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@yawlabs/tailscale-mcp

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TypeScript
Remote

Diff ACL access

tailscale_diff_acl_access
Read-onlyIdempotent

Compare a proposed Tailscale ACL against the live policy to see which users gain or lose access before applying it, catching silent revocations.

Instructions

Answer 'who loses access?' before applying an ACL change. Compares the CURRENT policy against a proposed one and reports, per user, which destinations they gain and lose. Run this before tailscale_update_acl -- validate_acl only checks syntax and the policy's own tests block, so a policy with no tests validates clean while revoking everyone. LIMITS, all reported in the response rather than left to be discovered. It compares USER principals only, so a revocation that runs through a tag or group can show a clean diff, and an empty result is never proof a change is safe. Posture DEFINITION changes ARE detected: posture names are resolved to their rules, so tightening posture:corp shows as a change -- except when a preview omits the definitions map, where it falls back to comparing names. It costs two preview requests per user, so it checks the first 25 by default and stops after 60 seconds regardless; either way it sets truncated, reports how many were skipped, and says which limit stopped it. Users whose preview fails are listed in failed and excluded from the compared count -- a failure is never reported as lost access, and if nothing could be compared the call fails rather than returning an empty diff.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
policyYesThe proposed ACL policy text to compare against the current live policy
principalsNoPrincipals to check, as they appear in `loginName` from tailscale_list_users. That is often an email, but on a GitHub or SSO tailnet it is not (e.g. 'alice@github') -- pass the loginName verbatim rather than an address you assume. Omit to enumerate the tailnet's users automatically. Pass an explicit list to bound the request count, or to check specific users beyond the cap.
maxPrincipalsNoMaximum users to check (default 25). Each costs two preview requests. Raising this on a large tailnet can be slow and may hit rate limits.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Addedv0.19.1

TDQS

A4.6/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

While annotations cover the safety profile (readOnly, idempotent, non-destructive), the description adds substantial behavioral context annotations cannot: cost model (two preview requests per user), a 25-user/60-second cap that sets `truncated`, a `failed` list excluded from the comparison, user-only principal coverage that can hide tag/group revocations, posture-definition resolution behavior, and a hard failure when nothing can be compared. This is exactly the kind of context that goes beyond structured fields.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Purpose and routing to the sibling are front-loaded, and every sentence carries information (limits, caveats, failure handling) with no filler. It is dense and runs long, but nothing is padding.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description carries the burden of explaining returns and does so: gain/loss per user, the `truncated` flag, the skip count, the stopping limit, and the `failed` list. An agent knows both the input contract and what the response will tell it.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so all three parameters are already well documented, including the loginName caveat and the maxPrincipals rate-limit warning. The description reinforces the 25-user default but adds no syntax or format detail beyond the schema, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

Names a specific verb (compares/diffs) and resource (ACL policy access), and frames the exact question it answers ('who loses access?'). It distinguishes itself from siblings by name (tailscale_update_acl, validate_acl), so an agent can select it without opening any schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly states when to use it ('Run this before tailscale_update_acl') and why the alternative is insufficient ('validate_acl only checks syntax... a policy with no tests validates clean while revoking everyone'). The when-not condition is spelled out, leaving nothing to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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