroes.
     pub totals: Vec<f64>,
@@ -35,8 +38,7 @@ pub struct GridBody {
 pub fn grid_body(
     model: &Model,
     layout: &GridLayout,
-    fmt_comma: bool,
-    fmt_decimals: u8,
+    plan: &FormatPlan,
     rows: std::ops::Range<usize>,
     cols: std::ops::Range<usize>,
 ) -> GridBody {
@@ -49,6 +51,11 @@ pub fn grid_body(
         false => Vec::new(),
     };
     let mut window: Vec<Vec<Option<CellValue>>> = Vec::with_capacity(rows.len());
+    // Off the COLUMN, not off row 0: a formula targeting a row-axis item is a
+    // row-local override, and the total has aggregated that row away.
+    let col_formats: Vec<NumberFormat> = (0..n)
+        .map(|ci| plan.at(layout.column_key(ci).as_ref()))
+        .collect();
 
     for ri in 0..layout.row_count() {
         let drawn = rows.contains(&ri);
@@ -57,10 +64,11 @@ pub fn grid_body(
             false => Vec::new(),
         };
         for ci in 0..n {
-            let value = cell_value(model, layout, records, ri, ci);
+            let (key, value) = cell_value(model, layout, records, ri, ci);
+            let fmt = plan.at(key.as_ref());
             let text = match records {
                 true => value.as_ref().map(|v| v.to_string()).unwrap_or_default(),
-                false => crate::format::format_value(value.as_ref(), fmt_comma, fmt_decimals),
+                false => fmt.value(value.as_ref()),
             };
             widen(&mut widths, ci, text.width());
             if let Some(sum) = totals.get_mut(ci) {
@@ -75,8 +83,8 @@ pub fn grid_body(
         }
     }
     for (ci, total) in totals.iter().enumerate() {
-        let width = format_f64(*total, fmt_comma, fmt_decimals).width();
-        widen(&mut widths, ci, width);
+        let fmt = col_formats.get(ci).copied().unwrap_or(plan.view_format());
+        widen(&mut widths, ci, fmt.number(*total).width());
     }
 
     GridBody {
@@ -84,6 +92,7 @@ pub fn grid_body(
             .into_iter()
             .map(|w| (w + 1).clamp(MIN_COL_WIDTH, MAX_COL_WIDTH))
             .collect(),
+        col_formats,
         totals,
         window,
     }
@@ -96,12 +105,16 @@ fn cell_value(
     records: bool,
     row: usize,
     col: usize,
-) -> Option<CellValue> {
+) -> (Option<CellKey>, Option<CellValue>) {
     match records {
-        true => layout.records_value(row, col),
-        false => layout
-            .cell_key(row, col)
-            .and_then(|key| model.evaluate_aggregated(&key, Aggregation::of(&layout.none_cats))),
+        true => (None, layout.records_value(row, col)),
+        false => {
+            let key = layout.cell_key(row, col);
+            let value = key
+                .as_ref()
+                .and_then(|k| model.evaluate_aggregated(k, Aggregation::of(&layout.none_cats)));
+            (key, value)
+        }
     }
 }
 
@@ -131,13 +144,8 @@ fn widen(widths: &mut [u16], col: usize, width: usize) {
 
 /// Adaptive column widths alone, for the keypress path, which needs no cells.
 /// It is [`grid_body`] with an empty window so there is one width rule.
-pub fn compute_col_widths(
-    model: &Model,
-    layout: &GridLayout,
-    fmt_comma: bool,
-    fmt_decimals: u8,
-) -> Vec<u16> {
-    grid_body(model, layout, fmt_comma, fmt_decimals, 0..0, 0..0).col_widths
+pub fn compute_col_widths(model: &Model, layout: &GridLayout, plan: &FormatPlan) -> Vec<u16> {
+    grid_body(model, layout, plan, 0..0, 0..0).col_widths
 }
 
 /// The total row-header width for a layout. The rule is the protocol's; this is the

crates/improvise-core/src/format.rs

@@ -1,4 +1,101 @@
-use crate::model::cell::CellValue;
+use crate::model::cell::{CellKey, CellValue};
+use crate::model::types::Model;
+use crate::view::types::View;
+
+/// Comma grouping and decimal places — the pair every render path used to
+/// thread as two scalars. `context/code/core.md`, "The number format".
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct NumberFormat {
+    pub comma: bool,
+    pub decimals: u8,
+}
+
+impl NumberFormat {
+    pub fn parse(fmt: &str) -> Self {
+        let (comma, decimals) = parse_number_format(fmt);
+        Self { comma, decimals }
+    }
+
+    pub fn value(&self, v: Option<&CellValue>) -> String {
+        format_value(v, self.comma, self.decimals)
+    }
+
+    pub fn number(&self, n: f64) -> String {
+        format_f64(n, self.comma, self.decimals)
+    }
+}
+
+/// One formula's claim on a cell: the coordinate it owns, and the format it
+/// asks for — `None` where it names none. `context/code/core.md`.
+#[derive(Clone, Debug)]
+struct TargetFormat {
+    category: String,
+    target: String,
+    format: Option<NumberFormat>,
+}
+
+/// Which format each cell is drawn with: the view's `format:`, unless the
+/// formula that OWNS the cell names its own (`context/code/core.md`).
+#[derive(Clone, Debug)]
+pub struct FormatPlan {
+    view: NumberFormat,
+    /// Every formula, in the model's own order, or **empty** where none names a
+    /// format — the ownership rule is `Model::formula_for`'s, not a hash's.
+    targets: Vec<TargetFormat>,
+}
+
+impl FormatPlan {
+    pub fn of(model: &Model, view: &View) -> Self {
+        let targets: Vec<TargetFormat> = model
+            .formulas()
+            .iter()
+            .map(|f| TargetFormat {
+                category: f.target_category.clone(),
+                target: f.target.clone(),
+                format: f.format.as_deref().map(NumberFormat::parse),
+            })
+            .collect();
+        Self {
+            view: NumberFormat::parse(&view.number_format),
+            // Where no formula names a format every answer is the view's, so
+            // the list is dropped rather than scanned once per cell.
+            targets: match targets.iter().any(|t| t.format.is_some()) {
+                true => targets,
+                false => Vec::new(),
+            },
+        }
+    }
+
+    /// One format for every cell — for the paths that have a format string and
+    /// no model to ask, and for tests that are not about the override.
+    pub fn uniform(fmt: &str) -> Self {
+        Self {
+            view: NumberFormat::parse(fmt),
+            targets: Vec::new(),
+        }
+    }
+
+    /// The view's own answer: what a cell no formula owns is drawn with.
+    pub fn view_format(&self) -> NumberFormat {
+        self.view
+    }
+
+    /// Asked of the *key*, never of the axes: a cell aggregating over `_Measure`
+    /// carries no target coordinate and so keeps the view's format.
+    pub fn at(&self, key: Option<&CellKey>) -> NumberFormat {
+        match key.filter(|_| !self.targets.is_empty()) {
+            None => self.view,
+            // The first match in the model's order — `formula_for`'s own rule,
+            // an owner naming no format included. `context/code/core.md`.
+            Some(key) => self
+                .targets
+                .iter()
+                .find(|t| key.get(&t.category) == Some(t.target.as_str()))
+                .and_then(|t| t.format)
+                .unwrap_or(self.view),
+        }
+    }
+}
 
 /// Format a CellValue for display with number formatting options.
 pub fn format_value(v: Option<&CellValue>, comma: bool, decimals: u8) -> String {
@@ -292,3 +389,131 @@ mod tests {
         assert_eq!(format_f64(f64::INFINITY, false, 2), "inf");
     }
 }
+
+#[cfg(test)]
+mod plan_tests {
+    use super::*;
+    use crate::formula::parse_formula;
+    use crate::model::cell::CellKey;
+    use crate::view::types::View;
+
+    fn ratio() -> CellValue {
+        CellValue::Number(2.0 / 3.0)
+    }
+
+    /// improvise-e0ex — see `context/test-evidence/core.md`.
+    #[test]
+    fn a_ratio_keeps_its_decimals_under_a_fresh_views_format() {
+        let plan = FormatPlan::uniform(&View::new("Default").number_format);
+        assert_eq!(plan.view_format().value(Some(&ratio())), "0.67");
+    }
+
+    fn model_with(raw: &str) -> Model {
+        let mut m = Model::new("m");
+        m.add_formula(parse_formula(raw, "_Measure").expect("parses"));
+        m
+    }
+
+    fn measure(item: &str) -> CellKey {
+        CellKey::new(vec![("_Measure".to_string(), item.to_string())])
+    }
+
+    /// improvise-e0ex — see `context/test-evidence/core.md`.
+    #[test]
+    fn a_formulas_own_format_wins_for_its_target_and_nowhere_else() {
+        let model = model_with("Margin = Profit / Revenue FORMAT .4");
+        let view = View::new("Default");
+        let plan = FormatPlan::of(&model, &view);
+
+        assert_eq!(
+            plan.at(Some(&measure("Margin"))).value(Some(&ratio())),
+            "0.6667"
+        );
+        // The raw measure beside it keeps the view's format, and so does a cell
+        // naming no measure at all — the two halves of "nowhere else".
+        assert_eq!(
+            plan.at(Some(&measure("Revenue"))).value(Some(&ratio())),
+            "0.67"
+        );
+        assert_eq!(plan.at(None).value(Some(&ratio())), "0.67");
+    }
+
+    fn two_owner_model(first: &str, second: &str) -> Model {
+        let mut m = Model::new("m");
+        m.add_category("Region").unwrap();
+        m.add_formula(parse_formula(first, "_Measure").expect("parses"));
+        m.add_formula(parse_formula(second, "Region").expect("parses"));
+        m
+    }
+
+    /// A key both formulas claim: `_Measure=Margin` and `Region=North`.
+    fn contested() -> CellKey {
+        CellKey::new(vec![
+            ("_Measure".to_string(), "Margin".to_string()),
+            ("Region".to_string(), "North".to_string()),
+        ])
+    }
+
+    /// improvise-e0ex — see `context/test-evidence/core.md`.
+    #[test]
+    fn the_formula_that_owns_a_cell_names_its_format_even_when_it_names_none() {
+        // Owner first, and it names NO format: the view answers, and the later
+        // formula's `.4` must not reach a cell whose value it does not produce.
+        let owner_unformatted = two_owner_model(
+            "Margin = Profit / Revenue",
+            "North = Profit / Revenue FORMAT .4",
+        );
+        let view = View::new("Default");
+        assert_eq!(
+            FormatPlan::of(&owner_unformatted, &view)
+                .at(Some(&contested()))
+                .value(Some(&ratio())),
+            "0.67",
+            "a later formula's format overrode the formula that owns the cell"
+        );
+
+        // The same two formulas the other way round: now the owner names `.4`.
+        let owner_formatted = two_owner_model(
+            "Margin = Profit / Revenue FORMAT .4",
+            "North = Profit / Revenue",
+        );
+        assert_eq!(
+            FormatPlan::of(&owner_formatted, &view)
+                .at(Some(&contested()))
+                .value(Some(&ratio())),
+            "0.6667",
+            "the owner's own format must win"
+        );
+    }
+
+    /// improvise-e0ex — see `context/test-evidence/core.md`.
+    #[test]
+    fn the_owner_is_the_one_model_order_names_and_not_the_one_a_hash_does() {
+        let model = two_owner_model(
+            "Margin = Profit / Revenue",
+            "North = Profit / Revenue FORMAT .4",
+        );
+        let view = View::new("Default");
+        // A `HashMap` gets a fresh `RandomState` per process, so one run cannot
+        // see hash order (gotcha 18); the plan is rebuilt to make that the point.
+        for _ in 0..5 {
+            assert_eq!(
+                FormatPlan::of(&model, &view)
+                    .at(Some(&contested()))
+                    .value(Some(&ratio())),
+                "0.67"
+            );
+        }
+    }
+
+    /// improvise-e0ex — see `context/test-evidence/core.md`.
+    #[test]
+    fn a_formula_naming_no_format_leaves_its_target_on_the_views() {
+        let model = model_with("Margin = Profit / Revenue");
+        let plan = FormatPlan::of(&model, &View::new("Default"));
+        assert_eq!(
+            plan.at(Some(&measure("Margin"))).value(Some(&ratio())),
+            "0.67"
+        );
+    }
+}

crates/improvise-core/src/view/layout.rs

@@ -547,15 +547,17 @@ impl GridLayout {
         model: &Model,
         row: usize,
         col: usize,
-        fmt_comma: bool,
-        fmt_decimals: u8,
+        plan: &crate::format::FormatPlan,
     ) -> String {
         if self.is_records_mode() {
             self.records_display(row, col).unwrap_or_default()
         } else {
             self.cell_key(row, col)
-                .and_then(|key| model.evaluate_aggregated(&key, Aggregation::of(&self.none_cats)))
-                .map(|v| crate::format::format_value(Some(&v), fmt_comma, fmt_decimals))
+                .and_then(|key| {
+                    let value =
+                        model.evaluate_aggregated(&key, Aggregation::of(&self.none_cats))?;
+                    Some(plan.at(Some(&key)).value(Some(&value)))
+                })
                 .unwrap_or_default()
         }
     }
@@ -584,6 +586,17 @@ impl GridLayout {
         Some(CellKey::new(coords))
     }
 
+    /// The coordinates a COLUMN answers for once its rows are aggregated away:
+    /// the page/filter constraints and the column's own tuple (core.md §6).
+    pub fn column_key(&self, col: usize) -> Option<CellKey> {
+        let col_item = data_items(&self.col_items).nth(col)?;
+        let mut coords = page_constraints(&self.page_coords);
+        for (cat, item) in self.col_cats.iter().zip(col_item.iter()) {
+            coords.push((cat.clone(), item.clone()));
+        }
+        Some(CellKey::new(coords))
+    }
+
     /// Visual index of the nth data row (skipping group headers).
     pub fn data_row_to_visual(&self, data_row: usize) -> Option<usize> {
         data_index_to_visual(&self.row_items, data_row)
@@ -809,7 +822,10 @@ mod tests {
             !layout.none_cats.is_empty(),
             "a default view must have None-axis categories for this to be the bug"
         );
-        assert_eq!(layout.display_text(&wb.model, 0, 0, false, 0), "pending");
+        assert_eq!(
+            layout.display_text(&wb.model, 0, 0, &crate::format::FormatPlan::uniform("")),
+            "pending"
+        );
     }
 
     #[test]

crates/improvise-core/src/view/types.rs

@@ -53,7 +53,9 @@ impl View {
             page_selections: HashMap::new(),
             hidden_items: HashMap::new(),
             collapsed_groups: HashMap::new(),
-            number_format: ",.0".to_string(),
+            // Decimals by default: a ratio is the case a format cannot be
+            // guessed for, and `.0` draws 2/3 as `1` (improvise-e0ex).
+            number_format: ",.2".to_string(),
             prune_empty: false,
         }
     }

crates/improvise-formula/src/ast.rs

@@ -57,6 +57,10 @@ pub struct Formula {
     pub expr: Expr,
     /// Optional WHERE filter
     pub filter: Option<Filter>,
+    /// How this formula's target is drawn, overriding the view's `format:` for
+    /// that one measure. `None` means the view answers — `context/code/core.md`.
+    #[serde(default)]
+    pub format: Option<String>,
 }
 
 impl Formula {
@@ -73,6 +77,14 @@ impl Formula {
             target_category: target_category.into(),
             expr,
             filter,
+            format: None,
         }
     }
+
+    /// The same formula drawn with `format`. Separate from [`Formula::new`] so
+    /// the five-argument constructor every caller already spells stays put.
+    pub fn with_format(mut self, format: Option<String>) -> Self {
+        self.format = format;
+        self
+    }
 }

crates/improvise-formula/src/formula.pest

@@ -13,7 +13,7 @@ WHITESPACE = _{ " " | "\t" }
 
 // ---- top-level ----------------------------------------------------------
 
-formula = { SOI ~ target ~ "=" ~ expr ~ where_clause? ~ EOI }
+formula = { SOI ~ target ~ "=" ~ expr ~ where_clause? ~ format_clause? ~ EOI }
 
 // The target is an ordinary identifier: parser.rs unquotes it exactly the
 // way it unquotes a `ref_expr`, so `|Total Revenue| = ...` and a reference
@@ -24,6 +24,11 @@ target = { identifier }
 // `WHEREAS x = y` reads as a WHERE on a category `AS`.
 where_clause = ${ ^"WHERE" ~ sp+ ~ identifier ~ sp* ~ "=" ~ sp* ~ filter_value }
 
+// How this formula's TARGET is drawn, overriding the view's `format:` for that
+// one measure. `sp+` for the same reason `WHERE` has it — `context/code/core.md`.
+format_clause = ${ ^"FORMAT" ~ sp+ ~ format_spec }
+format_spec   = @{ (ASCII_ALPHANUMERIC | "," | "." | "%")+ }
+
 // ---- expressions --------------------------------------------------------
 
 // Used by parse_expr() — forces a standalone expression to consume the

crates/improvise-formula/src/parser.rs

@@ -42,6 +42,7 @@ fn build_formula(pair: Pair<Rule>, raw: &str, target_category: &str) -> Formula
     let mut target = None;
     let mut expr = None;
     let mut filter = None;
+    let mut format = None;
     for inner in pair.into_inner() {
         match inner.as_rule() {
             // Unquoted exactly like `Expr::Ref`: the same spelling must name
@@ -49,6 +50,10 @@ fn build_formula(pair: Pair<Rule>, raw: &str, target_category: &str) -> Formula
             Rule::target => target = Some(identifier_to_string(first_inner(inner))),
             Rule::expr => expr = Some(build_expr(inner)),
             Rule::where_clause => filter = Some(build_filter(inner)),
+            // The spec, not the clause: `FORMAT ` is syntax and not the picture.
+            Rule::format_clause => {
+                format = Some(first_inner(inner).as_str().to_string());
+            }
             // Ignored as an ARM, so "ignored" is a decision and not a silence.
             Rule::EOI => {}
             Rule::WHITESPACE
@@ -77,6 +82,7 @@ fn build_formula(pair: Pair<Rule>, raw: &str, target_category: &str) -> Formula
             | Rule::pipe_quoted
             | Rule::bare_ident
             | Rule::filter_value
+            | Rule::format_spec
             | Rule::string
             | Rule::number => {
                 unreachable!("{GRAMMAR_INVARIANT}: formula part {:?}", inner.as_rule())
@@ -90,6 +96,7 @@ fn build_formula(pair: Pair<Rule>, raw: &str, target_category: &str) -> Formula
         expr.expect(GRAMMAR_INVARIANT),
         filter,
     )
+    .with_format(format)
 }
 
 fn build_expr(pair: Pair<Rule>) -> Expr {
@@ -166,6 +173,8 @@ fn build_unary(pair: Pair<Rule>) -> Expr {
         | Rule::pipe_quoted
         | Rule::bare_ident
         | Rule::filter_value
+        | Rule::format_clause
+        | Rule::format_spec
         | Rule::string
         | Rule::number => {
             unreachable!(
@@ -214,6 +223,8 @@ fn build_primary(pair: Pair<Rule>) -> Expr {
         | Rule::pipe_quoted
         | Rule::bare_ident
         | Rule::filter_value
+        | Rule::format_clause
+        | Rule::format_spec
         | Rule::string => {
             unreachable!(
                 "{GRAMMAR_INVARIANT}: primary alternative {:?}",
@@ -324,6 +335,8 @@ fn filter_value_to_string(pair: Pair<Rule>) -> String {
         | Rule::ref_expr
         | Rule::identifier
         | Rule::filter_value
+        | Rule::format_clause
+        | Rule::format_spec
         | Rule::number => {
             unreachable!(
                 "{GRAMMAR_INVARIANT}: filter_value alternative {:?}",

crates/improvise-formula/src/quote.rs

@@ -3,7 +3,9 @@
 
 /// Words the formula grammar gives a meaning of their own. A name spelled like
 /// one is pipe-quoted even though it is otherwise a legal bare identifier.
-const KEYWORDS: [&str; 8] = ["WHERE", "SUM", "AVG", "MIN", "MAX", "COUNT", "IF", "IS"];
+const KEYWORDS: [&str; 9] = [
+    "WHERE", "SUM", "AVG", "MIN", "MAX", "COUNT", "IF", "IS", "FORMAT",
+];
 
 /// Bare-writable: matches `[A-Za-z_][A-Za-z0-9_-]*` and is not a [`KEYWORDS`]
 /// entry. The keyword clause is canonical — `context/code/formula-and-io.md`.

crates/improvise-gp/src/dataset.rs

@@ -453,11 +453,10 @@ mod tests {
     /// `f64`, so a value the grid shows as "5" must reach the table as 5.25.
     #[test]
     fn a_cell_reaches_the_table_unrounded() {
-        let (mut model, view) = fixture();
-        assert_eq!(
-            view.number_format, ",.0",
-            "the fixture view rounds to integers"
-        );
+        let (mut model, mut view) = fixture();
+        // Pinned, not inherited: the default shows two decimals since
+        // improvise-e0ex, and 5.25 under it is not rounded at all.
+        view.number_format = ",.0".to_string();
         model.set_cell(
             record_key("North", "Web", "Q1", "n1", "Units"),
             CellValue::Number(5.25),

crates/improvise-io/src/persistence/export.rs

@@ -34,7 +34,7 @@ fn render_csv(workbook: &Workbook, view_name: &str) -> Result<Vec<u8>> {
     // writes rows the grid does not draw (improvise-iot6).
     let layout = GridLayout::with_frozen_records(&workbook.model, view, None);
     let model = &workbook.model;
-    let (fmt_comma, fmt_decimals) = crate::format::parse_number_format(&view.number_format);
+    let plan = crate::format::FormatPlan::of(model, view);
 
     let mut wtr = csv::WriterBuilder::new()
         .flexible(true)
@@ -66,10 +66,7 @@ fn render_csv(workbook: &Workbook, view_name: &str) -> Result<Vec<u8>> {
         if !row_label.is_empty() {
             record.push(row_label);
         }
-        record.extend(
-            (0..layout.col_count())
-                .map(|ci| layout.display_text(model, ri, ci, fmt_comma, fmt_decimals)),
-        );
+        record.extend((0..layout.col_count()).map(|ci| layout.display_text(model, ri, ci, &plan)));
         wtr.write_record(&record)?;
     }
 

crates/improvise-io/src/persistence/roundtrip_tests.rs

@@ -1121,3 +1121,40 @@ fn a_cell_with_no_coordinates_round_trips() {
     );
     assert_eq!(format_md(&back), text);
 }
+
+// ── parse_md: a formula's own number format (improvise-e0ex) ─────────
+
+/// The PARSED format, never the re-emitted line — `raw` cannot be the
+/// witness (gotcha 34). Evidence: `context/test-evidence/formula-and-io.md`.
+#[test]
+fn parse_md_round_trips_the_format_a_formula_carries() {
+    let mut m = two_cat_model();
+    m.model
+        .add_formula(parse_formula("Margin = Revenue / Cost FORMAT .4", "_Measure").unwrap());
+
+    let text = format_md(&m);
+    let loaded = parse_md(&text).unwrap();
+
+    let f = &loaded.model.formulas()[0];
+    assert_eq!(
+        f.format.as_deref(),
+        Some(".4"),
+        "the format must survive as a parsed field, not only inside `raw`"
+    );
+    // The two halves this test exists to keep apart: `raw` is byte-identical
+    // whether or not the walker kept the field, so it cannot be the witness.
+    assert_eq!(f.raw, "Margin = Revenue / Cost FORMAT .4");
+    assert_eq!(f.target, "Margin");
+}
+
+/// A formula naming no format reads back as `None`, so the field above is a
+/// decision the file carried. Evidence: `context/test-evidence/formula-and-io.md`.
+#[test]
+fn parse_md_reads_a_formula_with_no_format_as_none() {
+    let mut m = two_cat_model();
+    m.model
+        .add_formula(parse_formula("Margin = Revenue / Cost", "_Measure").unwrap());
+
+    let loaded = parse_md(&format_md(&m)).unwrap();
+    assert_eq!(loaded.model.formulas()[0].format, None);
+}

crates/improvise-wasm-client/tests/browser_round_trip.rs

@@ -120,7 +120,7 @@ fn a_browser_client_seeds_types_a_number_and_sees_it_come_back() {
     assert_eq!(model.col_levels[0], vec!["Revenue", "Cost"]);
     assert_eq!(
         model.rows[0].cells()[0].text,
-        "1,200",
+        "1,200.00",
         "the value arrives *formatted* — the separator is the server's view \
          format, and a client that re-derived the text would not have it"
     );
@@ -168,7 +168,7 @@ fn a_browser_client_seeds_types_a_number_and_sees_it_come_back() {
     );
     assert_eq!(
         client.model().rows[at.visual_row].cells()[at.col].text,
-        "7,500",
+        "7,500.00",
         "the page redraws from the server's formatting, never from the \
          \"7500\" the user typed"
     );

crates/improvise-worker-server/src/worker.rs

@@ -358,7 +358,7 @@ mod tests {
     fn a_wire_edit_reaches_the_document_and_comes_back_as_a_projection() {
         let (mut worker, _) = worker_over_storage();
         let mut grid = connect(&mut worker);
-        assert_eq!(grid.cells[0][0].text, "10");
+        assert_eq!(grid.cells[0][0].text, "10.00");
 
         let cell = addressed(&grid, 0, 0);
         let cmd = reduce::caught_up(
@@ -371,7 +371,7 @@ mod tests {
         let out = worker.on_frame(ClientFrame::command(cmd));
         let grid = folded(&mut grid, &out);
         assert_eq!(
-            grid.cells[0][0].text, "42",
+            grid.cells[0][0].text, "42.00",
             "the edit did not come back through the projection stream"
         );
     }

crates/improvise-worker-server/tests/post_message.rs

@@ -163,7 +163,7 @@ fn the_worker_loads_projects_edits_and_persists_over_post_message() {
         main.grid.row_frame.leaf_labels(),
         vec!["North", "South", "East"]
     );
-    assert_eq!(main.text_at(0, 0), "10");
+    assert_eq!(main.text_at(0, 0), "10.00");
 
     let cell = main.cell(2, 0);
     main.post(&main.caught_up(ModelCommand::SetCell {
@@ -172,7 +172,7 @@ fn the_worker_loads_projects_edits_and_persists_over_post_message() {
     }));
     assert_eq!(
         main.text_at(2, 0),
-        "31",
+        "31.00",
         "the edit must come back through the projection stream, not be assumed"
     );
 
@@ -227,7 +227,7 @@ fn a_reload_over_the_same_origin_reopens_what_was_saved() {
     reloaded.post(&ClientFrame::Viewport { rows: 10, cols: 4 });
     assert_eq!(
         reloaded.text_at(0, 0),
-        "77",
+        "77.00",
         "the reload re-seeded over the user's saved work"
     );
 }
@@ -258,7 +258,7 @@ fn a_batch_composed_against_a_moved_document_is_refused() {
     );
     assert_eq!(
         main.text_at(0, 0),
-        "2",
+        "2.00",
         "a refused batch changed the document"
     );
 }
@@ -336,7 +336,7 @@ fn the_shipped_demo_seeds_a_worker_whose_raw_cells_are_settable() {
         main.grid.col_frame.leaf_labels(),
         vec!["Revenue", "Cost", "Profit"]
     );
-    assert_eq!(main.text_at(0, 2), "12,200", "Profit is live on arrival");
+    assert_eq!(main.text_at(0, 2), "12,200.00", "Profit is live on arrival");
 
     // `cell` panics on anything but a stored cell, which is exactly the failure a
     // visitor meets as a cell that does not answer a click (ADR 002).
@@ -362,8 +362,8 @@ fn the_shipped_demo_seeds_a_worker_whose_raw_cells_are_settable() {
         cell: revenue,
         value: CellValue::Number(20000.0),
     }));
-    assert_eq!(main.text_at(0, 0), "20,000");
-    assert_eq!(main.text_at(0, 2), "13,800", "20,000 - 6,200");
+    assert_eq!(main.text_at(0, 0), "20,000.00");
+    assert_eq!(main.text_at(0, 2), "13,800.00", "20,000 - 6,200");
 }
 
 /// improvise-z754: `on_frame` built the client's frames and persisted AFTER, so

crates/improvise-ws-server/tests/socket.rs

@@ -133,7 +133,7 @@ fn a_client_on_a_socket_is_seeded_and_gets_its_edit_back() {
             waited += 1;
         }
         assert_eq!(
-            grid.cells[0][0].text, "12",
+            grid.cells[0][0].text, "12.00",
             "the edit never came back down the socket"
         );
 

harness/tests/contract.spec.js

@@ -159,7 +159,7 @@ test.describe("every field the render model projects reaches the screen", () =>
     // The totals strip. Q1 Revenue is 18400+24600+9200+11500 — a sum over
     // EVERY row, which is why it is projected and not added up here.
     await expect(page.locator("#grid tfoot th")).toHaveText("Total");
-    await expect(page.locator("#grid tfoot td").first()).toHaveText("63,700");
+    await expect(page.locator("#grid tfoot td").first()).toHaveText("63,700.00");
     // A total is not a cell anyone may type into.
     await expect(page.locator("#grid tfoot td.editable")).toHaveCount(0);
 
@@ -170,7 +170,7 @@ test.describe("every field the render model projects reaches the screen", () =>
     await expect(page.locator("#pages .page-axis")).toHaveText(/Quarter = Q2 \(2\/4\)/, {
       timeout: 10_000,
     });
-    await expect(page.locator("#grid tfoot td").first()).toHaveText("66,400");
+    await expect(page.locator("#grid tfoot td").first()).toHaveText("66,400.00");
   });
 });
 
@@ -280,7 +280,7 @@ test.describe("a pointer event reaches the client and its answer is painted", ()
     // The third edit of the same repair: `close` restores the DRAWN text, not
     // the seed. `repaint` early-returns while `editing` is set, so a `18400`
     // put back here stays on screen (improvise-0x9u).
-    await expect(cells.nth(0)).toHaveText("18,400");
+    await expect(cells.nth(0)).toHaveText("18,400.00");
 
     await page.keyboard.press("Escape");
     await expect(cells.nth(4)).toHaveClass(/selected/);

harness/tests/pages.spec.js

@@ -58,7 +58,7 @@ test.describe("a page axis can be picked from a menu", () => {
     // Espresso Revenue is 18,400 in Q1 and 20,300 in Q4, so a menu that moved
     // only the label would leave this unchanged.
     await expect(firstCell).not.toHaveText(q1 ?? "");
-    await expect(firstCell).toHaveText("20,300");
+    await expect(firstCell).toHaveText("20,300.00");
   });
 
   // The menu is operable without a mouse, and while it is open it owns the

harness/tests/shell.spec.js

@@ -273,8 +273,8 @@ test.describe("the standalone deployment", () => {
     await input.fill("20000");
     await input.press("Enter");
 
-    await expect(cell).toHaveText("20,000", { timeout: 10_000 });
-    expect(before).not.toBe("20,000");
+    await expect(cell).toHaveText("20,000.00", { timeout: 10_000 });
+    expect(before).not.toBe("20,000.00");
     // The editor is gone and did not come back: the half of the re-entry guard
     // that IS observable from the DOM.
     await expect(page.locator("#grid input")).toHaveCount(0);

src/ui/grid.rs

@@ -861,6 +861,9 @@ mod tests {
         m.add_item_in_group("Month", "Jan", "Q1").unwrap();
         m.add_item_in_group("Month", "Apr", "Q2").unwrap();
         m.add_item("Payer", "Alice").unwrap();
+        // Pinned so the byte-exact frames below characterize the LAYOUT and not
+        // the view default, which grew two decimals with improvise-e0ex.
+        m.active_view_mut().number_format = ",.0".to_string();
         for (r, v) in [("East", 100.0), ("West", 50.0)] {
             for mo in ["Jan", "Apr"] {
                 m.model.set_cell(

tests/bug_hunt_reproductions.rs

@@ -384,8 +384,7 @@ fn wide_cell_clamps_without_overflow() {
         let widths = improvise::ui::viewport::compute_col_widths(
             &app.model_state.workbook.model,
             &app.layout,
-            false,
-            0,
+            &improvise::format::FormatPlan::uniform(""),
         );
         assert_eq!(
             widths,

tests/web_demo_model.rs

@@ -76,5 +76,8 @@ fn the_web_demo_shows_a_formula_beside_its_inputs() {
         vec!["Espresso", "Latte", "Cold Brew", "Pastry"]
     );
     let profit = grid.cell(0, 2).expect("the Profit cell");
-    assert_eq!(profit.text, "12,200", "18,400 - 6,200, recomputed on load");
+    assert_eq!(
+        profit.text, "12,200.00",
+        "18,400 - 6,200, recomputed on load, under the decimal default"
+    );
 }

web/demo.improv

@@ -8,7 +8,7 @@ _Measure: column
 Quarter: page, Q1
 _Index: none
 _Dim: none
-format: ,.0
+format: ,.2
 
 ## View: Season
 Quarter: row
@@ -16,7 +16,7 @@ _Measure: column
 Product: page, Latte
 _Index: none
 _Dim: none
-format: ,.0
+format: ,.2
 
 ## Formulas
 - Profit = Revenue - Cost