Claude Code MCP Runtime

Claude Code MCP Runtime

After an MCP server connects, Claude Code adds its tools, resources, and prompts to the session. The names, the permission rules, and the limits decide what Claude can use and for how long. A tool from a server is named mcp__<server>__<tool>. For example, mcp__github__create_issue is the create_issue tool of the github server. The marks are explained on MCP.

Tools and Names

Mark	Item	What It Does	How It Works	Where It Can Be Used	Source
✅	mcp__<server>__<tool>	Name of an MCP tool	Characters outside A-Z a-z 0-9 _ - in the server name become _. A tool with a full name over 128 characters is left out	Permission rules, hooks, tools, allowed-tools	Connect Claude Code to Tools via MCP, Configure Permissions
✅	mcp__plugin_<plugin>_<server>__<tool>	Name of a plugin server tool	Hook matchers on the bare server key never fire for it. The server is named plugin:<plugin>:<server>	Same	-
✅	Patterns mcp__github, mcp__github__*, mcp__*	Select tools by server	mcp__github and mcp__github__* match one server. mcp__* matches all MCP tools	Subagent tools and disallowedTools	-
✅	Hook matcher mcp__memory__.*	Matches a server in hooks	A matcher with only exact-match characters, such as mcp__memory, matches no tool. The .* is required	PreToolUse, PostToolUse and others	-
✅	mcp_server hook input field	Gives the origin of the server	Object with name and source (plugin, sdk, user, project, ...)	Tool hooks	-
✅	mcp_tool hook type	Calls an MCP tool as a hook	Output is treated as command stdout. Skipped on Setup and at SessionStart before servers connect	Hooks	Connect Claude Code to Tools via MCP, Settings Reference
✅	updatedMCPToolOutput	Replaces an MCP tool result	PostToolUse hook output, MCP tools only	PostToolUse	-
✅	ToolSearch	Finds deferred MCP tools	On by default. permissions.deny: ["ToolSearch"] turns the tool off	Main session	Connect Claude Code to Tools via MCP
✅	WaitForMcpServers	Waits for a connecting server	Used when tool search is off. With tool search the wait is inside ToolSearch	Main session, subagents	Connect Claude Code to Tools via MCP
🔍	SearchMcpRegistry, SuggestConnectors, ListConnectors	Find and list MCP connectors	SearchMcpRegistry searches the connector registry by keyword. ListConnectors lists the connectors of the claude.ai org.	Main session	Bundle: tool descriptions

Resources and Prompts

Mark	Item	What It Does	How It Works	Where It Can Be Used	Source
✅	@server:protocol://path	Attaches a resource	Fetched as an attachment. Fuzzy search in the @ menu	Prompt input	MCP docs, Reference MCP resources
✅	ListMcpResourcesTool	Lists resources of all servers	The docs say Claude Code provides tools to list and read resources but give no names. Each entry has a server field. ui:// entries are hidden	Model	-
✅	ReadMcpResourceTool	Reads one resource	Parameters server and uri (both required)	Model	-
🔍	ReadMcpResourceDirTool	Lists children of a directory resource	Parameters server and uri. Uses resources/directory/read. Not recursive. Only for a server that declares directory support	Model	Bundle: tool text
🔍	Old tool names	Maps old names to new	ListMcpResources, ReadMcpResource, ReadMcpResourceDir map to the ...Tool names	Rules, hooks	Bundle: alias map
✅	/server:prompt (MCP) and /mcp__server__prompt args	Runs a server prompt	Arguments split on whitespace. Results go into the conversation. Needs the prompt capability	Slash menu	MCP docs, Execute MCP prompts
✅	MCP Apps resources	UI pages for a host	ui:// URI or media type text/html with the profile parameter mcp-app. Reading by URI still works	Resources	MCP docs, Use MCP resources
🔍	CLAUDE_CODE_MCP_APPS_HOST	Declares MCP Apps support	Adds the extension io.modelcontextprotocol/ui with the MIME type to the client capabilities	Env	Bundle: capability builder

Elicitation and Channels

Mark	Item	What It Does	How It Works	Where It Can Be Used	Source
✅	Elicitation dialog	Asks you for input	form mode shows fields. url mode asks to open a link. The escaped URL has a cap (about 8000 characters). Declared elicitation: {form: {}, url: {}}	Interactive session	MCP docs, Respond to MCP elicitation requests
✅	Elicitation hook	Answers a request	Input: mcp_server_name, message, mode, url, elicitation_id, requested_schema. Output action: accept (with content), decline, cancel	Hooks	Connect Claude Code to Tools via MCP
✅	ElicitationResult hook	Observes or overrides the answer	Input has action and content. Runs before the answer goes to the server	Hooks	-
✅	claude/channel capability	Lets a server push messages	You opt in with --channels. Not registered on a v2 connection that negotiated revision 2026-07-28	Servers	Connect Claude Code to Tools via MCP, Channels Reference
✅	claude/channel/permission	Channel permission relay	Notifications notifications/claude/channel/permission_request and .../permission.	Channel servers	Channels Reference
✅	channels plugin key	Binds a channel to a plugin server	Fields server, displayName, userConfig	plugin.json	-

Limits

Mark	Item	What It Does	How It Works	Where It Can Be Used	Source
✅	Output warning	Warns on large output	Fixed at 10000 tokens	MCP tool results	MCP docs, MCP output limits
✅	MAX_MCP_OUTPUT_TOKENS	Token limit of a result	Default 25000. Over the limit, Claude Code saves the result to a file in tool-results and shows the path. Images stay under this limit	MCP tool results	Connect Claude Code to Tools via MCP, Environment Variables
✅	50000 characters	File threshold for text	A successful text result over 50000 characters is saved to a file, whatever the token count	MCP tool results	MCP docs, MCP output limits
🔍	ENABLE_MCP_LARGE_OUTPUT_FILES	Switch for the file path	When false, Claude Code truncates instead of saving to a file	Env	Bundle: large-result handler
✅	_meta["anthropic/maxResultSizeChars"]	Raises the threshold for one tool	Up to 500000 characters. Text only	tools/list entry	Connect Claude Code to Tools via MCP
✅	_meta["anthropic/requiresUserInteraction"]	Always asks	Only JSON true counts. Prompt in all modes. dontAsk denies. --permission-prompt-tool allow becomes deny	tools/list entry	Connect Claude Code to Tools via MCP
✅	_meta["anthropic/alwaysLoad"]	Loads a tool upfront	false keeps it deferred when the user config sets alwaysLoad: true (2.1.285 and later)	tools/list entry	Connect Claude Code to Tools via MCP
✅	Error text	Shortens long errors	Over about 11000 characters, the first 5000 and last 5000 stay	isError: true results	MCP docs, MCP output limits
✅	16 MiB response	Cap of one response or message	HTTP and SSE stop reading. A WebSocket closes the connection	Remote servers	MCP docs, MCP output limits
✅	Description limits	Cuts long text	4096 characters for server instructions and an upfront tool. 16384 for a tool found by tool search. CLAUDE_CODE_MAX_MCP_DESCRIPTION_LENGTH replaces both	Tool search	MCP docs, Tool search for MCP server authors
✅	Schema checks	Drops bad tools	Excludes a tool with a bad top-level property name or an invalid 2020-12 schema. Flattens a root anyOf, oneOf, allOf	Tool load	MCP docs, Tools with invalid input schemas

Timing and Recovery

Mark	Item	What It Does	How It Works	Where It Can Be Used	Source
✅	Auto background	Moves a long call to a task	After 120000 ms in the main conversation. Not for subagents, IDE servers, or -p unless CLAUDE_AUTO_BACKGROUND_TASKS=1	Main session	MCP docs, Automatic backgrounding
✅	Idle timeout	Aborts a silent call	5 minutes for remote servers, 30 minutes for stdio. CLAUDE_CODE_MCP_TOOL_IDLE_TIMEOUT changes it. 0 turns it off	All but IDE and sdk	MCP docs, Tips
✅	Reconnection	Retries a dropped remote server	Five tries with backoff from 1 s. Stdio is not reconnected. First connect retries 3 times except ws and auth errors	Remote servers	MCP docs, Automatic reconnection
✅	list_changed	Refreshes tools, prompts, resources	Interactive sessions refresh all three. -p refreshes tools only. A failed refresh keeps the old lists	Connected servers	Connect Claude Code to Tools via MCP
✅	Stdio shutdown	Stops a stdio server	macOS and Linux: SIGINT, SIGTERM after 100 ms, SIGKILL after 400 ms more	Stdio servers	MCP docs, How Claude Code stops a stdio server
✅	roots/list	Gives allowed directories	Answers with the launch directory and added directories	Servers	Connect Claude Code to Tools via MCP
✅	Tool search modes	Controls deferral	ENABLE_TOOL_SEARCH: unset, true, auto, auto:N, false	Env	MCP docs, Configure tool search

See Also

• Connect Claude Code to Tools via MCP is the docs page.
• Tools explains tool search and deferral.