Rewrite

A Rewrite rule is a named list of edits applied to every request or response that matches a pattern. A breakpoint is for editing one message by hand. Rewrite is for an edit you want made every time, without you.

What one rule can do

Each edit picks a phase (request or response), a target, and an operation:

• Header: set, modify only if present, remove, or find and replace inside the value
• Body: replace the whole body, empty it, or find and replace inside it
• Status code: set any status on the response
• Query parameter: set, modify, remove, or find and replace one parameter on the request
• URL: set the path, or find and replace inside the path and query

Find and replace works on plain text or as a regular expression, with $1, $2 capture groups in the replacement. Edits run in the order you list them.

What it’s for

• Add an Authorization header to every request going to one host
• Swap an API key in a query string without touching the app’s config
• Turn every 200 from one endpoint into a 503 to test the retry path
• Rename a JSON field in a response to test the app against a proposed schema change

Details that matter

HTTPGlass asks the server for an uncompressed response when a rule needs to edit the response body, so a gzipped body doesn’t quietly defeat the rule. Hosts with a Rewrite rule use HTTP/1.1 so the rule can apply. Rewrite runs before scripts, so a script can always override what a rule did.

Rules of the road

A few behaviours worth knowing before you write your first rule:

• Set creates the value if it isn’t there. Modify if present only changes an existing one.
• Header names are matched case-insensitively, and setting a header that appears twice collapses it to one.
• A status code must be between 100 and 599.
• Header, status, query, and URL edits only need the message head, so responses keep streaming while they’re applied. A body edit needs the whole body first.
• When a body is replaced, a now-wrong Content-Encoding header is removed with it.
• When several rules match one message, they run in list order.

Adding a rule

Select Rewrite in the sidebar. A rule has a name, a pattern using the shared rule syntax, an optional method filter, and one or more actions. Header and query-parameter targets also take a name, and find and replace takes a match string.

Free and Pro

Rewrite is a Pro feature. If a Pro licence ends, your rules stay saved but stop applying, and they come back when you activate a licence again. Breakpoints stay available on Free for one-off edits.

Related

• Breakpoints let you edit a single message by hand
• Scripting covers edits that need logic instead of a fixed rule
• Rewrite reference lists every target and operation