replace-fetch-request

The replace-fetch-request snippet replaces the request body of fetch() calls before they are sent, if the body matches a given string. It is the fetch counterpart of replace-xhr-request.

Where replace-fetch-response rewrites what the server sends back, this snippet rewrites what the page sends out — useful when a site reports adblock state, consent flags or ad-slot requests to its backend and changes its behaviour based on the reply.

fetch is wrapped once, on the first filter. Further filters append rules, and every rule is applied in turn to the progressively modified body.

Parameters

Name	Description	Mandatory	Default
search	String or regex pattern to match in the request body. The pattern is globalised, so every occurrence is replaced. If the value starts with `jsonpath(`, it is treated as a JSONPath query instead — see below.	Yes	n.a.
replacement	The string to replace the matched pattern with. In JSONPath mode the value is parsed as JSON, so `false` becomes the boolean, `null` becomes null and `[]` becomes an empty array; if it is not valid JSON the raw string is used.	No	empty string
needle	Regex used to pre-filter on the body content. If specified, the replacement is only applied when the needle matches the request body.	No	null
mode	JSONPath operation mode. `replace` sets the matched property to the parsed replacement value. `append` adds to the existing value instead: it concatenates strings, pushes into or concatenates arrays, and merges objects. Ignored outside JSONPath mode.	No	`replace`

JSONPath mode

When search starts with jsonpath(, the body is parsed as JSON, the query is evaluated against it, every matching property is replaced or appended to, and the result is re-serialised. Use this when the value to change is a structured field rather than a text pattern.

If the body is not valid JSON, or the query fails to evaluate, the rule is skipped and the body is left unchanged rather than corrupted.

For the supported query syntax, see JSONPath evaluator.
Quotes inside a filter expression must be written \', or the parser strips them. See Snippets Overview for the full escaping rules.

Filter examples

Filter	Result
`replace-fetch-request '"adblock":true' '"adblock":false'`	Replaces every occurrence of `"adblock":true` with `"adblock":false` in the body of every outgoing fetch request.
`replace-fetch-request '/"adblock":\\s*true/' '"adblock":false'`	Same, tolerating whitespace after the colon. The backslash is doubled so it survives the filter parser.
`replace-fetch-request /trackingId/ blocked`	Replaces every occurrence of `trackingId` with `blocked`.
`replace-fetch-request /trackingId/ blocked analytics`	Same, but only for requests whose body also contains `analytics`.
`replace-fetch-request 'jsonpath($.ads.enabled)' false`	JSONPath mode. Sets `ads.enabled` to the boolean `false`.
`replace-fetch-request 'jsonpath($..trackingId)' null`	Sets every `trackingId` property at any depth to `null`.
`replace-fetch-request 'jsonpath($.tags)' '"blocked"' '' append`	Append mode: pushes the string `"blocked"` into the existing `tags` array instead of replacing it.

Debugging

Message	When	Definition
`fetch proxied`	After wrapping `fetch`	Outgoing fetch request bodies are now intercepted. Logged once per page, for the first filter.
`Invalid JSONPath query: <<search>>. Error: <<message>>`	Right after the snippet fires	The query could not be compiled, so the rule is not registered.
`'<<needle>>' found in fetch request body`	A body matched the needle	The rule will now be applied. There is no "not found" counterpart, so if this never appears the needle is not matching.
`'<<search>>' replaced with '<<replacement>>' in fetch request body`	Text mode, and the body actually changed	Only logged when the body genuinely changed, so it will not fire for a rule that matched nothing.
`JSONPath [<<mode>>] at [<<key>>] with <<replacement>>`	JSONPath mode, per matched property	A property matched the query and was replaced or appended to. One message per match.
`append has no meaning for the value at [<<key>>], replaced it instead`	`append` mode hit a value it cannot append to	The existing value was neither a string, array nor object. Usually a sign the query points one level too deep.
`JSONPath: skipping non-JSON body or evaluation error: <<message>>`	The body could not be handled	The body was not valid JSON, or the query threw. The body is left untouched.

A missing or empty search produces none of the messages above: the snippet throws before the rule is registered, so it installs nothing. The throw is still visible as a console error reading [replace-fetch-request]: Missing 'search' parameter.

Only string request bodies are rewritten. FormData, Blob, URLSearchParams and ArrayBuffer bodies pass through untouched, and GET and HEAD requests cannot carry a body at all.