diff --git a/.github/workflows/auto-close-duplicates.yml b/.github/workflows/auto-close-duplicates.yml
index e8092299..ba0b11d8 100644
--- a/.github/workflows/auto-close-duplicates.yml
+++ b/.github/workflows/auto-close-duplicates.yml
@@ -22,6 +22,9 @@ jobs:
         with:
           bun-version: latest
 
+      - name: Run auto-close-duplicates tests
+        run: bun test scripts/auto-close-duplicates
+
       - name: Auto-close duplicate issues
         run: bun run scripts/auto-close-duplicates.ts
         env:
diff --git a/package.json b/package.json
index 90070ec5..abddf802 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,8 @@
   },
   "license": "Apache-2.0",
   "scripts": {
-    "setup": "echo '⚠️  This npm setup is for the archived web application. For agent development, use: ./scripts/setup-python.sh' && bash scripts/setup.sh"
+    "setup": "echo '⚠️  This npm setup is for the archived web application. For agent development, use: ./scripts/setup-python.sh' && bash scripts/setup.sh",
+    "test:duplicates": "bun test scripts/auto-close-duplicates"
   },
   "devDependencies": {
     "@types/node": "^20.10.0",
diff --git a/scripts/AUTO_CLOSE_DUPLICATES_CROSS_VERIFY.md b/scripts/AUTO_CLOSE_DUPLICATES_CROSS_VERIFY.md
new file mode 100644
index 00000000..b0d5c75d
--- /dev/null
+++ b/scripts/AUTO_CLOSE_DUPLICATES_CROSS_VERIFY.md
@@ -0,0 +1,52 @@
+# Cross-verification: auto-close-duplicates changes
+
+## 1. Files changed (this PR / session)
+
+| File | Change |
+|------|--------|
+| `scripts/auto-close-duplicates.ts` | Circular-dup + self-ref prevention; extracted helpers (isDupeComment, isDupeCommentOldEnough, authorDisagreedWithDupe, getLastDupeComment, decideAutoClose); `if (import.meta.main)` guard |
+| `scripts/auto-close-duplicates.test.ts` | New: 23 unit tests for all exported helpers |
+| `package.json` | Added `"test:duplicates": "bun test scripts/auto-close-duplicates"` |
+| `.github/workflows/auto-close-duplicates.yml` | Added step "Run auto-close-duplicates tests" before auto-close step |
+
+**Not changed:** `core/`, `tools/`, main CI (`.github/workflows/ci.yml`), triage workflow logic (only the script that consumes its comments).
+
+---
+
+## 2. Dependencies / who uses what
+
+- **Only consumer of the script:** `.github/workflows/auto-close-duplicates.yml` runs `bun run scripts/auto-close-duplicates.ts`. No other code imports or runs it.
+- **Only consumer of the script’s exports:** `scripts/auto-close-duplicates.test.ts` imports the exported helpers for unit tests.
+- **Triage workflow contract:** `.github/workflows/claude-issue-triage.yml` instructs the bot to post: `"Found a possible duplicate of #<issue_number>: ..."`. The script still recognises this via `isDupeComment` (body includes `"possible duplicate"` and `comment.user.type === "Bot"`). **No change to that contract.**
+
+---
+
+## 3. Backward compatibility
+
+- **Happy path unchanged:** Issue A has a bot comment “possible duplicate of #B”, B is open, comment &gt; 12h old, no author thumbs-down → script still closes A as duplicate of B.
+- **New safeguards only:** (1) If B is already closed → skip (no close). (2) If comment says “duplicate of #A” on issue A (self) → skip. (3) If target fetch fails or returns non-`open` state → skip.
+- **Comment format:** Still `"possible duplicate"` + Bot; `extractDuplicateIssueNumber` still supports `#N` and GitHub issue URL. No change required in triage prompt.
+
+---
+
+## 4. What was not touched
+
+- **Core / tools / agent_builder:** No edits to `core/`, `tools/`, or `agent_builder_server.py`. Grep hits for `agent_builder` / `SESSIONS_DIR` are in other docs or local issue/notes, not in this script.
+- **Main CI:** `ci.yml` only runs Python (ruff + pytest). It does **not** run the TS script or `bun test`. So these changes do not affect main CI.
+- **Triage workflow:** Only the **comment text** is consumed by the script. We did not change the triage YAML or the bot’s instructions; we only changed how the auto-close script interprets comments and when it skips closing.
+
+---
+
+## 5. Verification performed
+
+- **Unit tests:** `bun test scripts/auto-close-duplicates` → 23 tests pass.
+- **Script entry:** `bun run scripts/auto-close-duplicates.ts` with fake env runs and fails at first API call (401), so the main path still runs and `import.meta.main` guard works.
+- **No other imports:** Only `scripts/auto-close-duplicates.test.ts` imports from `auto-close-duplicates.ts`.
+
+---
+
+## 6. Conclusion
+
+- **Impact:** Limited to the auto-close-duplicates workflow and its tests. No impact on core, tools, main CI, or triage prompt text.
+- **Behavior:** Same for the normal case; only adds skip conditions for circular duplicate, self-reference, and closed/unreachable target.
+- **Contract:** Triage comment format and 12h + author-reaction behaviour unchanged; script remains compatible with existing triage.
diff --git a/scripts/auto-close-duplicates.test.ts b/scripts/auto-close-duplicates.test.ts
new file mode 100644
index 00000000..7422cc50
--- /dev/null
+++ b/scripts/auto-close-duplicates.test.ts
@@ -0,0 +1,261 @@
+/**
+ * Tests for auto-close-duplicates script: comment filter, 12h check,
+ * author reaction, extractDuplicateIssueNumber, and decideAutoClose
+ * (circular-dup and self-ref prevention).
+ */
+import { describe, expect, test } from "bun:test";
+import {
+  authorDisagreedWithDupe,
+  decideAutoClose,
+  extractDuplicateIssueNumber,
+  getLastDupeComment,
+  isDupeComment,
+  isDupeCommentOldEnough,
+  type GitHubComment,
+  type GitHubIssue,
+  type GitHubReaction,
+} from "./auto-close-duplicates";
+
+describe("extractDuplicateIssueNumber", () => {
+  test("extracts #123 format", () => {
+    expect(
+      extractDuplicateIssueNumber("Found a possible duplicate of #1275: ...")
+    ).toBe(1275);
+    expect(extractDuplicateIssueNumber("Duplicate of #1")).toBe(1);
+    expect(extractDuplicateIssueNumber("See #1000")).toBe(1000);
+  });
+
+  test("extracts first #N when multiple present", () => {
+    expect(
+      extractDuplicateIssueNumber("Duplicate of #1000 and also #1275")
+    ).toBe(1000);
+  });
+
+  test("extracts GitHub issue URL format", () => {
+    expect(
+      extractDuplicateIssueNumber(
+        "Duplicate of https://github.com/adenhq/hive/issues/42"
+      )
+    ).toBe(42);
+  });
+
+  test("returns null when no issue number", () => {
+    expect(extractDuplicateIssueNumber("No number here")).toBe(null);
+    expect(extractDuplicateIssueNumber("")).toBe(null);
+  });
+});
+
+describe("isDupeComment", () => {
+  test("true when body has 'possible duplicate' and user is Bot", () => {
+    expect(
+      isDupeComment({
+        id: 1,
+        body: "Found a possible duplicate of #1000: same bug",
+        created_at: "",
+        user: { type: "Bot", id: 2 },
+      })
+    ).toBe(true);
+    expect(
+      isDupeComment({
+        id: 1,
+        body: "Possible duplicate of #1275",
+        created_at: "",
+        user: { type: "Bot", id: 2 },
+      })
+    ).toBe(true);
+  });
+
+  test("false when body lacks 'possible duplicate'", () => {
+    expect(
+      isDupeComment({
+        id: 1,
+        body: "Not a duplicate",
+        created_at: "",
+        user: { type: "Bot", id: 2 },
+      })
+    ).toBe(false);
+  });
+
+  test("false when user is not Bot", () => {
+    expect(
+      isDupeComment({
+        id: 1,
+        body: "Found a possible duplicate of #1000",
+        created_at: "",
+        user: { type: "User", id: 2 },
+      })
+    ).toBe(false);
+  });
+});
+
+describe("isDupeCommentOldEnough", () => {
+  test("true when comment date is before twelveHoursAgo", () => {
+    const twelveHoursAgo = new Date("2025-01-28T12:00:00Z");
+    const oldComment = new Date("2025-01-28T00:00:00Z");
+    expect(isDupeCommentOldEnough(oldComment, twelveHoursAgo)).toBe(true);
+  });
+
+  test("true when comment date equals twelveHoursAgo", () => {
+    const twelveHoursAgo = new Date("2025-01-28T12:00:00Z");
+    expect(isDupeCommentOldEnough(twelveHoursAgo, twelveHoursAgo)).toBe(true);
+  });
+
+  test("false when comment is after twelveHoursAgo (too recent)", () => {
+    const twelveHoursAgo = new Date("2025-01-28T12:00:00Z");
+    const recentComment = new Date("2025-01-28T18:00:00Z");
+    expect(isDupeCommentOldEnough(recentComment, twelveHoursAgo)).toBe(false);
+  });
+});
+
+describe("authorDisagreedWithDupe", () => {
+  test("true when issue author gave thumbs down", () => {
+    const issue = { number: 1275, title: "", state: "open", user: { id: 42 }, created_at: "" };
+    const reactions: GitHubReaction[] = [
+      { user: { id: 42 }, content: "-1" },
+    ];
+    expect(authorDisagreedWithDupe(reactions, issue)).toBe(true);
+  });
+
+  test("false when only other users reacted", () => {
+    const issue = { number: 1275, title: "", state: "open", user: { id: 42 }, created_at: "" };
+    const reactions: GitHubReaction[] = [
+      { user: { id: 99 }, content: "-1" },
+      { user: { id: 1 }, content: "+1" },
+    ];
+    expect(authorDisagreedWithDupe(reactions, issue)).toBe(false);
+  });
+
+  test("false when author gave +1 or other reaction", () => {
+    const issue = { number: 1275, title: "", state: "open", user: { id: 42 }, created_at: "" };
+    expect(authorDisagreedWithDupe([{ user: { id: 42 }, content: "+1" }], issue)).toBe(false);
+    expect(authorDisagreedWithDupe([{ user: { id: 42 }, content: "eyes" }], issue)).toBe(false);
+  });
+});
+
+describe("getLastDupeComment", () => {
+  test("returns null when no dupe comments", () => {
+    expect(
+      getLastDupeComment([
+        { id: 1, body: "Not a duplicate", created_at: "", user: { type: "User", id: 1 } },
+      ])
+    ).toBe(null);
+  });
+
+  test("returns the only dupe comment when one exists", () => {
+    const c: GitHubComment = {
+      id: 1,
+      body: "Found a possible duplicate of #1000",
+      created_at: "",
+      user: { type: "Bot", id: 2 },
+    };
+    expect(getLastDupeComment([c])).toBe(c);
+  });
+
+  test("returns the last dupe comment when multiple exist", () => {
+    const c1: GitHubComment = {
+      id: 1,
+      body: "Found a possible duplicate of #1000",
+      created_at: "",
+      user: { type: "Bot", id: 2 },
+    };
+    const c2: GitHubComment = {
+      id: 2,
+      body: "Found a possible duplicate of #1275",
+      created_at: "",
+      user: { type: "Bot", id: 2 },
+    };
+    const other: GitHubComment = {
+      id: 3,
+      body: "Some other comment",
+      created_at: "",
+      user: { type: "User", id: 3 },
+    };
+    expect(getLastDupeComment([other, c1, c2])).toBe(c2);
+  });
+});
+
+function issue(num: number, state = "open"): GitHubIssue {
+  return {
+    number: num,
+    title: `Issue ${num}`,
+    state,
+    user: { id: 1 },
+    created_at: new Date().toISOString(),
+  };
+}
+
+function comment(body: string): GitHubComment {
+  return {
+    id: 1,
+    body,
+    created_at: new Date().toISOString(),
+    user: { type: "Bot", id: 2 },
+  };
+}
+
+describe("decideAutoClose", () => {
+  test("returns null when comment has no extractable issue number", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Possible duplicate of something else"),
+      async () => ({ state: "open" })
+    );
+    expect(result).toBe(null);
+  });
+
+  test("returns null when duplicate target is self (same issue number)", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1275: same issue"),
+      async () => ({ state: "open" })
+    );
+    expect(result).toBe(null);
+  });
+
+  test("returns null when target issue is closed (avoids circular closure)", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1000"),
+      async (num) => (num === 1000 ? { state: "closed" } : { state: "open" })
+    );
+    expect(result).toBe(null);
+  });
+
+  test("returns null when getTargetIssue returns null", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1000"),
+      async () => null
+    );
+    expect(result).toBe(null);
+  });
+
+  test("returns null when getTargetIssue throws", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1000"),
+      async () => {
+        throw new Error("API error");
+      }
+    );
+    expect(result).toBe(null);
+  });
+
+  test("returns duplicateOf number when target is open (should close)", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1000: same bug"),
+      async (num) => (num === 1000 ? { state: "open" } : { state: "closed" })
+    );
+    expect(result).toBe(1000);
+  });
+
+  test("returns null when target state is not exactly 'open' (e.g. uppercase)", async () => {
+    const result = await decideAutoClose(
+      issue(1275),
+      comment("Found a possible duplicate of #1000"),
+      async () => ({ state: "OPEN" } as { state: string })
+    );
+    expect(result).toBe(null);
+  });
+});
diff --git a/scripts/auto-close-duplicates.ts b/scripts/auto-close-duplicates.ts
index ec46e16b..42461d70 100644
--- a/scripts/auto-close-duplicates.ts
+++ b/scripts/auto-close-duplicates.ts
@@ -6,21 +6,22 @@ declare global {
   };
 }
 
-interface GitHubIssue {
+export interface GitHubIssue {
   number: number;
   title: string;
+  state: string;
   user: { id: number };
   created_at: string;
 }
 
-interface GitHubComment {
+export interface GitHubComment {
   id: number;
   body: string;
   created_at: string;
   user: { type: string; id: number };
 }
 
-interface GitHubReaction {
+export interface GitHubReaction {
   user: { id: number };
   content: string;
 }
@@ -57,7 +58,41 @@ async function githubRequest<T>(
   return response.json();
 }
 
-function extractDuplicateIssueNumber(commentBody: string): number | null {
+/** True if comment is a bot "possible duplicate" detection (used for filtering). */
+export function isDupeComment(comment: GitHubComment): boolean {
+  const bodyLower = comment.body.toLowerCase();
+  return (
+    bodyLower.includes("possible duplicate") && comment.user.type === "Bot"
+  );
+}
+
+/** True if the duplicate comment is old enough to auto-close (>= 12h). */
+export function isDupeCommentOldEnough(
+  dupeCommentDate: Date,
+  twelveHoursAgo: Date
+): boolean {
+  return dupeCommentDate <= twelveHoursAgo;
+}
+
+/** True if the issue author reacted with thumbs down to the duplicate comment. */
+export function authorDisagreedWithDupe(
+  reactions: GitHubReaction[],
+  issue: GitHubIssue
+): boolean {
+  return reactions.some(
+    (r) => r.user.id === issue.user.id && r.content === "-1"
+  );
+}
+
+/** Returns the most recent duplicate-detection comment, or null if none. */
+export function getLastDupeComment(
+  comments: GitHubComment[]
+): GitHubComment | null {
+  const dupeComments = comments.filter(isDupeComment);
+  return dupeComments.length > 0 ? dupeComments[dupeComments.length - 1]! : null;
+}
+
+export function extractDuplicateIssueNumber(commentBody: string): number | null {
   // Try to match #123 format first
   let match = commentBody.match(/#(\d+)/);
   if (match) {
@@ -73,6 +108,30 @@ function extractDuplicateIssueNumber(commentBody: string): number | null {
   return null;
 }
 
+/**
+ * Decides whether to auto-close this issue as duplicate of another.
+ * Returns the target issue number to close as duplicate of, or null to skip.
+ * Used by the main loop and by tests.
+ */
+export async function decideAutoClose(
+  issue: GitHubIssue,
+  lastDupeComment: GitHubComment,
+  getTargetIssue: (issueNumber: number) => Promise<{ state: string } | null>
+): Promise<number | null> {
+  const duplicateIssueNumber = extractDuplicateIssueNumber(lastDupeComment.body);
+  if (duplicateIssueNumber === null) return null;
+
+  if (duplicateIssueNumber === issue.number) return null;
+
+  try {
+    const targetIssue = await getTargetIssue(duplicateIssueNumber);
+    if (!targetIssue || targetIssue.state !== "open") return null;
+    return duplicateIssueNumber;
+  } catch {
+    return null;
+  }
+}
+
 async function closeIssueAsDuplicate(
   owner: string,
   repo: string,
@@ -173,25 +232,18 @@ async function autoCloseDuplicates(): Promise<void> {
       `[DEBUG] Issue #${issue.number} has ${comments.length} comments`
     );
 
-    const dupeComments = comments.filter((comment) => {
-      const bodyLower = comment.body.toLowerCase();
-      return (
-        bodyLower.includes("possible duplicate") &&
-        comment.user.type === "Bot"
-      );
-    });
+    const lastDupeComment = getLastDupeComment(comments);
+    const dupeCount = comments.filter(isDupeComment).length;
     console.log(
-      `[DEBUG] Issue #${issue.number} has ${dupeComments.length} duplicate detection comments`
+      `[DEBUG] Issue #${issue.number} has ${dupeCount} duplicate detection comments`
     );
 
-    if (dupeComments.length === 0) {
+    if (lastDupeComment === null) {
       console.log(
         `[DEBUG] Issue #${issue.number} - no duplicate comments found, skipping`
       );
       continue;
     }
-
-    const lastDupeComment = dupeComments[dupeComments.length - 1];
     const dupeCommentDate = new Date(lastDupeComment.created_at);
     console.log(
       `[DEBUG] Issue #${
@@ -199,7 +251,7 @@ async function autoCloseDuplicates(): Promise<void> {
       } - most recent duplicate comment from: ${dupeCommentDate.toISOString()}`
     );
 
-    if (dupeCommentDate > twelveHoursAgo) {
+    if (!isDupeCommentOldEnough(dupeCommentDate, twelveHoursAgo)) {
       console.log(
         `[DEBUG] Issue #${issue.number} - duplicate comment is too recent, skipping`
       );
@@ -224,10 +276,7 @@ async function autoCloseDuplicates(): Promise<void> {
       `[DEBUG] Issue #${issue.number} - duplicate comment has ${reactions.length} reactions`
     );
 
-    const authorThumbsDown = reactions.some(
-      (reaction) =>
-        reaction.user.id === issue.user.id && reaction.content === "-1"
-    );
+    const authorThumbsDown = authorDisagreedWithDupe(reactions, issue);
     console.log(
       `[DEBUG] Issue #${issue.number} - author thumbs down reaction: ${authorThumbsDown}`
     );
@@ -239,12 +288,19 @@ async function autoCloseDuplicates(): Promise<void> {
       continue;
     }
 
-    const duplicateIssueNumber = extractDuplicateIssueNumber(
-      lastDupeComment.body
+    const duplicateOf = await decideAutoClose(
+      issue,
+      lastDupeComment,
+      (issueNumber) =>
+        githubRequest<GitHubIssue>(
+          `/repos/${owner}/${repo}/issues/${issueNumber}`,
+          token
+        ).then((i) => ({ state: i.state }))
     );
-    if (!duplicateIssueNumber) {
+
+    if (duplicateOf === null) {
       console.log(
-        `[DEBUG] Issue #${issue.number} - could not extract duplicate issue number from comment, skipping`
+        `[DEBUG] Issue #${issue.number} - skipping (invalid/self/closed target or fetch error)`
       );
       continue;
     }
@@ -254,17 +310,17 @@ async function autoCloseDuplicates(): Promise<void> {
 
     try {
       console.log(
-        `[INFO] Auto-closing issue #${issue.number} as duplicate of #${duplicateIssueNumber}: ${issueUrl}`
+        `[INFO] Auto-closing issue #${issue.number} as duplicate of #${duplicateOf}: ${issueUrl}`
       );
       await closeIssueAsDuplicate(
         owner,
         repo,
         issue.number,
-        duplicateIssueNumber,
+        duplicateOf,
         token
       );
       console.log(
-        `[SUCCESS] Successfully closed issue #${issue.number} as duplicate of #${duplicateIssueNumber}`
+        `[SUCCESS] Successfully closed issue #${issue.number} as duplicate of #${duplicateOf}`
       );
     } catch (error) {
       console.error(
@@ -278,6 +334,8 @@ async function autoCloseDuplicates(): Promise<void> {
   );
 }
 
-autoCloseDuplicates().catch(console.error);
+if (import.meta.main) {
+  autoCloseDuplicates().catch(console.error);
+}
 
 export {};