diff --git a/crates/ail/src/main.rs b/crates/ail/src/main.rs
index d00a4c0..3c7f30c 100644
--- a/crates/ail/src/main.rs
+++ b/crates/ail/src/main.rs
@@ -1029,6 +1029,23 @@ fn workspace_error_to_diagnostic(
                 "method": method,
             })),
         ),
+        W::OverridingNonExistentMethod {
+            class,
+            type_repr,
+            method,
+        } => Some(
+            ailang_check::Diagnostic::error(
+                "overriding-non-existent-method",
+                format!(
+                    "instance `{class} {type_repr}` provides body for method `{method}`, but class `{class}` does not declare it"
+                ),
+            )
+            .with_ctx(serde_json::json!({
+                "class": class,
+                "type": type_repr,
+                "method": method,
+            })),
+        ),
         W::KindMismatch {
             class,
             param,
diff --git a/crates/ailang-core/src/workspace.rs b/crates/ailang-core/src/workspace.rs
index 4fef528..443654f 100644
--- a/crates/ailang-core/src/workspace.rs
+++ b/crates/ailang-core/src/workspace.rs
@@ -200,6 +200,18 @@ pub enum WorkspaceLoadError {
         method: String,
     },
 
+    /// Iter 22b.2: an instance specifies a body for a method name
+    /// that the corresponding class does not declare. Symmetric to
+    /// `MissingMethod` but in the opposite direction.
+    #[error(
+        "instance `{class} {type_repr}` provides body for method `{method}`, but class `{class}` does not declare it"
+    )]
+    OverridingNonExistentMethod {
+        class: String,
+        type_repr: String,
+        method: String,
+    },
+
     /// Iter 22b.2: class-schema validation. The class parameter
     /// appears in applied position (e.g. as the head of a
     /// `Type::Con { name == param, args.len() > 0 }`) inside a method
@@ -420,6 +432,23 @@ fn build_registry(
                             });
                         }
                     }
+
+                    // Symmetric to MissingMethod: an instance must
+                    // not specify a body for a method name the class
+                    // never declared. Decision 11 forbids ad-hoc
+                    // additions to a class's method set at the
+                    // instance site.
+                    let declared: BTreeSet<&str> =
+                        class_def.methods.iter().map(|m| m.name.as_str()).collect();
+                    for inst_method in &inst.methods {
+                        if !declared.contains(inst_method.name.as_str()) {
+                            return Err(WorkspaceLoadError::OverridingNonExistentMethod {
+                                class: inst.class.clone(),
+                                type_repr: type_repr.clone(),
+                                method: inst_method.name.clone(),
+                            });
+                        }
+                    }
                 }
 
                 entries.insert(
@@ -890,6 +919,33 @@ mod tests {
         }
     }
 
+    /// Iter 22b.2: an instance that specifies a body for a method
+    /// name the class never declared must fire
+    /// `OverridingNonExistentMethod`. Symmetric counterpart to
+    /// `MissingMethod`: the latter fires when the class declares a
+    /// non-default method that the instance omits; this one fires
+    /// when the instance provides a body the class did not ask for.
+    /// Decision 11 forbids ad-hoc additions to a class's method set
+    /// at the instance site.
+    #[test]
+    fn instance_overriding_nonexistent_method_fires() {
+        let entry = std::path::PathBuf::from(
+            "../../examples/test_22b2_overriding_nonexistent.ail.json",
+        );
+        let err = load_workspace(&entry)
+            .expect_err("must fire overriding-non-existent-method");
+        match err {
+            WorkspaceLoadError::OverridingNonExistentMethod {
+                class, type_repr, method,
+            } => {
+                assert_eq!(class, "Eq");
+                assert_eq!(type_repr, "Int");
+                assert_eq!(method, "ne");
+            }
+            other => panic!("expected OverridingNonExistentMethod, got {other:?}"),
+        }
+    }
+
     /// Iter 22b.2: a class method's `Type::Forall` whose `constraints`
     /// reference a type variable that is neither bound by `Forall.vars`
     /// nor equal to the class's `param` must fire
diff --git a/examples/test_22b2_overriding_nonexistent.ail.json b/examples/test_22b2_overriding_nonexistent.ail.json
new file mode 100644
index 0000000..2a6dd5f
--- /dev/null
+++ b/examples/test_22b2_overriding_nonexistent.ail.json
@@ -0,0 +1,38 @@
+{
+  "schema": "ailang/v0",
+  "name": "test_22b2_overriding_nonexistent",
+  "imports": [],
+  "defs": [
+    {
+      "kind": "class",
+      "name": "Eq",
+      "param": "a",
+      "methods": [
+        {
+          "name": "eq",
+          "type": {
+            "k": "fn",
+            "params": [{ "k": "var", "name": "a" }, { "k": "var", "name": "a" }],
+            "ret": { "k": "con", "name": "Bool" },
+            "effects": []
+          }
+        }
+      ]
+    },
+    {
+      "kind": "instance",
+      "class": "Eq",
+      "type": { "k": "con", "name": "Int" },
+      "methods": [
+        {
+          "name": "eq",
+          "body": { "t": "lit", "lit": { "kind": "bool", "value": true } }
+        },
+        {
+          "name": "ne",
+          "body": { "t": "lit", "lit": { "kind": "bool", "value": false } }
+        }
+      ]
+    }
+  ]
+}