zcodium-project/zcodium/diagnosing-mcp

Diagnosing MCP Configuration

Goal: reduce any MCP problem to a single concrete file-field edit. A person inspects status from the client; an agent reads and edits the configuration files directly.

Key points that are often misunderstood: the user configuration file is ~/.zcode/cli/config.json; .agents/mcp.json is a compatibility fallback (read only when the same scope's .zcode has no MCP servers); and in the desktop client, MCP status and repair live under Settings → MCP.

1. Configuration locations and precedence

Within each scope, .zcode takes priority and .agents/mcp.json is a same-scope fallback: if that scope's .zcode defines any MCP server, its .agents/mcp.json is ignored entirely. Note the different key shape — .zcode uses nested mcp.servers, while .agents/mcp.json uses a top-level mcpServers.

Override order across scopes for a same-named server: CLI → environment → user → workspace → system. In short, user overrides workspace. Plugin-provided servers form the base layer and are overridden by explicit configuration.

Auto-connect: MCP servers from every scope — user, workspace, plugin, environment, and CLI — are trusted and connected automatically at session start. Workspace-scoped servers were previously untrusted (reported Project MCP server requires explicit connection before use.); they now connect by default like any other scope. Use Settings → MCP as the supported client surface for inspecting status and repairing configuration.

2. Configuration schema

• stdio: requires command; optional args[], cwd, env, enabled, timeoutMs.
• http / sse: requires url; optional headers, enabled, timeoutMs.
• Standard field names are env for stdio environment variables and headers for HTTP/SSE request headers. command is a string and args is an array of strings; do not paste OpenCode-style command: ["npx", "-y", "..."] into ZCode's JSON editor.
• When type is omitted it is inferred: a command implies stdio, a url implies http. Legacy forms are migrated automatically when the CLI reads config directly (type: "remote" → http, environment → env, enable → enabled, http_headers → headers). Desktop app-managed session creation may bypass part of that CLI file parser, so prefer canonical fields (env, headers, enabled, type: "http") in files that the desktop Settings → MCP page reads.
• The configuration-file server schema is strict: an unknown key causes the server to be dropped.
• Template variables `${...}` are expanded only for plugin-provided MCP servers (for example ${CLAUDE_PLUGIN_ROOT} / ${ZCODE_PLUGIN_ROOT}, ${CLAUDE_PROJECT_DIR}, ${user_config.KEY}). Configuration-file MCP servers do not expand templates — use absolute paths there.
• The default timeout is 30000 ms.

Canonical examples:

{
  "mcp": {
    "servers": {
      "mysql-local": {
        "type": "stdio",
        "command": "npx",
        "args": ["-y", "@benborla29/mcp-server-mysql"],
        "env": {
          "MYSQL_HOST": "127.0.0.1",
          "MYSQL_PORT": "3306"
        }
      },
      "remote-reader": {
        "type": "http",
        "url": "https://example.com/mcp",
        "headers": {
          "Authorization": "Bearer ..."
        }
      }
    }
  }
}

3. How to inspect status

• List and status: open Settings → MCP in the client. Each entry shows whether the server is connected, disabled, disconnected, or failed, with any error inline. Plugin-provided servers are marked as built-in. (untrusted is a legacy status that no longer appears for normally configured servers now that every scope auto-connects.)
• After edits: restart the affected session, or restart ZCode if the Settings page still shows stale data, then reopen Settings → MCP to confirm the server status.
• Standard-I/O error output (the root cause of most failures): a stdio server's captured error stream is written to the ZCode log. To see the full output, run the server's command with its arguments directly in a terminal.

4. Common pitfalls (symptom → cause → fix)

1. Workspace server does not connect — a server defined in <repo>/.zcode/config.json (or <repo>/.agents/mcp.json) is not connecting. Workspace servers now auto-connect like any other scope, so this is no longer a trust gate — the cause is a real config or startup problem. → Check its Settings → MCP status: failed → go to step 4; absent → the config was not loaded (pitfall 5/9) or it is in .agents/mcp.json but shadowed (pitfall 12).
2. `command not found` — the server shows failed with an error such as spawn npx ENOENT. The command is not on PATH, or a relative path was not resolved. → Use an absolute path, add cwd when needed, and on Windows point at the .cmd/.exe.
3. `${...}` reaches the process literally — configuration-file MCP servers do not expand templates. → Use concrete absolute paths (templates are a plugin-only feature).
4. Plugin is missing an environment value or secret — the plugin reports a missing variable. → Set the plugin's configuration value (Plugin Management → the plugin's advanced settings) or export the required environment variable; sensitive values may only be placed in env/headers, not in command/url.
5. Wrong transport type or unknown key — the server is silently dropped. → Make type match the fields, remove any extra top-level keys (the schema is strict), and ensure exactly one of command (stdio) or url (http/sse) is present.
6. Unexpected override — editing the workspace mcp.servers entry has no effect. A same-named server in the user configuration is shadowing it (user overrides workspace for MCP). → Edit the user entry, or rename one of the servers.
7. Connection or tool-listing timeout — failed ... timed out after 30000ms. → Add "timeoutMs": 60000 to that server, and address the slow startup.
8. Only `Connection closed` with no cause — the error stream was not surfaced in the status line. → Check the ZCode log for the captured error output, or run the command with its arguments in a terminal.
9. JSON syntax error in the configuration file — MCP servers (and possibly the whole file) go missing. → Validate the JSON, then fix the syntax.
10. Server shows as disabled — enabled: false (or legacy enable: false). → Set "enabled": true or remove the field.
11. A desktop-managed server list overrides the file — edits to configuration files have no effect because the client is supplying the MCP list. → Manage MCP through Settings → MCP in that context.
12. `.agents/mcp.json` edits have no effect, or use the wrong key — a server added to .agents/mcp.json never appears. Either the same scope's .zcode already defines MCP servers (so .agents/mcp.json is ignored entirely for that scope), or the servers were placed under mcp.servers instead of the top-level mcpServers that .agents/mcp.json expects. → Move the definition into the .zcode file for that scope, or ensure that scope's .zcode has no MCP servers and use the top-level mcpServers key in .agents/mcp.json.
13. Server name appears in logs but no MCP tools appear in the model request — the desktop app read the server entry and passed its name to the runtime, but the server failed during startup, so toolCount and registeredToolCount are zero. A common cause is a legacy environment field in ~/.zcode/cli/config.json: CLI direct config parsing can migrate it, but the desktop app-managed path currently expects env when converting to protocol mcpServers. → Rename environment to env, keep the values unchanged, restart ZCode, and reopen Settings → MCP.
14. Settings → MCP crashes after JSON editing with `command.trim is not a function` — the saved server has a non-string command, usually OpenCode-style command: ["npx", "-y", "server"]. → Edit ~/.zcode/cli/config.json manually: set "command": "npx" and move the rest into "args": ["-y", "server"], then restart the app.

5. Localization workflow (in order; stop when the cause is found)

1. Confirm MCP is enabled (it is by default).
2. Open Settings → MCP and read the status: disabled → pitfall 10; failed (<error>) → read the inline error and go to step 4; not listed at all → step 3. (untrusted should no longer appear for a normally configured server.)
3. Verify the configuration is loaded and valid: check the JSON validity of ~/.zcode/cli/config.json and <repo>/.zcode/config.json (and zcode.json) — pitfall 9. A server that is in the file but not listed failed schema validation (pitfall 5 — look for unknown keys, wrong type, or a missing command/url); a server defined only in .agents/mcp.json but not appearing points to the fallback shadowing or wrong-key issue (pitfall 12).
4. Diagnose a failed server: ENOENT → pitfall 2; timed out → pitfall 7; Connection closed → pitfall 8; an http/sse network error → check proxy/CA, URL reachability, and headers.
5. If Settings → MCP or service logs show mcpServerCount / server names but the model request has no mcp__... tools, check startup logs for mcp.startup.completed and mcp.tools.registered. If toolCount=0 with failed statuses, inspect the config field names first (env vs environment, string command vs array command) before treating it as a model-selection issue.
6. If edits have no effect → pitfall 6 (user overrides workspace) or pitfall 11 (desktop-managed list).
7. If a ${...} appears literally → pitfall 3 (configuration files do not expand templates) or pitfall 4 (an unset plugin variable).
8. Apply the concrete fix — most commonly editing ~/.zcode/cli/config.json at mcp.servers.<name> (command / args / cwd / env / headers / timeoutMs / enabled) — then restart the session (every scope auto-connects) and reopen Settings → MCP to confirm.