dsh-custom-tool

Custom tools for the DeepSeek Harness: users author their own JavaScript tools in the settings UI with a Monaco (VS Code) editor and TypeScript intellisense, and the model grows and prunes the same toolset itself through custom_tool_create / custom_tool_remove / custom_tools_list. Every tool is durable, hot-registered, and written into the model prompt on the next step.

Problems it solves

• Users cannot extend their agent: before this plugin, adding a capability meant shipping a harness package. Now a tool is a form in the settings UI — name, description, parameters, code — saved live and callable by the model immediately.
• The model cannot grow itself: custom_tool_create lets the model persist tools mid-session (hot-registered, visible on its next step), with the same validation gate the UI uses — the model cannot persist anything the settings UI would refuse.
• User-authored code runs in a restricted worker: every call executes in a fresh worker thread inside a node:vm realm with an explicit allowlist, Node Permission Model, and hard budgets. The worker receives no inherited environment variables, filesystem access outside its configured scope, or child-process capability.

Features

• Settings UI (Custom Tool section, own nav glyph): list, create, edit, enable/disable, delete. Model-created and workspace-scoped tools are badged. Every string follows the harness locale (Chinese / English) and switches with the language preference.
• Monaco editor: VS Code engine + TypeScript language service; args typed from the parameter schema, env/sandbox globals declared, completions and diagnostics live. Editor and TS workers are bundled inline — the client bundle is a single file.
• Durable store: tools live in the custom-tools settings namespace (schema defaults, composition base, user document — the ordinary settings layering). Edits apply live; restart restores them.
• Live registration: enabled tools register into ctx.tools the moment the settings write commits; disabled/removed tools unregister immediately. The harness assembles tool schemas into the system prompt automatically.
• Model self-service: custom_tool_create (upsert by name), custom_tools_list, and custom_tool_remove share the UI's validation gate and the ownership rules below.

Tool scopes and permission boundaries

Every tool declares one of two execution scopes. The boundary is the core security contract of this plugin:

Confinement rules for the workspace scope

• The root is the session workspace directory (the initiating agent's cwd), resolved at call time.
• Relative paths resolve from the root; absolute paths must stay inside it; any path that escapes the root is rejected with an explicit error.
• Outside a session (no initiator context) a workspace tool fails with no workspace root rather than running without a boundary.
• Confinement is lexical (resolve + prefix check). A symlink inside the workspace can still point out of it — workspace scope is trusted code, not a sandbox against a malicious host. The liveness budgets below apply to both scopes.

Storage locations

Every tool also declares where it EXISTS:

• location: 'global' — stored in the shared settings namespace; available in every workspace until removed.
• location: 'workspace' — stored in a per-workspace file under <dsh home>/workspace-tools/, keyed by the canonical workspace root; visible only to sessions of that workspace (registered into each matching agent's own tool scope).

The two dimensions combine freely: a global-location tool with workspace scope (a durable file task the user keeps everywhere, like a PDF reader) runs its fs on whichever workspace calls it.

Ownership and authorization rules for the model

• The model may create, list, and remove model-created tools (source: model).
• Creating a global-location tool requires the user's explicit approval: custom_tool_create raises a harness approval request (GUI prompt); a declined or unavailable answer fails the creation closed. Workspace-location tools are autonomous.
• The model may not remove user-created tools (source: user); custom_tool_remove refuses them and the system-prompt guidance says to ask the user to delete them in the settings UI.
• The settings UI manages everything: both sources, both scopes, both locations, enable/disable, delete.

Execution budgets (both scopes)

• One worker thread per call; the worker is terminated on timeout, abort, or completion.
• Wall-clock deadline (timeoutMs), heap cap (memoryLimitMb), result text bound (maxResultChars), code size bound (maxCodeBytes), stored-tool cap (maxTools).

Install

dsh plugin --profile web add https://github.com/omdsh-dev/dsh-custom-tool/archive/refs/tags/v0.1.2.tar.gz
dsh web   # restart the server to pick the plugin up

The package declares dsh.bundle.patch (mounts the host plugin) and dsh.client (serves the browser half at /plugins/dsh-custom-tool/client.js). lib/ is committed, so the GitHub tarball installs without a build step.

Harness requirement: the settings namespace is exposed to web configuration clients through the WEB_SETTINGS_NAMESPACES allowlist in packages/host/apiproxy/src/api-proxy.ts; the string 'custom-tools' must be present there (the upstream harness commit d6ea05b5 adds it). Without it the UI renders but saves are silently refused (settings-not-exposed).

Tool code contract

The code field is the body of an async function async (args, env) => value:

// args is typed from the parameter JSON Schema you declared.
const url = `https://api.example.com/weather?city=${encodeURIComponent(args.city)}`
const response = await fetch(url)
if (!response.ok) throw new Error(`upstream returned ${response.status}`)
return await response.json()

• Return a JSON value (string, number, boolean, null, array, or plain object); undefined or a non-JSON value fails the call.
• Parameters: object-rooted JSON Schema in the harness subset — type, properties, required, items, enum, const, oneOf, additionalProperties, description, title, default, examples.
• Globals: fetch (blocked when allowNetwork: false), console, TextEncoder/TextDecoder, URL/URLSearchParams, atob/btoa, structuredClone, AbortController, setTimeout/setInterval + clears. env is { tool, scope }. Workspace scope adds fs.

Configuration

All tunables are cordis.yml config fields of the dsh-custom-tool entry:

Development

pnpm install        # links the sibling dsh checkout for types and tests
pnpm run build      # worker bundles -> inline sources -> declarations -> bundles
pnpm run test       # builds workers first (pretest), then vitest
pnpm run typecheck
pnpm run lint
pnpm run check      # typecheck + lint + test + build

Node-env tests alias @deepseek-ai/dsh-client-runtime/client to source and monaco-editor to a mock (see vitest.config.ts).

Known Limitations and Deferred Work

• Custom tool names cannot shadow tools owned by other packages; a collision surfaces as a per-tool registration failure in custom_tools_list.
• Workspace confinement is lexical, not symlink-proof (see the scope table).
• No per-tool test-run button in the UI yet; tools are exercised through model calls or a headless run.