# Async Eval .then() Fix Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Fix `build_eval_js` to handle async script return values using `.then()` instead of `async IIFE`.

**Architecture:** Extract callback-sending logic into a `_s` helper function inside the generated JS. If the script returns a Promise, call `_s` via `.then()`; otherwise call `_s` synchronously. This keeps the outer IIFE synchronous for C++ injection compatibility.

**Tech Stack:** Rust, JavaScript

---

## Files

- Modify: `src/browser/callback_backend.rs:433-447` - `build_eval_js` function

---

### Task 1: Modify build_eval_js to support async via .then()

**Files:**
- Modify: `src/browser/callback_backend.rs:433-447`

- [ ] **Step 1: Replace build_eval_js implementation**

Replace the entire `build_eval_js` function body (lines 433-447) with:

```rust
fn build_eval_js(source_url: &str, script: &str) -> String {
    let escaped_source_url = escape_js_single_quoted(source_url);
    let callback = EVAL_CALLBACK_NAME;
    let events_url = escape_js_single_quoted(&events_endpoint_url(source_url));

    format!(
        "(function(){{try{{\
         var v=(function(){{return {script}}})();\
         function _s(v){{\
         var t=(typeof v==='string')?v:JSON.stringify(v);\
         try{{callBackJsToCpp('{escaped_source_url}@_@'+window.location.href+'@_@{callback}@_@sgBrowserExcuteJsCodeByDomain@_@'+(t??''))}}catch(_){{}}\
         var j=JSON.stringify({{type:'callback',callback:'{callback}',request_url:'{escaped_source_url}',payload:{{value:(t??'')}}}});\
         try{{var r=new XMLHttpRequest();r.open('POST','{events_url}',true);r.setRequestHeader('Content-Type','application/json');r.send(j)}}catch(_){{}}\
         try{{navigator.sendBeacon('{events_url}',new Blob([j],{{type:'application/json'}}))}}catch(_){{}}\
         }}\
         if(v&&typeof v.then==='function'){{v.then(_s).catch(function(){{}});}}else{{_s(v);}}\
         }}catch(e){{}}}})()"
    )
}
```

- [ ] **Step 2: Run tests**

Run: `cargo test browser_script_skill_tool --no-fail-fast`

Expected: All tests pass.

- [ ] **Step 3: Run full test suite**

Run: `cargo test`

Expected: All tests pass (except pre-existing `lineloss_period_resolver_prompts_for_missing_period` failure which is unrelated).

- [ ] **Step 4: Build**

Run: `cargo build`

Expected: Compiles with no errors.

- [ ] **Step 5: Commit**

```bash
git add src/browser/callback_backend.rs
git commit -m "fix: support async browser scripts via .then() in build_eval_js"
```