Error map
Unexpected character encountered while parsing value Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0
.NET · layer 4, Application & API
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)
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…
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).
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.
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.
SyntaxError: Unexpected end of input
The parser reached the end of the code (or JSON) while still expecting more — a missing } ) ] or empty JSON.
SyntaxError: Unexpected token < in JSON at position 0
`JSON.parse()` / `response.json()` received something that isn’t JSON — very often an HTML error page.
HttpMessageNotReadableException: JSON parse error / Required request body is missing
Spring couldn’t read the request body into your @RequestBody object.
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.
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
The text passed to json.loads() isn’t JSON — at char 0 it’s usually empty or HTML.
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.
TypeError: Converting circular structure to JSON
JSON.stringify met an object that references itself (directly or through children).
Two steps away (23)
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…
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
A lazy-loaded association was accessed after the Hibernate session/transaction closed.
TypeError: Object of type datetime is not JSON serializable
json.dumps() met a type it doesn’t know how to encode (datetime, Decimal, set, custom objects).
System.Text.Json.JsonException: The JSON value could not be converted to System.Int32. Path: $.id
System.Text.Json couldn’t map a JSON value to the target .NET type — e.g. "42" (string) to int, null to a non-nullable type, or w…
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
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.
HttpRequestException: The SSL connection could not be established, see inner exception. (AuthenticationException: The remote certificate is invalid)
HttpClient couldn’t complete the TLS handshake — untrusted/self-signed certificate, name mismatch, expired certificate, or TLS ve…
HttpRequestException: No connection could be made because the target machine actively refused it (localhost:5001)
Nothing is listening on the host/port the client called — the API isn’t running, uses another port, or the container/network addr…
TaskCanceledException: The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing
The HTTP call took longer than HttpClient.Timeout (100 s default), so .NET cancelled it.
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).
HTTP 404 Not Found
The server can’t find anything at the requested URL.
HTTP 500 Internal Server Error
The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.
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…
UnsatisfiedDependencyException / NoSuchBeanDefinitionException: No qualifying bean of type 'X' available
Spring couldn’t find (or choose) a bean to inject.
The dependencies of some of the beans in the application context form a cycle
Two or more Spring beans depend on each other through constructors, and Spring Boot 2.6+ forbids cycles by default.
Web server failed to start. Port 8080 was already in use.
Spring Boot couldn’t bind its embedded server port because another process uses it.
npm ERR! code EOVERRIDE — Override for x conflicts with direct dependency
An "overrides" entry sets a different version for a package you also depend on directly.
npm ERR! Missing script: "start"
The script name you ran isn’t defined in package.json "scripts".
npm ERR! Invalid Version: (semver parse error)
A package.json or lockfile contains a version string that isn’t valid semver.
Swift DecodingError: keyNotFound / typeMismatch / valueNotFound / dataCorrupted
JSONDecoder couldn’t map the JSON to your Codable type — a key is missing, a value has a different type or is null, or the data i…
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