feat(mcp): advertise maxResultSizeChars on every tool#380
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Injects an `anthropic/maxResultSizeChars` annotation into the `_meta` field of every tool returned by tools/list. Claude Code (and any other client that honors this annotation) raises its inline-response ceiling from the default 50k chars up to the declared value — so large upstream responses (multi-file reads, command output, query results) flow through inline instead of being spilled to disk as a 2KB preview, forcing the agent to waste 3-5 calls recovering the content. Implemented as a single `OnAfterListTools` hook registered in NewMCPProxyServer — zero touches at the tool definition sites, applies uniformly to built-in tools and forwarded upstream tools, and covers every routing mode (default, /mcp/call, direct, code execution). Configurable via the new `max_result_size_chars` field in `mcp_config.json`: - default: 500000 (Claude Code's documented hard max, `IU6=500000`) - 0 : annotation disabled entirely Context: spotted via luutuankiet/mcp-proxy-shim, which solves the same problem client-side by rewriting tools/list responses. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
Injects an
anthropic/maxResultSizeCharsannotation into the_metafield of every tool returned bytools/list. Claude Code (and any client that honors this annotation) raises its inline-response ceiling from the default 50k chars up to the declared value, so large upstream responses (multi-file reads, command output, query results) flow through inline instead of being spilled to disk as a 2KB preview that forces the agent to burn 3-5 extra calls recovering the content.Why
Claude Code has a hardcoded ~50,000 char ceiling (
Vb_=50000) for MCP tool results. Responses exceeding this are persisted to disk and replaced with a 2KB preview. Tools that declare_meta.anthropic/maxResultSizeCharsget the ceiling raised (Claude Code's own max isIU6=500000).Surfaced via luutuankiet/mcp-proxy-shim, which solves the same problem client-side by rewriting tools/list responses. This brings the fix server-side so every mcpproxy user benefits without running a shim.
Design
Single
OnAfterListToolshook registered inNewMCPProxyServer:/mcp,/mcp/call, direct, code execution — they all share the same Hooks)result.Toolsslice; idempotent and safeConfiguration
New
max_result_size_charsfield inmcp_config.json:500000(default) — Claude Code's documented hard max0— annotation disabled entirelyTests
New
internal/server/mcp_max_result_size_test.go:_metaon every tool, preserves existing fields, no-op when disabled, does not leak Meta pointers across invocations, survives JSON round-tripTest plan
go test ./internal/server/ -run 'TestAnnotateToolsWithMaxResultSize|TestRegisterMaxResultSizeHook'— 11 assertions passgo test ./internal/config/— config field picked upgo vet ./internal/...cleantools/listcarries_meta.anthropic/maxResultSizeChars: 500000on both/mcpand/mcp/callroutesmax_result_size_chars: 0): annotation absent🤖 Generated with Claude Code