What causes "%s: CDN upload failed after %d attempts: %w" in chenhg5/cc-connect?

Read the wrapped %w cause after 'attempts:' to see the actual failure (transport vs server error vs missing header)

%s: CDN upload failed after %d attempts: %w

Error message

%s: CDN upload failed after %d attempts: %w

What it means

Final error returned by uploadBufferToCDN after exhausting cdnUploadMaxRetries (3) attempts. It wraps the last underlying failure (lastErr) — a transport error, 5xx server error, or missing x-encrypted-param — so the root cause appears after 'attempts:'.

Source

Thrown at platform/weixin/cdn.go:223

		if resp.StatusCode != http.StatusOK {
			msg := resp.Header.Get("x-error-message")
			if msg == "" {
				msg = fmt.Sprintf("status %d", resp.StatusCode)
			}
			lastErr = fmt.Errorf("%s: CDN upload server error: %s", label, msg)
			slog.Warn("weixin: CDN upload server error", "label", label, "attempt", attempt, "error", lastErr)
			continue
		}
		dl := resp.Header.Get("x-encrypted-param")
		if dl == "" {
			lastErr = fmt.Errorf("%s: CDN response missing x-encrypted-param", label)
			slog.Warn("weixin: CDN upload bad response", "label", label, "attempt", attempt)
			continue
		}
		return dl, nil
	}
	if lastErr != nil {
		return "", fmt.Errorf("%s: CDN upload failed after %d attempts: %w", label, cdnUploadMaxRetries, lastErr)
	}
	return "", fmt.Errorf("%s: CDN upload failed after %d attempts", label, cdnUploadMaxRetries)
}

func md5Hex(b []byte) string {
	h := md5.Sum(b)
	return hex.EncodeToString(h[:])
}

func detectImageMime(b []byte) string {
	if len(b) >= 3 && b[0] == 0xFF && b[1] == 0xD8 && b[2] == 0xFF {
		return "image/jpeg"
	}
	if len(b) >= 8 && string(b[0:8]) == "\x89PNG\r\n\x1a\n" {
		return "image/png"
	}
	if len(b) >= 6 && (string(b[0:6]) == "GIF87a" || string(b[0:6]) == "GIF89a") {
		return "image/gif"

View on GitHub (pinned to 4000b2338a)

Solutions

Read the wrapped %w cause after 'attempts:' to see the actual failure (transport vs server error vs missing header)
If it's a network error, check connectivity/DNS/proxy to the CDN host and retry later
If it's 'server error', wait for the CDN side to recover or add caller-level backoff with longer delays
If it's 'missing x-encrypted-param' on every attempt, verify cdn_base configuration and API-version compatibility

Example fix

// before: no caller backoff
err := uploadToWeixinCDN(ctx, data, key)
// after: exponential backoff around the 3 built-in attempts
var err error
for d := time.Second; d <= 8*time.Second; d *= 2 {
    if err = uploadToWeixinCDN(ctx, data, key); err == nil { break }
    time.Sleep(d)
}

Defensive patterns

Strategy: retry

Validate before calling

// pre-flight network sanity check before large uploads
if _, err := net.LookupHost("novac2c.cdn.weixin.qq.com"); err != nil { /* postpone upload */ }

Try / catch

var err error
for d := time.Second; d <= 8*time.Second; d *= 2 {
    if err = uploadToWeixinCDN(ctx, data, key); err == nil { break }
    time.Sleep(d)
}
if err != nil { log.Printf("upload permanently failed: %v", err) }

Prevention

Run uploads on hosts with stable connectivity to the CDN
Parse the wrapped cause after 'attempts:' to route fixes (network vs server vs contract)
Queue and retry media uploads in the background instead of failing user messages synchronously
Cap media size well under 100 MiB to reduce upload failure windows

When it happens

Trigger: All 3 upload attempts failed with retryable errors: client.Do transport failures, HTTP >= 500, or 200 responses missing x-encrypted-param.

Common situations: Sustained CDN outage; network partition between the bot host and novac2c.cdn.weixin.qq.com; persistent response-contract mismatch (every 200 lacks the header).

Understand the failure class

Background: "API request failed": what wrapped HTTP errors from external APIs mean and how to find the real cause — this error's family across 29 libraries.

Related errors

AI-assisted analysis of chenhg5/cc-connect@4000b2338a (2026-09-06). Data as JSON: /api/errors/33e569032039df24. Report an issue: GitHub.