deepseek-harness/packages/mcp/mcp-client

@deepseek-ai/dsh-mcp-client

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MCP client bridge plugin: connects to external Model Context Protocol servers and registers their tools on ctx.tools, making them available to the model as native tools under server-qualified names (mcp__<serverName>__<rawName>).

Usage

One plugin instance per MCP server in cordis.yml:

- id: mcp-github
  name: '@deepseek-ai/dsh-mcp-client'
  config:
    serverName: github
    transport: stdio
    command: npx
    args: ['-y', '@modelcontextprotocol/server-github']
    env:
      GITHUB_TOKEN: !!js process.env.GITHUB_TOKEN

- id: mcp-web
  name: '@deepseek-ai/dsh-mcp-client'
  config:
    serverName: web
    transport: streamable-http
    url: http://localhost:3000/mcp
    headers:
      Authorization: !!js '`Bearer ${process.env.MCP_TOKEN}`'

The model sees mcp__github__create_issue, mcp__web__search, … — the same server-qualified shape Claude Code and Codex use. HMR hot-swaps: editing the entry triggers disconnect + reconnect without process restart; an unchanged serverName reproduces identical tool names.

Config

Field	Transport	Required	Description
transport	both	yes	"stdio" or "streamable-http"
serverName	both	yes	Namespace for this server's model-facing tool names; [A-Za-z0-9_-]{1,32}, unique across live instances
command	stdio	yes	Executable to spawn
args	stdio	no	Arguments passed to the command
env	stdio	no	Extra env vars merged on top of scrubbed ambient env
cwd	stdio	no	Working directory for the child process
url	http	yes	MCP server URL
headers	http	no	Extra headers (e.g. auth tokens)
toolCallTimeoutMs	both	no	Timeout per callTool invocation (default 60000)

Tool naming

Every MCP tool has two names: the raw MCP name (sent on the wire in tools/call) and the public name mcp__<serverName>__<rawName> registered on ctx.tools. Public names are normalized to the DeepSeek function-name contract (64 chars, [A-Za-z0-9_-]); when replacement or truncation changes the name, a deterministic 12-hex-char hash of (serverName, rawName) is appended so distinct tools never collapse into one name. Names are pure functions of (serverName, rawName) — connection order, re-syncs, and other servers never rename a tool.

• Two servers publishing the same raw name (e.g. search) coexist under their namespaces.
• A duplicate serverName across live instances fails the later plugin instance at load.
• A server listing the same tool name twice is rejected as an invalid tool list.
• A foreign registration squatting on this server's namespace rolls back the whole generation (never a partial set), with a loud error.

Behavior

• On connect: listTools() → registers each tool via ctx.tools.register() under its public name.
• Listens for notifications/tools/list_changed → re-syncs; a failed re-sync keeps the previous generation registered.
• Tool execute: client.callTool({ name: rawName, arguments }, { signal }) with timeout + abort support—the public name is never sent to the server.
• Canonical success is { content: JsonValue[], structuredContent? }; complete JSON MCP blocks survive for programmatic callers. A supported advertised outputSchema validates structuredContent; unsupported schema vocabulary falls back to unconstrained JsonValue.
• Native/model rendering keeps the existing text projection: text blocks join with newlines while image, audio, resource, and unsupported blocks become placeholders.
• On disconnect/crash: no auto-reconnect. Registered tools remain until plugin disposal or a successful re-sync, and calls can fail against the closed transport; reload with HMR or restart the Host to reconnect.

Services consumed

Service	Usage
ctx.tools	Register/unregister MCP tools

Model Experience

Discovered MCP tools

What the model sees

After initial discovery succeeds, each advertised MCP tool appears as a native tool named mcp__<serverName>__<rawName> (or its deterministic normalized form), with the server-provided description and input schema. A successful re-sync replaces the generation; plugin disposal removes it.

Token effect

Data-dependent schema cost is paid on every request while the tools are registered. Re-sync replaces rather than accumulates schemas, and the server-qualified name adds tokens to every tool definition and call.

KV Cache effect

Prefix-stable while the discovered tool set and schemas are unchanged. A re-sync that adds, removes, renames, or changes a tool replaces definitions and may invalidate reuse from the first changed schema token.

Tool-call history and results

What the model sees

The public tool name and JSON arguments remain in assistant history. Text result blocks are joined with newlines into one retained Native text result; image, audio, resource, and unsupported blocks become short placeholders there. Their full JSON blocks and optional structured content remain in the execution-local canonical value, and MCP isError rejects the call through the registry's error path.

Token effect

Arguments and mapped text are retained until compaction. Binary and resource payloads are discarded rather than added to context.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Known Limitations and Deferred Work

• Initial discovery is asynchronous — plugin load does not wait for connection and listTools(), so a turn started immediately after boot or HMR can assemble before the MCP tools are registered.
• Tools are the only bridged MCP capability — Resources and Prompts have no harness consumption surface and are deferred.
• Crash recovery is manual — transport closure does not auto-reconnect; registered tools can remain visible but fail against the closed transport until an HMR reload or Host restart.
• Native non-text rendering is lossy — image, audio, and resource payloads become placeholders in model context even though the execution-local canonical value preserves their JSON blocks. Richer Native multimedia projection is deferred.
• Unsupported MCP output schemas are not enforced — structuredContent falls back to JsonValue when the advertised schema uses vocabulary outside the harness subset.