feat(mcp): advertise native args object on call_tool_* variants#379
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feat(mcp): advertise native args object on call_tool_* variants#379
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Adds an `args` object parameter alongside `args_json` on call_tool_read,
call_tool_write, and call_tool_destructive. The handler already accepted
`args` as a fallback, but the schema did not advertise it — so LLM clients
always produced pre-serialized JSON strings, costing ~350 tokens per call
and producing frequent escaping bugs (see luutuankiet/mcp-proxy-shim for
context).
Clients can now send arguments as a native JSON object:
{ "name": "github:get_user", "args": { "username": "octocat" } }
instead of:
{ "name": "github:get_user", "args_json": "{\"username\":\"octocat\"}" }
`args_json` remains advertised and takes precedence when both are provided,
so existing clients continue to work unchanged.
The tool definitions for the default `/mcp` and `/mcp/call` routes were
duplicated across mcp.go and mcp_routing.go; factored them into a shared
`buildCallToolVariantTool(variant)` helper to keep the schema in sync.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
Adds a native
argsobject parameter alongside the legacyargs_jsonstring oncall_tool_read,call_tool_write, andcall_tool_destructive.Clients can now send arguments as a native JSON object:
{ "name": "github:get_user", "args": { "username": "octocat" } }instead of a pre-serialized string:
{ "name": "github:get_user", "args_json": "{\"username\":\"octocat\"}" }Motivation
Spotted via luutuankiet/mcp-proxy-shim — a client-side shim that transforms
args_jsonintoargsto eliminate escaping overhead. Shim benchmarks: ~350 tokens saved per call, frequent escaping bugs eliminated. This brings the fix server-side.The handler at
internal/server/mcp.go:1289already accepted anargsobject as a fallback — only the advertised schema was missing it, so LLM clients never discovered the native path.Changes
WithObject("args", ...)to the tool schemas (3 variants × 2 routes).mcp.go/mcp_routing.gointo a sharedbuildCallToolVariantTool(variant)helper.args_jsonremains advertised and wins when both are provided — fully backward-compatible.Tests
New
internal/server/mcp_call_tool_args_test.go:TestCallToolVariantSchemaAdvertisesArgsObject— schema contractTestHandleCallToolVariantAcceptsArgsObject— runtime extraction pathTestHandleCallToolVariantArgsJSONWinsWhenBothProvided— precedence / backward compatTest plan
go test ./internal/server/ -run 'TestCallTool|TestHandleCall|TestArgsJson|TestMCPProxyServer_'passesgo test ./internal/runtime/...passesgo vet ./internal/server/...cleantools/liston/mcpand/mcp/call— both routes showargs(object) +args_json(string) on all three variantstools/callwith nativeargsobject reaches upstream dispatch (errors on missing server, not arg parsing)🤖 Generated with Claude Code