diff --git a/src/bin/alpha_id.rs b/src/bin/alpha_id.rs
index a781f8c..e7d771b 100644
--- a/src/bin/alpha_id.rs
+++ b/src/bin/alpha_id.rs
@@ -1,4 +1,4 @@
-use alpha_id::model::{Config, Filter, Mode};
+use alpha_id::model::{normalize_chapters, Config, Filter, Mode};
 use alpha_id::pipeline::Pipeline;
 use clap::{Parser, Subcommand};
 use std::io::Read;
@@ -126,9 +126,16 @@ fn main() {
         Cmd::Suggest { input, mode, top, billable_only, valid_only, exclude_exotic, chapters, json } => {
             let p = Pipeline::load(&cfg).expect("load");
             let m: Mode = mode.parse().expect("mode");
+            let chapters = chapters.map(|c| {
+                let (padded, unknown) = normalize_chapters(&c);
+                for tok in &unknown {
+                    eprintln!("warning: --chapters token '{tok}' matches no known chapter (01..22); ignored");
+                }
+                padded
+            });
             let filter = Filter {
                 billable_only, valid_only, exclude_exotic,
-                chapters: chapters.map(|c| c.split(',').map(|s| s.trim().to_string()).collect()),
+                chapters,
             };
             let text = read_input(&input);
             let res = p.suggest(&text, m, &filter, top);
diff --git a/src/model.rs b/src/model.rs
index e6347ee..4c48fe2 100644
--- a/src/model.rs
+++ b/src/model.rs
@@ -42,6 +42,29 @@ pub struct Filter {
     pub exclude_exotic: bool,
 }
 
+/// Normalize a raw `--chapters` token list to the zero-padded chapter-tag form
+/// ("01".."22") that `Filter::accepts` compares against. Splits on ',', trims
+/// each token, and classifies: a token that parses as one of the 22 known
+/// ICD-10-GM chapters (1..=22) goes — zero-padded to two digits — into the first
+/// returned vec (in input order); every other non-empty token goes verbatim
+/// (trimmed) into the second (unrecognised) vec. Empty-after-trim tokens are
+/// skipped. Both "4" and "04" yield "04".
+pub fn normalize_chapters(raw: &str) -> (Vec<String>, Vec<String>) {
+    let mut known = Vec::new();
+    let mut unknown = Vec::new();
+    for tok in raw.split(',') {
+        let tok = tok.trim();
+        if tok.is_empty() {
+            continue;
+        }
+        match tok.parse::<u32>() {
+            Ok(n) if (1..=22).contains(&n) => known.push(format!("{n:02}")),
+            _ => unknown.push(tok.to_string()),
+        }
+    }
+    (known, unknown)
+}
+
 impl Filter {
     pub fn accepts(&self, t: &Tags) -> bool {
         if self.billable_only && !t.billable { return false; }