diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index 87636b9..677768c 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -24,7 +24,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.24"
       - name: Check formatting
         run: |
           cd go/sdk
@@ -51,7 +51,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.23", "1.24", "1.25"]
+        go: ["1.24", "1.25"]
     steps:
       - name: Check out code
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
diff --git a/go/sdk/go.mod b/go/sdk/go.mod
index e4a7abe..6a7718f 100644
--- a/go/sdk/go.mod
+++ b/go/sdk/go.mod
@@ -1,19 +1,22 @@
 module github.com/modelcontextprotocol/experimental-ext-variants/go/sdk
 
-go 1.23.0
+go 1.24.0
 
 toolchain go1.24.3
 
 require (
-	github.com/modelcontextprotocol/go-sdk v1.2.0
+	github.com/modelcontextprotocol/go-sdk v1.4.0
 	github.com/stretchr/testify v1.11.1
 )
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/google/jsonschema-go v0.3.0 // indirect
+	github.com/google/jsonschema-go v0.4.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/segmentio/asm v1.1.3 // indirect
+	github.com/segmentio/encoding v0.5.4 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
-	golang.org/x/oauth2 v0.30.0 // indirect
+	golang.org/x/oauth2 v0.34.0 // indirect
+	golang.org/x/sys v0.40.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
diff --git a/go/sdk/go.sum b/go/sdk/go.sum
index 2c0fd3d..b51fb72 100644
--- a/go/sdk/go.sum
+++ b/go/sdk/go.sum
@@ -1,23 +1,29 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/golang-jwt/jwt/v5 v5.2.2 h1:Rl4B7itRWVtYIHFrSNd7vhTiz9UpLdi6gZhZ3wEeDy8=
-github.com/golang-jwt/jwt/v5 v5.2.2/go.mod h1:pqrtFR0X4osieyHYxtmOUWsAWrfe1Q5UVIyoH402zdk=
+github.com/golang-jwt/jwt/v5 v5.3.0 h1:pv4AsKCKKZuqlgs5sUmn4x8UlGa0kEVt/puTpKx9vvo=
+github.com/golang-jwt/jwt/v5 v5.3.0/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
-github.com/google/jsonschema-go v0.3.0 h1:6AH2TxVNtk3IlvkkhjrtbUc4S8AvO0Xii0DxIygDg+Q=
-github.com/google/jsonschema-go v0.3.0/go.mod h1:r5quNTdLOYEz95Ru18zA0ydNbBuYoo9tgaYcxEYhJVE=
-github.com/modelcontextprotocol/go-sdk v1.2.0 h1:Y23co09300CEk8iZ/tMxIX1dVmKZkzoSBZOpJwUnc/s=
-github.com/modelcontextprotocol/go-sdk v1.2.0/go.mod h1:6fM3LCm3yV7pAs8isnKLn07oKtB0MP9LHd3DfAcKw10=
+github.com/google/jsonschema-go v0.4.2 h1:tmrUohrwoLZZS/P3x7ex0WAVknEkBZM46iALbcqoRA8=
+github.com/google/jsonschema-go v0.4.2/go.mod h1:r5quNTdLOYEz95Ru18zA0ydNbBuYoo9tgaYcxEYhJVE=
+github.com/modelcontextprotocol/go-sdk v1.4.0 h1:u0kr8lbJc1oBcawK7Df+/ajNMpIDFE41OEPxdeTLOn8=
+github.com/modelcontextprotocol/go-sdk v1.4.0/go.mod h1:Nxc2n+n/GdCebUaqCOhTetptS17SXXNu9IfNTaLDi1E=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/segmentio/asm v1.1.3 h1:WM03sfUOENvvKexOLp+pCqgb/WDjsi7EK8gIsICtzhc=
+github.com/segmentio/asm v1.1.3/go.mod h1:Ld3L4ZXGNcSLRg4JBsZ3//1+f/TjYl0Mzen/DQy1EJg=
+github.com/segmentio/encoding v0.5.4 h1:OW1VRern8Nw6ITAtwSZ7Idrl3MXCFwXHPgqESYfvNt0=
+github.com/segmentio/encoding v0.5.4/go.mod h1:HS1ZKa3kSN32ZHVZ7ZLPLXWvOVIiZtyJnO1gPH1sKt0=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=
 github.com/yosida95/uritemplate/v3 v3.0.2/go.mod h1:ILOh0sOhIJR3+L/8afwt/kE++YT040gmv5BQTMR2HP4=
-golang.org/x/oauth2 v0.30.0 h1:dnDm7JmhM45NNpd8FDDeLhK6FwqbOf4MLCM9zb1BOHI=
-golang.org/x/oauth2 v0.30.0/go.mod h1:B++QgG3ZKulg6sRPGD/mqlHQs5rB3Ml9erfeDY7xKlU=
-golang.org/x/tools v0.34.0 h1:qIpSLOxeCYGg9TrcJokLBG4KFA6d795g0xkBkiESGlo=
-golang.org/x/tools v0.34.0/go.mod h1:pAP9OwEaY1CAW3HOmg3hLZC5Z0CCmzjAF2UQMSqNARg=
+golang.org/x/oauth2 v0.34.0 h1:hqK/t4AKgbqWkdkcAeI8XLmbK+4m4G5YeQRrmiotGlw=
+golang.org/x/oauth2 v0.34.0/go.mod h1:lzm5WQJQwKZ3nwavOZ3IS5Aulzxi68dUSgRHujetwEA=
+golang.org/x/sys v0.40.0 h1:DBZZqJ2Rkml6QMQsZywtnjnnGvHza6BTfYFWY9kjEWQ=
+golang.org/x/sys v0.40.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/tools v0.41.0 h1:a9b8iMweWG+S0OBnlU36rzLp20z1Rp10w+IY2czHTQc=
+golang.org/x/tools v0.41.0/go.mod h1:XSY6eDqxVNiYgezAVqqCeihT4j1U2CCsqvH3WhQpnlg=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
diff --git a/go/sdk/variants/server.go b/go/sdk/variants/server.go
index 7405a97..66a1f46 100644
--- a/go/sdk/variants/server.go
+++ b/go/sdk/variants/server.go
@@ -378,8 +378,8 @@ func (s *Server) enrichInitResult(ctx context.Context, result mcp.Result, req mc
 //     one inner server supports it.
 //   - Completions, Logging: marker capabilities (empty structs). Advertised
 //     if any variant advertises them; the first non-nil value is kept.
-//   - Experimental: keys are merged into a single map. The first variant to
-//     register a given key wins; later duplicates are ignored.
+//   - Experimental, Extensions: keys are merged into a single map. The first
+//     variant to register a given key wins; later duplicates are ignored.
 func unionCapabilities(allCaps []*mcp.ServerCapabilities) *mcp.ServerCapabilities {
 	union := &mcp.ServerCapabilities{}
 
@@ -427,6 +427,17 @@ func unionCapabilities(allCaps []*mcp.ServerCapabilities) *mcp.ServerCapabilitie
 				}
 			}
 		}
+
+		if caps.Extensions != nil {
+			if union.Extensions == nil {
+				union.Extensions = make(map[string]any)
+			}
+			for k, v := range caps.Extensions {
+				if _, exists := union.Extensions[k]; !exists {
+					union.Extensions[k] = v
+				}
+			}
+		}
 	}
 
 	return union
diff --git a/go/sdk/variants/server_test.go b/go/sdk/variants/server_test.go
index 3110bc6..5f23a08 100644
--- a/go/sdk/variants/server_test.go
+++ b/go/sdk/variants/server_test.go
@@ -387,6 +387,71 @@ func TestIntegration_HTTP_Stateless(t *testing.T) {
 	assert.NotEmpty(t, result.Content)
 }
 
+// TestUnionCapabilities_MergesExtensions verifies that unionCapabilities
+// merges Extensions from multiple inner servers using first-wins semantics.
+func TestUnionCapabilities_MergesExtensions(t *testing.T) {
+	caps := []*mcp.ServerCapabilities{
+		{
+			Extensions: map[string]any{
+				"io.modelcontextprotocol/skills": map[string]any{},
+			},
+		},
+		{
+			Extensions: map[string]any{
+				"io.modelcontextprotocol/skills": map[string]any{"overridden": true},
+				"io.modelcontextprotocol/other":  map[string]any{"key": "val"},
+			},
+		},
+		{},
+	}
+
+	union := unionCapabilities(caps)
+
+	require.NotNil(t, union.Extensions)
+	assert.Contains(t, union.Extensions, "io.modelcontextprotocol/skills")
+	assert.Contains(t, union.Extensions, "io.modelcontextprotocol/other")
+
+	// First-wins: the first variant's value should be kept.
+	skillsExt, ok := union.Extensions["io.modelcontextprotocol/skills"].(map[string]any)
+	require.True(t, ok)
+	assert.Empty(t, skillsExt, "first variant's empty map should win over second variant's")
+}
+
+// TestIntegration_ExtensionsAdvertised verifies that extensions registered on
+// inner variant servers are advertised to clients in the InitializeResult.
+func TestIntegration_ExtensionsAdvertised(t *testing.T) {
+	codingServer := mcp.NewServer(
+		&mcp.Implementation{Name: "coding", Version: "v1"},
+		&mcp.ServerOptions{
+			Capabilities: func() *mcp.ServerCapabilities {
+				caps := &mcp.ServerCapabilities{}
+				caps.AddExtension("io.modelcontextprotocol/skills", nil)
+				return caps
+			}(),
+		},
+	)
+	mcp.AddTool(codingServer, &mcp.Tool{Name: "analyze_code", Description: "Static analysis"}, analyzeCode)
+
+	compactServer := mcp.NewServer(
+		&mcp.Implementation{Name: "compact", Version: "v1"},
+		nil,
+	)
+	mcp.AddTool(compactServer, &mcp.Tool{Name: "summarize", Description: "Summarize text"}, summarize)
+
+	vs := NewServer(&mcp.Implementation{Name: "test", Version: "1.0.0"}).
+		WithVariant(ServerVariant{ID: "coding", Description: "Coding", Status: Stable}, codingServer, 0).
+		WithVariant(ServerVariant{ID: "compact", Description: "Compact", Status: Stable}, compactServer, 1)
+
+	session := connectTestClient(t, vs, nil)
+
+	initResult := session.InitializeResult()
+	require.NotNil(t, initResult)
+	require.NotNil(t, initResult.Capabilities)
+	require.NotNil(t, initResult.Capabilities.Extensions,
+		"extensions from inner servers should be advertised in InitializeResult")
+	assert.Contains(t, initResult.Capabilities.Extensions, "io.modelcontextprotocol/skills")
+}
+
 // ---------------------------------------------------------------------------
 // Test tool handlers
 // ---------------------------------------------------------------------------