# MCP server and tool authoring

This document explains how MCP server definitions become callable `mcp__*` tools in coding-agent, and what operators should expect when configs are invalid, duplicated, disabled, or auth-gated.

## Architecture at a glance

```text
Config sources (.omp/.claude/.cursor/.vscode/mcp.json, mcp.json, etc.)
  -> discovery providers normalize to canonical MCPServer
  -> capability loader dedupes by server name (higher provider priority wins)
  -> loadAllMCPConfigs converts to MCPServerConfig + skips enabled:false
  -> MCPManager connects/listTools (with auth/header/env resolution)
  -> manager best-effort loads resources/prompts and subscribes to resource updates when enabled
  -> MCPTool/DeferredMCPTool bridge exposes tools as mcp__<server>_<tool>
  -> AgentSession.refreshMCPTools replaces live MCP tools immediately
```

## 1) Server config model and validation

`src/mcp/types.ts` defines the authoring shape used by MCP config writers and runtime:

- `stdio` (default when `type` missing): requires `command`, optional `args`, `env`, `cwd`
- `http`: requires `url`, optional `headers`
- `sse`: requires `url`, optional `headers` (kept for compatibility)
- shared fields: `enabled`, `timeout`, `auth`, `oauth`

`validateServerConfig()` (`src/mcp/config.ts`) enforces transport basics:

- rejects configs that set both `command` and `url`
- requires `command` for stdio
- requires `url` for http/sse
- rejects unknown `type`

`config-writer.ts` applies this validation for add/update operations and also validates server names:

- non-empty
- max 100 chars
- only `[a-zA-Z0-9_.:-]` (colon allows namespaced plugin server names, e.g. `cloudflare:cloudflare-api`)

### Transport pitfalls

- `type` omitted means stdio. If you intended HTTP/SSE but omitted `type`, `command` becomes mandatory.
- `sse` is still accepted but treated as HTTP transport internally (`createHttpTransport`).
- Validation is structural, not reachability: a syntactically valid URL can still fail at connect time.

## 2) Discovery, normalization, and precedence

### Capability-based discovery

`loadAllMCPConfigs()` (`src/mcp/config.ts`) loads canonical `MCPServer` items via `loadCapability(mcpCapability.id)`.

The capability layer (`src/capability/index.ts`) then:

1. loads providers in priority order
2. dedupes by `server.name` (first win = highest priority)
3. validates deduped items

Result: duplicate server names across sources are not merged. One definition wins; lower-priority duplicates are shadowed.

### `.mcp.json` and related files

The dedicated fallback provider in `src/discovery/mcp-json.ts` reads project-root `mcp.json` and `.mcp.json` (low priority).

In practice MCP servers also come from higher-priority providers (for example native `.omp/...` and tool-specific config dirs). Authoring guidance:

- Prefer `.omp/mcp.json` (project) or `~/.omp/agent/mcp.json` (user) for explicit control.
- Use root `mcp.json` / `.mcp.json` when you need fallback compatibility.
- Reusing the same server name in multiple sources causes precedence shadowing, not merge.

### Normalization behavior

`convertToLegacyConfig()` (`src/mcp/config.ts`) maps canonical `MCPServer` to runtime `MCPServerConfig`.

Key behavior:

- transport inferred as `server.transport ?? (command ? "stdio" : url ? "http" : "stdio")`
- disabled servers (`enabled === false`) and names in the user `disabledServers` list are dropped before connection
- optional fields are preserved when present

### Environment expansion during discovery

OMP-native MCP config (`.omp/mcp.json`, `~/.omp/agent/mcp.json`, plus their `.mcp.json` variants) expands `${VAR}` and `${VAR:-default}` placeholders recursively before converting to runtime config. It also accepts boolean/string forms for `enabled` (`true`, `false`, `1`, `0`) and numeric strings for `timeout`.

The standalone fallback provider in `src/discovery/mcp-json.ts` reads project-root `mcp.json` and `.mcp.json`, expands the same `${...}` placeholders, and type-checks `enabled`/`timeout` without coercing string values.

Invalid `enabled`/`timeout` values are ignored with warnings rather than failing the whole file.

## 3) Auth and runtime value resolution

`MCPManager.prepareConfig()`/`#resolveAuthConfig()` (`src/mcp/manager.ts`) is the final pre-connect pass.

### OAuth credential injection

If config has:

```ts
auth: { type: "oauth", credentialId: "..." }
```

and credential exists in auth storage:

- `http`/`sse`: injects `Authorization: Bearer <access_token>` header
- `stdio`: injects `OAUTH_ACCESS_TOKEN` env var

If credential lookup fails, manager logs a warning and continues with unresolved auth.

### Header/env value resolution

Before connect, manager resolves stdio `env` values and HTTP/SSE `headers` values via `resolveConfigValue()` (`src/config/resolve-config-value.ts`):

- value starting with `!` => execute shell command, use trimmed stdout (cached)
- failed, timed-out, or whitespace-only commands produce `undefined`, so that entry is omitted
- otherwise, treat value as environment variable name first (`process.env[name]`), fallback to literal value

Operational caveat: a mistyped `!` secret command can silently remove that header/env entry, producing downstream 401/403 or server startup failures. A mistyped environment variable name is sent literally unless that literal happens to be meaningful to the server.

## 4) Tool bridge: MCP -> agent-callable tools

`src/mcp/tool-bridge.ts` converts MCP tool definitions into `CustomTool`s.

### Naming and collision domain

Tool names are generated as:

```text
mcp__<sanitized_server_name>_<sanitized_tool_name>
```

Rules:

- lowercases
- non-`[a-z_]` chars become `_`
- repeated underscores collapse
- redundant `<server>_` prefix in tool name is stripped once

This avoids many collisions, but not all. Different raw names can still sanitize to the same identifier (for example `my-server` and `my.server` both sanitize similarly), and registry insertion is last-write-wins.

### Schema mapping

`tool-bridge.ts` passes each MCP `inputSchema` through `normalizeSchemaForMCP()` before registering it as a `CustomTool` schema.

### Execution mapping

`MCPTool.execute()` / `DeferredMCPTool.execute()`:

- calls MCP `tools/call`
- flattens MCP content into displayable text
- returns structured details (`serverName`, `mcpToolName`, provider metadata)
- maps server-reported `isError` to `Error: ...` text result
- attempts reconnect + one retry for retriable connection errors
- maps remaining thrown transport/runtime failures to `MCP error: ...`
- preserves abort semantics by translating AbortError into `ToolAbortError`

## 5) Operator lifecycle: add/edit/remove and live updates

Interactive mode exposes `/mcp` in `src/modes/controllers/mcp-command-controller.ts`.

Supported operations:

- `add` (wizard or quick-add)
- `remove` / `rm`
- `enable` / `disable`
- `test`
- `reauth` / `unauth`
- `reconnect`
- `reload`
- `resources`, `prompts`, `notifications`
- Smithery search/login/logout flows

Config writes are atomic (`writeMCPConfigFile`: temp file + rename).

After changes, controller calls `#reloadMCP()`:

1. `mcpManager.disconnectAll()`
2. `mcpManager.discoverAndConnect()`
3. `session.refreshMCPTools(mcpManager.getTools())`

`refreshMCPTools()` replaces all `mcp__` registry entries and immediately re-activates the latest MCP tool set, so changes take effect without restarting the session.

### Mode differences

- **Interactive/TUI mode**: `/mcp` gives in-app UX (wizard, OAuth flow, connection status text, immediate runtime rebinding).
- **SDK/headless integration**: `discoverAndLoadMCPTools()` (`src/mcp/loader.ts`) returns loaded tools + per-server errors; no `/mcp` command UX.

## 6) User-visible error surfaces

Common error strings users/operators see:

- add/update validation failures:
  - `Invalid server config: ...`
  - `Server "<name>" already exists in <path>`
- quick-add argument issues:
  - `Use either --url or -- <command...>, not both.`
  - `--token requires --url (HTTP/SSE transport).`
- connect/test failures:
  - `Failed to connect to "<name>": <message>`
  - timeout help text suggests increasing timeout
  - auth help text for `401/403`
- auth/OAuth flows:
  - `Authentication required ... OAuth endpoints could not be discovered`
  - `OAuth flow timed out. Please try again.`
  - `OAuth authentication failed: ...`
- disabled server usage:
  - `Server "<name>" is disabled. Run /mcp enable <name> first.`

Bad source JSON in discovery is generally handled as warnings/logs; config-writer paths throw explicit errors.

## 7) Practical authoring guidance

For robust MCP authoring in this codebase:

1. Keep server names globally unique across all MCP-capable config sources.
2. Prefer names that remain distinct after MCP tool-name sanitization to avoid generated `mcp__` collisions.
3. Use explicit `type` to avoid accidental stdio defaults.
4. Treat `enabled: false` as hard-off: server is omitted from runtime connect set.
5. For OAuth configs, store a valid `credentialId`; otherwise auth injection is skipped.
6. If using command-based secret resolution (`!cmd`), verify command output is stable and non-empty.

## Implementation files

- [`src/mcp/types.ts`](../packages/coding-agent/src/mcp/types.ts)
- [`src/mcp/config.ts`](../packages/coding-agent/src/mcp/config.ts)
- [`src/mcp/config-writer.ts`](../packages/coding-agent/src/mcp/config-writer.ts)
- [`src/mcp/tool-bridge.ts`](../packages/coding-agent/src/mcp/tool-bridge.ts)
- [`src/discovery/mcp-json.ts`](../packages/coding-agent/src/discovery/mcp-json.ts)
- [`src/modes/controllers/mcp-command-controller.ts`](../packages/coding-agent/src/modes/controllers/mcp-command-controller.ts)
- [`src/mcp/manager.ts`](../packages/coding-agent/src/mcp/manager.ts)
- [`src/capability/index.ts`](../packages/coding-agent/src/capability/index.ts)
- [`src/config/resolve-config-value.ts`](../packages/coding-agent/src/config/resolve-config-value.ts)
- [`src/mcp/loader.ts`](../packages/coding-agent/src/mcp/loader.ts)
