No way to set a tool's outputSchema: ToolDefinition has the field, the server builder has no setter

[azhur]

Summary

ToolDefinition carries outputSchema, but nothing on the server side can populate it — so it is always None on the wire.

Detail (chimp 0.4.0)

The protocol type has the field:

// chimp.protocol
final case class ToolDefinition(
    name: String,
    description: Option[String],
    inputSchema: Json,
    outputSchema: Option[Json],  // <- never set
    // ...
)

The server-side tool has no output counterpart, only an input schema and decoder:

// chimp.server
final case class ServerTool[I, F[_], C <: ServerContext[F]](
    name: String,
    description: Option[String],
    inputSchema: ToolSchema,
    inputDecoder: Decoder[I],
    annotations: Option[ToolAnnotations],
    logic: (I, C, Seq[Header]) => F[?]
)

and the builder offers no way to supply one:

// chimp.server
final case class PartialTool(...):
  def input[I: Schema: Decoder]: Tool[I]
  def inputJson(schema: Json): Tool[Json]
  // no .output / .outputJson

So a tool can describe what it accepts, but not what it returns:

tool("get_violations")
  .description("Top violated criteria, ranked by count")
  .input[ViolationsInput]      // schema advertised
  .serverLogic[F] { (in, _) =>
    ZIO.succeed(ToolResult.text(summary).withStructured(rows.asJson))
  }                            // structuredContent, but no schema describing it

Why it matters

ToolResult.withStructured already returns structuredContent, so clients get structured data — with no declared schema to validate it against. For tools returning tabular results that is the difference between data and a contract.

Spec context: outputSchema/structuredContent arrived in 2025-06-18, and 2026-07-28 loosened both inputSchema and outputSchema to allow any JSON Schema 2020-12 keywords — so it seems to be getting more load-bearing, not less.

Suggested shape

An .output mirroring .input, deriving the schema the same way:

tool("get_violations")
  .description("Top violated criteria, ranked by count")
  .input[ViolationsInput]
  .output[ViolationsOutput]    // populates ToolDefinition.outputSchema
  .serverLogic[F] { ... }

Happy to send a PR if that shape looks right.

Context

Possibly in scope for #144, but that issue is a general conformance target and does not name this, so filing separately. Found while building a server whose tools all return tabular data — everything else in chimp has been a pleasure to work with.

[kubinio123]

Definitely something we like to have in chimp, PRs are much welcome @azhur 👍

[azhur]

@kubinio123 created #208