mcp: typed tool arguments lose precision for large int64 values

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@jstar0 已经在做这个了。

开始于 2026年9月6日。

  • #1244 来自 @jstar0 —— 未关闭

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难度
3/5
预计耗时
1-2 天
新手友好度
74/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
go
领域
api

调研方向

从 mcp/tool.go 开始，使用提供的内存中复现作为首次检查，跟踪 AddTool 参数 schema 的应用和默认路径。添加一个覆盖 int64 值 9007199254740993 的回归测试，并验证 typed handler 接收到的值未发生变化，同时现有验证和尾随数据行为保持不变。

由索引模型根据 Issue 内容生成。

描述

Describe the bug

When using the typed mcp.AddTool API, an integer larger than the IEEE-754
safe integer range can be silently rounded before it reaches the tool handler.
The value is accepted as a JSON integer, but the handler receives a different
value.

To Reproduce

Environment:

  • Go MCP SDK: v1.7.0
  • Go: go1.25.7 linux/amd64

Save the following as main.go in a module that requires
github.com/modelcontextprotocol/go-sdk v1.7.0:

package main

import (
  "context"
  "fmt"
  "log"

  "github.com/modelcontextprotocol/go-sdk/mcp"
)

type input struct {
  ID int64 `json:"id"`
}

func main() {
  const want int64 = 9007199254740993 // 2^53 + 1

  server := mcp.NewServer(
    &mcp.Implementation{Name: "server", Version: "repro"}, nil)
  mcp.AddTool(server, &mcp.Tool{Name: "echo_id"},
    func(_ context.Context, _ *mcp.CallToolRequest, in input) (
      *mcp.CallToolResult, input, error) {
      fmt.Printf("handler_received=%d\n", in.ID)
      return nil, in, nil
    })

  clientTransport, serverTransport := mcp.NewInMemoryTransports()
  ctx := context.Background()

  serverSession, err := server.Connect(ctx, serverTransport, nil)
  if err != nil {
    log.Fatal(err)
  }
  defer serverSession.Close()

  client := mcp.NewClient(
    &mcp.Implementation{Name: "client", Version: "repro"}, nil)
  clientSession, err := client.Connect(ctx, clientTransport, nil)
  if err != nil {
    log.Fatal(err)
  }
  defer clientSession.Close()

  _, err = clientSession.CallTool(ctx, &mcp.CallToolParams{
    Name: "echo_id",
    Arguments: map[string]any{"id": want},
  })
  if err != nil {
    log.Fatal(err)
  }
  fmt.Printf("want=%d\n", want)
}

Run:

go mod init example.com/mcp-int64-repro
go get github.com/modelcontextprotocol/[email protected]
go run .

Observed output:

handler_received=9007199254740992
want=9007199254740993

The issue is reproducible with the in-memory transport and does not depend on
an external server.

Expected behavior

The typed tool handler should receive exactly the integer sent by the client:

handler_received=9007199254740993
want=9007199254740993

Values representable by Go int64 should not be silently changed during
schema validation or default application.

Logs

No error is returned. The value is silently rounded.

Additional context

The behavior appears to come from the schema application path in
mcp/tool.go: tool arguments are decoded into map[string]any, and the
generic JSON number is then represented as float64. Applying defaults causes
the value to be marshaled again, making the rounding observable before the
typed handler decodes it.

A possible implementation direction is to preserve JSON numbers (for example,
using Decoder.UseNumber()) while applying the schema and defaults, while
retaining strict validation of trailing JSON data. The low-level
Server.AddTool handler path does not have this particular schema round-trip,
but it also does not provide the typed validation/default behavior.

This affects any tool that accepts IDs or counters outside the IEEE-754 safe
integer range, even though those values are valid JSON integers and valid Go
int64 values.

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