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fix(agent): inject user-provided value into fillForm observe results #1791

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fix(agent): inject user-provided value into fillForm observe results #1791

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8 changes: 7 additions & 1 deletion packages/core/lib/v3/agent/tools/fillform.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -56,7 +56,13 @@ export const fillFormTool = (

const completed = [] as unknown[];
const replayableActions: Action[] = [];
for (const res of observeResults) {
for (let i = 0; i < observeResults.length; i++) {
const res = observeResults[i];

if (res.method === "fill" && fields[i] !== undefined) {
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@cubic-dev-ai cubic-dev-ai bot Mar 6, 2026 •
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P1: Form values are injected by observe-result index, which can misbind values to the wrong fill action when observe output ordering/count differs from input fields.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At packages/core/lib/v3/agent/tools/fillform.ts, line 62:

<comment>Form values are injected by observe-result index, which can misbind values to the wrong fill action when observe output ordering/count differs from input fields.</comment>

<file context>
@@ -56,7 +56,13 @@ export const fillFormTool = (
+        for (let i = 0; i < observeResults.length; i++) {
+          const res = observeResults[i];
+
+          if (res.method === "fill" && fields[i] !== undefined) {
+            res.arguments = [fields[i].value];
+          }
</file context>
Fix with Cubic

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@tiwalayo tiwalayo Mar 6, 2026

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This is theoretically valid but not a practical concern. The instruction sent to observe() lists actions in explicit order, and the LLM returns results corresponding to that ordered list.

The original code already had this same implicit assumption—it looped over observeResults and called act() without any correlation mechanism. This fix does not introduce a new ordering assumption.

There is no ID or correlation mechanism between fields and observe results in the existing API, so index-based matching is the only option without a larger architectural refactor, which is out of scope for this bug fix.

res.arguments = [fields[i].value];
}

const actOptions = variables
? { variables, timeout: toolTimeout }
: { timeout: toolTimeout };
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