Fix variable handling in actHandler

[miguelg719]

Variable values were only being replaced if the LLM replied with the percentage sign wrapped values %key%. This PR addresses that and replaces just key suggested values

---

Summary by cubic

Fix variable replacement in actHandler so plain keys are substituted, not only %key% tokens. This ensures suggested values populate element arguments reliably.

• Bug Fixes
  • Replace f"%{key}%" matching with f"{key}" when mapping variables into element_to_act_on.arguments.

^{Written for commit 1350abe. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

1 issue found across 3 files

Confidence score: 3/5

• Variable replacement in stagehand/handlers/act_handler.py now does bare substring matching, which can unintentionally corrupt arguments (e.g., "id" inside "valid"), creating a concrete risk of incorrect behavior at runtime.
• Severity is moderate-high (7/10) with a clear user-facing impact, so there’s some risk to merge without adjustment.
• Pay close attention to stagehand/handlers/act_handler.py - substring matching can alter unintended parts of arguments.

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="stagehand/handlers/act_handler.py">

<violation number="1" location="stagehand/handlers/act_handler.py:99">
P1: Removing the `%` delimiters from variable replacement causes bare substring matching, which can unintentionally corrupt arguments. For example, a variable key `"id"` would match inside `"valid"` → `"val123"`. Consider trying `%key%` first for backward compat, then falling back to bare `key`:
```python
replaced = str(arg)
if f"%{key}%" in replaced:
    replaced = replaced.replace(f"%{key}%", str(value))
else:
    replaced = replaced.replace(f"{key}", str(value))

(Note: the nested for key, value loop also produces len(args) × len(variables) items instead of applying all substitutions per arg — a pre-existing issue worth fixing alongside this.)

</details>

<details>
<summary>Architecture diagram</summary>

```mermaid
sequenceDiagram
    participant Client
    participant AH as ActHandler
    participant Browser as Browser Instance

    Client->>AH: act(options)
    
    Note over AH: Check for dynamic variables
    
    opt if variables provided in options
        loop for each argument in element_to_act_on.arguments
            loop for each key, value in variables
                AH->>AH: CHANGED: String replacement logic
                Note right of AH: Now matches "key" instead of "%key%"
                AH->>AH: Map variable value into argument string
            end
        end
    end

    AH->>Browser: Execute action with hydrated arguments
    Browser-->>AH: Action result
    AH-->>Client: Return ActResult (success/fail)

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}