diff --git a/crates/aura-campaign/src/exec.rs b/crates/aura-campaign/src/exec.rs
index 2e46410..6cf3c75 100644
--- a/crates/aura-campaign/src/exec.rs
+++ b/crates/aura-campaign/src/exec.rs
@@ -13,7 +13,7 @@ use aura_analysis::{FamilySelection, SelectionMode};
 use aura_core::{zip_params, Cell, ParamSpec, Scalar, Timestamp};
 use aura_engine::{
     sweep, walk_forward, GridSpace, ListSpace, RollMode, RunManifest, RunMetrics, RunReport,
-    Space, SweepFamily, SweepPoint, WindowBounds, WindowRoller, WindowRun,
+    Space, SweepFamily, SweepPoint, WalkForwardError, WindowBounds, WindowRoller, WindowRun,
 };
 use aura_registry::{
     optimize, optimize_deflated, optimize_plateau, sweep_member_reports,
@@ -508,7 +508,24 @@ fn run_walk_forward_stage(
         *step_ms as i64,
         roll_mode,
     )
-    .map_err(|e| ExecFault::Window { stage, detail: format!("{e:?}") })?;
+    .map_err(|e| ExecFault::Window {
+        stage,
+        // The detail string reaches user-facing seams verbatim — phrase both
+        // roller refusals in the doc's ms unit, never a Debug form.
+        detail: match e {
+            WalkForwardError::SpanTooShort { .. } => format!(
+                "in_sample_ms + out_of_sample_ms = {} ms exceeds the campaign window [{}, {}] ({} ms)",
+                in_sample_ms + out_of_sample_ms,
+                cell.window_ms.0,
+                cell.window_ms.1,
+                cell.window_ms.1 - cell.window_ms.0 + 1,
+            ),
+            // Zero lengths are doc-tier faults (preflight); phrased anyway.
+            WalkForwardError::NonPositiveLength { field, value } => {
+                format!("walk_forward {field} must be > 0 (got {value})")
+            }
+        },
+    })?;
     // A second identical roller enumerates the bounds up front: the engine's
     // walk_forward consumes its roller, and the parallel closure needs each
     // window's roll index for deterministic fault attribution.
diff --git a/crates/aura-campaign/src/lib.rs b/crates/aura-campaign/src/lib.rs
index 050ae9d..d31c289 100644
--- a/crates/aura-campaign/src/lib.rs
+++ b/crates/aura-campaign/src/lib.rs
@@ -861,8 +861,13 @@ mod wf_tests {
             ExecFault::Window { stage, detail } => {
                 assert_eq!(stage, 1, "walk_forward is pipeline stage 1 here");
                 assert!(
-                    detail.contains("SpanTooShort"),
-                    "detail carries the roller refusal: {detail}"
+                    detail.contains("in_sample_ms + out_of_sample_ms = 60 ms")
+                        && detail.contains("(50 ms)"),
+                    "detail phrases the refusal in doc-unit ms prose: {detail}"
+                );
+                assert!(
+                    !detail.contains("SpanTooShort") && !detail.contains("Timestamp("),
+                    "no Debug form leaks through the detail seam: {detail}"
                 );
             }
             other => panic!("expected ExecFault::Window, got {other:?}"),