René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Claude Code silently drops all tools due to MCP SDK 1.26.0 fields (instructions, annotations, execution)","articleBody":"## Description\n\nXcodeBuildMCP v2.0.7 tools are completely invisible in Claude Code 2.1.39 despite the server showing as \"Connected\" in `claude mcp list`. Zero tools appear — no error, no warning.\n\nThis is caused by fields added by **@modelcontextprotocol/sdk v1.26.0** that Claude Code's MCP client doesn't handle gracefully.\n\n## Affected Fields\n\n**In `initialize` response:**\n- `instructions` — large documentation string (~1500 chars)\n- `capabilities.tools.listChanged: true`\n- `capabilities.resources` and `capabilities.logging`\n\n**In `tools/list` response (per-tool):**\n- `execution: { taskSupport: \"forbidden\" }`\n- `annotations: { title: \"...\", readOnlyHint: true/false }`\n- `inputSchema.$schema: \"http://json-schema.org/draft-07/schema#\"`\n\n## How We Diagnosed\n\n1. Created a minimal canary MCP server with bare `{name, description, inputSchema}` and `capabilities: {tools: {}}` — **tools loaded** ✅\n2. XcodeBuildMCP proxy stripping only tool-level fields — **still dropped** ❌\n3. Proxy also normalizing the `initialize` response (removing `instructions`, simplifying capabilities) — **tools loaded** ✅\n\nThe `instructions` field and/or extra capabilities in the `initialize` response appear to be the primary trigger. Tool-level `annotations` and `execution` may also contribute.\n\n## Workaround\n\nNode.js proxy that normalizes MCP responses:\n\n```javascript\n#!/usr/bin/env node\nconst { spawn } = require('child_process');\n\nconst proc = spawn('xcodebuildmcp', ['mcp'], {\n  stdio: ['pipe', 'pipe', process.stderr],\n  cwd: process.env.XCODEBUILDMCP_CWD || process.cwd(),\n});\n\nlet buffer = '';\nproc.stdout.on('data', (data) =\u003e {\n  buffer += data.toString();\n  const lines = buffer.split('\\n');\n  buffer = lines.pop();\n  for (const line of lines) {\n    if (!line.trim()) { process.stdout.write('\\n'); continue; }\n    try {\n      const msg = JSON.parse(line);\n      // Clean initialize response\n      if (msg.result \u0026\u0026 msg.result.protocolVersion) {\n        delete msg.result.instructions;\n        if (msg.result.capabilities) {\n          msg.result.capabilities = { tools: {} };\n        }\n      }\n      // Clean tools/list response\n      if (msg.result \u0026\u0026 msg.result.tools \u0026\u0026 Array.isArray(msg.result.tools)) {\n        msg.result.tools = msg.result.tools.map(tool =\u003e {\n          const { execution, annotations, ...rest } = tool;\n          if (rest.inputSchema) delete rest.inputSchema['$schema'];\n          return rest;\n        });\n      }\n      process.stdout.write(JSON.stringify(msg) + '\\n');\n    } catch (e) {\n      process.stdout.write(line + '\\n');\n    }\n  }\n});\n\nprocess.stdin.pipe(proc.stdin);\nproc.on('exit', (code) =\u003e process.exit(code || 0));\nprocess.on('SIGTERM', () =\u003e proc.kill('SIGTERM'));\nprocess.on('SIGINT', () =\u003e proc.kill('SIGINT'));\n```\n\nSave as a proxy script, then in `.mcp.json`:\n```json\n{\n  \"XcodeBuildMCP\": {\n    \"type\": \"stdio\",\n    \"command\": \"node\",\n    \"args\": [\"/path/to/xcodebuildmcp-proxy.js\"],\n    \"env\": { \"XCODEBUILDMCP_CWD\": \"/path/to/your/project\" }\n  }\n}\n```\n\n## Suggested Fix\n\nConsider one or more of:\n\n1. **Make `instructions` opt-in** via config (e.g. `enableInstructions: false` in `.xcodebuildmcp/config.yaml`)\n2. **Strip `instructions` from initialize** when the client doesn't advertise support\n3. **Make `annotations` and `execution` optional** per MCP spec — only include them if the client negotiates support during `initialize`\n\n## Related\n\n- Claude Code issue: https://github.com/anthropics/claude-code/issues/25081\n- Issue #210 in this repo (connection failures — may be same root cause)\n\n## Environment\n\n- XcodeBuildMCP: 2.0.7\n- @modelcontextprotocol/sdk: 1.26.0\n- Claude Code: 2.1.39\n- macOS: Darwin 24.6.0 (Apple Silicon)","author":{"url":"https://github.com/greenstevester","@type":"Person","name":"greenstevester"},"datePublished":"2026-02-12T22:43:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/220/XcodeBuildMCP/issues/220"}