You paid for your own API key. You pointed Claude Code at your own provider. You assumed the conversation was between you and your endpoint. It isn’t — not entirely. Here’s what still flies back to Anthropic, captured off the wire, and the one line of config that shuts it up.

Point-in-time snapshot — capture performed 2026-02-04. Claude Code changes fast; the client version behind this capture is a moment in time, and some internals below have already shifted (e.g. the flag backend moved from Statsig to GrowthBook). Treat this as a dated field report, not a description of current behavior. The [DOC] claims are re-verifiable against live docs; the [WIRE] observations reflect what the client did on 2026-02-04.

Ethics & method. Everything here comes from watching my own machine’s HTTPS traffic with mitmproxy — the same lawful, on-your-own-device inspection any developer can run. Nothing was exfiltrated, decrypted from anyone else, or accessed without authorization. This is transparency research, in the same spirit as my RECon 2024 GPT-reversing work. I flag every claim as [DOC] (Anthropic-documented), [WIRE] (observed in my own 2026-02-04 capture), or [FLAG] (community-observed internal detail, not guaranteed).

The uncomfortable one-liner

When you run Claude Code in Bring-Your-Own-Key mode:

export ANTHROPIC_BASE_URL="https://your-provider.example/v1"
export ANTHROPIC_AUTH_TOKEN="your_api_key"

…your prompts and code go to your provider. Good. But by default, Claude Code still opens side-channels to Anthropic-controlled infrastructure and streams operational telemetry, error traces, and feature-flag lookups — while computing an estimated dollar cost for API calls it isn’t even billing you for. [DOC][WIRE]

None of it contains your source code. All of it is enough to fingerprint you, time you, and profile how you work.

And here’s the part most "privacy" write-ups miss: custom-base-URL BYOK is treated like the direct Anthropic API — telemetry defaults ON. Bedrock, Vertex, and Foundry users get it off by default. Roll your own gateway, and you inherit the chattier posture. [DOC]

What I actually saw on the wire

Capture setup: mitmproxy, HTTPS interception, cert pinning disabled, one trivial task — "create a C hello world, build it with CMake, run it." One boring task. Look how much came back.

They know your machine — persistently

Every session carries a stable device fingerprint plus a per-session ID: [WIRE]

// [WIRE] observed shape
{ "device_id": "50xx… (persistent SHA-256)", "session_id": "xxxxxxxx-xxxx-…" }

The session_id rotates. The device_id does not. It’s a durable hash that ties every session you ever run — across projects, across providers, across months — back to the same box. On your own key, with your own provider, you are still a stable, trackable identity to Anthropic. That’s the finding.

They know how you work (not what you write)

The telemetry is metadata-dense and content-free — sizes, types, durations, verdicts. A representative slice of what streams out during a session: [WIRE]

• Every tool call: name, duration, success/failure, result size — Bash, file writes, todo updates, the lot.
• Bash commands, classified: not the string, but the type — mkdir, cmake, git, cd — plus exit code and output byte-length.
• File operations: create/write/delete, hashed path, lines added/removed, char count.
• Token + cost: input/output/cache tokens and a computed cost_usd — estimated on their side even though your provider is doing the billing. [DOC][WIRE]
• Timing: time-to-first-token and total duration per call — a latency fingerprint of your provider.
• BYOK tell: your base_url and model selection can ride along in request telemetry — i.e. Anthropic can see which alternative provider you defected to. [WIRE]

Illustrative shape (structure, not a guaranteed schema):

// ILLUSTRATIVE reconstruction of an observed event
{
  "event": "tengu_tool_use",
  "properties": { "tool_name": "Bash", "duration_ms": 2531, "success": true, "result_length_chars": 45 }
}

Stitch these together and you don’t get my code — you get a behavioral cast of my session: how many tools, how fast, how often I failed, how big my outputs were, which provider I used, and what it cost. Anonymous it is not, given that persistent device_id.

The full event zoo

A single session lit up dozens of distinct tengu_* event types [WIRE][FLAG] — session lifecycle (tengu_session_started / _ended, tengu_request_initialized), API (tengu_api_request, tengu_api_usage, streaming events), tools (tengu_tool_use, tool-result and rate-limit events), bash (tengu_bash_tool_command_executed, tengu_bash_tool_timeout), files (tengu_file_tool_applied, read events), conversation, errors and retries.

"Tengu" — the client’s own nervous system

[FLAG] tengu_ is Claude Code’s internal codename, wired into its runtime feature-flag system (originally Statsig; migrated to GrowthBook after Statsig’s Sept-2025 acquisition). None of it is officially documented; all of it is visible if you watch the client evaluate flags on startup. Community-observed gate names are the spicy part — they read like a changelog of things the client is quietly deciding about you:

• tengu_attribution_header — request attribution tagging
• tengu_anti_distill_fake_tool_injection — a countermeasure whose very name suggests anti-distillation / anti-scraping defenses baked into the client
• tengu_penguins_off — an internal kill-switch of some flavor

You don’t have to speculate about intent to find it notable that your local CLI ships remote-controlled behavior flags with names like "anti-distill fake tool injection." These names are community-observed rather than documented, and they churn between releases — hence the [FLAG] marker on this section.

The redemption arc: it’s all opt-out

Here’s what keeps this fair — and, honestly, more useful than pure outrage. None of this is mandatory. Anthropic documents the switches, and a single one silences telemetry, error reporting, auto-update, feature-flag lookups, and model discovery in one shot: [DOC]

export CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1   # the wire goes quiet

Set it and the Datadog / flag / error chatter drops away, leaving only your provider on the wire. The full, tiered hardening config — plus the two settings people think belong here but don’t — is in Lock it down at the end.

What’s private vs. what leaks (default BYOK)

Stays with you / your provider only [DOC][WIRE]

• Prompts and model responses · file contents and diffs · full command strings (only a coarse type leaks) · real file paths (only hashes leak)

Anthropic’s own docs back the content-exclusion, for the record: [DOC]

"Metrics never include your code, prompts, or file paths."

Leaves to Anthropic-side infra by default [WIRE]

• Persistent device_id + session IDs · tool-usage metadata · bash command types + exit codes · token counts and estimated cost · TTFT/total timing · your provider base URL + model · redacted error traces

Bottom line

1. BYOK protects your content, not your metadata. Your code is safe; your behavior and identity are not, by default.
2. The provider leak is the sharpest finding for BYOK users — Anthropic can see which alternative endpoint you’re routing to.
3. You are persistently fingerprinted via a non-rotating device_id, even on your own key.
4. It’s all opt-out — one documented env var silences it. Any critique that omits this is doing outrage, not research.
5. Trust your capture, not the codenames. The tengu_* internals are undocumented and mutate between releases; publish what your own wire shows.

Lock it down: the hardened BYOK config

Everything above is opt-out. Here’s the config to run. Two equivalent forms — the env block in ~/.claude/settings.json, or plain shell exports (settings.json simply injects these as environment variables). [DOC]

The 80/20, if you read nothing else:

export CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1   # the master kill-switch

That one flag already covers telemetry, error reporting, auto-update, feature flags, and model discovery. Everything below is belt-and-suspenders — explicit, self-documenting, and resilient if the master flag’s coverage ever changes.

Tier 1 — Privacy: stop the phone-home

Tier 2 — Quality of life (noise, not telemetry)

These quiet the UI; they don’t change what leaves your machine. Bundled in for a clean, distraction-free setup — just don’t mistake them for privacy controls.

Drop-in ~/.claude/settings.json

{
  "env": {
    "CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC": "1",
    "DISABLE_TELEMETRY": "1",
    "DISABLE_ERROR_REPORTING": "1",
    "DISABLE_AUTOUPDATER": "1",
    "DISABLE_BUG_COMMAND": "1",
    "DISABLE_NON_ESSENTIAL_MODEL_CALLS": "1",

    "DISABLE_COST_WARNINGS": "1",
    "DISABLE_PROMOTIONAL_MESSAGES": "1",
    "CLAUDE_CODE_DISABLE_TERMINAL_TITLE": "1"
  }
}

Same thing as shell exports (no file)

bash / zsh:

# Tier 1 — privacy
export CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1
export DISABLE_TELEMETRY=1
export DISABLE_ERROR_REPORTING=1
export DISABLE_AUTOUPDATER=1
export DISABLE_BUG_COMMAND=1
export DISABLE_NON_ESSENTIAL_MODEL_CALLS=1
# Tier 2 — quality of life
export DISABLE_COST_WARNINGS=1
export DISABLE_PROMOTIONAL_MESSAGES=1
export CLAUDE_CODE_DISABLE_TERMINAL_TITLE=1

PowerShell:

$env:CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC = "1"
$env:DISABLE_TELEMETRY                        = "1"
$env:DISABLE_ERROR_REPORTING                  = "1"
$env:DISABLE_AUTOUPDATER                      = "1"
$env:DISABLE_BUG_COMMAND                      = "1"
$env:DISABLE_NON_ESSENTIAL_MODEL_CALLS        = "1"
$env:DISABLE_COST_WARNINGS                    = "1"
$env:DISABLE_PROMOTIONAL_MESSAGES             = "1"
$env:CLAUDE_CODE_DISABLE_TERMINAL_TITLE       = "1"

Toggle-on-presence gotcha. For these flags, any non-empty value — including "0" — turns the disable ON. To re-enable a feature, remove the variable entirely; setting it to "0" will not do what you’d expect.

Tier 3 — Handle with care (you’ll see these in the wild)

Two settings get copy-pasted into "privacy" configs where they don’t belong. Listed here explicitly so you know why they’re not in the block above — not dropped, just quarantined:

• CLAUDE_CODE_ENABLE_GATEWAY_MODEL_DISCOVERY=1 — this is an enable, and it adds traffic: the client queries your gateway to discover available models. It directly opposes CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC, which switches discovery off. Only set it if your BYOK gateway genuinely needs runtime model discovery to function — otherwise leave it out for a quieter wire.
• DISABLE_PROMPT_CACHING=1 — not a telemetry or privacy control. It turns off prompt caching, which just makes sessions slower and pricier; in BYOK the caching happens at your provider anyway, so it buys you zero privacy from Anthropic. Set it only for a specific reason (e.g. a provider that mishandles cache headers).

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Sources for the [DOC] claims: Data usage · Environment variables · Settings · Network configuration · Security.

Lawful, on-device transparency research — a dated snapshot from 2026-02-04. Some internals will have changed since; re-verify against live docs before relying on the specifics.

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Originally published in TheBigPromptLibrary on GitHub (view source).
⭐ Star the library on GitHub for more system prompts and research.