diff --git a/hooks/pi/README.md b/hooks/pi/README.md
index 0094227129..9ae767c1f2 100644
--- a/hooks/pi/README.md
+++ b/hooks/pi/README.md
@@ -16,7 +16,7 @@ with other Pi extensions.
 
 - TypeScript extension using Pi's `ExtensionAPI` (not a shell hook, no `zx` dependency)
 - Subscribes to `tool_call` event, narrows to `bash` tool via `isToolCallEventType`
-- Calls `rtk rewrite` via `pi.exec`; mutates `event.input.command` in-place if rewrite differs
+- Calls `rtk rewrite` via `pi.exec`; returns a revised `{ input }` (not an in-place mutation of `event.input`) when the rewrite differs, per `ToolCallEventResult`
 - All error paths return `undefined` (pass through); RTK never blocks execution
 - Version guard at load time: checks `rtk >= 0.23.0`; warns and registers no-op if too old or missing
 - Installed to `.pi/extensions/rtk.ts` by `rtk init --agent pi` (project-local) or `~/.pi/agent/extensions/rtk.ts` by `rtk init --agent pi --global`
diff --git a/hooks/pi/rtk.ts b/hooks/pi/rtk.ts
index 32cb2b5f11..0267acf3cc 100644
--- a/hooks/pi/rtk.ts
+++ b/hooks/pi/rtk.ts
@@ -6,9 +6,9 @@
 // To add or change rewrite rules, edit the Rust registry — not this file.
 //
 // Exit code contract for `rtk rewrite`:
-//   0 + stdout  Rewrite found → mutate command
+//   0 + stdout  Rewrite found → revised command returned to the caller
 //   1           No RTK equivalent → pass through unchanged
-//   3 + stdout  Rewrite (advisory) → mutate command
+//   3 + stdout  Rewrite (advisory) → revised command returned to the caller
 
 import type { ExtensionAPI } from "@earendil-works/pi-coding-agent"
 import { isToolCallEventType } from "@earendil-works/pi-coding-agent"
@@ -68,9 +68,14 @@ export default async function (pi: ExtensionAPI) {
 
       // Delegate to RTK.
       const rewritten = await rewriteCommand(pi, cmd, ctx.signal)
-      if (rewritten && rewritten !== cmd) {
-        event.input.command = rewritten
-      }
+      if (!rewritten || rewritten === cmd) return
+
+      // The Pi/OMP dispatcher builds the args a tool actually executes with
+      // from a separate copy of `event.input`; it only substitutes a handler's
+      // RETURNED `{ input }` (see ToolCallEventResult), so mutating
+      // `event.input.command` here has no effect on execution — it silently
+      // rewrites a value nothing reads.
+      return { input: { ...event.input, command: rewritten } }
     } catch (err) {
       // Fail open: never block execution on an unexpected error.
       console.warn("[rtk] unexpected error in tool_call handler; passing through command", err)
diff --git a/src/hooks/init.rs b/src/hooks/init.rs
index 439dbfefa6..121b407caa 100644
--- a/src/hooks/init.rs
+++ b/src/hooks/init.rs
@@ -7168,6 +7168,25 @@ mod tests {
         });
     }
 
+    #[test]
+    fn test_pi_plugin_returns_revised_input_instead_of_mutating_in_place() {
+        // Regression test: the Pi/OMP `tool_call` dispatcher builds the args a
+        // tool actually executes with from a separate copy of `event.input`;
+        // it only substitutes a handler's RETURNED `{ input }`. A handler that
+        // mutates `event.input.command` and returns nothing silently rewrites
+        // a value nothing reads, so the bash command never actually changes.
+        assert!(
+            PI_PLUGIN.contains("return { input:"),
+            "tool_call handler must return the revised input (ToolCallEventResult), \
+             not just mutate event.input in place"
+        );
+        assert!(
+            !PI_PLUGIN.contains("event.input.command = rewritten"),
+            "tool_call handler must not rely on mutating event.input in place; \
+             the dispatcher never observes it"
+        );
+    }
+
     #[test]
     fn test_run_pi_mode_local_installs_plugin() {
         let tmp = TempDir::new().unwrap();