Missing items definition for array parameters in generated tool schemas

[gambtho]

Description

When exporting tool schemas via GoMCP's built-in generator (either with --export-schemas or the HTTP /schema endpoint), any parameter declared as a Go slice or array (e.g. []string) produces a JSON Schema entry with:

{
  "type": "array",
  "description": "…"
}

but no corresponding "items" definition. This violates the JSON Schema specification and breaks downstream validators (notably the GitHub Copilot extension), which refuse to load the schema:

Failed to validate tool xxx_tool:
Error: tool parameters array type must have items

Steps to Reproduce

1. Define a tool with an array field:

type ToolArgs struct {
  SomeSlice []string `json:"some_slice"`
  // …
}

server.Tool(
  "mcp_xxx_tool",
  "Some Tool",
  ToolArgs{},
  ToolResult{},
  handlerFunc,
)

2. Build and export schemas:

go build -o mcp-server
./mcp-server --export-schemas

3. Inspect the output for some_slice:

"some_slice": {
  "type": "array",
  "description": "List of strings…"
  // ← missing "items": { "type": "string" }
}

4. Observe that a JSON Schema validator (e.g. GitHub Copilot) rejects it:

Error: tool parameters array type must have items

Expected Behavior

Every parameter with "type": "array" in the generated schema must include an "items" property that accurately describes the element type. For example:

"some_slice": {
  "type": "array",
  "items": {
    "type": "string"
  },
  "description": "List of srings…"
}

Proposed Solutions

Option 1: Post-processing approach

Post-process exported schemas in cmd/mcp-server/main.go (and the HTTP handler) to walk the raw map[string]interface{} and inject missing "items" blocks.

Option 2: Core integration

Integrate the fix into GoMCP's core generator so that slice and array types always emit an items schema entry by default—eliminating the need for external post-processing.

Implementation sketch (post-processor approach):

// util/schema_utils.go
func AddMissingArrayItems(schema map[string]interface{}) {
  // Recursively traverse schema maps/arrays…
  // When encountering {"type":"array"} with no "items", inject:
  schema["items"] = map[string]interface{}{"type":"string"}
}

Environment

• GoMCP version: main (commit SHA…)
• Go version: go1.24

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Please let me know if you'd like additional context and if you're open to a PR for this. Thank you!

[localrivet]

Perfect explanation of the issue. I'll fix it ASAP.