JSON Mask
Paste a JSON document and a field mask like kind,items(title,stats/length) to get back the same document with everything else pruned away — paths, sub-selections, wildcards and array mapping included. Invert the mask to strip tokens or PII instead. Runs entirely in your browser, no server, no sign-up.
About this tool
A field mask is the little language Google's Partial Response fields= parameter uses:
kind,items(title,stats/length). It says which parts of a JSON document you care about, and
the document comes back with everything else pruned away.
That last part is what makes a mask different from JSONPath, jq or JSONata. Those return a flat list of the nodes that matched. A mask preserves the shape of the document: the same object, at the same nesting, minus the branches you didn't ask for. If you are trimming an API response down to the handful of fields a client actually reads, the shape is exactly what you want to keep.
Mask syntax
| Form | Meaning |
|---|---|
a | keep a and its entire subtree |
a,b,c | keep several siblings |
a/b/c | follow a path — keep c inside b inside a |
a(b,c) | sub-selection — from a, keep only b and c |
* | wildcard — match every key of an object |
\, \/ \( \) \* \\ | escape a literal special character inside a key name |
These nest and combine freely. Arrays are transparent: a selection applied to an array is applied to every element, and the array's length and order survive untouched.
Worked example
Mask kind,items(title,stats/length) over
{
"kind": "list",
"etag": "W/\"9a1\"",
"items": [
{ "title": "First", "author": "Ada", "stats": { "length": 120, "views": 3 } },
{ "title": "Second", "author": "Grace", "stats": { "length": 80, "views": 9 } }
]
}
gives
{
"kind": "list",
"items": [
{ "title": "First", "stats": { "length": 120 } },
{ "title": "Second", "stats": { "length": 80 } }
]
}
etag, author and views are gone; kind and both array elements stay where they were.
Beyond the standard grammar
The four selects add the things a one-shot tool needs that a library doesn't:
- Remove instead of keep. Switch the mode and the mask becomes a delete list — everything
it names is dropped and the rest is returned.
users(token,password)in remove mode is a one-line PII/secret stripper. - Missing fields. By default a field the document doesn't have is silently skipped, which
is what every implementation of this grammar does. You can instead emit it as
nullso a list of records all come out the same shape, or fail loudly with the offending path — the fastest way to catch a typo'd mask that would otherwise return a nearly-empty result. - Emptied objects. When an element matches nothing, masking leaves a
{}husk behind. Keep them (the compatible default) or drop them recursively. - Compact output for pasting into another tool, or pretty 2-space indent for reading.
Everything runs in WebAssembly inside your browser tab. The document you paste is never uploaded, and key order is preserved exactly as you wrote it.
FAQ
How is a field mask different from JSONPath or jq?
JSONPath and jq are query languages: they evaluate an expression and hand back the nodes that matched, usually as a flat list. A field mask is a pruning language: the result is the original document with the unselected branches removed, at the same paths and in the same order. Use a mask when the consumer still expects the document's shape — trimming an API response, for instance — and jq when you want to reshape or compute.
Why did `a` keep everything under `a` instead of just `a` itself?
That is the defined behaviour of the grammar: a bare name selects the field and its whole
subtree. To prune inside it you have to say so with a sub-selection — a(x,y) — or a path
like a/x. So items keeps every field of every element, while items(title) keeps only
each element's title.
Do arrays need an index in the mask?
No. Arrays are transparent to a mask. Write the selection as though the array were a single object and it is applied to every element, preserving the array's order and length. There is no way to select element 3 specifically — that is a query, not a mask, and jq or JSONPath is the right tool for it.
My mask returned `{}` or almost nothing. What went wrong?
Almost always a name that doesn't exist in the document — a typo, or the wrong case. Absent fields are omitted silently by default, so a completely wrong mask looks like a working mask with nothing to show. Set the missing-fields option to "Fail with an error" and the tool names the offending field and the path it was looked for at.
Can I use this to strip secrets or PII out of a payload?
Yes — that is what remove mode is for. Write a mask that names the fields you want gone,
switch the mode to remove, and you get the whole document back minus exactly those fields:
token, users(email,phone), */password. Because the tool runs locally in your browser,
the payload never leaves the machine while you do it.
Is there a size limit?
The document can be up to 5,000,000 bytes and the mask up to 4,000 bytes, nested at most 64 levels deep. Those ceilings exist to keep a pathological input from hanging the tab; ordinary API responses are nowhere near them.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool json-mask '{"kind":"list","etag":"W/\"9a1\"","items":[{"title":"First","author":"Ada","stats":{"length":120,"views":3}},{"title":"Second","author":"Grace","stats":{"length":80,"views":9}}]}' 'mask=kind,items(title,stats/length)'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/json-mask/?json=%7B%22kind%22%3A%22list%22%2C%22etag%22%3A%22W%2F%5C%229a1%5C%22%22%2C%22items%22%3A%5B%7B%22title%22%3A%22First%22%2C%22author%22%3A%22Ada%22%2C%22stats%22%3A%7B%22length%22%3A120%2C%22views%22%3A3%7D%7D%2C%7B%22title%22%3A%22Second%22%2C%22author%22%3A%22Grace%22%2C%22stats%22%3A%7B%22length%22%3A80%2C%22views%22%3A9%7D%7D%5D%7D&mask=kind%2Citems%28title%2Cstats%2Flength%29&mode=keep&format=pretty&on_missing=omit&empty=keepMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.