geminicli-mcp

Stateless stdio MCP server wrapping the headless Gemini CLI.

• stdio transport, spawn-per-client

• three tools: gemini_prompt, gemini_prompt_with_context, gemini_prompt_structured

• no sessions, no working_dir, no timeout enforcement

• runs in the server process's cwd

• child env inherits the server's env (so GEMINI_API_KEY flows through) and pins NO_COLOR=1, TERM=dumb; prompt is always a single argv element — never interpolated into a shell string

• single runtime dependency: @modelcontextprotocol/sdk

Prerequisites

• Node.js ≥ 20

• @google/gemini-cli ≥ 0.6.1 — this server invokes the CLI with --output-format json, which is only recognised by gemini-cli 0.6.1 and newer (upstream PR #8119).

Install from npm

npm install -g geminicli-mcp

This exposes the geminicli-mcp binary on your PATH. Point your MCP client at it:

// e.g. ~/.config/<your-client>/mcp.json
{
  "mcpServers": {
    "gemini": {
      "command": "geminicli-mcp"
    }
  }
}

You also need the headless Gemini CLI installed and authenticated:

npm install -g @google/gemini-cli
gemini  # one-time interactive auth

Install from source

git clone https://github.com/trevoraspencer/geminicli-mcp.git
cd geminicli-mcp
npm install
npm run build
node dist/index.js

Tools

gemini_prompt

Returns the CLI's response text.

gemini_prompt_with_context

Returns the CLI's response text.

gemini_prompt_structured

The server appends an explicit "respond with JSON conforming to this schema" instruction to your prompt, extracts JSON from the model's response (it tolerates surrounding prose or a ```json fence), and validates the result against schema. On success the tool returns the canonical JSON string. On failure the result is isError: true with a diagnostic that includes both the validation errors and the raw response.

A minimal schema looks like:

{
  "type": "object",
  "required": ["title", "tags"],
  "properties": {
    "title": { "type": "string" },
    "tags": { "type": "array", "items": { "type": "string" } }
  },
  "additionalProperties": false
}

The bundled validator supports a useful subset of JSON Schema: type (with all primitive types plus array and object), required, properties, items, additionalProperties, enum, const, minLength/maxLength, minItems/maxItems, minimum/maximum, pattern, and the anyOf / oneOf / allOf combinators. It is deliberately small — if you need full JSON Schema 2020-12, pre-validate on the caller side.

Examples

The examples/ directory contains runnable JSON-RPC request files you can pipe into the server over stdio. See examples/README.md for a one-liner that lists tools or invokes a tool from the command line.

Environment

• GEMINI_CLI_BIN — override the gemini binary path (default: gemini on PATH).

• GEMINICLI_MCP_MAX_OUTPUT_BYTES — cap (per stream) on bytes accumulated from the gemini child's stdout/stderr before the server kills it (default: 33554432 / 32 MiB). Guards against a runaway child OOM-ing the server.

• GEMINICLI_MCP_DEBUG — when set to 1, true, or yes, emits structured JSON diagnostics to stderr (invocation args, exit codes, durations, stderr content, kill-failure markers). Safe for MCP use: diagnostics go to stderr only and never corrupt the stdio protocol on stdout. Leave unset in production; enable only for targeted debugging.

• Child process env inherits the server's environment, plus forced NO_COLOR=1 / TERM=dumb. Anything the server can read — including GEMINI_API_KEY and friends — the gemini CLI can read.

Exit codes

The server surfaces the gemini CLI's exit code via the errorKind / exit= prefix on error responses (e.g. [error exit=1] ...).

Security caveats

• --approval-mode yolo is always on. The server hardcodes this flag so it can run non-interactively. Any tool calls or file edits the gemini CLI performs are auto-approved. Do not expose this MCP server to MCP clients you don't trust.

• No quota controls. A caller that can reach this server can make unlimited gemini API calls at your expense. Gate access at the MCP-client layer.

• Child inherits the server's env. Don't place secrets in the server's env that you don't want the gemini CLI (and its plugins) to see.

Troubleshooting

Unknown arguments: output-format / Unknown arguments: outputFormat

Your @google/gemini-cli is older than 0.6.1, which is when --output-format json was added (upstream PR #8119). Upgrade:

npm install -g @google/gemini-cli@latest
gemini --version  # confirm ≥ 0.6.1

"Gemini CLI binary not found" (exit 127)

The server could not spawn the gemini executable. Either:

• Install the Google headless Gemini CLI (npm install -g @google/gemini-cli), then verify with which gemini && gemini --version; or

• Set GEMINI_CLI_BIN=/absolute/path/to/gemini in the server's environment.

If you launch the MCP server from a GUI (Claude Desktop, IDE), it may not inherit your shell PATH. Either pass an absolute path via GEMINI_CLI_BIN or configure your MCP client's env block, e.g.:

{
  "mcpServers": {
    "gemini": {
      "command": "geminicli-mcp",
      "env": {
        "PATH": "/usr/local/bin:/usr/bin",
        "GEMINI_CLI_BIN": "/usr/local/bin/gemini"
      }
    }
  }
}

"Auth not configured" / authentication errors

The Gemini CLI handles its own auth (Google account, API key, or Vertex AI). This server does not touch credentials. Run the CLI directly once to authenticate:

gemini       # follow the interactive prompts
echo "ping" | gemini -p "Reply with the single word: ping"

If you're using an API key, make sure the relevant env vars (e.g. GEMINI_API_KEY, GOOGLE_APPLICATION_CREDENTIALS) are present in the MCP server's env block — children inherit the server's environment but the server's environment is whatever your MCP client gave it.

"Gemini response did not match schema" (gemini_prompt_structured)

The error text includes both the validation errors and the raw response so you can see what the model produced. Common fixes:

• Tighten the prompt so it can't ramble (the server already appends a JSON-only instruction).

• Make the schema friendlier — e.g. allow additionalProperties: true for fields you don't need to constrain.

• Pick a stronger model (model: "gemini-2.5-pro").

Design invariants

These are intentional, not omissions. See CONTRIBUTING.md for the full list.

• Stateless: no session IDs, no --resume, no working_dir.

• Spawn-per-client: one server process per MCP client.

• No timeout enforcement in the server — the client and CLI are authoritative.

• No shell interpolation.

• One runtime dependency (@modelcontextprotocol/sdk).

License

MIT