fix(tools): handle null values in IssueTagValuesSchema (#708)

## Summary

Fixes ZodError validation failures in the `get_issue_tag_values` tool
when Sentry API returns null values for `name` and `value` fields in the
`topValues` array.

**Issue:** https://sentry.sentry.io/issues/7181206453/

### Key Changes

- Made `name`, `value`, and `key` fields nullable in
`IssueTagValueSchema` to match actual API responses
- Added null fallback in handler to display `"(null)"` for null values
- Added test case to verify null value handling

### Root Cause

The Sentry API can return `null` for tag value entries (observed in
production with 19 occurrences affecting 14 users). The Zod schema
expected strings, causing validation failures.

Fixes MCP-SERVER-EZR

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: dcramer

packages/mcp-core/src/api-client/schema.ts

@@ -672,9 +672,9 @@ export const EventAttachmentListSchema = z.array(EventAttachmentSchema);
  * Represents a single value's occurrence count and percentage within a tag.
  */
 export const IssueTagValueSchema = z.object({
-  key: z.string().optional(),
-  name: z.string().optional(),
-  value: z.string(),
+  key: z.string().nullable().optional(),
+  name: z.string().nullable().optional(),
+  value: z.string().nullable(),
   count: z.number(),
   lastSeen: z.string().datetime().nullable().optional(),
   firstSeen: z.string().datetime().nullable().optional(),

packages/mcp-core/src/tools/get-issue-tag-values.test.ts

@@ -1,4 +1,6 @@
 import { describe, it, expect } from "vitest";
+import { http, HttpResponse } from "msw";
+import { mswServer } from "@sentry/mcp-server-mocks";
 import getIssueTagValues from "./get-issue-tag-values.js";
 import { getServerContext } from "../test-setup.js";
 import { UserInputError } from "../errors.js";
@@ -132,4 +134,55 @@ describe("get_issue_tag_values", () => {
       ),
     ).rejects.toThrow();
   });
+
+  it("handles null values in topValues gracefully", async () => {
+    // Override the handler to return null values (which can occur with certain tag types)
+    mswServer.use(
+      http.get(
+        "https://sentry.io/api/0/organizations/:org/issues/:issueId/tags/:tagKey/",
+        () => {
+          return HttpResponse.json({
+            key: "custom_tag",
+            name: "Custom Tag",
+            totalValues: 2,
+            topValues: [
+              {
+                key: "custom_tag",
+                name: "valid_value",
+                value: "valid_value",
+                count: 10,
+                lastSeen: "2024-01-15T10:30:00.000Z",
+                firstSeen: "2024-01-10T08:00:00.000Z",
+              },
+              {
+                key: "custom_tag",
+                name: null,
+                value: null,
+                count: 5,
+                lastSeen: "2024-01-14T09:00:00.000Z",
+                firstSeen: "2024-01-12T14:00:00.000Z",
+              },
+            ],
+          });
+        },
+      ),
+    );
+
+    const result = await getIssueTagValues.handler(
+      {
+        organizationSlug: "sentry-mcp-evals",
+        issueId: "CLOUDFLARE-MCP-41",
+        tagKey: "custom_tag",
+        regionUrl: null,
+        issueUrl: undefined,
+      },
+      getServerContext(),
+    );
+
+    // Verify the output handles null values by displaying "(null)"
+    expect(result).toContain("# Tag Distribution: Custom Tag");
+    expect(result).toContain("`valid_value`");
+    expect(result).toContain("`(null)`");
+    expect(result).toContain("| 5 |"); // The null value entry should have count 5
+  });
 });

packages/mcp-core/src/tools/get-issue-tag-values.ts

@@ -154,11 +154,11 @@ export default defineTool({
       const lastSeen = value.lastSeen
         ? new Date(value.lastSeen).toISOString().split("T")[0]
         : "-";
+      // Handle null values (can occur with certain tag types)
+      const rawValue = value.value ?? "(null)";
       // Truncate long values for readability
       let displayValue =
-        value.value.length > 60
-          ? `${value.value.substring(0, 57)}...`
-          : value.value;
+        rawValue.length > 60 ? `${rawValue.substring(0, 57)}...` : rawValue;
       // Escape markdown table special characters (backslashes first)
       displayValue = displayValue
         .replace(/\\/g, "\\\\")