diff --git a/.pi/extensions/format-on-edit.ts b/.pi/extensions/format-on-edit.ts
new file mode 100644
index 00000000..918f8bb3
--- /dev/null
+++ b/.pi/extensions/format-on-edit.ts
@@ -0,0 +1,53 @@
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import { resolve } from "node:path";
+
+const ELIXIR_EXTENSIONS = [".ex", ".exs", ".heex"];
+
+function isPiTsFile(path: string, cwd: string): boolean {
+  if (!path.endsWith(".ts")) return false;
+  const piDir = resolve(cwd, ".pi");
+  return resolve(cwd, path).startsWith(piDir);
+}
+
+async function formatElixir(
+  pi: ExtensionAPI,
+  path: string,
+  signal: AbortSignal | undefined,
+) {
+  await pi.exec("mix", ["format", path], { signal, timeout: 10_000 });
+}
+
+async function formatTypeScript(
+  pi: ExtensionAPI,
+  path: string,
+  signal: AbortSignal | undefined,
+) {
+  await pi.exec("prettier", ["--write", path], { signal, timeout: 10_000 });
+}
+
+export default function (pi: ExtensionAPI) {
+  pi.on("tool_result", async (event, ctx) => {
+    // Only hook into file-modifying tools, and only on success
+    if (event.toolName !== "edit" && event.toolName !== "write") return;
+    if (event.isError) return;
+
+    // Get the file path from the tool input
+    const path = event.input?.path;
+    if (typeof path !== "string") return;
+
+    // Determine formatter based on file type
+    if (ELIXIR_EXTENSIONS.some((ext) => path.endsWith(ext))) {
+      try {
+        await formatElixir(pi, path, ctx.signal);
+      } catch {
+        /* ignore */
+      }
+    } else if (isPiTsFile(path, ctx.cwd)) {
+      try {
+        await formatTypeScript(pi, path, ctx.signal);
+      } catch {
+        /* ignore */
+      }
+    }
+  });
+}
diff --git a/mise.lock b/mise.lock
index 75f54475..2f1cdb53 100644
--- a/mise.lock
+++ b/mise.lock
@@ -167,6 +167,10 @@ url = "https://nodejs.org/dist/v25.9.0/node-v25.9.0-win-x64.zip"
 version = "1.45.0"
 backend = "npm:backlog.md"
 
+[[tools."npm:prettier"]]
+version = "3.8.3"
+backend = "npm:prettier"
+
 [[tools.shellcheck]]
 version = "0.11.0"
 backend = "aqua:koalaman/shellcheck"
diff --git a/mise.toml b/mise.toml
index 5909f82d..62caa9e6 100644
--- a/mise.toml
+++ b/mise.toml
@@ -31,6 +31,7 @@ shellcheck = 'latest'
 fd = 'latest'
 litestream = 'latest'
 "npm:backlog.md" = 'latest'
+"npm:prettier" = 'latest'
 
 [task_config]
 includes = ["scripts"]