What is browser-session-defense?

A guide for protecting the boundary between an agent-controlled browser and a person's ordinary browser session. It explains which pages and actions the agent can actually inspect.

Use it when working with Guardian-managed browser tools, deciding between read-only and interactive browser actions, or explaining what cannot be seen in a user's normal browser.

It prevents incorrect assumptions about browser visibility and encourages safer handling of uploads, stored login data, and scripts.

browser-session-defense — Skill by Threat-Vector-Security

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add Threat-Vector-Security/guardian-agent --skill browser-session-defense

Clone the repo

git clone --depth 1 https://github.com/Threat-Vector-Security/guardian-agent

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/threat-vector-security/guardian-agent/browser-session-defense/github.svg)](https://agentmods.dev/skills/threat-vector-security/guardian-agent/browser-session-defense)

Your own site

<a href="https://agentmods.dev/skills/threat-vector-security/guardian-agent/browser-session-defense"><img src="https://agentmods.dev/badge/skills/threat-vector-security/guardian-agent/browser-session-defense/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/threat-vector-security/guardian-agent/browser-session-defense"><img src="https://agentmods.dev/badge/skills/threat-vector-security/guardian-agent/browser-session-defense.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 0 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 418 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00000	$0.00418
Opus 5.5	$0.00000	$0.00167
Sonnet 5.5	$0.00000	$0.00084
Haiku 4.5	$0.00000	$0.00042

Measured 2mo ago against content hash 548431bbd07d, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

browser-session-defense scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/browser-session-defense/SKILL.md · 43 lines

How it starts

The opening of the file, as written. The whole thing — 43 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Browser Session Defense

Use this when the work is about security boundaries around Guardian-managed browser tools rather than generic web UI testing.

Core Boundary

GuardianAgent can directly control and reason about browsing that happens through its managed browser tools.
GuardianAgent does not get rich page-level visibility into the user's normal browser sessions unless additional components exist outside the current runtime.

Workflow

Clarify whether the browsing is:
- Guardian-managed
- user-driven in a normal browser
For Guardian-managed browsing:
- prefer read-only page understanding first
- use the lowest-power browser tool that answers the question
- treat uploads, storage-state operations, and script execution as higher-risk
For user-driven browsing:
- explain the visibility boundary clearly
- fall back to host and network signals when appropriate

Tooling Guidance

Read-only page work: prefer browser_read, browser_links, and browser_extract.
Interactive browser work: use browser_state plus browser_act, or Playwright tools directly when you need lower-level control.
Do not use browser_run_code.
Treat browser_evaluate, file uploads, and storage-state operations as approval-worthy actions.

Boundaries

Use webapp-testing for functional UI verification and bug reproduction.
Use host-firewall-defense when the only available evidence is indirect host or network drift around browser activity.
Use security-mode-escalation when browser risk needs to be translated into monitor, guarded, or lockdown recommendations.

Gotchas

Do not imply GuardianAgent can see arbitrary tabs or page contents in the user's own browser when it cannot.
Do not default to the most powerful raw MCP tool for basic page reading.
Do not recommend storage-state or upload actions as if they were harmless read-only inspection.

Read references/visibility-boundary.md when the user is confused about what GuardianAgent can and cannot observe in browser activity.

Read the full file on GitHub · 43 lines

Files

What ships with it

2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

2mo ago First seen · 43 lines · 0 tokens per session scan A 548431bbd07d

Subscribe to this mod's changes

browser-session-defense is a skill published in the GitHub repository Threat-Vector-Security/guardian-agent (13 stars, last pushed 1mo ago), licensed Apache-2.0. It costs nothing until one of its globs matches a file; then it loads 418 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Browser automation Security TypeScript aws cybersecurity endpoint-security mcp self-hosted

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