fix: prevent assign_to_agent from posting error comments on PRs

.github/workflows/issue-monster.md

@@ -596,6 +596,8 @@ safeoutputs/assign_to_agent(issue_number=<issue_number>, agent="copilot")
 
 Use the exact field name `issue_number` (underscore). Do **not** use `issue-number` (hyphen), which is invalid and will fail safe-output validation.
 
+**Important**: Only call `assign_to_agent` for **issues**, never for pull requests. The pre-fetched list already contains only issues, so never pass a PR number here. If you are ever unsure whether a number refers to an issue or a PR, call `issue_read` with `method: get` and check: if the response includes a `pull_request` URL field, skip that item.
+
 Do not use GitHub tools for this assignment. The `assign_to_agent` tool will handle the actual assignment.
 
 The Copilot coding agent will:
@@ -641,6 +643,7 @@ Issue Monster runs frequently (every 30 minutes), so keeping each run lean is cr
 - ✅ **Always report outcome**: If no issues are assigned, use the `noop` tool to explain why
 - ✅ **Skip integrity-blocked issues**: If `issue_read` is blocked by integrity policy, skip that issue and continue — never call `missing_data` for integrity errors
 - ❌ **Don't force batching**: If only 1-2 clearly separate issues exist, assign only those
+- ❌ **Never assign pull requests**: `assign_to_agent` is for issues only — never pass a PR number
 
 ## skill: `issue-monster-report-formatting`
 ---

actions/setup/js/assign_agent_helpers.cjs

@@ -275,6 +275,11 @@ async function getIssueDetails(owner, repo, issueNumber, githubClient = github)
       core.error("Could not get issue data");
       return null;
     }
+    // GitHub's issues API returns pull requests too; reject them here so callers
+    // never accidentally treat a PR as an assignable issue.
+    if (issue.pull_request) {
+      throw Object.assign(new Error(`#${issueNumber} is a pull request, not an issue — use pull_number instead of issue_number to assign to a pull request`), { isPullRequest: true });
+    }
     const currentAssignees = (issue.assignees || []).map(assignee => ({
       id: String(assignee.id),
       login: assignee.login,
@@ -290,7 +295,9 @@ async function getIssueDetails(owner, repo, issueNumber, githubClient = github)
     };
   } catch (error) {
     const errorMessage = getErrorMessage(error);
-    core.error(`Failed to get issue details: ${errorMessage}`);
+    if (!(/** @type {any} */ error.isPullRequest)) {
+      core.error(`Failed to get issue details: ${errorMessage}`);
+    }
     throw error;
   }
 }

actions/setup/js/assign_to_agent.cjs

@@ -392,6 +392,24 @@ async function main(config = {}) {
     } catch (error) {
       let errorMessage = getErrorMessage(error);
 
+      // When the agent specified an issue_number that turns out to be a PR, skip
+      // silently without posting a comment — error comments on PRs are confusing.
+      if (/** @type {any} */ error.isPullRequest) {
+        core.warning(`Skipping assign_to_agent for #${number}: target is a pull request, not an issue.`);
+        _allResults.push({
+          issue_number: issueNumber,
+          pull_number: pullNumber,
+          agent: agentName,
+          owner: effectiveOwner,
+          repo: effectiveRepo,
+          pull_request_repo: effectivePullRequestRepoSlug,
+          success: false,
+          skipped: true,
+          error: errorMessage,
+        });
+        return { success: false, skipped: true, error: errorMessage };
+      }
+
       const isAuthError = ["Bad credentials", "Not Authenticated", "Resource not accessible", "Insufficient permissions", "requires authentication"].some(msg => errorMessage.includes(msg));
       const isAvailabilityError = errorMessage.includes("coding agent is not available for this repository");
 

actions/setup/js/assign_to_agent.test.cjs

@@ -1212,6 +1212,34 @@ describe("assign_to_agent", () => {
     expect(mockGithub.rest.issues.createComment).not.toHaveBeenCalled();
   });
 
+  it("should skip silently and not post a comment when issue_number resolves to a pull request", async () => {
+    setAgentOutput({
+      items: [{ type: "assign_to_agent", issue_number: 99, agent: "copilot" }],
+      errors: [],
+    });
+
+    // Simulate the issues.get API returning a PR (has pull_request field)
+    mockGithub.rest.issues.get.mockResolvedValueOnce({
+      data: {
+        id: 99999,
+        number: 99,
+        assignees: [],
+        html_url: "https://github.com/test-owner/test-repo/pull/99",
+        title: "Some PR",
+        body: "",
+        pull_request: { url: "https://api.github.com/repos/test-owner/test-repo/pulls/99" },
+      },
+    });
+
+    await eval(`(async () => { ${assignToAgentScript}; ${STANDALONE_RUNNER} })()`);
+
+    // Should warn about the PR but not post a comment on it
+    expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("pull request, not an issue"));
+    expect(mockGithub.rest.issues.createComment).not.toHaveBeenCalled();
+    // Should not call setFailed — skipping a PR target is not a workflow failure
+    expect(mockCore.setFailed).not.toHaveBeenCalled();
+  });
+
   it("should post failure comment on single failed assignment", async () => {
     setAgentOutput({
       items: [{ type: "assign_to_agent", issue_number: 11, agent: "copilot" }],