cozyvtt-mcp

Character Update

character_update

Destructive

Patch an existing VTT character sheet while preserving unspecified fields, merging nested data after a read and replacing arrays or scalars. Use for partial sheet changes, not new sheets.

Instructions

Patch an existing sheet while preserving unspecified fields (owner or campaign DM). Use character_get first; use token_hp_update for a signed HP change and character_create for a new sheet. data is a request-field object, e.g. {"data":{"hp":{"current":5}}}. Only name/data/tokenImageUrl are allowed at the top level. Nested data dictionaries merge recursively after a GET; arrays/scalars replace whole values and null is an explicit value, not deletion. The caller calculates rules values using the sheet's own gameSystem. Updates are locked only within this process; concurrent browser saves may be lost outside this tool's GET-to-PUT window. On servers supporting updatedAt guards, changes after this tool's GET produce a visible conflict; this does not protect stale intent from an earlier character_get. Upstream parsed validation may discard undeclared fixed-system fields even though this bridge preserves them outbound. REST errors retain diagnostics. Returns {ok:true,data} on success; tool-body failures return {ok:false,error} with optional diagnostic data. Argument-schema errors are MCP errors.

Input Schema

Name	Required	Description	Default
`data`	Yes	Nonempty request patch containing only name, data, tokenImageUrl. Sheet changes go inside data as an object; arrays replace, null is explicit.
`character_id`	Yes	Existing sheet ID to modify; upstream enforces owner or campaign DM access.

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedv0.3.0Sep 20, 2026

addedInput schema / properties / character_id / description

Added value: +"Existing sheet ID to modify; upstream enforces owner or campaign DM access."

addedInput schema / properties / data / description

Added value: +"Nonempty request patch containing only name, data, tokenImageUrl. Sheet changes go inside data as an object; arrays replace, null is explicit."

First observedv0.1.0Sep 5, 2026

TDQS

A4.8/5.0

Behavior5/5

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

Goes far beyond the destructive/openWorld/idempotent annotations: recursive merge semantics, array/scalar replacement, null-as-explicit-value, in-process locking scope, lost concurrent browser saves, updatedAt guard conflicts, and stale-intent caveat. It also discloses which fields upstream validation may discard and how errors surface.

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?

Front-loads purpose, prerequisites, and alternatives before the caveats. Dense and long, but nearly every sentence carries distinct, non-redundant operational information; only the concurrency/validation sentences could be tightened.

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?

An output schema exists, yet the description still summarizes return shapes ({ok:true,data} vs {ok:false,error}) and distinguishes tool-body failures from MCP argument-schema errors. Combined with the merge and concurrency rules, nothing material is missing for correct invocation.

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 baseline is 3, but the description adds real meaning: the 'request-field object' shape with an example, the top-level allowlist (name/data/tokenImageUrl), and the recursive merge rule for nested dictionaries. The example `{"data":{"hp":{"current":5}}}` is not in the schema.

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 and resource ('Patch an existing sheet') plus the defining behavioral trait ('preserving unspecified fields'). Names the siblings it is not (token_hp_update, character_create) so an agent can distinguish it without opening schemas.

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 prescribes prerequisites and alternatives: 'Use character_get first', 'use token_hp_update for a signed HP change', 'character_create for a new sheet'. When-to-use and when-to-use-something-else are both covered.

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