Combine multiple strings into a single string using a specified delimiter. Useful for building CSV lines, paths, or concatenating text with separators.
Join a list of strings with a delimiter.
| Name | Required | Description | Default |
|---|---|---|---|
| items | Yes | ||
| delimiter | Yes |
| Name | Required | Description | Default |
|---|---|---|---|
| result | Yes |
Changes observed during successful MCP inspections.
v1.0.4Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description carries the full burden, but this is a pure, side-effect-free string operation with little behavior to disclose. It still omits edge-case behavior such as empty lists, empty delimiters, or handling of null/None elements, which are the realistic failure modes here.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
A single sentence of nine words that front-loads the verb and resource with zero waste or filler.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
An output schema exists, so the return value need not be explained, and the tool is a trivial two-parameter pure function. The description supplies everything needed to invoke it correctly.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema description coverage is 0%, so the description must compensate, and it does name both required inputs: the string list to be joined and the delimiter to join with. It adds no format or constraint detail (e.g., must delimiter be non-empty), but both parameters are semantically covered.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description uses a specific verb ("Join") and resource ("a list of strings") with the delimiter mechanism named, so the operation is unambiguous. It does not, however, distinguish itself from related siblings such as string_split or collection_merge, which the agent must infer on its own.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
There is no when-to-use or when-not-to-use guidance, and no alternative tool is named. For a simple primitive the intended use is inferable, but nothing in the text routes the agent between this and string_split/collection_merge.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.