Dynamic Interceptors

APXY provides Dynamic Interceptors — a programmable system for matching traffic and applying transformations using a DSL (Domain-Specific Language). Unlike static mock rules, interceptors support conditional logic and can modify requests and responses on-the-fly.

Paid feature — requires a license with interceptor editing capability.

Web UI

Open Interceptors from the Modify group. The page provides:

A form to create interceptors with Name, Match Expression (DSL syntax), Action dropdown (mock / modify / observe), and optional Transform fields for headers, status, body, and delay
A list of active interceptors with enable/disable toggles
Delete buttons for each interceptor

CLI

Create an interceptor


apxy rules interceptor set --name "debug-staging" \
  --match "host == staging.example.com" \
  --action observe

Flag	Default	Description
`--name`	—	Interceptor name (required)
`--match`	—	Match expression in DSL syntax (required)
`--action`	observe	Action: `mock`, `modify`, `observe`
`--description`	""	Human-readable description

List interceptors


apxy rules interceptor list

Remove an interceptor


apxy rules interceptor remove --id <id>

Match Expression DSL

Match expressions filter traffic based on request/response properties:


host == api.example.com
path contains /api/v2/
method == POST
status >= 400
url matches .*\.json$
header:Content-Type contains json

Combine with && (AND) and || (OR):


host == api.example.com && method == POST
path contains /users || path contains /accounts

Special values: * or true match everything.

Actions

Action	Description
`mock`	Return a fake response without forwarding to the server
`modify`	Forward to the server but alter the request or response
`observe`	Forward normally and log the match (no transformation)

Transform Options

When using mock or modify actions, you can specify transforms:

Field	Description
`set_request_headers`	Overwrite request headers
`set_response_headers`	Overwrite response headers
`set_response_status`	Override response status code
`set_response_body`	Override response body
`add_request_headers`	Add headers without overwriting
`remove_headers`	Header names to remove
`delay_ms`	Delay before responding

Use Cases

Add debug headers to staging requests


apxy rules interceptor set --name "debug-staging" \
  --match "host == staging.example.com" \
  --action modify

Then configure the transform to add X-Debug-Proxy: true to matching requests.

Log all POST requests


apxy rules interceptor set --name "log-posts" \
  --match "method == POST" \
  --action observe \
  --description "Track all form submissions and API writes"

Mock errors for specific conditions


apxy rules interceptor set --name "error-auth" \
  --match "path contains /api/auth && method == POST" \
  --action mock