vscode-mcp-tool-development

name: vscode-mcp-tool-development description: End-to-end workflow for adding, modifying, or debugging VSCode MCP tools in this repo — IPC zod schema and EventMap, bridge service registration, core ToolDefinition and tool constants, MCP/CLI adapter behavior, workspace_path semantics, CLI flag generation, validation, and regression testing.

VSCode MCP Tool Development

Use this when a change touches tool contracts, service behavior, tool output, or adapter exposure under packages/vscode-mcp-{ipc,bridge,core,server,cli}/src/.

Golden Path

IPC schema/EventMap -> bridge service -> core ToolDefinition/constants -> validation

Server and CLI are thin adapters around vscode-mcp-core. Do not edit them for ordinary tools unless adapter behavior itself changes.

1. IPC Contract

Add or update the event in packages/vscode-mcp-ipc/src/events/.

• Use a kebab-case file name and camelCase IPC event name.
• Export InputSchema, OutputSchema, payload type, and result type.
• Use .strict() on object schemas.
• Put shared path/location/position fields in packages/vscode-mcp-ipc/src/common.ts when they are reused.
• Update packages/vscode-mcp-ipc/src/events/index.ts:
  • import payload/result types
  • export the event module
  • add the event to EventMap

Keep ToolDefinition.schema compatible with .shape. If a schema needs .refine() or another wrapper that hides .shape, expose a base z.object(...).strict() schema for core/adapter usage and apply refined validation in the layer that needs it.

2. Bridge Service

Implement the VSCode API behavior in packages/vscode-mcp-bridge/src/services/.

• Type service payloads as EventParams<'eventName'> and results as EventResult<'eventName'>.
• Register the service in packages/vscode-mcp-bridge/src/extension.ts with payload and result schemas.
• Keep VSCode API objects inside the bridge layer. Return plain JSON-compatible data that passes the IPC output schema.
• Throw errors for true failures. Include cause when it helps debugging.
• For command-style operations that intentionally surface command failure as data, follow the existing executeCommand pattern.

3. Core Tool

Create or update packages/vscode-mcp-core/src/tools/<tool-name>.ts.

• Reuse the IPC input schema directly: schema: SomeInputSchema.
• Use snake_case for MCP name, kebab-case for CLI cliName, and kebab-case file names.
• Set requiresWorkspace: true unless the tool is genuinely global, like list_workspaces.
• In handler, create a dispatcher with createDispatcher(workspacePath), dispatch the camelCase IPC event, and return the final human-readable text.
• Let unexpected errors throw. MCP and CLI adapters add the ❌ <title> failed wrapper.
• Use errorTips only for extra operator guidance, such as extension installation or version mismatch hints.

Then update:

• packages/vscode-mcp-core/src/tools/index.ts: export the tool and add it to getAllTools({ clientVersion }).
• packages/vscode-mcp-core/src/constants.ts: add the tool name to VscodeMcpToolName so server filtering validates it.

4. CLI Exposure

The CLI auto-generates one flag per zod object field.

• Simple string/number/boolean/enum/array-of-primitive fields need no CLI code.
• Arrays of objects and other non-flat shapes require tool.cli.options plus tool.cli.transform.
• Boolean defaults of true become --no-<flag>.
• __NOT_RECOMMEND__filePaths becomes --not-recommend-file-paths; the CLI rebuilds the original zod key before parsing.

5. Descriptions And Agent Steering

Tool descriptions are prompt surface. Include only operational guidance that helps an agent use the tool correctly.

• Say that workspace_path selects a VSCode socket, not an arbitrary repository root.
• Tell agents to run list_workspaces when unsure which workspace path to use.
• For git-modified diagnostics, preserve the existing empty-array behavior of __NOT_RECOMMEND__filePaths.
• Use __NOT_RECOMMEND__ field names for params that are technically configurable but usually should be left at defaults.
• Mark dangerous tools accurately. execute_command is destructive/open-ended unless the specific command is proven safe.

6. Validation

Prefer targeted validation matching the layer changed.

# IPC schema/type changes
cd packages/vscode-mcp-ipc && bun run build

# Bridge service or extension registration
cd packages/vscode-mcp-bridge && bun run compile:test

# Core tool definitions, descriptions, constants, or formatting
cd packages/vscode-mcp-core && bun run build

# Adapter behavior changes
cd packages/vscode-mcp-server && bun run build
cd packages/vscode-mcp-cli && bun run build

For repo diagnostics, run from the VSCode workspace root:

vscode-cli get-diagnostics

If the extension is available, smoke-test the changed tool through vscode-cli because it exercises the same core handler and socket path as MCP. Use vscode-cli list-workspaces first when the workspace path is uncertain.

Regression Tests

• Add or update a regression test when fixing a bug.
• Prefer the smallest testable unit. For bridge VSCode API behavior, use the existing extension test harness only when the logic cannot be isolated.
• Avoid changing server/CLI tests for a new ordinary tool unless adapter behavior changes; core registration plus a CLI smoke test is usually enough.

Common Failure Modes

• Unknown method: missing EventMap entry or bridge socketServer.register.
• Tool not available through server filtering: missing VscodeMcpToolName entry.
• CLI crashes during registration: schema field is not representable as flat flags and needs cli.options/cli.transform.
• Connection points at the wrong VSCode window: workspace_path is a child folder or submodule path instead of the open workspace.
• Result validation fails: bridge service returned a VSCode object or non-JSON shape instead of schema-compatible data.