Invalid JSON: common causes

October 30, 2026 · 12 min read

Invalid JSON blocks deploys, breaks webhooks, and wastes hours when a log line is almost JSON but not quite. RFC 8259 is strict: no comments, no trailing commas, double-quoted keys, and precise escaping. This article lists causes parsers report most often and how to fix them before schema validation.

Missing braces, brackets, or quotes

A missing } or ] makes the parser reach EOF still inside a structure. Copy-paste from Slack often truncates the tail. Paste the blob into the JSON validator - the error position points near the unclosed block.

// Invalid - missing closing brace
{ "ok": true, "items": [1, 2]

// Valid
{ "ok": true, "items": [1, 2] }

Trailing commas and lone commas

JavaScript allows [1,2,]; JSON does not. Generators that emit trailing commas from template loops need a final-element flag. Likewise, double commas ,, or a comma before } fail immediately.

Unescaped characters in strings

Control characters must be escaped as \n, \t, \uXXXX.
Backslashes and quotes inside strings need \\ and \".
Single-quoted strings are invalid - JSON requires double quotes.

BOM, smart quotes, and truncated UTF-8

Word processors insert curly quotes that parsers reject. A UTF-8 BOM at file start can confuse strict parsers. Ensure files are UTF-8 without BOM for APIs; convert legacy exports before ingestion.

A practical fix workflow

Validate syntax first, then schema. Minify only after valid - use JSON formatter to pretty-print once clean. Store golden fixtures in CI so invalid JSON never merges.

FAQ

Why does JSON.parse fail on valid-looking config?: Often trailing commas, comments, or single quotes from JavaScript config styles.
Can JSON have comments?: Standard JSON no. Use JSONC in editors or separate comment fields in config.
Are NaN and Infinity allowed?: No in RFC 8259. Use null or strings if you must represent non-finite values.
Does order of keys matter for validity?: No for parsing; duplicate keys are discouraged and last-wins in many parsers.

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