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horizOn MCP Server

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by ProjectMakersDE
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TypeScript
Remote

Set Item Equipped State

horizon_equip_item
DestructiveIdempotent

Sets an item instance's equipped state using the exact reviewed inventory revision and player session, returning updated inventory data.

Instructions

Sets Item Equipped State using the reviewed inventory revision. Use when: the player explicitly requests this inventory action. Not for: automatic retries, client grants, changing instance IDs or revisions to force an uncertain action through. Requires: userId and the player's session: sign in with horizon_signin_email or horizon_signin_anonymous and pass its accessToken as sessionToken. An exact reviewed inventory revision and instance UUID. Side effects: Sets this instance's equipped boolean; it does not enforce game-specific slot rules. One HTTP attempt only. Returns: Complete {userId,revision,instances,limits,serverTime} inventory. Replace only the matching host/project/player cache. Instances carry server-generated UUIDs, properties, equipped and createdAt. Errors: 409 REVISION_CONFLICT or an uncertain response: use horizon_get_items and reconcile intent, never silently substitute a new revision and repeat. 404 ITEM_NOT_FOUND means this instance no longer exists. 422 ITEM_PROPERTY_NOT_ALLOWED/ITEM_PROPERTY_LIMIT_REACHED rejects the entire adjustment. 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 UUID matching the session
equippedYesExplicit desired equipped state
instanceIdYesExact owned item instance UUID from the reviewed inventory
sessionTokenYesPlayer accessToken sent only as Authorization Bearer
expectedRevisionYesExact reviewed revision; never refresh and substitute automatically

Schema Changelog

Changes observed during successful MCP inspections.

  1. Addedv1.9.1

TDQS

A4.8/5.0
Behavior5/5

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

Annotations declare the safety profile (destructive, openWorld, idempotent), and the description adds substantial context beyond them: the write is limited to the equipped boolean and does not enforce slot rules, it makes exactly one HTTP attempt, the returned inventory shape and cache-replacement rule, plus a full error taxonomy (409/404/422/401/403/429) with prescribed recovery rather than silent retry. The idempotentHint and 'one attempt only' detail are complementary rather than contradictory given the expectedRevision guard.

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?

Labeled sections (Use when / Not for / Requires / Side effects / Returns / Errors) make a long block scannable and front-load the routing decision. It is dense but nearly every sentence carries actionable information; only the duplicated 401/rate-limit diagnostics edge toward verbosity.

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 mutating, open-world, revision-guarded tool with no output schema, it covers prerequisites (API key, session flow), side-effect scope, the returned inventory shape, and every foreseeable failure with recovery guidance. Nothing needed to invoke it correctly is missing.

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 the baseline is 3; the description adds real meaning on top by explaining how sessionToken is obtained (horizon_signin_email/horizon_signin_anonymous accessToken), that userId must match the signed-in session, and that expectedRevision must be the exact reviewed revision with no silent refresh. It stops short of documenting the equipped/instanceId semantics beyond what the schema already says.

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?

States a specific verb+resource (set an item instance's equipped boolean) and the precondition that makes it distinct: it operates on an exact reviewed inventory revision and instance UUID, unlike horizon_adjust_item or horizon_discard_item. An agent can distinguish it from its inventory siblings 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?

Explicit 'Use when' (player explicitly requests the inventory action) and 'Not for' (automatic retries, client grants, forcing through by swapping revision/instance IDs). It also routes to the concrete alternative for the uncertain case (horizon_get_items to reconcile intent), satisfying the when/when-not/alternatives bar.

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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