diff --git a/docs/public/openapi.json b/docs/public/openapi.json
index 7489bc142..c1cfec904 100644
--- a/docs/public/openapi.json
+++ b/docs/public/openapi.json
@@ -6215,7 +6215,7 @@
       "get": {
         "operationId": "campaigns_send_plan",
         "summary": "Today's sending plan",
-        "description": "What the campaign sends today and every limit that decided it, worked out through the scheduler's own gates. Nothing is stored or written. Scope READ_CAMPAIGNS, org permission view_campaigns.",
+        "description": "What the campaign sends today and every limit that decided it, worked out through the scheduler's own gates. An active campaign is served from a snapshot computed in the background (`computed_at` says when, `stale` says a refresh is pending); a campaign with no snapshot yet is computed on the request. Scope READ_CAMPAIGNS, org permission view_campaigns.",
         "tags": [
           "campaigns"
         ],
@@ -30540,13 +30540,14 @@
       },
       "CampaignSendPlan": {
         "type": "object",
-        "description": "Today's sending plan for a campaign, derived through the scheduler's gates on every read.",
+        "description": "Today's sending plan for a campaign, derived through the scheduler's gates and served from a background snapshot for an active campaign.",
         "required": [
           "campaign_id",
           "status",
           "day",
           "timezone",
           "computed_at",
+          "stale",
           "configured_ceiling",
           "projected_today",
           "sent_today",
@@ -30574,7 +30575,12 @@
           },
           "computed_at": {
             "type": "string",
-            "format": "date-time"
+            "format": "date-time",
+            "description": "When this plan was computed. For a snapshot, the moment it was last walked."
+          },
+          "stale": {
+            "type": "boolean",
+            "description": "True when the snapshot has been outrun by an edit, a start or stop, or a new budget day, or has grown older than the snapshot's maximum age even though nothing changed, and a refreshed plan is already being computed. Always false for a plan computed on the request."
           },
           "configured_ceiling": {
             "type": "integer",
diff --git a/internal/repository/pg_campaign.go b/internal/repository/pg_campaign.go
index b1a9e914d..e73c90b1c 100644
--- a/internal/repository/pg_campaign.go
+++ b/internal/repository/pg_campaign.go
@@ -1338,7 +1338,7 @@ func (r *campaignRepository) Update(ctx context.Context, orgID, campaignID strin
 	}()
 
 	var query string
-	if argPos > 3 {
+	if argPos >= 3 {
 		query = fmt.Sprintf(`
 			UPDATE campaigns
 			SET %s
@@ -2183,6 +2183,12 @@ func (r *campaignRepository) ReplaceCampaignSenders(ctx context.Context, campaig
 		return nil, xerr
 	}
 
+	// The send-plan snapshot is keyed on updated_at, so a sender edit must move it.
+	if _, err := tx.Exec(ctx, `UPDATE campaigns SET updated_at = now() WHERE id = $1`, campaignID); err != nil {
+		db.CaptureError(err, "campaign updated_at", []any{campaignID}, "exec")
+		return nil, errx.InternalError()
+	}
+
 	if err := tx.Commit(ctx); err != nil {
 		db.CaptureError(err, "", nil, "commit")
 		return nil, errx.InternalError()
diff --git a/internal/repository/pg_segment.go b/internal/repository/pg_segment.go
index 8b5a0f52c..51ed90639 100644
--- a/internal/repository/pg_segment.go
+++ b/internal/repository/pg_segment.go
@@ -655,10 +655,18 @@ func setForCampaignTx(ctx context.Context, tx pgx.Tx, orgID, campaignID uuid.UUI
 		return "", change, xerr
 	}
 	if len(segmentIDs) > 0 {
-		if _, err := tx.Exec(ctx, `INSERT INTO campaign_segments (campaign_id, segment_id) SELECT $1, unnest($2::uuid[]) ON CONFLICT DO NOTHING`, campaignID, segmentIDs); err != nil {
+		tag, err := tx.Exec(ctx, `INSERT INTO campaign_segments (campaign_id, segment_id) SELECT $1, unnest($2::uuid[]) ON CONFLICT DO NOTHING`, campaignID, segmentIDs)
+		if err != nil {
 			db.CaptureError(err, "campaign segments insert", nil, "exec")
 			return "", change, errx.InternalError()
 		}
+		// New links change the send plan the snapshot is keyed on.
+		if tag.RowsAffected() > 0 {
+			if _, err := tx.Exec(ctx, `UPDATE campaigns SET updated_at = NOW() WHERE id = $1`, campaignID); err != nil {
+				db.CaptureError(err, "campaign updated_at", nil, "exec")
+				return "", change, errx.InternalError()
+			}
+		}
 		// A live audience is the reason to keep running: linking turns the
 		// setting on, and the owner can turn it off again in preferences.
 		if _, err := tx.Exec(ctx, `UPDATE campaigns SET continuous = true, updated_at = NOW() WHERE id = $1 AND NOT continuous`, campaignID); err != nil {
@@ -666,6 +674,13 @@ func setForCampaignTx(ctx context.Context, tx pgx.Tx, orgID, campaignID uuid.UUI
 			return "", change, errx.InternalError()
 		}
 	}
+	// Detaching withdraws leads, which changes the send plan the snapshot is keyed on.
+	if len(detached) > 0 {
+		if _, err := tx.Exec(ctx, `UPDATE campaigns SET updated_at = NOW() WHERE id = $1`, campaignID); err != nil {
+			db.CaptureError(err, "campaign updated_at", nil, "exec")
+			return "", change, errx.InternalError()
+		}
+	}
 	return status, change, nil
 }