From a0e254a681b7a799fdb4b7bb69668ce7168990e6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=B0=8F=E6=A9=98?= <xiaoju@shazhou.work>
Date: Sat, 6 Jun 2026 01:12:13 +0000
Subject: [PATCH] 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
---
 .changeset/const-prompt-literal.md            |  9 ++++++
 .../build-output-format-instruction.test.ts   | 30 +++++++++++++++++++
 .../src/build-output-format-instruction.ts    |  9 ++++++
 3 files changed, 48 insertions(+)
 create mode 100644 .changeset/const-prompt-literal.md

diff --git a/.changeset/const-prompt-literal.md b/.changeset/const-prompt-literal.md
new file mode 100644
index 0000000..465cdc1
--- /dev/null
+++ b/.changeset/const-prompt-literal.md
@@ -0,0 +1,9 @@
+---
+"@united-workforce/util-agent": patch
+---
+
+fix: render const values as literals in output format instruction (#129)
+
+Previously `buildOutputFormatInstruction` rendered `const: greeted` as
+`$status: <string>`, causing agents to output `$status: const` instead of
+the actual value. Now const fields render as `$status: greeted  # required | fixed value`.
diff --git a/packages/util-agent/__tests__/build-output-format-instruction.test.ts b/packages/util-agent/__tests__/build-output-format-instruction.test.ts
index a975743..1e8509c 100644
--- a/packages/util-agent/__tests__/build-output-format-instruction.test.ts
+++ b/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");
+  });
 });
diff --git a/packages/util-agent/src/build-output-format-instruction.ts b/packages/util-agent/src/build-output-format-instruction.ts
index 2df7e73..b83465b 100644
--- a/packages/util-agent/src/build-output-format-instruction.ts
+++ b/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);
-- 
2.43.0