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.
npx skills add operasoftware/opera-devtools-mcp --skill cookie-debugginggit clone --depth 1 https://github.com/operasoftware/opera-devtools-mcpWrote 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.
[](https://agentmods.dev/skills/operasoftware/opera-devtools-mcp/cookie-debugging)<a href="https://agentmods.dev/skills/operasoftware/opera-devtools-mcp/cookie-debugging"><img src="https://agentmods.dev/badge/skills/operasoftware/opera-devtools-mcp/cookie-debugging/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.
<a href="https://agentmods.dev/skills/operasoftware/opera-devtools-mcp/cookie-debugging"><img src="https://agentmods.dev/badge/skills/operasoftware/opera-devtools-mcp/cookie-debugging.svg" alt="Reviewed on agentmods" width="80" height="20"></a>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 | $0.00071 | $0.02175 |
| Opus 5.5 | $0.00028 | $0.00870 |
| Sonnet 5.5 | $0.00014 | $0.00435 |
| Haiku 4.5 | $0.00007 | $0.00217 |
Grade A, and why
cookie-debugging scanned grade A with 1 finding 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 1mo 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.
Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
- Call `evaluate_script` with `() => fetch(window.location.href)` This is a copy
100% identical to cookie-debugging — 13 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 150 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Core Concepts
HttpOnly vs Client-Side Storage
Cookies marked HttpOnly cannot be accessed or modified by client-side JavaScript (cookieStore or document.cookie). However, the browser automatically attaches active HttpOnly cookies to outgoing HTTP request headers (Cookie).
- To inspect current
HttpOnlyvalues: Look at theCookierequest header of any outgoing HTTP request viaget_network_request. - To inspect how cookies were created or configured: Look at the
Set-Cookieresponse header of login/auth responses. - To inspect non-
HttpOnlycookies: Useevaluate_scriptwith the moderncookieStoreAPI (async () => await cookieStore.getAll()).
Session Strategy: Live Tab vs Isolated Context
Choose the right session environment to avoid state contamination (e.g., residual analytics or auth tokens):
| Strategy | When to Use | Setup / Teardown |
|---|---|---|
| Live Tab (Active Page) | Diagnosing an active user session, live 401/403 error, or current state. | Operates directly on the currently selected page. |
Clean-Slate (isolatedContext) |
Testing cookie consent banners, first-time visits, or zero-cookie guarantees. | Call new_page with a unique isolatedContext (e.g. "consent-audit-1"). When finished, call close_page. |
Client-Side Capabilities & Limitations
| Action | Client JavaScript (cookieStore / document.cookie) |
DevTools Network & Context Tools |
|---|---|---|
| Read Non-HttpOnly | ✅ async () => await cookieStore.getAll() |
✅ get_network_request (Request Cookie) |
| Read HttpOnly | ❌ Blocked by browser security | ✅ get_network_request (Request Cookie) |
Inspect Attributes (Domain, Path, SameSite, Expires) |
✅ async () => await cookieStore.getAll() |
✅ get_network_request (Response Set-Cookie) |
| Modify / Delete Non-HttpOnly | ✅ async () => await cookieStore.set(...) |
N/A |
| Modify / Delete HttpOnly | ❌ Silent failure in JavaScript | ✅ Use new_page(isolatedContext: ...) for clean state |
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.
- 1mo ago First seen · 150 lines · 71 tokens per session scan A eeb526491ce5
cookie-debugging is a skill published in the GitHub repository operasoftware/opera-devtools-mcp (14 stars, last pushed yesterday), licensed Apache-2.0. It adds 71 tokens to every session and 2,175 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 1 finding (makes network calls). It is 100% identical to cookie-debugging, differing in 13 lines, and is treated as a copy.
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