Proxy test of 90 MCP agent trials finds client-side failures masked as model errors

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A developer ran 90 structured trials across three MCP servers and two AI clients — Claude Code and Gemini CLI — using a proxy on the stdio pipe to log every frame. The suite covered filesystem, Playwright, and GitHub servers across 15 scripted tasks, yielding 87 successes and 3 failures, all from Gemini CLI. A key pre-run discovery found one client was silently failing tool calls internally before any data reached the server, making the issue appear as poor model reasoning rather than a client bug. Token costs varied sharply between clients on the same server: GitHub tasks cost Claude Code a median 1,698 call tokens versus 223 for Gemini CLI, largely because Claude Code received verbose metadata blocks with each response that Gemini CLI did not. The divergence was traced to the two clients negotiating different MCP protocol revisions, illustrating that per-call token costs depend on client behavior, not just the server.

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ProgrammingDEV Community ·

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