# Mapping And Variables

Use mapping when the upstream needs transformed headers or query params rather than the original client request. The mapping block lets you select specific values from the request, JWT, or config variables and forward them as upstream headers or query parameters. This page gives an end-to-end overview of the mapping system.

**Last reviewed:** 2026-03-06

## When to use this

Use the mapping and variables reference when you need to transform, inject, or replace values in upstream requests. Mapping lets you forward selected headers, query parameters, JWT claims, and config variables to the upstream without modifying your backend code.

## Key concepts

* The `mapping` block on a path entry defines which headers and query parameters to send to the upstream. Only mapped values are forwarded -- unmapped client headers and query params are dropped.
* Mapping sources include `$request.headers.*`, `$request.query.*`, `$request.jwt.*` (any JWT claim), `$config.*` (global variables), and `$route.*` (route-level variables).
* Global `variables` are defined at the top level of the config and available to all routes. Route-level `variables` are defined inside a path entry and override global variables with the same name.
* `$env.*` and `$secrets.*` placeholders are replaced at config load time, not at request time. This means environment variables and secrets are baked into the parsed config once at startup.
* If a mapping source resolves to `null` or `undefined`, the gateway sends an empty string for that header or query parameter. Use the debugging guide to trace missing values.

## Repo-grounded example

```json
{
  "variables": { "region": "eu-west-1" },
  "paths": [
    {
      "method": "GET",
      "path": "/proxy/{.+}",
      "auth": true,
      "integration": { "type": "http_proxy", "server": "upstream" },
      "mapping": {
        "headers": {
          "x-user-id": "$request.jwt.sub",
          "x-region": "$config.region"
        },
        "query": {
          "source": "$request.query.source"
        }
      },
      "variables": { "api_key": "internal-key" }
    }
  ]
}
```

This snippet defines a `mapping` block with header and query mappings. The `x-user-id` header is set from the JWT `sub` claim, the `x-region` header from a config variable, and the `source` query parameter from the original request query string.

## Troubleshooting

* If a mapped header arrives empty at the upstream, verify the source exists: check that the JWT claim is present in the token, the request header was sent by the client, or the variable is defined in config.
* If `$request.jwt.sub` is null on a route without `auth: true`, remember that JWT claims are only available after successful token validation -- add `auth: true` to the route.
* If route variables are not overriding global variables, confirm the variable name matches exactly (case-sensitive) and that the route-level `variables` block is inside the path entry, not at the top level.
* Use a tool like jwt.io to decode your test token and confirm the claim names match what your mapping expects (e.g., `sub` vs `user_id`).

## Related docs

* [README](/configuration/variable-mapping.md)
* [priority variables](/configuration/priority-variables.md)
* [gateway troubleshooting matrix](/troubleshooting/wrangler-deploy-guide/gateway-troubleshooting-matrix.md)


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