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horizOn MCP Server

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by ProjectMakersDE
OverviewSchemaSourceRelated ServersDiscussionsAdmin
TypeScript
Remote

Start Match

horizon_start_match
DestructiveIdempotent

Starts an asynchronous multiplayer match at the exact original revision using a player session, match ID, and explicit participant turn order; reread and reconcile on revision conflicts.

Instructions

Executes start match under the exact original revision. Use when: explicit asynchronous multiplayer actions after reading horizon://docs/matches. Not for: automatic polling, page traversal, timeout progression, invented winners or automatic rewards. Reconcile with horizon_get_matches and horizon_get_match. Consent and game consensus do not validate honest gameplay. Requires: userId and the player's session: sign in with horizon_signin_email or horizon_signin_anonymous and pass its accessToken as sessionToken. Side effects: One HTTP attempt. Start requires the creator and every joined UUID exactly once. Turn requires the current unexpired actor. Timeout advances only one turn. Active leave forfeits without selecting a winner. Lobby cancel is creator-only. Full Inbox retains a durable notice without undoing a committed move. Returns: {match,pollAfterSeconds,serverTime}; departing lobby actors receive metadata only and lose point-read access. Errors: 404 hides private/missing/expired matches. 409 REVISION_CONFLICT or STATE_CONFLICT: explicitly reread and reconcile the original intent, never replace the revision or automatically resubmit. RESULT_CONFLICT preserves the original report. RUN_EXPIRED/RUN_ALREADY_USED never allow replacement ranked runs. 501 FEATURE_NOT_SUPPORTED is explicit for simpleServer ranked mode. 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
userIdYesActive player matching the session and project
matchIdYesOriginal server match UUID in the selected project
turnOrderYesEvery joined participant in the explicit turn order
sessionTokenYesPlayer accessToken sent only as Bearer
expectedRevisionYesOriginal reviewed revision; never substitute a newer revision after an uncertain write

Schema Changelog

Changes observed during successful MCP inspections.

  1. Addedv1.9.1

TDQS

A4.4/5.0
Behavior5/5

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

Far beyond the annotations, it discloses the sign-in prerequisite, HORIZON_API_KEY requirement, one-HTTP-attempt behavior, revision-conflict handling, rate-limit response, 401/403 session semantics, and return payload. It also states consequential behavior such as non-replacement of ranked runs and preservation of original results on conflict.

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?

It is well front-loaded and many sentences carry real operational value, especially the error recovery guidance. But it is very long and the side-effects paragraph describes behaviors belonging to other match tools, so not every sentence earns its place in this tool's definition.

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?

For a complex five-required-parameter mutation with no output schema, it covers authentication, side effects, return shape, and detailed error recovery. Nothing an agent needs in order to invoke it correctly appears to be missing, even though the description is broader than necessary.

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

Parameters4/5

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

Schema coverage is 100%, so parameter basics are already documented. The description still adds useful meaning: sessionToken must be the accessToken from horizon_signin_email or horizon_signin_anonymous, userId must match the signer, and expectedRevision must never be replaced after an uncertain write.

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?

The opening sentence, 'Executes start match under the exact original revision,' names a specific verb and resource and is distinguishable from list/get siblings. However, the later 'Use when: explicit asynchronous multiplayer actions' and the side-effects paragraph describing turn, timeout, leave, and lobby-cancel semantics blur whether this tool also performs those actions, which are separate siblings.

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?

It gives explicit 'Use when' and 'Not for' guidance, excluding automatic polling, timeout progression, invented winners, and automatic rewards. It also names concrete alternatives for reconciliation, horizon_get_matches and horizon_get_match, and tells the agent to read horizon://docs/matches first.

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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