How an 8MB MCP Tool Response Crashes Claude Code — And How to Debug Prompt Cache Misses Across Agents

A developer released three open-source, zero-telemetry tools for debugging Model Context Protocol (MCP) failures in coding agents such as Claude Code, OpenAI Codex CLI, Cursor and VS Code Copilot. The tools target three silent failure modes: oversized tools/list responses that exhaust context, tools/call outputs large enough to tear down the stdio pipe (Claude Code's 8 MiB limit, Codex's 16 MiB), and prompt-cache misses caused by mid-session config edits. The packages are @framy2/ctxwise (context firewall, multi-editor MCP auditor and CI budget guard), @framy2/mcp-diet (a stdio MCP proxy that offloads large results and redacts secrets) and @framy2/agent-wire-tap (a wire contract tester with a --passthrough prompt-prefix cache-miss debugger).

If you use Claude Code , OpenAI Codex CLI , Cursor , or VS Code Copilot with Model Context Protocol MCP servers, your context window and prompt cache are exposed to three silent failure modes: tools/list : github , postgres , playwright injects tools/call : <error Connection closed</error anthropics/claude-code 105703 openai/codex 60383 cache control , or mid-session "disableAllHooks": true edits anthropics/claude-code 106225 We built three zero-telemetry open-source tools to inspect and prevent all three: @framy2/ctxwise v1.2.0 — Context Firewall, Multi-Editor MCP Auditor & CI Budget Guard Apache-2.0 @framy2/mcp-diet v0.1.1 — Stdio MCP Proxy with MIT @framy2/agent-wire-tap v0.2.0 — Local Wire Contract Tester & Live --passthrough Prompt-Prefix Cache-Miss Debugger { "mcpServers": { "github": { "command": "npx", "args": "-y", "@framy2/mcp-diet@0.1.1", "--read-only", "--max-desc", "160", "--max-result-kb", "512", "--redact-secrets", "--", "npx", "-y", "@modelcontextprotocol/server-github" } } } mcp-diet sits on stdio between the host CLI and the upstream MCP server: = 512 KiB tool outputs to a local temp file and returns a compact < 2 KB preview so Claude Code 8 MiB and Codex 16 MiB never tear down the pipe. sk-ant- , sk-proj- , ghp , AKIA , DB passwords before they reach the model transcript. Probe stdio MCP tools safely works on Windows without shell: true & profile token weight npx @framy2/ctxwise mcp-xray --probe-stdio Check CLAUDE.md, .claude/rules/ .md, AGENTS.md, .cursor/mcp.json, and .vscode/mcp.json parity npx @framy2/ctxwise agents --fail-on-divergence Scan for leaked secrets, Trojan Unicode, disableAllHooks, and unpinned npx -y MCP runners npx @framy2/ctxwise shield --strict --sarif ctxwise.sarif Enforce ctxwise.budget.yaml across Claude, Codex, and Cursor without double-counting synced rules npx @framy2/ctxwise budget CI contract test at $0.00 API cost mock mode npx @framy2/agent-wire-tap \ --assert-contains "AGENTS.md" \ --assert-not-contains "sk-ant-" \ --assert-max-bytes 32768 \ --assert-max-tools 12 \ -- codex exec "ping" Live transparent pass-through proxy streams SSE & detects prefix divergence block-by-block npx @framy2/agent-wire-tap --passthrough https://api.anthropic.com --port 8787 --passthrough keeps credentials strictly in memory REDACTED in traces/logs while hashing each prefix segment model , tools , system , messages and flagging the exact firstDivergentLayer / firstDivergentIndex whenever a mutation breaks a cache control breakpoint. Paste your .mcp.json , .vscode/mcp.json , CLAUDE.md , or AGENTS.md right in the browser 100% local, zero uploads :