diff --git a/crates/ail/src/main.rs b/crates/ail/src/main.rs
index 08d7a5e..e42aad1 100644
--- a/crates/ail/src/main.rs
+++ b/crates/ail/src/main.rs
@@ -69,6 +69,20 @@ enum Cmd {
         #[arg(long)]
         json: bool,
     },
+    /// Semantischer Modul-Diff per Def-Hash.
+    ///
+    /// Vergleicht zwei Module rein strukturell auf Top-Level-Defs:
+    /// pro Name werden die Hashes der canonical Bytes verglichen. Das
+    /// Diff funktioniert auch, wenn ein Modul gerade nicht typecheckt —
+    /// nur das Schema und die JSON-Form müssen ladbar sein.
+    ///
+    /// Exit-Code: 0 wenn keine Änderungen (außer `unchanged`), sonst 1.
+    Diff {
+        a: PathBuf,
+        b: PathBuf,
+        #[arg(long)]
+        json: bool,
+    },
 }
 
 fn main() -> Result<()> {
@@ -258,6 +272,20 @@ fn main() -> Result<()> {
                 }
             }
         }
+        Cmd::Diff { a, b, json } => {
+            let ma = ailang_core::load_module(&a)?;
+            let mb = ailang_core::load_module(&b)?;
+            let report = build_diff(&ma, &mb);
+            if json {
+                let v = diff_report_to_json(&report);
+                println!("{}", serde_json::to_string_pretty(&v)?);
+            } else {
+                print!("{}", render_diff_text(&report));
+            }
+            if !report.is_identical() {
+                std::process::exit(1);
+            }
+        }
         Cmd::Deps { path, of, json } => {
             let m = ailang_core::load_module(&path)?;
             let mut entries = Vec::new();
@@ -357,3 +385,210 @@ fn walk_term(t: &ailang_core::Term, out: &mut std::collections::BTreeSet<String>
         }
     }
 }
+
+// --- ail diff -------------------------------------------------------------
+
+/// Rein struktureller Modul-Diff. Top-Level-Defs werden per `name`
+/// identifiziert und per BLAKE3-16-Hex der canonical Bytes verglichen.
+struct DiffReport {
+    module_a: String,
+    module_b: String,
+    added: Vec<DiffEntry>,
+    removed: Vec<DiffEntry>,
+    changed: Vec<ChangedEntry>,
+    unchanged: Vec<DiffEntry>,
+}
+
+struct DiffEntry {
+    name: String,
+    hash: String,
+    kind: &'static str,
+}
+
+struct ChangedEntry {
+    name: String,
+    hash_a: String,
+    hash_b: String,
+    kind_a: &'static str,
+    kind_b: &'static str,
+}
+
+impl DiffReport {
+    fn is_identical(&self) -> bool {
+        self.added.is_empty() && self.removed.is_empty() && self.changed.is_empty()
+    }
+}
+
+fn build_diff(a: &ailang_core::Module, b: &ailang_core::Module) -> DiffReport {
+    use std::collections::BTreeMap;
+
+    let map_a: BTreeMap<&str, &ailang_core::Def> =
+        a.defs.iter().map(|d| (ailang_core::def_name(d), d)).collect();
+    let map_b: BTreeMap<&str, &ailang_core::Def> =
+        b.defs.iter().map(|d| (ailang_core::def_name(d), d)).collect();
+
+    let mut added = Vec::new();
+    let mut removed = Vec::new();
+    let mut changed = Vec::new();
+    let mut unchanged = Vec::new();
+
+    // Removed + (un)changed: alles aus A.
+    for (name, def_a) in &map_a {
+        let hash_a = ailang_core::def_hash(def_a);
+        let kind_a = ailang_core::def_kind(def_a);
+        match map_b.get(*name) {
+            None => removed.push(DiffEntry {
+                name: (*name).to_string(),
+                hash: hash_a,
+                kind: kind_a,
+            }),
+            Some(def_b) => {
+                let hash_b = ailang_core::def_hash(def_b);
+                let kind_b = ailang_core::def_kind(def_b);
+                if hash_a == hash_b {
+                    unchanged.push(DiffEntry {
+                        name: (*name).to_string(),
+                        hash: hash_a,
+                        kind: kind_a,
+                    });
+                } else {
+                    changed.push(ChangedEntry {
+                        name: (*name).to_string(),
+                        hash_a,
+                        hash_b,
+                        kind_a,
+                        kind_b,
+                    });
+                }
+            }
+        }
+    }
+
+    // Added: was nur in B vorkommt.
+    for (name, def_b) in &map_b {
+        if !map_a.contains_key(*name) {
+            added.push(DiffEntry {
+                name: (*name).to_string(),
+                hash: ailang_core::def_hash(def_b),
+                kind: ailang_core::def_kind(def_b),
+            });
+        }
+    }
+
+    // BTreeMap-Iteration ist bereits alphabetisch — keine extra-Sortierung
+    // nötig, aber explizit absichern, falls die Reihenfolge der Quelle
+    // jemals umgestellt wird.
+    added.sort_by(|x, y| x.name.cmp(&y.name));
+    removed.sort_by(|x, y| x.name.cmp(&y.name));
+    changed.sort_by(|x, y| x.name.cmp(&y.name));
+    unchanged.sort_by(|x, y| x.name.cmp(&y.name));
+
+    DiffReport {
+        module_a: a.name.clone(),
+        module_b: b.name.clone(),
+        added,
+        removed,
+        changed,
+        unchanged,
+    }
+}
+
+fn diff_report_to_json(r: &DiffReport) -> serde_json::Value {
+    let entry = |e: &DiffEntry| {
+        serde_json::json!({
+            "name": e.name,
+            "hash": e.hash,
+            "kind": e.kind,
+        })
+    };
+    let changed = |c: &ChangedEntry| {
+        serde_json::json!({
+            "name": c.name,
+            "hash_a": c.hash_a,
+            "hash_b": c.hash_b,
+            "kind_a": c.kind_a,
+            "kind_b": c.kind_b,
+        })
+    };
+    serde_json::json!({
+        "module_a": r.module_a,
+        "module_b": r.module_b,
+        "added":     r.added.iter().map(entry).collect::<Vec<_>>(),
+        "removed":   r.removed.iter().map(entry).collect::<Vec<_>>(),
+        "changed":   r.changed.iter().map(changed).collect::<Vec<_>>(),
+        "unchanged": r.unchanged.iter().map(entry).collect::<Vec<_>>(),
+    })
+}
+
+fn render_diff_text(r: &DiffReport) -> String {
+    use std::fmt::Write;
+
+    let mut out = String::new();
+    let _ = writeln!(out, "diff: {} -> {}", r.module_a, r.module_b);
+
+    if r.is_identical() && r.unchanged.is_empty() {
+        let _ = writeln!(out, "no changes");
+        return out;
+    }
+
+    // Einheitliche Spaltenbreite für Namens-/Kind-Spalte, damit Hashes
+    // visuell aligned sind. Längster Name bestimmt die Breite.
+    let name_width = r
+        .added
+        .iter()
+        .map(|e| e.name.len())
+        .chain(r.removed.iter().map(|e| e.name.len()))
+        .chain(r.changed.iter().map(|e| e.name.len()))
+        .chain(r.unchanged.iter().map(|e| e.name.len()))
+        .max()
+        .unwrap_or(0);
+
+    for e in &r.added {
+        let _ = writeln!(
+            out,
+            "+ {:<width$} ({})  {}",
+            e.name,
+            e.kind,
+            e.hash,
+            width = name_width
+        );
+    }
+    for e in &r.removed {
+        let _ = writeln!(
+            out,
+            "- {:<width$} ({})  {}",
+            e.name,
+            e.kind,
+            e.hash,
+            width = name_width
+        );
+    }
+    for c in &r.changed {
+        // Wenn sich der Kind geändert hat (z. B. const → fn), beide zeigen.
+        let kind = if c.kind_a == c.kind_b {
+            c.kind_a.to_string()
+        } else {
+            format!("{} -> {}", c.kind_a, c.kind_b)
+        };
+        let _ = writeln!(
+            out,
+            "~ {:<width$} ({})  {} -> {}",
+            c.name,
+            kind,
+            c.hash_a,
+            c.hash_b,
+            width = name_width
+        );
+    }
+    for e in &r.unchanged {
+        let _ = writeln!(
+            out,
+            "  {:<width$} ({})  {} (unchanged)",
+            e.name,
+            e.kind,
+            e.hash,
+            width = name_width
+        );
+    }
+    out
+}
diff --git a/crates/ail/tests/e2e.rs b/crates/ail/tests/e2e.rs
index cbe19aa..7369a58 100644
--- a/crates/ail/tests/e2e.rs
+++ b/crates/ail/tests/e2e.rs
@@ -64,6 +64,129 @@ fn list_sum_via_match() {
     assert_eq!(stdout.trim(), "42");
 }
 
+/// Schützt `ail diff`: ein modifizierter Body ändert den Hash von `sum`,
+/// während `main` unverändert bleibt. Erwartet Exit-Code 1, `changed`
+/// enthält genau `sum`, `unchanged` enthält `main`, `added`/`removed` leer.
+#[test]
+fn diff_detects_changed_def() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src_a = workspace.join("examples").join("sum.ail.json");
+
+    // Variante: lade sum.ail.json, mutiere den `then`-Zweig (statt 0 nun 1)
+    // in der `sum`-Definition. `main` bleibt bitidentisch.
+    let raw = std::fs::read(&src_a).expect("read sum.ail.json");
+    let mut module: serde_json::Value = serde_json::from_slice(&raw).expect("parse sum.ail.json");
+    {
+        let defs = module
+            .get_mut("defs")
+            .and_then(|d| d.as_array_mut())
+            .expect("defs array");
+        for def in defs.iter_mut() {
+            if def.get("name").and_then(|n| n.as_str()) == Some("sum") {
+                // Ersetze den then-Zweig literal 0 → literal 1.
+                let new_then = serde_json::json!({
+                    "t": "lit",
+                    "lit": { "kind": "int", "value": 1 }
+                });
+                def["body"]["then"] = new_then;
+            }
+        }
+    }
+
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_diff_changed_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let path_b = tmp.join("sum_v2.ail.json");
+    std::fs::write(&path_b, serde_json::to_vec_pretty(&module).unwrap())
+        .expect("write sum_v2.ail.json");
+
+    let output = Command::new(ail_bin())
+        .args([
+            "diff",
+            src_a.to_str().unwrap(),
+            path_b.to_str().unwrap(),
+            "--json",
+        ])
+        .output()
+        .expect("ail diff failed to run");
+
+    let code = output.status.code().expect("process terminated by signal");
+    assert_eq!(
+        code,
+        1,
+        "expected exit code 1 for differing modules; stderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    let v: serde_json::Value =
+        serde_json::from_str(stdout.trim()).expect("stdout must be valid JSON");
+
+    let added = v["added"].as_array().expect("added array");
+    let removed = v["removed"].as_array().expect("removed array");
+    let changed = v["changed"].as_array().expect("changed array");
+    let unchanged = v["unchanged"].as_array().expect("unchanged array");
+
+    assert!(added.is_empty(), "expected added empty: {added:?}");
+    assert!(removed.is_empty(), "expected removed empty: {removed:?}");
+    assert_eq!(changed.len(), 1, "expected one changed entry: {changed:?}");
+    assert_eq!(
+        changed[0].get("name").and_then(|n| n.as_str()),
+        Some("sum")
+    );
+    assert_ne!(
+        changed[0].get("hash_a").and_then(|n| n.as_str()),
+        changed[0].get("hash_b").and_then(|n| n.as_str()),
+        "hash_a and hash_b must differ for a changed def"
+    );
+
+    assert!(
+        unchanged.iter().any(|e| e.get("name").and_then(|n| n.as_str()) == Some("main")),
+        "expected `main` in unchanged: {unchanged:?}"
+    );
+}
+
+/// Diff eines Moduls mit sich selbst: Exit 0, alle Listen außer `unchanged` leer.
+#[test]
+fn diff_no_changes_exit_zero() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace.join("examples").join("sum.ail.json");
+
+    let output = Command::new(ail_bin())
+        .args([
+            "diff",
+            src.to_str().unwrap(),
+            src.to_str().unwrap(),
+            "--json",
+        ])
+        .output()
+        .expect("ail diff failed to run");
+
+    let code = output.status.code().expect("process terminated by signal");
+    assert_eq!(
+        code,
+        0,
+        "expected exit code 0 for identical modules; stderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    let v: serde_json::Value =
+        serde_json::from_str(stdout.trim()).expect("stdout must be valid JSON");
+
+    assert!(v["added"].as_array().unwrap().is_empty());
+    assert!(v["removed"].as_array().unwrap().is_empty());
+    assert!(v["changed"].as_array().unwrap().is_empty());
+    assert!(
+        !v["unchanged"].as_array().unwrap().is_empty(),
+        "self-diff should report unchanged defs"
+    );
+}
+
 /// Schützt das `--json`-Diagnostic-Format für Tooling-Konsumenten.
 /// `broken_unbound.ail.json` referenziert eine nicht-existente Variable;
 /// erwartet wird Exit-Code 1 und mindestens ein Diagnostic mit
diff --git a/crates/ailang-core/src/ast.rs b/crates/ailang-core/src/ast.rs
index 8fa8273..44a99f1 100644
--- a/crates/ailang-core/src/ast.rs
+++ b/crates/ailang-core/src/ast.rs
@@ -36,6 +36,22 @@ impl Def {
     }
 }
 
+/// Externer Helper: Name einer Definition (für Tools wie `ail diff`,
+/// die den Def-Knoten entkoppelt vom Methoden-Aufruf konsumieren).
+pub fn def_name(def: &Def) -> &str {
+    def.name()
+}
+
+/// Externer Helper: Diskriminator-Tag einer Definition (`fn`, `const`, `type`).
+/// Identisch mit dem `kind`-Feld in der JSON-Repräsentation.
+pub fn def_kind(def: &Def) -> &'static str {
+    match def {
+        Def::Fn(_) => "fn",
+        Def::Const(_) => "const",
+        Def::Type(_) => "type",
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct TypeDef {
     pub name: String,
diff --git a/crates/ailang-core/src/lib.rs b/crates/ailang-core/src/lib.rs
index 09651c9..1a4005f 100644
--- a/crates/ailang-core/src/lib.rs
+++ b/crates/ailang-core/src/lib.rs
@@ -8,7 +8,9 @@ pub mod canonical;
 pub mod hash;
 pub mod pretty;
 
-pub use ast::{ConstDef, Def, FnDef, Import, Literal, Module, Term, Type};
+pub use ast::{
+    def_kind, def_name, ConstDef, Def, FnDef, Import, Literal, Module, Term, Type,
+};
 pub use hash::def_hash;
 
 #[derive(Debug, thiserror::Error)]