> ## Documentation Index
> Fetch the complete documentation index at: https://code.claude.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Connect to external tools with MCP

> Configure MCP servers to extend your agent with external tools. Covers transport types, tool search for large tool sets, authentication, and error handling.

The [Model Context Protocol (MCP)](https://modelcontextprotocol.io/docs/getting-started/intro) is an open standard for connecting AI agents to external tools and data sources. With MCP, your agent can query databases, integrate with APIs like Slack and GitHub, and connect to other services without writing custom tool implementations.

MCP servers can run as local processes, connect over HTTP, or execute directly within your SDK application.

> [!NOTE]
>
> This page covers MCP configuration for the Agent SDK. To add MCP servers to the Claude Code CLI so they load in every project, see [MCP installation scopes](https://code.claude.com/docs/en/mcp.md#mcp-installation-scopes).

## Quickstart

This example connects to the [Claude Code documentation](https://code.claude.com/docs) MCP server using [HTTP transport](#http%2Fsse-servers) and uses [`allowedTools`](#allow-mcp-tools) with a wildcard to permit all tools from the server.

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({
  prompt: "Use the docs MCP server to explain what hooks are in Claude Code",
  options: {
    mcpServers: {
      "claude-code-docs": {
        type: "http",
        url: "https://code.claude.com/docs/mcp"
      }
    },
    allowedTools: ["mcp__claude-code-docs__*"]
  }
})) {
  if (message.type === "result" && message.subtype === "success") {
    console.log(message.result);
  }
}
```

```python Python theme={null}
import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, ResultMessage


async def main():
    options = ClaudeAgentOptions(
        mcp_servers={
            "claude-code-docs": {
                "type": "http",
                "url": "https://code.claude.com/docs/mcp",
            }
        },
        allowed_tools=["mcp__claude-code-docs__*"],
    )

    async for message in query(
        prompt="Use the docs MCP server to explain what hooks are in Claude Code",
        options=options,
    ):
        if isinstance(message, ResultMessage) and message.subtype == "success":
            print(message.result)


asyncio.run(main())
```

The agent connects to the documentation server, searches for information about hooks, and returns the results.

## Add an MCP server

You can configure MCP servers in code when calling `query()`, or in a `.mcp.json` file loaded via [`settingSources`](#from-a-config-file).

### Add a server in code

Pass MCP servers directly in the `mcpServers` option. This example starts a local filesystem MCP server for `/Users/me/projects`. Replace that path with a directory on your machine:

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({
  prompt: "List files in my project",
  options: {
    mcpServers: {
      filesystem: {
        command: "npx",
        args: ["-y", "@modelcontextprotocol/server-filesystem", "/Users/me/projects"]
      }
    },
    allowedTools: ["mcp__filesystem__*"]
  }
})) {
  if (message.type === "result" && message.subtype === "success") {
    console.log(message.result);
  }
}
```

```python Python theme={null}
import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, ResultMessage


async def main():
    options = ClaudeAgentOptions(
        mcp_servers={
            "filesystem": {
                "command": "npx",
                "args": [
                    "-y",
                    "@modelcontextprotocol/server-filesystem",
                    "/Users/me/projects",
                ],
            }
        },
        allowed_tools=["mcp__filesystem__*"],
    )

    async for message in query(prompt="List files in my project", options=options):
        if isinstance(message, ResultMessage) and message.subtype == "success":
            print(message.result)


asyncio.run(main())
```

### Add a server from a config file

Create a `.mcp.json` file at your project root. The file is picked up when the `project` setting source is enabled, which it is for default `query()` options. If you set `settingSources` explicitly, include `"project"` for this file to load. Replace `/Users/me/projects` with a directory on your machine:

```json theme={null}
{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "/Users/me/projects"]
    }
  }
}
```

## Connection timing

Claude Code starts connecting your MCP servers when the session starts, and sends the [init message](#error-handling) once the first-turn wait, if any, ends. How long the first turn waits for a server depends on where you configured it. If you set `permissionPromptToolName`, the first turn always waits for that tool's server, up to `MCP_TIMEOUT`, wherever you configured it.

For a server in `options.mcpServers`, find its type in the first column to see how long the first turn waits for it.

| Server type | Delays the first turn? | First-turn wait timeout |
| :- | :- | :- |
| stdio server, or HTTP/SSE server without a cached tool list | Yes, until it connects | [`MCP_TIMEOUT`](https://code.claude.com/docs/en/env-vars), 30 seconds by default; the connection fails at that deadline |
| Remote server with a cached tool list, saved by Claude Code from a previous connection | No; the cached tools are available from the first turn | None; connects on its first tool call, and that deferred connect has its own timeout |
| In-process [SDK server](#sdk-mcp-servers) | Yes, until it connects and lists its tools | [`MCP_TIMEOUT`](https://code.claude.com/docs/en/env-vars), 30 seconds by default, per connect attempt; the connection fails at that deadline |

A server has a cached tool list only when the [MCP discovery cache](https://code.claude.com/docs/en/mcp.md#server-status-detail) is on.

Servers from [settings files](#from-a-config-file) such as `.mcp.json`, or from plugins, often still show `pending` in the init message. How long the first turn waits for them depends on `options.mcpServers` and tool search:

* **`options.mcpServers` includes a stdio, HTTP, or SSE server**: the first turn waits for the pending settings-file and plugin servers too, up to `MCP_TIMEOUT`.
* **Otherwise, when `ANTHROPIC_BASE_URL` points to a [gateway](https://code.claude.com/docs/en/mcp.md#tool-search-through-a-gateway) and [`ENABLE_TOOL_SEARCH`](https://code.claude.com/docs/en/mcp#configure-tool-search) is unset**: the first turn waits up to 2 seconds for every pending settings-file and plugin server.
* **Otherwise, with [tool search](https://code.claude.com/docs/en/agent-sdk/tool-search.md) on, which is the default**: the first turn waits up to 2 seconds, and only for pending servers that set [`alwaysLoad: true`](https://code.claude.com/docs/en/mcp#exempt-a-server-from-deferral) or that you [named](#named-servers). The rest keep connecting in the background, and Claude [reaches their tools](https://code.claude.com/docs/en/mcp.md#tool-availability) once they connect.
* **Otherwise, with tool search off**: the first turn waits up to 2 seconds for every pending settings-file and plugin server. Tool search is off when your [`ENABLE_TOOL_SEARCH` value or model](https://code.claude.com/docs/en/agent-sdk/tool-search#configure-tool-search) turns it off, or when you exclude the `ToolSearch` tool, for example through `disallowedTools`.

### Named servers

You name a server by listing any of its tools, such as `mcp__github__list_issues` or `mcp__github__*`, in [`allowedTools`](#allow-mcp-tools) or in a `permissions.allow` rule in the `settings` option. An [`mcp_tool` hook](https://code.claude.com/docs/en/hooks#mcp-tool-hook-fields) also names the server it calls when you configure it on a prompt-submission, tool-use, permission, subagent, task, elicitation, or worktree event. Before v2.1.284, naming a server had no effect on the first-turn wait.

### Set the first-turn wait yourself

To choose the first-turn wait yourself, set `CLAUDE_CODE_MCP_STARTUP_WAIT_MS` in the [`env` option](https://code.claude.com/docs/en/agent-sdk/configuration#set-environment-variables), for example `CLAUDE_CODE_MCP_STARTUP_WAIT_MS: "5000"`. The first turn then waits up to that many milliseconds for pending settings-file and plugin servers, whether or not tool search is on. The same limit replaces the `MCP_TIMEOUT` wait for the stdio, HTTP, and SSE servers in `options.mcpServers`.

Three waits apply whatever value you set, including `0`:

* **A `permissionPromptToolName` server**: up to `MCP_TIMEOUT`.
* **An in-process [SDK server](#sdk-mcp-servers)**: up to `MCP_TIMEOUT` per connect attempt.
* **A still-connecting [named server](#named-servers)**: at least up to 2 seconds, so a value below `2000` doesn't shorten its wait.

Set the variable to `0` to skip the wait for every other server. Servers still pending when the wait ends keep connecting in the background. `CLAUDE_CODE_MCP_STARTUP_WAIT_MS` requires Claude Code v2.1.274 or later.

### Make startup wait for servers

Two settings make startup itself wait, before Claude Code sends the init message:

* **[`MCP_CONNECTION_NONBLOCKING`](https://code.claude.com/docs/en/env-vars) set to `0`**: startup waits for the whole connection batch, up to 5 seconds by default. Set [`MCP_CONNECT_TIMEOUT_MS`](https://code.claude.com/docs/en/env-vars), in milliseconds, to change that cap. Servers still pending at the cap keep connecting in the background.
* **`alwaysLoad: true` on a server's config**: startup waits up to the same cap for that server's tools, so the first turn has them at full schema, [exempt from tool search deferral](https://code.claude.com/docs/en/mcp.md#exempt-a-server-from-deferral). A remote server with a cached tool list supplies them without connecting or holding startup.

The init message reports each server's status as of the moment Claude Code sends it.

## Allow MCP tools

MCP tools require explicit permission before Claude can use them. Without permission, Claude will see that tools are available but won't be able to call them.

### Tool naming convention

MCP tools are named `mcp__<server-name>__<tool-name>`, where `<server-name>` is the name you register the server under, such as its key in `mcpServers`.

Characters in the server name such as `.` and spaces are replaced by `_`. This table shows the full name of a `list_issues` tool for two server keys:

| Key in `mcpServers` | Full tool name |
| :- | :- |
| `github` | `mcp__github__list_issues` |
| `my.server` | `mcp__my_server__list_issues` |

Use the full tool names in `allowedTools` and in [MCP permission rules](https://code.claude.com/docs/en/permissions.md#mcp). For a server that a [plugin](https://code.claude.com/docs/en/agent-sdk/plugins.md) provides, `<server-name>` takes the [plugin-scoped form](https://code.claude.com/docs/en/mcp.md#plugin-provided-mcp-servers) and starts with `plugin_`.

### Auto-approve with allowedTools

Use `allowedTools` to auto-approve specific MCP tools:

```typescript TypeScript hidelines={1,-1} theme={null}
const _ = {
  options: {
    mcpServers: {
      // your servers
    },
    allowedTools: [
      "mcp__github__*", // All tools from the github server
      "mcp__db__query", // Only the query tool from db server
      "mcp__slack__send_message" // Only send_message from slack server
    ]
  }
};
```

```python Python theme={null}
options = ClaudeAgentOptions(
    mcp_servers={
        # your servers
    },
    allowed_tools=[
        "mcp__github__*",  # All tools from the github server
        "mcp__db__query",  # Only the query tool from db server
        "mcp__slack__send_message",  # Only send_message from slack server
    ],
)
```

Wildcards (`*`) let you allow all tools from a server without listing each one individually.

> [!NOTE]
>
> **Prefer `allowedTools` over permission modes for MCP access.** `permissionMode: "acceptEdits"` does not auto-approve MCP tools (only file edits and filesystem Bash commands). `permissionMode: "bypassPermissions"` does auto-approve MCP tools but also disables most other safety prompts, which is broader than necessary; see [How permissions are evaluated](https://code.claude.com/docs/en/agent-sdk/permissions.md#how-permissions-are-evaluated) for the prompts that remain. A wildcard in `allowedTools` grants exactly the MCP server you want and nothing more. See [Permission modes](https://code.claude.com/docs/en/agent-sdk/permissions.md#permission-modes) for a full comparison.

### Discover available tools

To see what tools an MCP server provides, check the server's documentation or inspect the `tools` array in the `system` init message. MCP tool names start with `mcp__`.

Claude Code emits the init message after the [first-turn connection wait](#connection-timing) for servers passed in `options.mcpServers`, so the `tools` array lists the `mcp__` tools of each server that has connected by then, plus those of servers with a [cached tool list](#connection-timing), which connect on first use. Tools of any other server that hasn't connected are absent; see [Error handling](#error-handling) for reading each server's status.

This filter prints the MCP tool names:

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

const options = {
  mcpServers: {
    // your servers
  },
};

for await (const message of query({ prompt: "...", options })) {
  if (message.type === "system" && message.subtype === "init") {
    const mcpTools = message.tools.filter((name) => name.startsWith("mcp__"));
    console.log("Available MCP tools:", mcpTools);
  }
}
```

```python Python theme={null}
import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, SystemMessage


async def main():
    options = ClaudeAgentOptions(
        mcp_servers={
            # your servers
        },
    )
    async for message in query(prompt="...", options=options):
        if isinstance(message, SystemMessage) and message.subtype == "init":
            mcp_tools = [t for t in message.data.get("tools", []) if t.startswith("mcp__")]
            print("Available MCP tools:", mcp_tools)


asyncio.run(main())
```

You can also ask Claude to list the tools available from a server.

## Transport types

MCP servers communicate with your agent using different transport protocols. Check the server's documentation to see which transport it supports:

* If the docs give you a **command to run** (like `npx @modelcontextprotocol/server-filesystem`), use stdio
* If the docs give you a **URL**, use HTTP or SSE
* If you're building your own tools in code, use an SDK MCP server

### stdio servers

Local processes that communicate via stdin/stdout. Use this for MCP servers you run on the same machine. For the `.mcp.json` form, use the same fields shown at [Add a server from a config file](#from-a-config-file). In code, pass the command and its arguments. Replace `/Users/me/projects` with a directory on your machine:

```typescript TypeScript hidelines={1,-1} theme={null}
const _ = {
  options: {
    mcpServers: {
      filesystem: {
        command: "npx",
        args: ["-y", "@modelcontextprotocol/server-filesystem", "/Users/me/projects"]
      }
    },
    allowedTools: ["mcp__filesystem__read_file", "mcp__filesystem__list_directory"]
  }
};
```

```python Python theme={null}
options = ClaudeAgentOptions(
    mcp_servers={
        "filesystem": {
            "command": "npx",
            "args": [
                "-y",
                "@modelcontextprotocol/server-filesystem",
                "/Users/me/projects",
            ],
        }
    },
    allowed_tools=["mcp__filesystem__read_file", "mcp__filesystem__list_directory"],
)
```

### HTTP/SSE servers

Use HTTP or SSE for cloud-hosted MCP servers and remote APIs. For the `.mcp.json` form, use the same fields as the example at [HTTP headers for remote servers](#http-headers-for-remote-servers), with `"type": "sse"` for an SSE server. In code, pass the server's URL:

```typescript TypeScript hidelines={1,-1} theme={null}
const _ = {
  options: {
    mcpServers: {
      "remote-api": {
        type: "sse",
        url: "https://api.example.com/mcp/sse",
        headers: {
          Authorization: `Bearer ${process.env.API_TOKEN}`
        }
      }
    },
    allowedTools: ["mcp__remote-api__*"]
  }
};
```

```python Python theme={null}
options = ClaudeAgentOptions(
    mcp_servers={
        "remote-api": {
            "type": "sse",
            "url": "https://api.example.com/mcp/sse",
            "headers": {"Authorization": f"Bearer {os.environ['API_TOKEN']}"},
        }
    },
    allowed_tools=["mcp__remote-api__*"],
)
```

For the streamable HTTP transport, use `"type": "http"` instead. In `.mcp.json` and other JSON config files, `"streamable-http"` is accepted as an alias for `"http"`. The SDKs' `McpHttpServerConfig` type declares only `"http"`, so use `"http"` for servers you pass in code.

### SDK MCP servers

Define custom tools directly in your application code instead of running a separate server process. See the [custom tools guide](https://code.claude.com/docs/en/agent-sdk/custom-tools.md) for implementation details.

An SDK MCP server registered by an [`initialize` control request](https://code.claude.com/docs/en/agent-sdk/typescript#sdkcontrolinitializeresponse) begins connecting as soon as Claude Code processes the request.

## MCP tool search

When you have many MCP tools configured, tool definitions can consume a significant portion of your context window. Tool search solves this by withholding tool definitions from context and loading only the ones Claude needs for each turn.

Tool search is enabled by default. See [Tool search](https://code.claude.com/docs/en/agent-sdk/tool-search.md) for configuration options, best practices, and using tool search with custom SDK tools.

## Authentication

Most MCP servers require authentication to access external services. Pass credentials through environment variables in the server configuration.

### Pass credentials via environment variables

Use the `env` field to pass API keys, tokens, and other credentials to the MCP server:

> [tab] In code
>
> ```typescript TypeScript hidelines={1,-1} theme={null}
> const _ = {
>   options: {
>     mcpServers: {
>       "api-server": {
>         command: "npx",
>         args: ["-y", "@your-org/api-mcp-server"],
>         env: {
>           API_KEY: process.env.API_KEY
>         }
>       }
>     },
>     allowedTools: ["mcp__api-server__*"]
>   }
> };
> ```
>
> ```python Python theme={null}
> options = ClaudeAgentOptions(
>     mcp_servers={
>         "api-server": {
>             "command": "npx",
>             "args": ["-y", "@your-org/api-mcp-server"],
>             "env": {"API_KEY": os.environ["API_KEY"]},
>         }
>     },
>     allowed_tools=["mcp__api-server__*"],
> )
> ```

> [tab] .mcp.json
>
> ```json theme={null}
> {
>   "mcpServers": {
>     "api-server": {
>       "command": "npx",
>       "args": ["-y", "@your-org/api-mcp-server"],
>       "env": {
>         "API_KEY": "${API_KEY}"
>       }
>     }
>   }
> }
> ```
>
> The `${API_KEY}` syntax expands environment variables at runtime.

### HTTP headers for remote servers

For HTTP and SSE servers, pass authentication headers directly in the server configuration:

> [tab] In code
>
> ```typescript TypeScript hidelines={1,-1} theme={null}
> const _ = {
>   options: {
>     mcpServers: {
>       "secure-api": {
>         type: "http",
>         url: "https://api.example.com/mcp",
>         headers: {
>           Authorization: `Bearer ${process.env.API_TOKEN}`
>         }
>       }
>     },
>     allowedTools: ["mcp__secure-api__*"]
>   }
> };
> ```
>
> ```python Python theme={null}
> options = ClaudeAgentOptions(
>     mcp_servers={
>         "secure-api": {
>             "type": "http",
>             "url": "https://api.example.com/mcp",
>             "headers": {"Authorization": f"Bearer {os.environ['API_TOKEN']}"},
>         }
>     },
>     allowed_tools=["mcp__secure-api__*"],
> )
> ```

> [tab] .mcp.json
>
> ```json theme={null}
> {
>   "mcpServers": {
>     "secure-api": {
>       "type": "http",
>       "url": "https://api.example.com/mcp",
>       "headers": {
>         "Authorization": "Bearer ${API_TOKEN}"
>       }
>     }
>   }
> }
> ```
>
> The `${API_TOKEN}` syntax expands environment variables at runtime.

For a complete working example of a remote server authenticated with headers, see [List issues from a repository](#list-issues-from-a-repository).

### OAuth2 authentication

The [MCP specification supports OAuth 2.1](https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization) for authorization. The SDK doesn't open a browser or run an interactive OAuth flow. When a configured server returns an authorization challenge and Claude Code has no token to send, the SDK reports the server as `needs-auth` or `failed`:

* **`needs-auth`**: Claude Code found no sign-in it can use for the server. Pass a token in `headers`, or [sign in from an interactive Claude Code session](https://code.claude.com/docs/en/mcp.md#authenticate-with-remote-mcp-servers) on the same machine before the run.
* **`failed`**, with an `error` in its [`McpServerStatus`](https://code.claude.com/docs/en/agent-sdk/typescript#mcpserverstatus) entry that starts with `Couldn't read the stored credentials` or `A request to MCP server`: Claude Code couldn't read its credential store. For what to do, see [Couldn't read the stored credentials for MCP server](https://code.claude.com/docs/en/errors.md#couldnt-read-the-stored-credentials-for-mcp-server). Before Claude Code v2.1.307, the status was `needs-auth` instead when a Mac's login keychain was locked.

In either case, the agent run continues without that server's tools. The `mcp_servers` array of the [system init message](https://code.claude.com/docs/en/agent-sdk/typescript.md#sdksystemmessage) may still show `pending` for that server when it's emitted. To confirm whether a server needs credentials, poll `mcpServerStatus()` in the TypeScript SDK or [`get_mcp_status()`](https://code.claude.com/docs/en/agent-sdk/python#methods) in Python.

To supply credentials, complete the OAuth flow in your own application and pass the resulting access token in the server's `headers`:

```typescript TypeScript theme={null}
// After completing OAuth flow in your app.
// Implement getAccessTokenFromOAuthFlow for your OAuth provider.
const accessToken = await getAccessTokenFromOAuthFlow();

const options = {
  mcpServers: {
    "oauth-api": {
      type: "http",
      url: "https://api.example.com/mcp",
      headers: {
        Authorization: `Bearer ${accessToken}`
      }
    }
  },
  allowedTools: ["mcp__oauth-api__*"]
};
```

```python Python theme={null}
# After completing OAuth flow in your app.
# Implement get_access_token_from_oauth_flow for your OAuth provider.
access_token = await get_access_token_from_oauth_flow()

options = ClaudeAgentOptions(
    mcp_servers={
        "oauth-api": {
            "type": "http",
            "url": "https://api.example.com/mcp",
            "headers": {"Authorization": f"Bearer {access_token}"},
        }
    },
    allowed_tools=["mcp__oauth-api__*"],
)
```

## Examples

### List issues from a repository

This example connects to the remote [GitHub MCP server](https://github.com/github/github-mcp-server) to list recent issues. The example includes debug logging to verify the MCP connection and tool calls.

Before running, create a [GitHub personal access token](https://github.com/settings/personal-access-tokens) with read access to the repositories you want to query and set it as an environment variable:

```bash theme={null}
export GITHUB_TOKEN=YOUR_GITHUB_PAT
```

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({
  prompt: "List the 3 most recent issues in anthropics/claude-code",
  options: {
    mcpServers: {
      github: {
        type: "http",
        url: "https://api.githubcopilot.com/mcp/",
        headers: {
          Authorization: `Bearer ${process.env.GITHUB_TOKEN}`
        }
      }
    },
    allowedTools: ["mcp__github__list_issues"]
  }
})) {
  // Verify MCP server connected successfully
  if (message.type === "system" && message.subtype === "init") {
    console.log("MCP servers:", message.mcp_servers);
  }

  // Log when Claude calls an MCP tool
  if (message.type === "assistant") {
    for (const block of message.message.content) {
      if (block.type === "tool_use" && block.name.startsWith("mcp__")) {
        console.log("MCP tool called:", block.name);
      }
    }
  }

  // Print the final result
  if (message.type === "result" && message.subtype === "success") {
    console.log(message.result);
  }
}
```

```python Python theme={null}
import asyncio
import os
from claude_agent_sdk import (
    query,
    ClaudeAgentOptions,
    ResultMessage,
    SystemMessage,
    AssistantMessage,
)


async def main():
    options = ClaudeAgentOptions(
        mcp_servers={
            "github": {
                "type": "http",
                "url": "https://api.githubcopilot.com/mcp/",
                "headers": {"Authorization": f"Bearer {os.environ['GITHUB_TOKEN']}"},
            }
        },
        allowed_tools=["mcp__github__list_issues"],
    )

    async for message in query(
        prompt="List the 3 most recent issues in anthropics/claude-code",
        options=options,
    ):
        # Verify MCP server connected successfully
        if isinstance(message, SystemMessage) and message.subtype == "init":
            print("MCP servers:", message.data.get("mcp_servers"))

        # Log when Claude calls an MCP tool
        if isinstance(message, AssistantMessage):
            for block in message.content:
                if hasattr(block, "name") and block.name.startswith("mcp__"):
                    print("MCP tool called:", block.name)

        # Print the final result
        if isinstance(message, ResultMessage) and message.subtype == "success":
            print(message.result)


asyncio.run(main())
```

In the `MCP servers:` line, a `status` of `connected` for `github` confirms the token works. If Claude Code has a [cached tool list](#connection-timing) for the server, the status can read `pending` instead and the server connects on its first tool call. If the status is `failed` or `needs-auth`, see [Error handling](#error-handling) before trusting the result, since Claude can fall back to built-in tools when the server is unavailable.

### Query a database

This example uses [DBHub](https://github.com/bytebase/dbhub) to query a Postgres database. The agent automatically discovers the database schema, writes the SQL query, and returns the results.

DBHub's `execute_sql` tool runs whatever SQL the agent emits, including writes, unless you restrict it. Setting `readonly = true` in the [DBHub configuration file](https://dbhub.ai/config/toml) makes DBHub reject `INSERT`, `UPDATE`, `DELETE`, and DDL statements, so the example cannot modify your data even if the agent emits a write. DBHub resolves `${DATABASE_URL}` from the process environment when it loads the config, so the connection string stays out of the file. Create this `dbhub.toml` next to your script:

```toml dbhub.toml theme={null}
[[sources]]
id = "production"
dsn = "${DATABASE_URL}"

[[tools]]
name = "execute_sql"
source = "production"
readonly = true
```

The script then points DBHub at the config file instead of passing a connection string directly. Before running, set the `DATABASE_URL` environment variable to your connection string. Replace the placeholder values with your own database details:

```bash theme={null}
export DATABASE_URL=postgresql://user:password@localhost:5432/mydb
```

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({
  // Natural language query - Claude writes the SQL
  prompt: "How many users signed up last week? Break it down by day.",
  options: {
    mcpServers: {
      postgres: {
        command: "npx",
        // dbhub.toml sets readonly = true, so execute_sql rejects writes
        args: ["-y", "@bytebase/dbhub", "--config", "dbhub.toml"]
      }
    },
    allowedTools: ["mcp__postgres__execute_sql"]
  }
})) {
  if (message.type === "result" && message.subtype === "success") {
    console.log(message.result);
  }
}
```

```python Python theme={null}
import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, ResultMessage


async def main():
    options = ClaudeAgentOptions(
        mcp_servers={
            "postgres": {
                "command": "npx",
                # dbhub.toml sets readonly = true, so execute_sql rejects writes
                "args": [
                    "-y",
                    "@bytebase/dbhub",
                    "--config",
                    "dbhub.toml",
                ],
            }
        },
        allowed_tools=["mcp__postgres__execute_sql"],
    )

    # Natural language query - Claude writes the SQL
    async for message in query(
        prompt="How many users signed up last week? Break it down by day.",
        options=options,
    ):
        if isinstance(message, ResultMessage) and message.subtype == "success":
            print(message.result)


asyncio.run(main())
```

## Error handling

MCP servers can fail to connect for various reasons: the server process might not be installed, credentials might be invalid, or a remote server might be unreachable.

Claude Code emits a `system` message with subtype `init` at the start of each query. This message includes the connection status for each MCP server. The `status` field can be `"pending"`, `"connected"`, `"failed"`, `"needs-auth"`, or `"disabled"`. Claude Code emits the init message after the [first-turn connection wait](#connection-timing) for servers passed in `options.mcpServers`, so such a server that connected within the wait shows `"connected"`.

In the init message, don't treat `"pending"` as a failure on its own. It can mean any of these:

* The server hasn't connected yet. See [how long Claude Code waits for it before the first turn](#connection-timing)
* The server's tool list was [served from the cache](#connection-timing), with a connection made on first use
* The connection deadline expired. Such a server reports `"pending"` or `"failed"` depending on timing

Check for `"failed"` or `"needs-auth"` to detect servers that won't be usable:

```typescript TypeScript theme={null}
import { query } from "@anthropic-ai/claude-agent-sdk";

try {
  for await (const message of query({
    prompt: "Process data",
    options: {
      mcpServers: {
        // Replace dataServer with your server configuration
        "data-processor": dataServer
      }
    }
  })) {
    if (message.type === "system" && message.subtype === "init") {
      const unavailableServers = message.mcp_servers.filter(
        (s) => s.status === "failed" || s.status === "needs-auth"
      );

      if (unavailableServers.length > 0) {
        console.warn("Unavailable MCP servers:", unavailableServers);
      }
    }

    if (message.type === "result" && message.subtype === "error_during_execution") {
      console.error("Execution failed");
    }
  }
} catch (error) {
  // A single-shot query() throws after yielding an error result. If the
  // failure was an error result, the error subtype branch above has
  // already run; a failure to start or reach the Claude Code process
  // yields no result message. MCP servers that fail to connect don't
  // throw: use the status check above, and note that servers still
  // "pending" at init need a later status check.
  console.log(`Session ended with an error: ${error}`);
}
```

```python Python theme={null}
import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, SystemMessage, ResultMessage


async def main():
    # Replace data_server with your server configuration
    options = ClaudeAgentOptions(mcp_servers={"data-processor": data_server})

    try:
        async for message in query(prompt="Process data", options=options):
            if isinstance(message, SystemMessage) and message.subtype == "init":
                unavailable_servers = [
                    s
                    for s in message.data.get("mcp_servers", [])
                    if s.get("status") in ("failed", "needs-auth")
                ]

                if unavailable_servers:
                    print(f"Unavailable MCP servers: {unavailable_servers}")

            if (
                isinstance(message, ResultMessage)
                and message.subtype == "error_during_execution"
            ):
                print("Execution failed")
    except Exception as error:
        # A single-shot query() raises after yielding an error result. If the
        # failure was an error result, the error subtype branch above has
        # already run; a failure to start or reach the Claude Code process
        # yields no result message. MCP servers that fail to connect don't
        # raise: use the status check above, and note that servers still
        # "pending" at init need a later status check.
        print(f"Session ended with an error: {error}")


asyncio.run(main())
```

A remote server's status can also change after it reports `"connected"`. When the connection to it drops mid-session, Claude Code moves the server back to `"pending"` while [reconnecting](https://code.claude.com/docs/en/mcp.md#automatic-reconnection). A later `mcpServerStatus()` call in TypeScript, or [`ClaudeSDKClient.get_mcp_status()`](https://code.claude.com/docs/en/agent-sdk/python#methods) in Python, can then report `"pending"` for a server you saw connected earlier, with no configuration change on your side.

After five reconnection attempts fail, the server reports `"failed"`, or `"needs-auth"` when it needs authorizing again. A `"failed"` HTTP or SSE server can still return to `"connected"` without a call from you, because Claude Code keeps [retrying in the background](https://code.claude.com/docs/en/mcp.md#mid-session-drops-of-a-remote-server) after a transient error. To retry manually, call [`reconnectMcpServer()`](https://code.claude.com/docs/en/agent-sdk/typescript#methods) in TypeScript or [`ClaudeSDKClient.reconnect_mcp_server()`](https://code.claude.com/docs/en/agent-sdk/python#methods) in Python.

## Troubleshooting

### Server shows "failed" status

Check the `init` message to see which servers failed to connect:

```typescript TypeScript theme={null}
if (message.type === "system" && message.subtype === "init") {
  for (const server of message.mcp_servers) {
    if (server.status === "failed") {
      console.error(`Server ${server.name} failed to connect`);
    }
  }
}
```

```python Python theme={null}
if isinstance(message, SystemMessage) and message.subtype == "init":
    for server in message.data.get("mcp_servers", []):
        if server.get("status") == "failed":
            print(f"Server {server['name']} failed to connect")
```

A `"pending"` status doesn't mean the server failed. See [Error handling](#error-handling) for the cases it covers at init. To get updated statuses later in the session, call the query's `mcpServerStatus()` method in the TypeScript SDK, or [`ClaudeSDKClient.get_mcp_status()`](https://code.claude.com/docs/en/agent-sdk/python#methods) in Python.

Common causes:

* **Missing environment variables**: Ensure required tokens and credentials are set. For stdio servers, check the `env` field matches what the server expects.
* **Server not installed**: For `npx` commands, verify the package exists and Node.js is in your PATH.
* **Invalid connection string**: For database servers, verify the connection string format and that the database is accessible.
* **Network issues**: For remote HTTP/SSE servers, check the URL is reachable and any firewalls allow the connection.

### Tools not being called

If Claude sees tools but doesn't use them, check that you've granted permission with `allowedTools`:

```typescript TypeScript hidelines={1,-1} theme={null}
const _ = {
  options: {
    mcpServers: {
      // your servers
    },
    allowedTools: ["mcp__servername__*"] // Auto-approve calls from this server
  }
};
```

```python Python theme={null}
options = ClaudeAgentOptions(
    mcp_servers={
        # your servers
    },
    allowed_tools=["mcp__servername__*"],  # Auto-approve calls from this server
)
```

### A tool is missing from an SDK MCP server

In the TypeScript SDK, when a tool's input schema can't be converted to JSON Schema, the server you created with [`createSdkMcpServer()`](https://code.claude.com/docs/en/agent-sdk/typescript#createsdkmcpserver) leaves that tool out when it lists its tools. The SDK emits a warning at that point. Under Node.js the warning is a process warning with code `CLAUDE_SDK_MCP_TOOL_SCHEMA_UNCONVERTIBLE`, and it starts with this text:

```text theme={null}
Tool "<name>" on SDK MCP server "<server>" was left out of the server's tool list, because its input schema cannot be converted to JSON Schema
```

The rest of the warning gives the conversion error's message when it has one, then says what to check and change.

Before TypeScript Agent SDK v0.3.286, one unconvertible schema made the server's whole tool listing fail without this warning, so none of that server's tools reached Claude.

### Connection timeouts

MCP server connections time out after 30 seconds by default. To change how long a running tool call may take, set [`MCP_TOOL_TIMEOUT`](https://code.claude.com/docs/en/env-vars). If your server takes longer to start, the connection fails. Raise the connection limit with the [`MCP_TIMEOUT`](https://code.claude.com/docs/en/env-vars) environment variable, in milliseconds. For servers that need more startup time, also consider:

* Using a lighter-weight server if available
* Pre-warming the server before starting your agent
* Checking server logs for slow initialization causes

In TypeScript, you can set the tool-call limit for a single [SDK MCP server](#sdk-mcp-servers) by passing [`timeout` to `createSdkMcpServer()`](https://code.claude.com/docs/en/agent-sdk/typescript#createsdkmcpserver).

### Tool output exceeds maximum allowed tokens

The SDK applies the same MCP output limits as Claude Code. When a successful tool result with no image content is larger than 25,000 tokens, Claude Code saves the output to a file and replaces the tool result with an error message that names the file path, so the agent can read the output back in portions.

To change the token limit, set the [`MAX_MCP_OUTPUT_TOKENS`](https://code.claude.com/docs/en/env-vars) environment variable. Unless the tool declares `anthropic/maxResultSizeChars`, a successful text result longer than 50,000 characters is saved to a file regardless of the token limit. See [MCP output limits and warnings](https://code.claude.com/docs/en/mcp.md#mcp-output-limits-and-warnings) for the full behavior, including how a server declares that annotation.

## Related resources

* **[Custom tools guide](https://code.claude.com/docs/en/agent-sdk/custom-tools.md)**: Build your own MCP server that runs in-process with your SDK application
* **[Permissions](https://code.claude.com/docs/en/agent-sdk/permissions.md)**: Control which MCP tools your agent can use with `allowedTools` and `disallowedTools`
* **[TypeScript SDK reference](https://code.claude.com/docs/en/agent-sdk/typescript.md)**: Full API reference including MCP configuration options
* **[Python SDK reference](https://code.claude.com/docs/en/agent-sdk/python.md)**: Full API reference including MCP configuration options
* **[MCP server directory](https://github.com/modelcontextprotocol/servers)**: Browse available MCP servers for databases, APIs, and more