Gemini function calling rejects tool schemas due to "contentEncoding" field

[RaviTharuma]

Problem

When agentmail MCP tools are registered with Google Gemini models (via OpenCode/AI SDK), Gemini's function calling API rejects the tool schemas with:

Invalid JSON payload received. Unknown name "contentEncoding" at
'request.tools[0].function_declarations[131].parameters.properties[4].value.items.properties[2].value': Cannot find field.

This affects 3 tools: send_message, reply_to_message, forward_message -- all of which have attachments[].content defined with a contentEncoding: "base64" field in the JSON Schema.

Root Cause

The attachment content field likely uses Zod's z.string().base64() which, when serialized to JSON Schema via zod-to-json-schema, produces:

{
  "content": {
    "type": "string",
    "contentEncoding": "base64",
    "format": "base64",
    "pattern": "^$|^(?:[0-9a-zA-Z+/]{4})*..."
  }
}

contentEncoding is valid JSON Schema (RFC draft-07+) but Gemini's function calling API only supports a subset of JSON Schema and does not recognize this field. Gemini supports: type, description, enum, items, properties, required, format, nullable -- but NOT contentEncoding, contentMediaType, pattern, etc.

Affected Models

• All Google Gemini models (gemini-2.5-, gemini-3-, gemini-3.1-*)
• Any provider that uses Gemini's function calling schema validation

Suggested Fix

Use zod-to-json-schema with base64Strategy: "format:binary" instead of the default "contentEncoding:base64":

import { zodToJsonSchema } from 'zod-to-json-schema';

zodToJsonSchema(schema, { base64Strategy: 'format:binary' });

Or alternatively, define the field as a plain z.string() with a description noting it expects base64:

content: z.string().describe("Base64 encoded content")

Environment

• OpenCode with Oh My OpenCode (OmO) orchestration
• AI SDK provider routing through CLIProxyAPI/Quotio
• agentmail MCP registered via mcpm.sh

[RaviTharuma]

Workaround: MCP stdio proxy wrapper

Until this is fixed upstream, here's a working workaround using an MCP stdio proxy that strips contentEncoding and contentMediaType from tool schemas before they reach the client (and thus Gemini).

mcp-strip-content-encoding.ts

#!/usr/bin/env bun
// MCP stdio proxy that strips contentEncoding from tool schemas.
// Gemini doesn't support contentEncoding in function declarations,
// causing 400 errors when tools like AgentMail use z.base64().
// Usage: wrap any MCP server command with this script.
// Example: bun run mcp-strip-content-encoding.ts -- bunx -y agentmail-mcp

import { spawn } from "child_process";

const args = process.argv.slice(2);
const dashIdx = args.indexOf("--");
const cmd = dashIdx >= 0 ? args.slice(dashIdx + 1) : args;
if (!cmd.length) {
  console.error("Usage: bun run mcp-strip-content-encoding.ts -- <command> [args...]");
  process.exit(1);
}

function strip(obj: unknown): unknown {
  if (obj === null || typeof obj !== "object") return obj;
  if (Array.isArray(obj)) return obj.map(strip);
  const out: Record<string, unknown> = {};
  for (const [k, v] of Object.entries(obj as Record<string, unknown>)) {
    if (k === "contentEncoding" || k === "contentMediaType") continue;
    out[k] = strip(v);
  }
  return out;
}

const child = spawn(cmd[0], cmd.slice(1), {
  env: process.env,
  stdio: ["pipe", "pipe", "inherit"],
});

let buf = "";
child.stdout.on("data", (chunk: Buffer) => {
  buf += chunk.toString();
  let nl: number;
  while ((nl = buf.indexOf("\n")) !== -1) {
    const line = buf.slice(0, nl);
    buf = buf.slice(nl + 1);
    if (!line.trim()) { process.stdout.write("\n"); continue; }
    try {
      const msg = JSON.parse(line);
      if (msg.result?.tools && Array.isArray(msg.result.tools)) {
        for (const tool of msg.result.tools) {
          if (tool.inputSchema) tool.inputSchema = strip(tool.inputSchema);
        }
      }
      process.stdout.write(JSON.stringify(msg) + "\n");
    } catch {
      process.stdout.write(line + "\n");
    }
  }
});

process.stdin.pipe(child.stdin);
child.on("exit", (code) => process.exit(code ?? 0));

Usage with mcpm

Update the agentmail entry in ~/.config/mcpm/servers.json:

{
  "agentmail": {
    "command": "bun",
    "args": [
      "run",
      "/path/to/mcp-strip-content-encoding.ts",
      "--",
      "bunx",
      "-y",
      "agentmail-mcp"
    ],
    "env": {
      "AGENTMAIL_API_KEY": "your-api-key"
    }
  }
}

Verified

• All 3 affected tools (send_message, reply_to_message, forward_message) have contentEncoding stripped
• E2E tested with Gemini 3 Flash — responds successfully with 0 schema errors
• Works transparently with mcpm, Claude Code, and OpenCode