diff --git a/src/runtime-js.ts b/src/runtime-js.ts
index 2ddfafc..e4faf9c 100644
--- a/src/runtime-js.ts
+++ b/src/runtime-js.ts
@@ -6,6 +6,7 @@ import type { AST } from './ast'
 import { measure } from './ui';
 
 const STORAGE_KEY = 'cg-definitions';
+const CHANGELOG_KEY = 'cg-changelog';
 
 export const store: Record<string, any> = {};
 
@@ -170,6 +171,8 @@ export const _rt = {
                 if (defs.length > 0) {
                     const def = defs[0];
                     recompile(def.name, def.body);
+                    const source = prettyPrint({ kind: 'definition', name: def.name, body: def.body });
+                    appendChangeLog(def.name, source);
                     saveDefinitions();
                     return { _tag: 'Defined', _0: def.name };
                 }
@@ -306,6 +309,8 @@ function valueToAst(value: any): AST {
 export function syncToAst(name: string) {
     if (definitions.has(name)) {
         definitions.set(name, valueToAst(store[name]));
+        const source = prettyPrint({ kind: 'definition', name, body: definitions.get(name)! });
+        appendChangeLog(name, source);
         saveDefinitions();
     }
 }
@@ -323,3 +328,25 @@ function deepEqual(a: any, b: any): boolean {
     if (keysA.length !== keysB.length) return false;
     return keysA.every(k => deepEqual(a[k], b[k]));
 }
+
+function appendChangeLog(name: string, source: string) {
+    try {
+        const data = localStorage.getItem(CHANGELOG_KEY);
+        const log: Array<{ name: string, source: string, timestamp: number }> = data
+            ? JSON.parse(data)
+            : [];
+
+        if (log.length > 0 && log[log.length - 1].name === name) {
+            log[log.length - 1] = { name, source, timestamp: Date.now() };
+        } else {
+            log.push({ name, source, timestamp: Date.now() });
+        }
+
+        if (log.length > 200) {
+            log.splice(0, log.length - 200);
+        }
+        localStorage.setItem(CHANGELOG_KEY, JSON.stringify(log));
+    } catch (e) {
+        console.error('Failed to append changelog:', e);
+    }
+}