REST API & Webhook Errors

Understanding API errors

API integration errors are usually a contract mismatch: the client and the server disagree about authentication, encoding, size limits, or which version of the API is in play. Because most API clients swallow the response body and surface only the status code, the single highest-value debugging habit here is to log the full response, headers included, before you touch any code.

How to debug API errors

1. Reproduce the failing call with curl -v outside your application. If curl succeeds and your client fails, the bug is in your client's header or serialisation layer, not on the server.
2. Print the exact Authorization header being sent (redact the secret, keep the prefix). A missing Bearer  prefix and a trailing newline from an environment file are the two most common causes of a mystery 401.
3. Check for an intermediate proxy. A 413 or 502 from nginx, Cloudflare or an API gateway looks identical to one from your application, but is fixed in completely different config.
4. For webhooks, verify the signature against the raw request body. Any middleware that parses and re-serialises JSON before your handler runs will silently break HMAC verification.
5. Confirm the API version. Deprecation errors and unexpected schema changes almost always trace back to a pinned version header that was never updated, or one that was never pinned at all.

Tools worth reaching for

• curl -v
• httpie
• Postman/Bruno
• mitmproxy
• ngrok (for webhooks)

All 14 API errors

• API Gateway 413: Payload too largeRequest body exceeds API Gateway/Lambda payload limits (10MB REST, 6MB HTTP API by default).
• API: Bearer token missing or malformedAPI requires Bearer token in Authorisation header but token is missing or invalid format.
• API: CORS preflight request failedBrowser sent OPTIONS preflight but server returned error. Need to handle OPTIONS requests.
• API: Endpoint deprecatedAPI endpoint is deprecated and will be removed. Need to migrate to new version.
• API: HTTP method override not workingSome proxies/firewalls block PUT/PATCH/DELETE. Need to use X-HTTP-Method-Override header.
• API: Idempotency key conflictSame idempotency key used with different request parameters. Indicates duplicate or conflicting request.
• API: JSON schema validation failedRequest body doesn't match expected JSON schema. Missing required fields or wrong types.
• API: Rate limit exceeded (per minute)Too many API requests in the last minute. Need to implement rate limiting or backoff.
• API: Request payload too largeAPI request body exceeds maximum allowed size. Need to reduce payload or use chunking.
• API: Webhook signature verification failedWebhook payload signature doesn't match computed signature. Possible tampering or wrong secret.
• GitHub API: You have exceeded a secondary rate limit NewSeparate from the hourly primary quota, GitHub applies secondary limits on burst concurrency, content-creating…
• GraphQL: Query complexity exceededGraphQL query too complex and exceeds server limits. Reduce nesting or field count.
• Stripe: card_declinedThe issuing bank refused the charge. The decline_code provides the actionable reason, such as insufficient_funds…
• Stripe: Webhook signature verification failedThe Stripe-Signature header does not match the raw request body and endpoint secret. Usually JSON middleware modified…

Other categories

• AI 35Rate limits, context windows, GPU memory and model-serving failures.
• Ansible 10Unreachable hosts, become passwords, undefined variables and Jinja2 failures.
• Apple 10Command line tools, dyld, Homebrew permissions, notarisation and Keychain.
• Auth 11OIDC, SAML, Auth0, Okta, Keycloak, passkeys and MFA failures.
• BigData 11Spark, Kafka, Airflow, Snowflake, Flink and Databricks failures.
• C# 12NuGet restore, null references, EF Core migrations, async deadlocks and Blazor…
• C++ 11Segfaults, linker errors, memory corruption and template deduction failures.
• Caching 10Cache stampedes, stale content, Varnish and CloudFront failures.
• CI/CD 18GitHub Actions, GitLab CI, Jenkins, CircleCI: permissions, runners and…
• Client 21CORS, mixed content, module resolution, memory limits and framework runtime…
• Cloud 25IAM permissions, quotas, service limits and credential failures.
• Dart 10Null safety, pub version solving and build toolchain problems.
• Database 41Connections, deadlocks, constraints, replication and memory limits.
• DNS 10NXDOMAIN, SERVFAIL, timeouts, propagation and delegation problems.
• Docker 27Daemon connectivity, disk space, image pulls, ports and architecture mismatches.
• Elixir 9GenServer timeouts, supervision failures and Mix compilation problems.
• Email 8Delivery failures, relay denial, authentication, SPF, DKIM and DMARC.
• Frontend 23Hydration mismatches, bundler resolution, layout shift and font loading.
• Git 20Merge conflicts, rejected pushes, detached HEAD, LFS and repository corruption.
• Go 19Nil map assignment, concurrent map access, context cancellation and deadlocks.
• GraphQL 13Validation, depth limits, N+1 queries and fragment problems.
• gRPC 10Status codes, deadlines, message limits, TLS and HTTP/2 transport failures.
• HTTP 494xx client errors, 5xx server errors, redirects, headers and protocol problems.
• ICMP 23Destination unreachable, time exceeded, fragmentation needed and redirects.
• Ingress 8404 default backend, missing TLS secrets, IngressClass and path matching.
• Java 19Class loading, dependency resolution, connection pools and JVM version…
• JavaScript 42npm resolution, async pitfalls, hydration, memory limits and runtime type…
• Kubernetes 34CrashLoopBackOff, ImagePullBackOff, OOMKilled, RBAC, scheduling and storage.
• Logging 9Log4j, Logback, Fluentd, Logstash and CloudWatch ingestion problems.
• MessageQueue 14Kafka, RabbitMQ, SQS, NATS and Celery: lag, rebalancing and poison messages.
• Mobile 17Gradle, CocoaPods, Xcode signing, Metro bundler and toolchain mismatches.
• Monitoring 12Prometheus scrapes, Grafana data sources, OpenTelemetry exporters and agent…
• Network 35Refused connections, timeouts, resets, MTU problems and port exhaustion.
• Performance 6GC pauses, thread pool starvation and event loop blocking.
• PHP 11Memory limits, execution timeouts, autoloading, Composer and PDO connections.
• Proxy 17nginx, Envoy, HAProxy, Traefik, Caddy and Cloudflare upstream failures.
• Python 35Imports, virtual environments, encoding, concurrency and dependency conflicts.
• Regex 8Catastrophic backtracking, back references, escaping and engine differences.
• Ruby 11Bundler, migrations, native extensions, encoding and asset compilation.
• Rust 19Borrow checker, ownership moves, trait bounds and lifetime mismatches.
• Scala 6Dependency resolution, binary compatibility and type inference failures.
• Security 25JWT validation, CSRF, OAuth grants, SELinux, SSH host keys and CSP.
• Serverless 11Lambda timeouts, package size limits, VPC networking and cold starts.
• Shell 13Command not found, permissions, quoting, expansion and Makefile syntax.
• Storage 13S3 permissions, NFS mounts, quotas, signed URLs and volume attachment.
• Svelte 10Store subscriptions, load functions and server/client boundaries.
• System 26Disk space, systemd units, file descriptors, OOM killer and scheduled jobs.
• Terraform 18State locks, provider auth, drift, dependency cycles and plan-time unknowns.
• Testing 18Jest, pytest, JUnit, Cypress and Playwright: fixtures, snapshots and timeouts.
• TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.
• TypeScript 19Assignability, missing declarations, strict null checks and generic constraints.
• Virtualization 8VirtualBox, VMware, Hyper-V, WSL, KVM and hypervisor conflicts.
• Web3 7Gas estimation, nonce management and reverted transactions.
• WebAssembly 7Compile errors, memory bounds and host binding mismatches.
• WebServer 10nginx, Apache, IIS and Caddy: binding, permissions, rewrites and TLS.
• Windows 10Installer failures, missing runtimes, update errors and permission problems.