diff --git a/test/comments-common.test.ts b/test/comments-common.test.ts
new file mode 100644
index 000000000..b37dec97f
--- /dev/null
+++ b/test/comments-common.test.ts
@@ -0,0 +1,74 @@
+import { describe, test, expect } from "bun:test";
+import {
+  SPINNER_HTML,
+  createJobRunLink,
+  createBranchLink,
+  createCommentBody,
+} from "../src/github/operations/comments/common";
+import { GITHUB_SERVER_URL } from "../src/github/api/config";
+
+describe("comments/common", () => {
+  describe("createJobRunLink", () => {
+    test("builds a markdown link to the workflow run", () => {
+      const result = createJobRunLink("anthropics", "claude-code-action", "42");
+      expect(result).toBe(
+        `[View job run](${GITHUB_SERVER_URL}/anthropics/claude-code-action/actions/runs/42)`,
+      );
+    });
+
+    test("honors GITHUB_SERVER_URL (GHES) rather than hardcoding github.com", () => {
+      // The link is built from the configured server URL, so it must point at
+      // whatever GITHUB_SERVER_URL resolves to (github.com by default, a GHES
+      // host in enterprise setups).
+      expect(createJobRunLink("o", "r", "1")).toContain(GITHUB_SERVER_URL);
+    });
+  });
+
+  describe("createBranchLink", () => {
+    test("builds a leading-newline markdown link to the branch tree", () => {
+      const result = createBranchLink(
+        "anthropics",
+        "claude-code-action",
+        "feature/x",
+      );
+      expect(result).toBe(
+        `\n[View branch](${GITHUB_SERVER_URL}/anthropics/claude-code-action/tree/feature/x)`,
+      );
+    });
+
+    test("prefixes the link with a newline so it renders on its own line", () => {
+      expect(createBranchLink("o", "r", "main").startsWith("\n")).toBe(true);
+    });
+  });
+
+  describe("createCommentBody", () => {
+    test("includes the spinner, the working message, and the job run link", () => {
+      const jobRunLink = createJobRunLink("o", "r", "7");
+      const body = createCommentBody(jobRunLink);
+
+      expect(body).toContain(SPINNER_HTML);
+      expect(body).toContain("Claude Code is working…");
+      expect(body).toContain(jobRunLink);
+    });
+
+    test("omits the branch link when none is provided (defaults to empty)", () => {
+      const body = createCommentBody(createJobRunLink("o", "r", "7"));
+      expect(body).not.toContain("View branch");
+      // No trailing branch content: body ends with the job run link.
+      expect(body.endsWith(")")).toBe(true);
+    });
+
+    test("appends the branch link when provided", () => {
+      const jobRunLink = createJobRunLink("o", "r", "7");
+      const branchLink = createBranchLink("o", "r", "feature/x");
+      const body = createCommentBody(jobRunLink, branchLink);
+
+      expect(body).toContain(jobRunLink);
+      expect(body).toContain(branchLink);
+      // The branch link (with its leading newline) comes after the job run link.
+      expect(body.indexOf(branchLink)).toBeGreaterThan(
+        body.indexOf(jobRunLink),
+      );
+    });
+  });
+});
diff --git a/test/create-prompt.test.ts b/test/create-prompt.test.ts
index cf5a44ae5..8ff510676 100644
--- a/test/create-prompt.test.ts
+++ b/test/create-prompt.test.ts
@@ -6,8 +6,10 @@ import {
   getEventTypeAndContext,
   buildAllowedToolsString,
   buildDisallowedToolsString,
+  prepareContext,
 } from "../src/create-prompt";
 import type { PreparedContext } from "../src/create-prompt";
+import { createMockContext } from "./mockContext";
 
 beforeAll(() => {
   process.env.GITHUB_ACTION_PATH = "/test/action/path";
@@ -1270,3 +1272,83 @@ describe("buildDisallowedToolsString", () => {
     expect(result).toBe("BadTool1,BadTool2");
   });
 });
+
+describe("prepareContext validation errors", () => {
+  const commentId = "12345";
+
+  test("throws on an unsupported event type", () => {
+    const context = createMockContext({
+      eventName: "deployment_status" as any,
+    });
+
+    expect(() => prepareContext(context, commentId)).toThrow(
+      "Unsupported event type: deployment_status",
+    );
+  });
+
+  test("pull_request event requires a PR number (isPR must be true)", () => {
+    const context = createMockContext({
+      eventName: "pull_request",
+      eventAction: "opened",
+      isPR: false,
+    });
+
+    expect(() => prepareContext(context, commentId)).toThrow(
+      "PR_NUMBER is required for pull_request event",
+    );
+  });
+
+  test("pull_request_review event requires a PR number", () => {
+    const context = createMockContext({
+      eventName: "pull_request_review",
+      isPR: false,
+      payload: {
+        review: { body: "please fix", user: { login: "user1" } },
+      } as any,
+    });
+
+    expect(() => prepareContext(context, commentId)).toThrow(
+      "PR_NUMBER is required for pull_request_review event",
+    );
+  });
+
+  test("issues event requires an event action", () => {
+    const context = createMockContext({
+      eventName: "issues",
+      eventAction: "",
+      isPR: false,
+      payload: { issue: { user: { login: "user1" } } } as any,
+    });
+
+    expect(() => prepareContext(context, commentId)).toThrow(
+      "GITHUB_EVENT_ACTION is required for issues event",
+    );
+  });
+
+  test("issues event rejects an unsupported action", () => {
+    const context = createMockContext({
+      eventName: "issues",
+      eventAction: "deleted",
+      isPR: false,
+      payload: { issue: { user: { login: "user1" } } } as any,
+    });
+
+    expect(() =>
+      prepareContext(context, commentId, "main", "claude/issue-1"),
+    ).toThrow("Unsupported issue action: deleted");
+  });
+
+  test("issue_comment on an issue requires a claude branch", () => {
+    const context = createMockContext({
+      eventName: "issue_comment",
+      isPR: false,
+      payload: {
+        comment: { id: 999, body: "@claude help", user: { login: "user1" } },
+      } as any,
+    });
+
+    expect(() => prepareContext(context, commentId)).toThrow(
+      "CLAUDE_BRANCH is required for issue_comment event",
+    );
+  });
+});