Gemini models fail with 400 for any MCP tool whose array `items` has a union type (e.g. ["object","null"]); GPT/Claude unaffected

[Atales]

gemini-mcp-repro.js

Describe the bug

Summary

Any MCP server that publishes a tool whose array items schema uses a union type containing
"object" — e.g. {"type": ["object", "null"]} — makes every Gemini model fail with
400 Bad Request. The same schema works fine on GPT and Claude.

A single non-conformant tool, anywhere in the loaded MCP set, poisons the entire session. The error
names no tool, no server, and no field, so it is effectively undiagnosable without bisection.

Minimal reproduction

No third-party servers needed. Save as gemini-mcp-repro.js:

// node gemini-mcp-repro.js bad   (or: good)
const bad = (process.argv[2] || 'bad') === 'bad';

const inputSchema = {
  type: 'object',
  properties: {
    rows: {
      type: 'array',
      description: 'a list of row objects',
      // ONLY difference between the two modes:
      items: bad
        ? { type: ['object', 'null'], properties: { a: { type: 'string' } } }
        : { type: 'object',           properties: { a: { type: 'string' } } }
    }
  }
};

let buf = '';
process.stdin.on('data', d => {
  buf += d;
  let i;
  while ((i = buf.indexOf('\n')) >= 0) {
    const line = buf.slice(0, i).trim(); buf = buf.slice(i + 1);
    if (!line) continue;
    let m; try { m = JSON.parse(line); } catch { continue; }
    const send = r => process.stdout.write(JSON.stringify(r) + '\n');
    if (m.method === 'initialize') send({ jsonrpc:'2.0', id:m.id, result:{ protocolVersion:'2024-11-05', capabilities:{tools:{}}, serverInfo:{name:'repro',version:'1'} } });
    else if (m.method === 'tools/list') send({ jsonrpc:'2.0', id:m.id, result:{ tools:[{ name:'repro_tool', description:'repro', inputSchema }] } });
    else if (m.method === 'tools/call') send({ jsonrpc:'2.0', id:m.id, result:{ content:[{type:'text',text:'ok'}] } });
    else if (m.id !== undefined) send({ jsonrpc:'2.0', id:m.id, result:{} });
  }
});

Run:

CFG='{"mcpServers":{"repro":{"type":"stdio","command":"node","args":["./gemini-mcp-repro.js","bad"],"tools":["*"]}}}'

# FAILS
copilot -p "hi" --model gemini-3.7-flash --allow-all-tools \
  --disable-builtin-mcps --additional-mcp-config "$CFG"

# Same command with "good" instead of "bad" -> succeeds

Result matrix

	bad (items.type: ["object","null"])	good (items.type: "object")
gemini-3.7-flash	❌ 400	✅ ok
gemini-3.1-pro-preview	❌ 400	✅ ok
claude-sonnet-4.6	✅ ok	✅ ok
gpt-5.4	✅ ok	✅ ok

Gemini-specific, and isolated to a single JSON-Schema node.

Scope of the trigger

I bisected each construct with the same one-tool harness. A union type is only fatal on an
object-typed node:

Schema shape	Result
{type: "string"}	✅
{type: ["string","null"]}	✅
{type: ["string","null"], default: null}	✅
{description: "..."} — no type at all	✅
{enum: [...], type: ["string","null"]}	✅
{type: "string", format: "uuid"}	✅
type: ["array","null"] + items.type: "object"	✅
type: "array" + items.type: ["string","null"]	✅
type: "array" + items.type: ["object","null"]	❌ 400

The CLI already strips $schema and normalizes several unsupported keywords before dispatch, so the
sanitization layer exists — it just does not recurse into items for object-typed unions. Correct
OpenAPI 3.0 output would be "type": "object" + "nullable": true.

Second, independent trigger

A boolean schema — "properties": { "value": true }, valid JSON Schema 2020-12 meaning "accept
anything" — also produces a Gemini-only 400 via the same harness. Likely the same root fix.

Why this matters

• MCP servers commonly emit ["object","null"] — it is what several popular schema generators
  produce for a nullable list-of-objects parameter. I hit it from two unrelated servers
  independently.
• The failure is total: one bad tool out of ~180 loaded tools broke every Gemini model for me for
  six days across every session on the machine.
• It arrived via a silent auto-update of an MCP server, with no config change on my side.
• The user-visible symptom is 400 Bad Request with no further detail, so the natural user response
  is "Gemini is broken" → switch back to Claude. I suspect that is why this is widely experienced but
  rarely reported. See CLI constantly getting 400 errors for invalid request body #1274, Gemini models return 400 Bad Request in Copilot CLI #4155, [Bug] MCP Integration fails with API validation error (400) and "malformed function call" unless Yolo mode is enabled google-gemini/gemini-cli#22179 (Atlassian/Jira MCP, same
  trigger), gh-aw#20833, Gemini Bad Request with Github Copilot anomalyco/opencode#8417 — all plausibly the same defect.

Asks

1. Normalize object-typed unions and boolean schemas in the existing MCP schema sanitizer, so a
   non-conformant server degrades gracefully instead of bricking a model family.
2. Failing that, make the error actionable — name the offending server, tool, and JSON path in
   the 400. Today there is no way to find it short of an O(n) bisection over every tool.

(2) is arguably the more important fix: it converts an undiagnosable outage into a one-line message.

Environment

• Copilot CLI 1.0.81-x, Windows 11
• Repro is platform-independent (plain Node stdio server)
• Request IDs from my failures: <paste your own>

Affected version

GitHub Copilot CLI 1.0.81-9.

Steps to reproduce the behavior

All the repro details are captured in the above blob. This is a 100% repro and Claude help me craft that minimal repro

Few more related/similar issues:
#4155
#1274
google-gemini/gemini-cli#22179
github/gh-aw#20833
anomalyco/opencode#8417

Expected behavior

No response

Additional context

No response