SchemaForm crashes with Mantine "Duplicate options … value """ when array items use anyOf of object schemas

[lneveu]

Which version line?

v2 — current (@modelcontextprotocol/inspector@latest)

Which client?

Web

Inspector version

2.2.0

Node version

v22.22.2

Operating system (and browser, for the web client)

No response

Transport

Streamable HTTP

MCP server under inspection

Custom MCP server

• Transport: Streamable HTTP
• SDK: @modelcontextprotocol/sdk ^1.29.0
• Input schemas defined with Zod 4
• Trigger: any tool whose inputSchema has an array property whose items are an anyOf of object schemas (no top-level scalar const on each branch), e.g. Zod z.array(z.union([z.object(...), z.object(...)]))

Steps to reproduce

1. Connect Inspector (Web) to an MCP server that exposes a tool with an inputSchema shaped like:

{
  "type": "object",
  "properties": {
    "items": {
      "type": "array",
      "items": {
        "anyOf": [
          {
            "type": "object",
            "properties": {
              "type": { "type": "string", "const": "A" },
              "payload": { "type": "object" }
            },
            "required": ["type", "payload"]
          },
          {
            "type": "object",
            "properties": {
              "type": { "type": "string", "const": "B" },
              "payload": { "type": "object" }
            },
            "required": ["type", "payload"]
          }
        ]
      }
    }
  },
  "required": ["items"]
}

Equivalent Zod shape:

   z.object({
     items: z.array(
       z.union([
         z.object({ type: z.literal("A"), payload: z.object({}) }),
         z.object({ type: z.literal("B"), payload: z.object({}) }),
       ])
     ),
   })

2. Open the Tools tab and select that tool.

3. Observe the tool detail / parameter form render.

Expected behavior

The form should render without crashing. For an array of object unions (anyOf branches without a top-level scalar const), SchemaForm should fall back to the JSON editor (JsonInput), same as other unsupported/complex schemas — not treat the field as a string MultiSelect.

Actual behavior

The UI greys out / the tool panel crashes. The browser console throws:

Uncaught Error: [@mantine/core] Duplicate options are not supported. Option with value "" was provided more than once

Root cause in clients/web/src/components/groups/SchemaForm/SchemaForm.tsx:

// array with items having anyOf
if (fieldSchema.type === "array" && fieldSchema.items?.anyOf) {
const data = fieldSchema.items.anyOf.map((item) => ({
value: String(item.const ?? ""),
label: item.title ?? String(item.const ?? ""),
}));
return <MultiSelect ... data={data} />;
}

For object-union branches there is no top-level const, so every option becomes value "". Mantine Select/MultiSelect requires unique values and throws.

Logs, errors, or screenshots

Console:

Uncaught Error: [@mantine/core] Duplicate options are not supported. Option with value "" was provided more than once
at ... (@mantine/core Select/MultiSelect option validation)
at SchemaForm render for the tool parameter form

Already prototyped a fix?

No response

Before you submit

• I searched existing issues and this is not a duplicate.
• This is not a security vulnerability report (those go through the private advisory process).

[cliffhall]

Triage: Priority High (total 10)

• Severity 4 — a core workflow is unusable: the tool detail panel throws during render, so any tool with this schema shape cannot be tested from the Tools tab at all, with no in-app workaround. Root cause confirmed as reported at clients/web/src/components/groups/SchemaForm/SchemaForm.tsx:337 — String(item.const ?? "") collapses every object-union branch to "", and Mantine rejects duplicate option values.
• Urgency 5 — actively hurting users on a published version right now (reported against released 2.2.0; z.array(z.union([z.object(…), …])) is an ordinary schema shape).
• Bonuses: +1 bug label. (No milestone — this is a pass-1 triage; no comments or reactions; unassigned; not a sub-issue.)

Board: #28, Status Incoming (awaiting maintainer review — no milestone yet).