fix: render const values as literals in output format instruction (#129)

buildOutputFormatInstruction now renders const fields with their actual
value (e.g. $status: greeted) instead of the type placeholder (<string>).
Also adds early return in resolvePropertySchema for const properties.

Fixes #129

packages/util-agent/__tests__/build-output-format-instruction.test.ts

@@ -225,4 +225,34 @@ describe("buildOutputFormatInstruction", () => {
     const result = buildOutputFormatInstruction({});
     expect(result).toContain("Focus exclusively on YOUR role");
   });
+
+  test("renders const value as literal in flat schema example", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        $status: { type: "string", const: "greeted" },
+        message: { type: "string" },
+      },
+      required: ["$status", "message"],
+    };
+    const result = buildOutputFormatInstruction(schema);
+    expect(result).toContain("$status: greeted");
+    expect(result).toContain("fixed value");
+    expect(result).not.toContain("$status: <string>");
+  });
+
+  test("renders const value for non-string types", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        count: { type: "number", const: 42 },
+        done: { type: "boolean", const: true },
+      },
+      required: ["count", "done"],
+    };
+    const result = buildOutputFormatInstruction(schema);
+    expect(result).toContain("count: 42");
+    expect(result).toContain("done: true");
+    expect(result).toContain("fixed value");
+  });
 });

packages/util-agent/src/build-output-format-instruction.ts

@@ -74,6 +74,10 @@ function collectObjectSchemas(schema: JSONSchema): JSONSchema[] {
 }
 
 function resolvePropertySchema(prop: JSONSchema): JSONSchema {
+  if (prop.const !== undefined) {
+    return prop;
+  }
+
   if (Array.isArray(prop.enum) && prop.enum.length > 0) {
     return prop;
   }
@@ -113,6 +117,11 @@ function buildPropertyExampleLine(prop: SchemaProperty): string {
     commentParts.push("required");
   }
 
+  if (resolved.const !== undefined) {
+    commentParts.push("fixed value");
+    return `${prop.name}: ${formatYamlScalar(resolved.const)}${buildPropertyComment(commentParts)}`;
+  }
+
   if (Array.isArray(resolved.enum) && resolved.enum.length > 0) {
     const enumValues = resolved.enum.map((v) => String(v));
     commentParts.push(...enumValues);