HTML received where JSON was expected
The response body is an HTML page, not JSON. The server returned an error page, a login redirect, a proxy or captive-portal interstitial, or a rate-limit page — and the client parsed it without checking the status code or content type.
The input
<!DOCTYPE html>
<html><body>500</body></html>{"error": "internal"}How to fix it
Check the HTTP status and the Content-Type header before parsing. Log the first 200 characters of the body when parsing fails so the actual page is visible in your logs.
An error mentioning `<` at position 0 essentially always means this. The JSON is not malformed — you did not receive JSON at all.
What each parser reports
Every message below was captured by running this exact input through the parser named. 21 of 21 parsers rejected it.
| Parser | Exact message |
|---|---|
| CPython json.loads CPython 3.12.3 | Expecting value: line 1 column 1 (char 0)what this message means → |
| encoding/json Unmarshal Go go1.22.2 | invalid character '<' looking for beginning of valuewhat this message means → |
| Gson Java 21.0.12 | Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 12 path $what this message means → |
| Gson JsonParser (lenient off) Java 21.0.12 | Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 12 path $what this message means → |
| hjson Node.js 22.22.2 | Found EOF while looking for a key name (check your syntax) at line 2,30 >>><html><body>500</bod ...what this message means → |
| Jackson ObjectMapper Java 21.0.12 | Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')what this message means → |
| JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1) | JSON Parse error: Unrecognized token '<'what this message means → |
| json-bigint Node.js 22.22.2 | Unexpected '<' |
| JSON5 Node.js 22.22.2 | JSON5: invalid character '<' at 1:1what this message means → |
| JSON::PP Perl 5.038002 | malformed JSON string, neither array, object, number, string or atom, at character offset 0 (before "<!DOCTYPE html>\x{a}...")what this message means → |
| json_decode PHP 8.3.6 | Syntax error |
| Newtonsoft.Json JsonConvert Newtonsoft.Json 13.0.3 on .NET 8.0.30 | Unexpected character encountered while parsing value: <. Path '', line 0, position 0.what this message means → |
| Newtonsoft.Json JsonTextReader Newtonsoft.Json 13.0.3 on .NET 8.0.30 | Unexpected character encountered while parsing value: <. Path '', line 0, position 0.what this message means → |
| orjson CPython 3.12.3 | unexpected character, expected a JSON value: line 1 column 1 (char 0)what this message means → |
| Ruby JSON.parse Ruby 3.2.3 | unexpected token at '<!DOCTYPE html>
<html><body>500</body></html>' |
| serde_json::from_str Rust (serde_json 1.x) | expected value at line 1 column 1what this message means → |
| SpiderMonkey JSON.parse SpiderMonkey 115 | JSON.parse: unexpected character at line 1 column 1 of the JSON datawhat this message means → |
| System.Text.Json Deserialize .NET 8.0.30 | '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.what this message means → |
| System.Text.Json JsonDocument .NET 8.0.30 | '<' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.what this message means → |
| ujson CPython 3.12.3 | Expected object or value |
| V8 JSON.parse Node.js 22.22.2 | Unexpected token '<', "<!DOCTYPE "... is not valid JSONwhat this message means → |
What the specification says
The exact sections that govern this error. Descriptions are our paraphrase; follow the links for the normative text.
| Body | Section |
|---|---|
| IETF | A JSON text is a single value with optional surrounding whitespace ( JSON-text = ws value ws). The token set is six structural characters, strings, numbers and three literal names — nothing else. |
| IETF | Registers the application/json media type — the Content-Type a JSON response is supposed to carry. |
| IETF | Defines Content-Type, the header that tells you a response is not JSON before you parse it. |
Other errors in this category
Empty string passed to a JSON parser
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Whitespace-only input
The document contains only spaces, tabs or newlines. Same family as empty input — a file was created but never filled.…
Extra data after a JSON document
Two complete documents concatenated. Usually a file appended to rather than overwritten, or several API responses written to the s…
Trailing characters after a JSON document
Non-whitespace content after the closing brace — a stray word, a shell prompt captured with the output, or a log line appended to …
The literal string 'undefined' as a JSON document
A JavaScript variable holding `undefined` was interpolated into a string and sent as the body. `String(undefined)` is `"undefined"…
Bare unquoted word as a JSON document
A single token with no quotes. Either a plain-text response being parsed as JSON, or a string value that was never quoted.…
Whole JSON document wrapped in quotes
The document is double-encoded — a JSON string containing JSON, or a shell variable that kept its surrounding quotes.…
NDJSON fed to a whole-document JSON parser
Newline-delimited JSON: one complete document per line. Each line is valid on its own, but the file as a whole is not a single JSO…