> For the complete documentation index, see [llms.txt](https://docs.serverlessapigateway.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.serverlessapigateway.com/reference/mapping-overview/mapping-debugging-null-values.md).

# Mapping: Debugging Null Values

Diagnose missing mapped values by checking token claims, request headers, and variable scope order. When a mapped value arrives as null or empty at the upstream, the issue is usually a missing source: the JWT claim does not exist, the request header was not sent, or the variable name has a typo. This page walks through the debugging steps.

**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 maps values from JWT claims, config variables, and request query parameters. If any source is missing at request time, the gateway forwards an empty string. Check the JWT payload, the `variables` block, and the actual client request to find the missing source.

## 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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