diff --git a/internal/jsonrpc2/decimal_id_test.go b/internal/jsonrpc2/decimal_id_test.go
new file mode 100644
index 000000000..dfad87755
--- /dev/null
+++ b/internal/jsonrpc2/decimal_id_test.go
@@ -0,0 +1,115 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package jsonrpc2
+
+import (
+	"encoding/json"
+	"math/big"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestDecodeDecimalIntegerIDs(t *testing.T) {
+	for _, tc := range []struct {
+		wire string
+		want int64
+	}{
+		{"9007199254740993", 9007199254740993},
+		{"9007199254740993.0", 9007199254740993},
+		{"90071992547409930e-1", 9007199254740993},
+		{"-9007199254740993.0", -9007199254740993},
+		{"9223372036854775807.0", 9223372036854775807},
+		{"-9223372036854775808.0", -9223372036854775808},
+		{"9.007199254740993e15", 9007199254740993},
+		{"9007199254740993000E-3", 9007199254740993},
+		{"0.00042e5", 42},
+		{"-0e999999999999999999999", 0},
+		{"42.0", 42},
+		{"420e-1", 42},
+	} {
+		t.Run(tc.wire, func(t *testing.T) {
+			for _, suffix := range []string{`,"method":"m"}`, `,"result":{}}`} {
+				wire := []byte(`{"jsonrpc":"2.0","id":` + tc.wire + suffix)
+				msg, err := DecodeMessage(wire)
+				if err != nil {
+					t.Fatal(err)
+				}
+				var id ID
+				switch m := msg.(type) {
+				case *Request:
+					id = m.ID
+				case *Response:
+					id = m.ID
+				}
+				if id != Int64ID(tc.want) {
+					t.Errorf("wire %s: got %v, want %d", wire, id.Raw(), tc.want)
+				}
+			}
+			id, err := MakeID(json.Number(tc.wire))
+			if err != nil || id != Int64ID(tc.want) {
+				t.Errorf("MakeID: got %v, err %v, want %d", id.Raw(), err, tc.want)
+			}
+		})
+	}
+}
+
+func TestNumericIDLegacyFallback(t *testing.T) {
+	for _, value := range []string{"1.5", "-1.5", "9223372036854775808", "-9223372036854775809", "1e100", "1e-100", "1e99999999999999999999", "invalid"} {
+		f, oldErr := strconv.ParseFloat(value, 64)
+		got, err := MakeID(json.Number(value))
+		if oldErr != nil {
+			if err == nil {
+				t.Errorf("MakeID(%q) unexpectedly succeeded", value)
+			}
+		} else if err != nil || got != Int64ID(int64(f)) {
+			t.Errorf("MakeID(%q) = %v, %v; want legacy %v", value, got.Raw(), err, int64(f))
+		}
+	}
+	for _, f := range []float64{1.5, -1.5, 9007199254740992} {
+		got, err := MakeID(f)
+		if err != nil || got != Int64ID(int64(f)) {
+			t.Errorf("MakeID(%v) = %v, %v", f, got.Raw(), err)
+		}
+	}
+}
+
+func FuzzExactIntegerID(f *testing.F) {
+	for _, s := range []string{"9007199254740993.0", "90071992547409930e-1", "9e18", "-9223372036854775808.0", "1.5", "0", "0e99999999999999999999", "1e-9999999999999999999"} {
+		f.Add(s)
+	}
+	f.Fuzz(func(t *testing.T, s string) {
+		if len(s) > 256 || !json.Valid([]byte(s)) || strings.TrimSpace(s) != s {
+			return
+		}
+		// Bound the oracle's exponent; the production parser has no such input cap.
+		if pos := strings.IndexAny(s, "eE"); pos >= 0 {
+			e, err := strconv.Atoi(s[pos+1:])
+			if err != nil || e < -300 || e > 300 {
+				return
+			}
+		}
+		want, ok := new(big.Rat).SetString(s)
+		if !ok {
+			return
+		}
+		got, exact := exactIntegerID(s)
+		valid := want.IsInt() && want.Num().IsInt64()
+		if exact != valid || (exact && got != want.Num().Int64()) {
+			t.Fatalf("exactIntegerID(%q) = %d, %v; want %s, %v", s, got, exact, want, valid)
+		}
+	})
+}
+
+func BenchmarkMakeNumericID(b *testing.B) {
+	for _, s := range []string{"9007199254740993", "9007199254740993.0", "90071992547409930e-1", "1e99999999999999999999"} {
+		b.Run(s, func(b *testing.B) {
+			b.ReportAllocs()
+			for b.Loop() {
+				_, _ = MakeID(json.Number(s))
+			}
+		})
+	}
+}
diff --git a/internal/jsonrpc2/messages.go b/internal/jsonrpc2/messages.go
index dc445445c..a2d3483d9 100644
--- a/internal/jsonrpc2/messages.go
+++ b/internal/jsonrpc2/messages.go
@@ -9,6 +9,8 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"strconv"
+	"strings"
 
 	internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
 )
@@ -21,8 +23,9 @@ type ID struct {
 
 // MakeID coerces the given Go value to an ID. The value should be the JSON
 // decoding of a Request identifier: nil, float64, json.Number, or string.
-// Decoding with json.Number keeps integer IDs above 2^53 exact; a json.Number
-// must be an integer literal in the int64 range.
+// Decoding with json.Number keeps integer IDs in the int64 range exact,
+// including decimal and exponent notation. Other numbers retain the legacy
+// float64 conversion behavior.
 //
 // Returns an error if the value type was not a valid Request ID type.
 //
@@ -39,6 +42,9 @@ func MakeID(v any) (ID, error) {
 		if i, err := v.Int64(); err == nil {
 			return Int64ID(i), nil
 		}
+		if i, ok := exactIntegerID(string(v)); ok {
+			return Int64ID(i), nil
+		}
 		// Fallback to maintain backward compatibility
 		if f, err := v.Float64(); err == nil {
 			return Int64ID(int64(f)), nil
@@ -50,6 +56,63 @@ func MakeID(v any) (ID, error) {
 	return ID{}, fmt.Errorf("%w: invalid ID type %T", ErrParse, v)
 }
 
+// exactIntegerID normalizes decimal digits without converting through float64
+// or allocating storage proportional to the exponent.
+func exactIntegerID(s string) (int64, bool) {
+	if !json.Valid([]byte(s)) || len(s) == 0 {
+		return 0, false
+	}
+	negative := s[0] == '-'
+	if negative {
+		s = s[1:]
+	}
+	mantissa, exponent := s, "0"
+	if i := strings.IndexAny(s, "eE"); i >= 0 {
+		mantissa, exponent = s[:i], s[i+1:]
+	}
+	fractional := 0
+	if i := strings.IndexByte(mantissa, '.'); i >= 0 {
+		fractional = len(mantissa) - i - 1
+		mantissa = mantissa[:i] + mantissa[i+1:]
+	}
+	for _, c := range mantissa {
+		if c < '0' || c > '9' {
+			return 0, false
+		}
+	}
+	if len(mantissa) == 0 {
+		return 0, false
+	}
+	digits := strings.TrimLeft(mantissa, "0")
+	if digits == "" {
+		return 0, true
+	}
+	exp, err := strconv.Atoi(exponent)
+	if err != nil || exp > len(s)+19 || exp < -len(s) {
+		return 0, false
+	}
+	// Remove trailing zeroes before applying the exponent, so a negative scale
+	// can only describe a genuinely fractional number.
+	significant := strings.TrimRight(digits, "0")
+	scale := exp - fractional + len(digits) - len(significant)
+	if scale < 0 || scale > 19 || len(significant) > 19-scale {
+		return 0, false
+	}
+	var buf [20]byte
+	n := 0
+	if negative {
+		buf[n] = '-'
+		n++
+	}
+	n += copy(buf[n:], significant)
+	for range scale {
+		buf[n] = '0'
+		n++
+	}
+	i, err := strconv.ParseInt(string(buf[:n]), 10, 64)
+	return i, err == nil
+}
+
 // Message is the interface to all jsonrpc2 message types.
 // They share no common functionality, but are a closed set of concrete types
 // that are allowed to implement this interface. The message types are *Request
diff --git a/jsonrpc/jsonrpc.go b/jsonrpc/jsonrpc.go
index bbc2efe4d..4b401c62a 100644
--- a/jsonrpc/jsonrpc.go
+++ b/jsonrpc/jsonrpc.go
@@ -23,8 +23,9 @@ type (
 
 // MakeID coerces the given Go value to an ID. The value should be the JSON
 // decoding of a Request identifier: nil, float64, json.Number, or string.
-// Decoding with json.Number keeps integer IDs above 2^53 exact; a json.Number
-// must be an integer literal in the int64 range.
+// Decoding with json.Number keeps integer IDs in the int64 range exact,
+// including decimal and exponent notation. Other numbers retain the legacy
+// float64 conversion behavior.
 //
 // Returns an error if the value type was not a valid Request ID type.
 func MakeID(v any) (ID, error) {
diff --git a/mcp/decimal_cancel_test.go b/mcp/decimal_cancel_test.go
new file mode 100644
index 000000000..43f05a0ad
--- /dev/null
+++ b/mcp/decimal_cancel_test.go
@@ -0,0 +1,70 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
+)
+
+func TestCancelDecimalIntegerID(t *testing.T) {
+	for _, id := range []string{"9007199254740993.0", "90071992547409930e-1"} {
+		t.Run(id, func(t *testing.T) {
+			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+			a, b := net.Pipe()
+			transport, peer := newIOConn(a), newIOConn(b)
+			started := make(chan struct{})
+			canceled := make(chan struct{})
+			preempt := &canceller{logger: slog.New(slog.NewTextHandler(io.Discard, nil))}
+			conn := jsonrpc2.NewConnection(ctx, jsonrpc2.ConnectionConfig{
+				Reader: transport, Writer: transport, Closer: transport, Preempter: preempt,
+				Bind: func(c *jsonrpc2.Connection) jsonrpc2.Handler {
+					preempt.conn = c
+					return jsonrpc2.HandlerFunc(func(callCtx context.Context, req *jsonrpc2.Request) (any, error) {
+						if req.Method != "wait" {
+							return nil, jsonrpc2.ErrNotHandled
+						}
+						close(started)
+						select {
+						case <-callCtx.Done():
+							close(canceled)
+							return nil, callCtx.Err()
+						case <-ctx.Done():
+							return nil, ctx.Err()
+						}
+					})
+				},
+			})
+			t.Cleanup(func() { cancel(); peer.Close(); conn.Close() })
+			if err := peer.Write(ctx, &jsonrpc2.Request{ID: jsonrpc2.Int64ID(9007199254740993), Method: "wait"}); err != nil {
+				t.Fatal(err)
+			}
+			select {
+			case <-started:
+			case <-ctx.Done():
+				t.Fatal(ctx.Err())
+			}
+			if err := peer.Write(ctx, &jsonrpc2.Request{Method: notificationCancelled, Params: json.RawMessage(`{"requestId":` + id + `}`)}); err != nil {
+				t.Fatal(err)
+			}
+			select {
+			case <-canceled:
+			case <-ctx.Done():
+				t.Fatal("decimal requestId did not cancel the matching integer request")
+			}
+			if _, err := peer.Read(ctx); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}