fix(mattermost): address Greptile review - isRetryableError ordering and jitter

- Fix isRetryableError: check 5xx BEFORE 4xx to prevent misclassification
  when 5xx error detail contains 4xx substring (e.g., '503: upstream 404')
- Fix jitter: use proportional jitter (full-jitter pattern) instead of
  hardcoded 1000ms. Jitter is now 0-100% of exponential delay
- Update tests to reflect new jitter behavior
- Add test for 5xx with 4xx substring in error message

extensions/mattermost/src/mattermost/client.retry.test.ts

@@ -239,12 +239,12 @@ describe("createMattermostDirectChannelWithRetry", () => {
     });
 
     expect(delays).toHaveLength(2);
-    // First retry should be around initialDelayMs (100ms) + jitter
+    // First retry: exponentialDelay = 100ms, jitter = 0-100ms, total = 100-200ms
     expect(delays[0]).toBeGreaterThanOrEqual(100);
-    expect(delays[0]).toBeLessThanOrEqual(1000);
-    // Second retry should be around initialDelayMs * 2 (200ms) + jitter
+    expect(delays[0]).toBeLessThanOrEqual(200);
+    // Second retry: exponentialDelay = 200ms, jitter = 0-200ms, total = 200-400ms
     expect(delays[1]).toBeGreaterThanOrEqual(200);
-    expect(delays[1]).toBeLessThanOrEqual(1000);
+    expect(delays[1]).toBeLessThanOrEqual(400);
   });
 
   it("respects maxDelayMs cap", async () => {
@@ -344,4 +344,27 @@ describe("createMattermostDirectChannelWithRetry", () => {
     expect(capturedSignal).toBeDefined();
     expect(capturedSignal).toBeInstanceOf(AbortSignal);
   });
+
+  it("retries on 5xx even if error message contains 4xx substring", async () => {
+    // This tests the fix for the ordering bug: 503 with "upstream 404" should be retried
+    mockFetch
+      .mockRejectedValueOnce(new Error("Mattermost API 503: upstream returned 404 Not Found"))
+      .mockResolvedValueOnce({
+        ok: true,
+        status: 201,
+        headers: new Headers({ "content-type": "application/json" }),
+        json: async () => ({ id: "dm-channel-5xx-with-404" }),
+      } as Response);
+
+    const client = createMockClient();
+
+    const result = await createMattermostDirectChannelWithRetry(client, ["user-1", "user-2"], {
+      maxRetries: 3,
+      initialDelayMs: 10,
+    });
+
+    // Should retry and succeed on second attempt
+    expect(mockFetch).toHaveBeenCalledTimes(2);
+    expect(result.id).toBe("dm-channel-5xx-with-404");
+  });
 });

extensions/mattermost/src/mattermost/client.ts

@@ -235,11 +235,12 @@ export async function createMattermostDirectChannelWithRetry(
         throw lastError;
       }
 
-      // Calculate exponential backoff delay with jitter
-      const delayMs = Math.min(
-        initialDelayMs * Math.pow(2, attempt) + Math.random() * 1000,
-        maxDelayMs,
-      );
+      // Calculate exponential backoff delay with full-jitter
+      // Jitter is proportional to the exponential delay, not a fixed 1000ms
+      // This ensures backoff behaves correctly for small delay configurations
+      const exponentialDelay = initialDelayMs * Math.pow(2, attempt);
+      const jitter = Math.random() * exponentialDelay;
+      const delayMs = Math.min(exponentialDelay + jitter, maxDelayMs);
 
       if (onRetry) {
         onRetry(attempt + 1, delayMs, lastError);
@@ -261,6 +262,13 @@ function isRetryableError(error: Error): boolean {
     return true;
   }
 
+  // Retry on 5xx server errors FIRST (before checking 4xx)
+  // This prevents misclassification when a 5xx error detail contains a 4xx substring
+  // e.g., "503 Service Unavailable: upstream returned 404"
+  if (/\b5\d{2}\b/.test(message)) {
+    return true;
+  }
+
   // Check for explicit 4xx status codes - these are client errors and should NOT be retried
   // (except 429 which is handled above)
   const clientErrorMatch = message.match(/\b4\d{2}\b/);
@@ -271,11 +279,6 @@ function isRetryableError(error: Error): boolean {
     }
   }
 
-  // Retry on 5xx server errors
-  if (/\b5\d{2}\b/.test(message)) {
-    return true;
-  }
-
   // Retry on network/transient errors only if no explicit HTTP status code is present
   // This avoids false positives like "400 Bad Request: connection timed out"
   const hasExplicitStatusCode = /\b\d{3}\b/.test(message);