diff --git a/actions/setup/js/setup_comment_memory_files.cjs b/actions/setup/js/setup_comment_memory_files.cjs
index 147795ee5f..ba2e6a075f 100644
--- a/actions/setup/js/setup_comment_memory_files.cjs
+++ b/actions/setup/js/setup_comment_memory_files.cjs
@@ -4,7 +4,9 @@ require("./shim.cjs");
 
 const fs = require("fs");
 const path = require("path");
+const { ERR_VALIDATION } = require("./error_codes.cjs");
 const { getErrorMessage } = require("./error_helpers.cjs");
+const { parseAllowedRepos, validateTargetRepo } = require("./repo_helpers.cjs");
 const {
   COMMENT_MEMORY_DIR,
   COMMENT_MEMORY_MAX_SCAN_PAGES,
@@ -74,6 +76,23 @@ async function collectCommentMemoryFiles(githubClient, commentMemoryConfig) {
     return [];
   }
 
+  const contextRepoSlug = `${context.repo.owner}/${context.repo.repo}`;
+  const normalizedTargetRepoSlug = targetRepo.slug.toLowerCase();
+  const normalizedContextRepoSlug = contextRepoSlug.toLowerCase();
+  const isCrossRepo = normalizedTargetRepoSlug !== normalizedContextRepoSlug;
+  if (isCrossRepo) {
+    const allowedRepos = parseAllowedRepos(commentMemoryConfig?.allowed_repos);
+    if (allowedRepos.size === 0) {
+      throw new Error(`${ERR_VALIDATION}: E004: Cross-repository comment-memory setup to '${targetRepo.slug}' is not permitted. No allowlist is configured. Define 'allowed_repos' to enable cross-repository access.`);
+    }
+
+    const repoValidation = validateTargetRepo(targetRepo.slug, contextRepoSlug, allowedRepos);
+    if (!repoValidation.valid) {
+      throw new Error(`${ERR_VALIDATION}: E004: ${repoValidation.error}`);
+    }
+    core.info(`comment_memory setup: cross-repo allowlist check passed for ${targetRepo.slug}`);
+  }
+
   core.info(`comment_memory setup: loading managed comment memory from ${targetRepo.slug}#${targetNumber}`);
   const memoryMap = new Map();
   let emptyPageCount = 0;
diff --git a/actions/setup/js/setup_comment_memory_files.test.cjs b/actions/setup/js/setup_comment_memory_files.test.cjs
index 9167cc70e6..1580880cf3 100644
--- a/actions/setup/js/setup_comment_memory_files.test.cjs
+++ b/actions/setup/js/setup_comment_memory_files.test.cjs
@@ -96,4 +96,122 @@ describe("setup_comment_memory_files", () => {
     expect(fs.readFileSync(memoryFile, "utf8")).toBe("Late memory\n");
     expect(listComments).toHaveBeenCalledTimes(6);
   });
+
+  it("rejects cross-repo comment-memory setup when no allowlist is configured", async () => {
+    fs.writeFileSync(CONFIG_PATH, JSON.stringify({ "comment-memory": { target: "triggering", "target-repo": "other-org/other-repo" } }));
+    const listComments = vi.fn().mockResolvedValue({ data: [] });
+    global.github = {
+      rest: {
+        issues: {
+          listComments,
+        },
+      },
+    };
+
+    const module = await import("./setup_comment_memory_files.cjs");
+    await module.main();
+
+    expect(listComments).not.toHaveBeenCalled();
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("E004"));
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("No allowlist is configured"));
+  });
+
+  it("rejects cross-repo comment-memory setup when target repo is not in allowlist", async () => {
+    fs.writeFileSync(
+      CONFIG_PATH,
+      JSON.stringify({
+        "comment-memory": {
+          target: "triggering",
+          "target-repo": "other-org/other-repo",
+          allowed_repos: ["other-org/different-repo"],
+        },
+      })
+    );
+    const listComments = vi.fn().mockResolvedValue({ data: [] });
+    global.github = {
+      rest: {
+        issues: {
+          listComments,
+        },
+      },
+    };
+
+    const module = await import("./setup_comment_memory_files.cjs");
+    await module.main();
+
+    expect(listComments).not.toHaveBeenCalled();
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("E004"));
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("not in the allowed-repos list"));
+  });
+
+  it("allows cross-repo comment-memory setup when target repo is in allowlist", async () => {
+    fs.writeFileSync(
+      CONFIG_PATH,
+      JSON.stringify({
+        "comment-memory": {
+          target: "triggering",
+          "target-repo": "other-org/other-repo",
+          allowed_repos: ["other-org/other-repo"],
+        },
+      })
+    );
+    const listComments = vi.fn().mockResolvedValue({
+      data: [{ body: '<gh-aw-comment-memory id="default">\nCross repo memory\n</gh-aw-comment-memory>' }],
+    });
+    global.github = {
+      rest: {
+        issues: {
+          listComments,
+        },
+      },
+    };
+
+    const module = await import("./setup_comment_memory_files.cjs");
+    await module.main();
+
+    expect(listComments).toHaveBeenCalledWith(
+      expect.objectContaining({
+        owner: "other-org",
+        repo: "other-repo",
+        issue_number: 42,
+      })
+    );
+    const memoryFile = path.join(COMMENT_MEMORY_DIR, "default.md");
+    expect(fs.existsSync(memoryFile)).toBe(true);
+    expect(fs.readFileSync(memoryFile, "utf8")).toBe("Cross repo memory\n");
+  });
+
+  it("treats target-repo as same repo when slug differs only by case", async () => {
+    fs.writeFileSync(
+      CONFIG_PATH,
+      JSON.stringify({
+        "comment-memory": {
+          target: "triggering",
+          "target-repo": "Octo/Repo",
+        },
+      })
+    );
+    const listComments = vi.fn().mockResolvedValue({
+      data: [{ body: '<gh-aw-comment-memory id="default">\nSame repo memory\n</gh-aw-comment-memory>' }],
+    });
+    global.github = {
+      rest: {
+        issues: {
+          listComments,
+        },
+      },
+    };
+
+    const module = await import("./setup_comment_memory_files.cjs");
+    await module.main();
+
+    expect(listComments).toHaveBeenCalledWith(
+      expect.objectContaining({
+        owner: "Octo",
+        repo: "Repo",
+        issue_number: 42,
+      })
+    );
+    expect(global.core.warning).not.toHaveBeenCalledWith(expect.stringContaining("E004"));
+  });
 });