deepseek-harness/docs/subsystems/mcp.md

MCP

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Summary

Model Context Protocol (MCP) connects the model to tools supplied by external servers. Each configured server contributes ordinary harness tools with cancellation, permission checks, recorded results, and supported image output. Shared tools discover and read resources when a server is configured in the caller's scope, while server instructions join the logged system prompt. The official SDK negotiates modern or supported legacy protocol revisions. This reference covers the MCP package group's responsibilities, scope, and composition choices; the client README owns server configuration.

Table of Contents

• Configuration
• Responsibilities and scope
• Protocol and results
• Resources and instructions
• Resource provider types
• Limits
• Further reading

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Configuration

MCP servers are opt-in. Configure one @deepseek-ai/dsh-mcp-client entry per server in the intended Cordis scope. Every shipped profile supplies the tool registry and mounts the shared resource service once; users configure only client entries. Callers with no visible configured server receive no MCP prompt text or tools in native or PTC mode.

The Web and Desktop profiles also mount the MCP server manager once. It runs servers configured through Settings and servers that installed plugins register for their own lifetime, and serves their status, restart, and probe operations to Remote clients.

Choice	Configuration owner
Server identity, local process or HTTP endpoint, credentials, and process environment	Client configuration
Tool and resource request timeout, startup failure policy, and reconnection	Client configuration
Resource discovery and reading	The MCP resource service is included in shipped profiles; it has no configuration fields
Server instruction size limit	Client maxInstructionBytes; the composition supplies system-prompt assembly
Permission decisions and supported image output	Tool execution and attachments

Protocol negotiation follows the SDK's supported revisions; there is no product setting that forces a protocol revision. The configuration catalog lists accepted client fields and defaults.

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Responsibilities and scope

The client is a per-server connection plugin and a consumer of the harness tool registry. It does not publish a shared ctx.mcp service; the manager publishes ctx.mcpManager for the Host-global servers it mounts. The external server implements MCP operations; the SDK owns protocol exchange; the client adapts discovered tools to harness execution.

mcp-resources owns the shared resource tools and selects providers in the caller's scope. Each MCP client supplies resource operations through its own connection. The first provider in a scope enables the local shared tools, and removing the last removes them; inherited providers remain visible. The service owns these tool registrations independently of any one client. Connection failures do not remove shared resource tools while a visible client entry remains active.

Configured serverName identifies a server in its registration scope. Two entries in that scope cannot reserve the same name; separate Agent scopes can reuse it. Public tool names include the configured server name, so equally named tools from different servers remain distinct. Registration effects own names and discovered tools; plugin disposal closes the connection and removes its contributions.

The native Cua Driver provider shares the client's exported result adapter without opening an MCP connection. Desktop provider selection belongs to the computer-use subsystem.

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Protocol and results

Both stdio and Streamable HTTP use the official SDK's negotiation, discovery, protocol validation, and cancellation. Tool-list changes trigger discovery through legacy notifications or a modern subscription. A failed refresh retains the previous tool generation; connection recovery follows the client lifecycle.

The result adapter retains canonical MCP JSON for programmatic callers and prepares ordinary tool content. Supported images use the attachment system; unsupported rich content produces explicit text diagnostics. The tool registry remains authoritative for policy failures and replaced results. The tool contracts own recording and final presentation; the client result reference owns MCP-specific projection details.

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Resources and instructions

Resource calls require an explicit configured server name. When system-prompt assembly is available, the resource service lists caller-visible names from the same registry used for dispatch, including servers with no tools or instructions. The shared registry resolves that name in the calling Agent's scope before dispatch; unavailable servers fail without a network request. Discovery and reads are on demand, including for servers that expose resources without tools. The resource package owns pagination and content rendering; its generated tool schemas live in the tool catalog.

Resource providers remain connection-owned. Scope disposal removes registrations; the MCP client controls cancellation and recovery. Canonical results retain complete JSON for programmatic callers, while the text projection replaces binary blobs with descriptions. Returned text enters ordinary tool history; content is not fetched merely because a server connects.

When system-prompt assembly is composed, the client publishes nonblank server instructions as a scoped, server-attributed section. Instructions remain literal text and pass the configured size limit before publication. A replacement connection publishes instructions only after discovery succeeds; absent instructions add no section. The system-prompt subsystem owns assembly and recording.

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Resource provider types

The connection provider receives one operation and the original tool execution, including its caller and cancellation signal.

/** One supported resource operation, with server-owned cursors and URIs. */
type McpResourceRequest =
  | { method: 'resources/list' | 'resources/templates/list'; cursor?: string }
  | { method: 'resources/read'; uri: string }

/** One configured server's resource access, owned by its MCP connection plugin. */
interface McpResourceProvider {
  /**
   * Run an operation against one live connection generation.
   * @param request - MCP resource method and parameters.
   * @param exec - caller identity and cancellation for this invocation.
   * @returns the protocol result as lossless JSON.
   */
  request(request: McpResourceRequest, exec: ToolExecution): Promise<JsonValue>
}

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Limits

MCP prompt templates, human-input elicitation, task-based execution, and resource subscriptions are unsupported. Resource tools require a caller-visible configured server; binary resources remain programmatic data with text descriptions for the model. Servers without a tools capability connect with an empty tool set. Connection and discovery timeouts follow the SDK; the client has no separate settings for them.

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Further reading

• MCP package group — package entry points.
• MCP resources — shared tools and resource-provider semantics.
• MCP manager — Settings-configured and plugin-registered servers, status, and Remote operations.
• Resource visibility reference — shared profile mounting and visibility from configured servers.
• Third-party memory servers — product configuration guide.
• Protocol negotiation reference — SDK ownership and compatibility decisions.

Cordis API

Generated from source by scripts/gen-cordis-catalog.ts (verified fresh by pnpm run verify-cordis-catalog in doc-sync; regenerate with pnpm run gen-cordis-catalog) — the language sides differ only in locale-specific paired document paths. Signature blocks use a ts cordis-catalog fence and keep the original source JSDoc; dispatch modes are defined in the primer, and the framework-inherited ctx API lives in cordis-api/inherited.md.

ctx.mcpManager — McpServerManager

The manager service. Reconciliation and restart serialize instance replacement. Probes run independently and are cancelled before replacement or disposal waits for their cleanup.

/**
 * Read configured and registered servers and retained runtime records without mounting or probing.
 * @returns configured entries in document order, then registered entries, then retained records for removed entries.
 */
list(): McpManagedServerView[]

/**
 * Contribute one server for the calling plugin's lifetime. A registered
 * entry is enabled while registered; its removal awaits the instance's
 * unmount, so plugin disposal withdraws the server's tools before settling.
 * @param name - server key and tool namespace, unique across configured and registered servers.
 * @param entry - transport, endpoint, and budgets, validated like a configured entry.
 * @returns an idempotent disposer that removes the registration.
 * @throws when the name is taken, the entry is invalid, or the manager is disposed.
 */
register(name: string, entry: McpServerRegistration): () => void

/**
 * Stream server views for a management surface: the current list on
 * subscription, then the complete list again after each status or entry change.
 * @param signal - subscription lifetime; aborting ends only this stream.
 * @returns coalesced server lists until the subscriber or the manager stops.
 */
@Remote({ mode: 'stream' }) async* watch(signal: AbortSignal): AsyncIterable<McpManagedServerView[]>

/**
 * Rebuild one server from its current entry: dispose the live instance (if
 * any) and mount a fresh one with a fresh reconnect budget. Incomplete
 * cleanup blocks replacement until the Host restarts.
 * @param name - the configured or registered server key.
 * @throws when the manager is disposed, no such entry exists, or cleanup is incomplete.
 */
@Remote async restart(name: string): Promise<void>

/**
 * Probe one server for a management surface; see {@link McpServerManager.probe}.
 * @param name - the configured or registered server key.
 * @param signal - caller cancellation.
 * @returns the stage-classified probe outcome and independent cleanup result.
 */
@Remote('probe') probeFromClient(name: string, signal: AbortSignal): Promise<McpProbeResult>

/**
 * Probe one configured server without touching its live instance: resolve
 * the entry's templates, connect, list tools, close. A disabled entry can be
 * probed — that is how a management surface tests it before enabling.
 * Configuration changes and manager disposal cancel the probe.
 * @param name - the configured or registered server key.
 * @param options - probe budget, listing caps, and abort signal.
 * @returns the stage-classified probe outcome and independent cleanup result.
 * @throws when the manager is disposed, the entry is absent or changing, cleanup is
 * incomplete, options are invalid, or a required credentials provider is missing.
 */
async probe(name: string, options?: McpProbeOptions): Promise<McpProbeResult>

Source: packages/mcp/mcp-manager/src/index.ts

ctx.mcpResources — McpResourceRuntime

Scoped resource access plus three tools shared by configured MCP servers.

/**
 * Register one server and expose resource tools while that scope has providers.
 * @param server - configured server name, unique in this scope.
 * @param provider - connection-owned resource operations.
 * @returns the effect disposer for this exact registration.
 */
register(server: string, provider: McpResourceProvider): () => void

Source: packages/mcp/mcp-resources/src/index.ts

mcp-manager/* events

mcp-manager/status — parallel

One managed server's status transitioned. Emitted for every published change while the manager is live and either the entry is configured or registered, or a runtime record remains. Removed entries may emit cleanup failures; watch and list() report removals. Lifecycle operations do not wait for listeners. Synchronous throws and returned promise rejections are logged without skipping other listeners.

/**
 * One managed server's status transitioned. Emitted for every published
 * change while the manager is live and either the entry is configured or
 * registered, or a runtime record remains. Removed entries may emit cleanup
 * failures; `watch` and `list()` report removals.
 * Lifecycle operations do not wait for listeners. Synchronous throws and
 * returned promise rejections are logged without skipping other listeners.
 * @param name - the configured `servers` key or registered name.
 * @param status - the status now current for that server.
 * @mode parallel
 */
'mcp-manager/status'(name: string, status: McpManagedServerStatus): void | Promise<void>

Source: packages/mcp/mcp-manager/src/types.ts