dedup: length guard against corrupted output + --chapter retry

One chapter in a dedup run had the model emit the chapter twice
(18.7k -> 32.6k chars). A surgical dedup only nudges length, so
the pass now rejects any output wildly off the input length —
marks that chapter's run failed, leaves the prose untouched, and
moves on rather than silently corrupting it. New --chapter flag
narrows a run to one chapter so a skipped one can be retried.

skald/src/dedup.rs

@@ -22,6 +22,7 @@ pub async fn run(
     database_url: &str,
     story_id: Uuid,
     author_slug: Option<&str>,
+    chapter_filter: Option<i32>,
 ) -> anyhow::Result<()> {
     let cfg = load_forge_config()?;
     tracing::info!(base_url = %cfg.base_url, model = %cfg.model, "forge configured");
@@ -62,7 +63,7 @@ pub async fn run(
         "dedup starting",
     );
 
-    let chapters: Vec<(Uuid, i32)> = sqlx::query_as(
+    let mut chapters: Vec<(Uuid, i32)> = sqlx::query_as(
         "SELECT id, n FROM chapters WHERE story_id = $1 ORDER BY n",
     )
     .bind(story_id)
@@ -71,7 +72,16 @@ pub async fn run(
     if chapters.is_empty() {
         bail!("story {story_id} has no chapters");
     }
+    // --chapter narrows the run to one chapter — used to retry a
+    // chapter the length guard skipped on an earlier run.
+    if let Some(target) = chapter_filter {
+        chapters.retain(|(_, n)| *n == target);
+        if chapters.is_empty() {
+            bail!("chapter {target} not found in story {story_id}");
+        }
+    }
 
+    let mut skipped = 0usize;
     for (chapter_id, n) in &chapters {
         let body_md: String =
             sqlx::query_scalar("SELECT body_md FROM chapters WHERE id = $1")
@@ -107,6 +117,34 @@ pub async fn run(
         };
 
         let deduped = pass_text(&out)?;
+
+        // Sanity guard: a surgical dedup nudges a chapter's length by
+        // a little. An output wildly off the input means the model
+        // duplicated or ballooned the chapter — reject it, leave the
+        // chapter untouched, move on. A re-run with --chapter retries
+        // just the skipped one.
+        let before = body_md.len();
+        let after = deduped.len();
+        if after > before * 3 / 2 || after < before * 3 / 5 {
+            sqlx::query(
+                "UPDATE generation_runs SET status='failed', error=$1, ended_at=$2 WHERE id=$3",
+            )
+            .bind(format!(
+                "rejected: dedup output {after}c is wildly off input {before}c \
+                 — likely a duplicated or ballooned output"
+            ))
+            .bind(Utc::now())
+            .bind(run_id)
+            .execute(&pool)
+            .await?;
+            skipped += 1;
+            println!(
+                "SKIPPED chapter {n}: dedup returned {after}c from {before}c \
+                 — chapter left untouched (retry with --chapter {n})"
+            );
+            continue;
+        }
+
         // Overwrite body_md and clear body_md_tts — the chapter must be
         // re-prepped before it is narrated again. body_md_original is
         // left untouched (it belongs to the rewrite pass).
@@ -121,19 +159,25 @@ pub async fn run(
             .execute(&pool)
             .await?;
 
-        let before = body_md.len();
-        let after = deduped.len();
         println!(
             "deduped chapter {n} ({before}c -> {after}c) in {:.1}s",
             elapsed.as_secs_f32(),
         );
     }
 
+    if skipped > 0 {
         println!(
-        "dedup complete: \"{title}\" — {} chapter(s), {} finding(s) applied",
+            "dedup complete: \"{title}\" — {} chapter(s) processed, {skipped} SKIPPED \
+             (retry each with --chapter)",
+            chapters.len(),
+        );
+    } else {
+        println!(
+            "dedup complete: \"{title}\" — {} chapter(s) deduped against {} finding(s)",
             chapters.len(),
             findings.len(),
         );
+    }
     Ok(())
 }
 

skald/src/main.rs

@@ -199,6 +199,10 @@ enum Cmd {
         /// bound author if omitted.
         #[arg(long)]
         author: Option<String>,
+        /// Restrict the run to a single chapter number — used to
+        /// retry a chapter the length guard skipped.
+        #[arg(long)]
+        chapter: Option<i32>,
     },
 }
 
@@ -289,8 +293,8 @@ async fn run() -> anyhow::Result<()> {
             rewrite::run(&cli.database_url, chapter, author.as_deref()).await
         }
         Cmd::Audit { story } => audit::run(&cli.database_url, story).await,
-        Cmd::Dedup { story, author } => {
+        Cmd::Dedup { story, author, chapter } => {
-            dedup::run(&cli.database_url, story, author.as_deref()).await
+            dedup::run(&cli.database_url, story, author.as_deref(), chapter).await
         }
     }
 }