diff --git a/crates/aura-cli/src/main.rs b/crates/aura-cli/src/main.rs
index 6638f9f..ff5903c 100644
--- a/crates/aura-cli/src/main.rs
+++ b/crates/aura-cli/src/main.rs
@@ -2365,11 +2365,15 @@ enum Command {
     Graph(GraphCmd),
     /// Sweep a parameter grid over a strategy or a loaded blueprint.
     Sweep(SweepCmd),
-    /// Walk-forward validation over a strategy or a loaded blueprint.
+    /// Walk-forward validation over a strategy or a loaded blueprint. Over --real, the
+    /// fixed 90/30-day roller fits to a --from/--to window shorter than it, preserving
+    /// the 3:1 IS:OOS ratio, instead of refusing the window outright.
     Walkforward(WalkforwardCmd),
     /// Grade one candidate across multiple instruments.
     Generalize(GeneralizeCmd),
-    /// Monte-Carlo over synthetic draws, an R-bootstrap, or a loaded blueprint.
+    /// Monte-Carlo over synthetic draws, an R-bootstrap, or a loaded blueprint. Over
+    /// --real, the fixed 90/30-day walk-forward roller fits to a --from/--to window
+    /// shorter than it, preserving the 3:1 IS:OOS ratio, instead of refusing outright.
     Mc(McCmd),
     /// List or inspect recorded run families.
     Runs(RunsCmd),
@@ -2686,6 +2690,32 @@ fn wf_ms_sizes() -> (u64, u64, u64) {
     )
 }
 
+/// Fit the fixed real-archive walk-forward roller (`wf_ms_sizes`, 90/30/30 days)
+/// to a resolved campaign window `(from_ms, to_ms)` (#239). When the fixed
+/// IS+OOS span fits inside the window, the sizes come back byte-identical (every
+/// existing anchor/e2e over a year-plus window pins this branch). When the window
+/// is shorter than IS+OOS, the roller scales DOWN to the window, preserving the
+/// fixed 3:1 IS:OOS ratio, with `step_ms == oos_ms` (a window of exactly IS+OOS
+/// then yields exactly one roll — never zero). Pure and unit-testable; both
+/// `dispatch_walkforward`/`dispatch_mc` consume it in place of the unconditional
+/// `wf_ms_sizes()` stamp. The executor's own fit check (`campaign_run.rs`) stays
+/// untouched — it still validates an AUTHORED document's declared window as-is.
+fn fit_wf_ms_sizes(from_ms: i64, to_ms: i64) -> (u64, u64, u64) {
+    let (is_ms, oos_ms, step_ms) = wf_ms_sizes();
+    let span_ms = to_ms.saturating_sub(from_ms).max(0) as u64;
+    if is_ms + oos_ms <= span_ms {
+        return (is_ms, oos_ms, step_ms);
+    }
+    // Preserve the fixed IS:OOS ratio derived from the constants themselves (not a
+    // bare literal): OOS = span/(ratio+1) (floor), IS = ratio*OOS, so
+    // IS+OOS = (ratio+1)*OOS <= span always holds, and step == OOS (one roll over
+    // the fit window, matching the fixed-size branch's `step_ms == oos_ms`).
+    let ratio = is_ms / oos_ms;
+    let oos_fit = span_ms / (ratio + 1);
+    let is_fit = oos_fit * ratio;
+    (is_fit, oos_fit, oos_fit)
+}
+
 /// Resolve a single optional stop knob (`--stop-length`/`--stop-k`): an absent flag
 /// defaults to `default`; a present one parses via [`parse_csv_list`] and refuses
 /// unless it holds exactly one value (the stop is a risk regime, not a swept axis).
@@ -3421,7 +3451,7 @@ fn dispatch_walkforward(a: WalkforwardCmd, env: &project::Env) {
                     DataChoice::Real { symbol: symbol.clone(), from_ms: from, to_ms: to },
                     env,
                 );
-                let (is_ms, oos_ms, step_ms) = wf_ms_sizes();
+                let (is_ms, oos_ms, step_ms) = fit_wf_ms_sizes(from_ms, to_ms);
                 let inv = verb_sugar::SugarInvocation {
                     axes: &raw_axes,
                     name,
@@ -3536,7 +3566,7 @@ fn dispatch_mc(a: McCmd, env: &project::Env) {
                     DataChoice::Real { symbol: symbol.clone(), from_ms: from, to_ms: to },
                     env,
                 );
-                let (is_ms, oos_ms, step_ms) = wf_ms_sizes();
+                let (is_ms, oos_ms, step_ms) = fit_wf_ms_sizes(from_ms, to_ms);
                 let inv = verb_sugar::SugarInvocation {
                     axes: &raw_axes,
                     name,
@@ -4709,6 +4739,33 @@ mod tests {
         assert_eq!(WF_REAL_STEP_NS, 30 * day_ns);
     }
 
+    /// Property: a window that already fits the fixed 90/30/30-day roller passes
+    /// it through byte-identical (the year-plus anchor/e2e grade pins rely on this
+    /// branch never perturbing the fixed sizes).
+    #[test]
+    fn fit_wf_ms_sizes_passes_through_when_the_window_already_fits() {
+        let day_ms: i64 = 24 * 60 * 60 * 1_000;
+        let from_ms = 0;
+        let to_ms = 121 * day_ms; // > 90 + 30 days
+        assert_eq!(fit_wf_ms_sizes(from_ms, to_ms), wf_ms_sizes());
+    }
+
+    /// Property: a window shorter than IS+OOS scales the roller DOWN to the
+    /// window, preserving the fixed 3:1 IS:OOS ratio, with `step_ms == oos_ms`
+    /// (one roll over the fit window) and `is_ms + oos_ms <= span_ms` always.
+    #[test]
+    fn fit_wf_ms_sizes_scales_down_to_a_short_window_at_3_to_1() {
+        let day_ms: i64 = 24 * 60 * 60 * 1_000;
+        let from_ms = 0;
+        let to_ms = 30 * day_ms; // far shorter than the fixed 90+30-day roller
+        let span_ms = (to_ms - from_ms) as u64;
+        let (is_ms, oos_ms, step_ms) = fit_wf_ms_sizes(from_ms, to_ms);
+        assert_eq!(is_ms, oos_ms * 3, "the fit preserves the fixed 3:1 IS:OOS ratio");
+        assert_eq!(step_ms, oos_ms, "one roll over the fit window: step == oos");
+        assert!(is_ms + oos_ms <= span_ms, "the fit roller must fit inside the window");
+        assert!(oos_ms > 0, "a 30-day window must yield a non-degenerate fit");
+    }
+
     #[test]
     fn sim_optimal_manifest_renders_per_instrument_pip() {
         let m = sim_optimal_manifest(vec![], (Timestamp(1), Timestamp(2)), 0, 1.0);