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

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.

npx @framy2/ctxwise mcp-xray --probe-stdio

npx @framy2/ctxwise agents --fail-on-divergence

npx @framy2/ctxwise shield --strict --sarif ctxwise.sarif

npx @framy2/ctxwise budget
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"

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):