Skip to main content

​
Overview

The Fiddler Claude Code Plugin (fiddler-coding-agent-plugin) instruments Claude Code sessions and delivers per-turn OpenTelemetry traces to Fiddler. It runs as a set of Claude Code hooks (a small binary that fires on session events) and captures each turn as a nested trace with the user prompt, LLM response content, tool input/output, and token usage. Plugin traces group into the same Fiddler session as native Claude Code traces, so the two can be used together.
This capability is currently in private preview and is not yet available to all customers. Contact your Fiddler Customer Success Manager to request access.
The plugin fails open: a hook error, a missing binary, or an unreachable endpoint never breaks or slows your Claude Code session. At worst, a turn’s trace is skipped.

​
Prerequisites

  • Claude Code 2.1.120 or later. Full trace capture depends on hooks and payload fields introduced across the 2.1.x line, the most recent being the duration_ms tool-timing field added in 2.1.119. On older versions the plugin fails open, capturing less rather than breaking your session.
  • gh CLI, authenticated (gh auth login). During private preview the plugin repository is private, so the plugin downloads its binary from a private GitHub Release using your gh authentication. Each machine that runs the plugin needs gh installed and authenticated with access to fiddler-labs/fiddler-coding-agent-plugin before the first session. Request access through your Fiddler account team.
  • A Fiddler GenAI application, with its application ID and an API key.

​
Installation

1

Add the marketplace

2

Install the plugin

3

Configure and activate

Add the configuration values below. The plugin activates on install; if Claude Code prompts you to, run /reload-plugins to apply it without restarting. The plugin downloads its binary on the first session.
The binary is downloaded once per version and cached, so the first session after installing (or upgrading) does the download and later sessions reuse it.

​
Upgrading

New versions are published to the marketplace repository. Third-party marketplaces do not auto-update by default, so upgrades are manual. To move to a newer version, re-run the install command; it refreshes the marketplace and installs the latest version:
1

Update the plugin

Optionally refresh the marketplace first with /plugin marketplace update fiddler-plugins, though the install command already does this.
2

Activate the new version

Run /reload-plugins if Claude Code prompts you; no restart is needed. On the next session the plugin downloads the binary matching the new version (again using your gh authentication) and caches it.
Check the installed version any time with /plugin list.
Because the versioned binary is fetched from a private GitHub Release, gh must still be authenticated with repository access when you upgrade, the same requirement as the first install.

​
Configuration

Set three values under "env" in your Claude Code settings, either ~/.claude/settings.json (user-level) or .claude/settings.local.json (project-level):
Configuration values are read when a session starts, so start a new Claude Code session after changing any of them. All three values must be set for the plugin to send traces.
Do not commit your API key. For project-level configuration use .claude/settings.local.json (git-ignored by default) rather than a committed settings.json.

​
Optional Settings

These are read from the process environment (not from "env" in settings):

​
What You Get

Each user turn produces one trace in Fiddler, nested under a root agent span, with:
  • The user prompt and the assistant’s response for the turn.
  • One LLM span per model call, with token usage (including cache tokens), model, and finish reason.
  • One tool span per tool call, with the tool name, arguments, and result, plus the permission decision for the call.
  • Error, refusal, and interrupt outcomes marked on the relevant spans.
Because the plugin stamps the Claude Code session ID on every span, a session’s turns group into a single Fiddler session with no extra configuration. Once traces arrive, you can score them with Fiddler evaluators. Because the plugin captures both input and output content, evaluators that need the response (such as faithfulness) can run on these traces.

​
Data Capture and Privacy

In the current preview build the plugin always captures prompt, response, and tool input/output content. Content-redaction controls are not yet enforced. Deploy the plugin only where capturing this content is acceptable, and scope the Fiddler application and API key accordingly. To limit the developer identity that accompanies traces, set FIDDLER_OMIT_USER_INFO=true.
The plugin makes only outbound OTLP/HTTP calls to your configured Fiddler endpoint. The auth token is delivered through Claude Code’s plugin configuration (marked sensitive), is never logged, and is never written to plaintext project config.

​
Troubleshooting

​
No Plugin Traces Appearing in Fiddler

  1. After installing or upgrading, make sure the plugin is active (run /reload-plugins if Claude Code prompts you), and confirm gh auth status shows an authenticated account with access to fiddler-labs/fiddler-coding-agent-plugin (needed to download the binary).
  2. Confirm all three configuration values are set (CLAUDE_PLUGIN_OPTION_OTLP_URL, CLAUDE_PLUGIN_OPTION_APP_ID, CLAUDE_PLUGIN_OPTION_AUTH_TOKEN). If any is empty, the plugin sends nothing.
  3. Confirm CLAUDE_PLUGIN_OPTION_APP_ID is a valid application ID; Fiddler drops spans without one.
  4. Confirm the auth token is the bare token, with no Bearer prefix.

​
Some Turns Are Missing Token Usage or the Response

Turns that were interrupted, or that were in flight when the session ended, are recovered with the prompt but without token usage or response text, which are available only when a turn completes normally.