fix: extract balanced JSON arrays from prose responses

Previously extractBalancedJsonObject only scanned for '{...}' objects.
Agents with array output schemas (s.array(...)) would fail to parse
responses where the model embedded the JSON array in surrounding prose,
triggering an unnecessary repair turn.

Rename to extractBalancedJson and pick whichever container character
({  or [) appears first, so both objects and arrays are handled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

skills/rig/rig.ts

@@ -1284,7 +1284,7 @@ function parseJson(text: string): { ok: true; value: unknown } | { ok: false; er
     }
   }
 
-  const objectText = extractBalancedJsonObject(text);
+  const objectText = extractBalancedJson(text);
   if (objectText === undefined) {
     return { ok: false, error: "No JSON value found." };
   }
@@ -1329,16 +1329,24 @@ function extractFencedJson(text: string): string | undefined {
   return text.slice(cursor, fenceEnd);
 }
 
-function extractBalancedJsonObject(text: string): string | undefined {
+function extractBalancedJson(text: string): string | undefined {
   const objectStart = text.indexOf("{");
-  if (objectStart === -1) {
+  const arrayStart = text.indexOf("[");
+  const start =
+    objectStart === -1 ? arrayStart :
+    arrayStart === -1 ? objectStart :
+    Math.min(objectStart, arrayStart);
+  if (start === -1) {
     return undefined;
   }
 
+  const openChar = text[start] as "{" | "[";
+  const closeChar = openChar === "{" ? "}" : "]";
+
   let depth = 0;
   let inString = false;
   let escaping = false;
-  for (let index = objectStart; index < text.length; index += 1) {
+  for (let index = start; index < text.length; index += 1) {
     const char = text[index]!;
     if (inString) {
       if (escaping) {
@@ -1356,12 +1364,12 @@ function extractBalancedJsonObject(text: string): string | undefined {
       continue;
     }
 
-    if (char === "{") {
+    if (char === openChar) {
       depth += 1;
-    } else if (char === "}") {
+    } else if (char === closeChar) {
       depth -= 1;
       if (depth === 0) {
-        return text.slice(objectStart, index + 1);
+        return text.slice(start, index + 1);
       }
     }
   }

src/rig.test.ts

@@ -422,6 +422,17 @@ describe("agent invocation", () => {
     await expect(reviewer("go")).resolves.toEqual({ text: "hello" });
   });
 
+  it("parses a JSON array embedded in surrounding text", async () => {
+    mocks.setSendAndWaitImpl(async () => 'Here are the results:\n["alpha","beta"]\nDone.');
+
+    const lister = agent({
+      name: "lister",
+      output: s.array(s.string),
+    });
+
+    await expect(lister("go")).resolves.toEqual(["alpha", "beta"]);
+  });
+
   it("retries validation failures with addon-customized repair prompts", async () => {
     const prompts: string[] = [];
     let calls = 0;