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horizOn MCP Server

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Remote

join group

horizon_join_group
Idempotent

Joins an open group with a confirmed player session and reviewed revision, checking member/player limits and returning updated group state. Reconcile on revision conflicts; no automatic retry.

Instructions

Applies the explicit join group action using the reviewed group revision. Use when: the player explicitly confirms this group action. Not for: replacing reviewed revisions/request IDs, bypassing a leader's block, or retrying automatically after uncertainty. Requires: userId and the player's session: sign in with horizon_signin_email or horizon_signin_anonymous and pass its accessToken as sessionToken. Current group revision and the exact pending requestId where applicable. Side effects: Joins an open group; checks member and player limits. No automatic retry. Same-revision replay cannot create a second effect. Returns: {groups:MyGroupsView,group:GroupView|null}. groups is the actor's complete snapshot, never the target's private state. group is null after leave/delete; nonmembers never receive private data. Replace only the matching host/project/player cache. Errors: Use horizon_get_groups and horizon_get_group_pending to reconcile state. 409 REVISION_CONFLICT or uncertain response: read and reconcile intent, never refresh the revision and repeat automatically. Pending decisions require the original requestId; REQUEST_NOT_FOUND means it was removed/replaced. INBOX_FULL rolls back the entire action. 403 rejects group role, session or bilateral block violations. 429 is reported without automatic retry. 401 means the session is missing or expired: sign in again and retry with the new accessToken. 403 for a session means the token belongs to another user: pass the userId that signed in. Needs HORIZON_API_KEY. Every failure returns an error result (isError) with the HTTP status and body: a 401 that horizon_test_connection also returns means HORIZON_API_KEY is wrong, 429 means the account's per-minute rate limit was reached (wait a minute, then retry).

Input Schema

NameRequiredDescriptionDefault
userIdYesSigned-in player matching the Bearer session
groupIdYesExact group UUID in the current project; never an authorization credential
sessionTokenYesPlayer accessToken, sent only in Authorization
expectedRevisionYesExact reviewed revision; never replace automatically after an uncertain response

Schema Changelog

Changes observed during successful MCP inspections.

  1. Addedv1.9.1

TDQS

A4/5.0
Behavior5/5

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

Goes well beyond the annotations: states it joins an open group, checks member and player limit constraints, performs no automatic retry, and that same-revision replay cannot double-effect (consistent with idempotentHint=true). It also documents 409 REVISION_CONFLICT, REQUEST_NOT_FOUND, INBOX_FULL rollback, 403 block violations, 429 rate limits and 401 session recovery — genuinely actionable failure behavior.

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

Conciseness3/5

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

The Use when / Not for / Requires / Side effects / Returns / Errors labels front-load well, but the block is very long and mixes tool-local guidance with account-level API key and rate-limit explanation. Several clauses (e.g. the 401/horizon_test_connection cross-check) could be trimmed without loss.

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

Completeness4/5

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

With no output schema, describing the return shape ({groups: MyGroupsView, group: GroupView|null}) and privacy scoping is valuable and present, as are cache-invalidation and error-reconciliation instructions. The stray requestId reference is the one gap, since this tool takes no requestId.

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 coverage is 100%, so baseline is 3; the description does add value by explaining how to obtain sessionToken via horizon_signin_email/anon and that expectedRevision must never be auto-replaced. However, it references 'the exact pending requestId where applicable', which does not exist in this tool's schema, muddying parameter expectations.

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

Purpose4/5

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

States a specific verb and resource ('Applies the explicit join group action using the reviewed group revision') and the 'explicit' qualifier distinguishes it from horizon_request_group_join and horizon_accept_group_invite. The phrase 'join group action' is slightly jargon-heavy but an agent can select it correctly.

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

Usage Guidelines4/5

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

Explicit 'Use when' (player explicitly confirms) and 'Not for' clauses (replacing revisions, bypassing leader block, automatic retry) give concrete inclusion and exclusion criteria. It never names the specific sibling to use instead for the open-vs-request-join decision, so it falls short of a 5.

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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