fix(flue): Fallback for older gh duplicate closure

Check whether the installed gh CLI supports --duplicate-of before linking duplicate closures. Fall back to --reason duplicate on older runners so duplicate triage can still close the issue instead of degrading to human review.

Co-Authored-By: GPT-5 Codex <noreply@openai.com>

Author: dcramer

.flue/agents/issue-triage.ts

@@ -354,6 +354,27 @@ async function runGhCommand(
   }
 }
 
+export function hasDuplicateOfFlag(helpText: string) {
+  return helpText.includes("--duplicate-of");
+}
+
+let duplicateOfFlagSupported: boolean | undefined;
+
+async function supportsDuplicateOfFlag(session: FlueSession) {
+  if (duplicateOfFlagSupported !== undefined) {
+    return duplicateOfFlagSupported;
+  }
+
+  const result = await session.shell("gh issue close --help", {
+    commands: [gh],
+    timeout: 60_000,
+  });
+  duplicateOfFlagSupported =
+    result.exitCode === 0 && hasDuplicateOfFlag(`${result.stdout}\n${result.stderr}`);
+
+  return duplicateOfFlagSupported;
+}
+
 async function withGhBodyFile<T>(
   prefix: string,
   body: string,
@@ -524,9 +545,13 @@ async function closeDuplicate(
         "Closing issue as not planned",
       );
     } else {
+      const canLinkDuplicate = await supportsDuplicateOfFlag(session);
+      const duplicateOfArg = canLinkDuplicate
+        ? ` --duplicate-of ${duplicate.number}`
+        : "";
       await runGhCommand(
         session,
-        `gh issue close ${context.issueNumber}${repoArg(context.repository)} --reason duplicate --duplicate-of ${duplicate.number}`,
+        `gh issue close ${context.issueNumber}${repoArg(context.repository)} --reason duplicate${duplicateOfArg}`,
         "Closing duplicate issue",
       );
     }

.flue/tests/issue-triage.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 
 import {
   buildDuplicateClosureComment,
+  hasDuplicateOfFlag,
   hasIssueTriageBotIntro,
   wasClosedAsNotPlanned,
   withIssueTriageBotIntro,
@@ -74,4 +75,11 @@ describe("duplicate closure", () => {
       "already closed as not planned",
     );
   });
+
+  it("detects whether gh can link duplicate closures", () => {
+    expect(hasDuplicateOfFlag("      --duplicate-of int   Issue number")).toBe(
+      true,
+    );
+    expect(hasDuplicateOfFlag("      --reason string      Reason")).toBe(false);
+  });
 });