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Jul 18, 2025
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Document snippet option #14263

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Document snippet option
trask Jul 17, 2025
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Merge branch 'main' into doc
breedx-splk Jul 17, 2025
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19 changes: 19 additions & 0 deletions docs/advanced-configuration-options.md
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Expand Up @@ -36,3 +36,22 @@ This option can be used to let agent run with all privileges without being affec

[1] Disclaimer: agent can provide application means for escaping security manager sandbox. Do not use
this option if your application relies on security manager to run untrusted code.

## JavaScript snippet injection

This experimental feature allows you to inject JavaScript code into HTML responses from servlet applications. The agent will look for the `</head>` tag in HTML responses, and inject the configured JavaScript snippet before the closing `</head>` tag.

This feature is designed for integrating client-side monitoring.
We plan to integrate OpenTelemetry's own client-side monitoring solution by default once it's available
(see the [browser instrumentation proposal](https://github.com/open-telemetry/community/blob/main/projects/browser-phase-1.md)).

| System property | Environment variable | Purpose |
|-----------------------------------------|-----------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| otel.experimental.javascript-snippet | OTEL_EXPERIMENTAL_JAVASCRIPT_SNIPPET | JavaScript code to inject into HTML responses before the closing `</head>` tag. The value should be a complete JavaScript snippet including `<script>` tags if needed, e.g. `-Dotel.experimental.javascript-snippet="<script>console.log('Hello world!');</script>"` |

**Important notes:**

- This only works with servlet-based applications currently
- The snippet is injected only into HTML responses that contain a `</head>` tag
- The agent will attempt to preserve the original character encoding of the response
- If the response already has a `Content-Length` header, it will be updated to reflect the additional content
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