Error map
SyntaxError: JSON SyntaxError: Unexpected token < in JSON at position 0
JavaScript · layer 1, On your device
Centre: this error. First ring: errors it is linked with (12). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (12)
HTTP 500 Internal Server Error
The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
HTTP 404 Not Found
The server can’t find anything at the requested URL.
HTTP 200 OK (but something is still wrong)
The request succeeded at the HTTP level — yet the page is blank, the data is wrong, or an error is hidden inside the response bod…
TypeError: Failed to fetch / NetworkError when attempting to fetch resource
The browser couldn’t complete the request at the network level — CORS, DNS, offline, blocked mixed content, or the server refused…
HTTP 302 Found (Temporary Redirect)
The resource is temporarily at another URL; clients should keep using the original URL for future requests.
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class X and no properties discovered
Jackson can’t serialize the class — it has no public getters/fields, or it’s a Hibernate proxy, or Java 8 date types without the…
API Error: Malformed JSON in request body (entity.parse.failed / JSON parse error)
The server couldn’t parse the request body as JSON, so it rejected the request before looking at any field.
API/app requests get 403 with cf-mitigated: challenge (Bot Fight Mode / challenge on non-browser clients)
Cloudflare is answering API calls, webhooks or mobile-app requests with a challenge page (HTML 403) because bot protection treats…
System.Text.Json: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0
The response wasn’t JSON — usually an HTML error page, login redirect, or an empty body was passed to the JSON deserializer.
WordPress: Updating failed. The response is not a valid JSON response.
The block editor saved via the REST API but got HTML or an error page back — broken permalinks, a security plugin/WAF, or PHP out…
SyntaxError: Unexpected end of input
The parser reached the end of the code (or JSON) while still expecting more — a missing } ) ] or empty JSON.
Two steps away (36)
HTTP 502 Bad Gateway
A proxy or gateway (Nginx, load balancer, CDN) got an invalid response — or no response — from the upstream server behind it.
HTTP 503 Service Unavailable
The server is temporarily unable to handle the request — overloaded, in maintenance, or has no healthy backends.
PHP Fatal error: Allowed memory size of X bytes exhausted
The script tried to use more memory than PHP’s `memory_limit` allows.
HTTP 401 Unauthorized
The request has no valid authentication credentials — you need to log in or send a valid token.
HTTP 405 Method Not Allowed
The URL exists, but it doesn’t accept the HTTP method you used (e.g. POST to a GET-only route).
HTTP 413 Content Too Large (Request Entity Too Large)
The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.
HTTP 403 Forbidden
The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…
HTTP 410 Gone
The resource used to exist but was intentionally and permanently removed, with no forwarding address.
HTTP 301 Moved Permanently
The resource has a new permanent URL; clients and search engines should use the address in the Location header from now on.
GraphQL: Cannot query field "x" on type "Y"
The query asks for a field that doesn’t exist in the server’s schema (or isn’t available on that type).
HTTP 103 Early Hints
An interim response that sends Link headers early so the browser can preload CSS, fonts or scripts while the server is still buil…
HTTP 303 See Other
A redirect telling the client to fetch the result from another URL with GET — used after a form POST so refreshing doesn’t resubm…
Error: connect ECONNREFUSED 127.0.0.1:5432
Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.
CORS Error: No 'Access-Control-Allow-Origin' header is present
The browser blocked a cross-origin request because the server’s response didn’t include the CORS headers allowing your site’s ori…
SSL: CERTIFICATE_VERIFY_FAILED / unable to get local issuer certificate
The client couldn’t verify the server’s TLS certificate chain, so it refused the HTTPS connection.
HTTP 508 Loop Detected (Resource Limit Is Reached)
Either the server detected an infinite loop (WebDAV) or — much more often — a cPanel/CloudLinux host is saying your account hit i…
HTTP 307 Temporary Redirect
A temporary redirect that keeps the original method and body — a POST stays a POST at the new URL.
HTTP 308 Permanent Redirect
A permanent redirect that, unlike 301, guarantees the method and body are kept.
JsonException: A possible object cycle was detected
System.Text.Json found a reference loop (e.g. Order → Customer → Orders → …) while serialising, usually from EF Core navigation p…
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList` from Object value
Jackson found JSON whose shape doesn’t match the target Java type (object vs array, string vs number, missing constructor).
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "x", not marked as ignorable
The JSON contains a field your Java class doesn’t have, and Jackson is configured to fail on unknown properties.
HTTP 415 Unsupported Media Type
The server refuses the request because the body’s Content-Type isn’t one it accepts.
HTTP 411 Length Required
The server refuses a request with a body but no Content-Length header.
HttpMessageNotReadableException: JSON parse error / Required request body is missing
Spring couldn’t read the request body into your @RequestBody object.
HTTP 418 I'm a teapot
An April Fools’ status from RFC 2324 — some sites and WAFs return it on purpose to tell bots or scrapers to go away.
Nginx 444 No Response (connection closed without response)
An Nginx-specific code: the server deliberately closed the connection without sending anything — a rule is dropping the request.
Cloudflare Error 1005: Access denied — The owner of this website has banned the autonomous system number (ASN) your IP address is in
The site owner blocked your network provider (ASN) — common for VPNs, cloud/data-center IPs and some mobile carriers.
TypeError: Converting circular structure to JSON
JSON.stringify met an object that references itself (directly or through children).
npm ERR! code EJSONPARSE — Failed to parse json / Unexpected token in JSON at position
package.json (or package-lock.json) contains invalid JSON.
json_decode() returns null / JSON_ERROR_SYNTAX: Syntax error
The string passed to json_decode isn’t valid JSON, so PHP returns null.
Warning: Cannot modify header information - headers already sent
You called `header()`, `setcookie()` or `session_start()` after output had already been sent to the browser.
WordPress: There Has Been a Critical Error on This Website
A PHP fatal error (usually from a plugin or theme) stopped WordPress; the real error is in the debug log or recovery-mode email.
WordPress: Are you sure you want to do this? Please try again. (nonce check failed)
A WordPress security nonce didn’t validate — the page was open too long, cached, or a plugin/theme form is broken.
SyntaxError: missing ) after argument list
A function call’s parentheses are unbalanced — often an unescaped quote or a missing comma/plus.
SyntaxError: Identifier 'x' has already been declared
The same let/const/class name was declared twice in the same scope.
SyntaxError: await is only valid in async functions and the top level bodies of modules
await was used inside a function that isn’t async (or in a classic, non-module script).
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