diff --git a/design/design.md b/design/design.md
index 3791f0f..083fb49 100644
--- a/design/design.md
+++ b/design/design.md
@@ -329,9 +329,8 @@ corresponding to the union of all properties for union elements.
 For brevity, only a few examples are shown here:
 
 ```go
-type CallToolParams struct {
-	Arguments map[string]json.RawMessage `json:"arguments,omitempty"`
-	Name      string                     `json:"name"`
+type ReadResourceParams struct {
+	URI string `json:"uri"`
 }
 
 type CallToolResult struct {
@@ -428,10 +427,7 @@ transport := mcp.NewCommandTransport(exec.Command("myserver"))
 session, err := client.Connect(ctx, transport)
 if err != nil { ... }
 // Call a tool on the server.
-content, err := session.CallTool(ctx, &CallToolParams{
-  Name: "greet",
-  Arguments: map[string]any{"name": "you"} ,
-})
+content, err := session.CallTool(ctx, "greet", map[string]any{"name": "you"}, nil})
 ...
 return session.Close()
 ```
@@ -479,17 +475,20 @@ documentation.
 
 ### Spec Methods
 
-As we saw above, the `ClientSession` method for the specification's
-`CallTool` RPC takes a context and a params pointer as arguments, and returns a
-result pointer and error:
+In our SDK, RPC methods that are defined in the specification take a context and
+a params pointer as arguments, and return a result pointer and error:
 
 ```go
-func (*ClientSession) CallTool(context.Context, *CallToolParams) (*CallToolResult, error)
+func (*ClientSession) ListTools(context.Context, *ListToolsParams) (*ListToolsResult, error)
 ```
 
-Our SDK has a method for every RPC in the spec, and their signatures all share
-this form. To avoid boilerplate, we don't repeat this signature for RPCs
-defined in the spec; readers may assume it when we mention a "spec method."
+Our SDK has a method for every RPC in the spec, and except for `CallTool`,
+their signatures all share this form. To avoid boilerplate, we don't repeat this
+signature for RPCs defined in the spec; readers may assume it when we mention a
+"spec method."
+
+`CallTool` is the only exception: for convenience, it takes the tool name and
+arguments, with an options truct for additional request fields.
 
 Why do we use params instead of the full request? JSON-RPC requests consist of a method
 name and a set of parameters, and the method is already encoded in the Go method name.
@@ -564,7 +563,7 @@ can cancel an operation by cancelling the associated context:
 
 ```go
 ctx, cancel := context.WithCancel(ctx)
-go session.CallTool(ctx, "slow", map[string]any{})
+go session.CallTool(ctx, "slow", map[string]any{}, nil)
 cancel()
 ```
 
@@ -687,7 +686,7 @@ type Tool struct {
 	Name string                    `json:"name"`
 }
 
-type ToolHandler func(context.Context, *ServerSession, map[string]json.RawMessage) (*CallToolResult, error)
+type ToolHandler func(context.Context, *ServerSession, *CallToolParams) (*CallToolResult, error)
 
 type ServerTool struct {
 	Tool    Tool
diff --git a/mcp/client.go b/mcp/client.go
index d4dfba6..0a24930 100644
--- a/mcp/client.go
+++ b/mcp/client.go
@@ -174,7 +174,8 @@ func (c *Client) ListTools(ctx context.Context, params *ListToolsParams) (*ListT
 }
 
 // CallTool calls the tool with the given name and arguments.
-func (c *Client) CallTool(ctx context.Context, name string, args map[string]any) (_ *CallToolResult, err error) {
+// Pass a [CallToolOptions] to provide additional request fields.
+func (c *Client) CallTool(ctx context.Context, name string, args map[string]any, opts *CallToolOptions) (_ *CallToolResult, err error) {
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("calling tool %q: %w", name, err)
@@ -196,6 +197,13 @@ func (c *Client) CallTool(ctx context.Context, name string, args map[string]any)
 	return standardCall[CallToolResult](ctx, c.conn, "tools/call", params)
 }
 
+// NOTE: the following struct should consist of all fields of callToolParams except name and arguments.
+
+// CallToolOptions contains options to [Client.CallTools].
+type CallToolOptions struct {
+	ProgressToken any // string or int
+}
+
 // ListResources lists the resources that are currently available on the server.
 func (c *Client) ListResources(ctx context.Context, params *ListResourcesParams) (*ListResourcesResult, error) {
 	return standardCall[ListResourcesResult](ctx, c.conn, "resources/list", params)
diff --git a/mcp/cmd_test.go b/mcp/cmd_test.go
index addc59e..fd048f4 100644
--- a/mcp/cmd_test.go
+++ b/mcp/cmd_test.go
@@ -52,7 +52,7 @@ func TestCmdTransport(t *testing.T) {
 	if err := client.Start(ctx); err != nil {
 		log.Fatal(err)
 	}
-	got, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"})
+	got, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/mcp/mcp_test.go b/mcp/mcp_test.go
index 2d494b3..b487483 100644
--- a/mcp/mcp_test.go
+++ b/mcp/mcp_test.go
@@ -160,7 +160,7 @@ func TestEndToEnd(t *testing.T) {
 			t.Fatalf("tools/list mismatch (-want +got):\n%s", diff)
 		}
 
-		gotHi, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"})
+		gotHi, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -171,7 +171,7 @@ func TestEndToEnd(t *testing.T) {
 			t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)
 		}
 
-		gotFail, err := c.CallTool(ctx, "fail", map[string]any{})
+		gotFail, err := c.CallTool(ctx, "fail", map[string]any{}, nil)
 		// Counter-intuitively, when a tool fails, we don't expect an RPC error for
 		// call tool: instead, the failure is embedded in the result.
 		if err != nil {
@@ -317,12 +317,12 @@ func TestServerClosing(t *testing.T) {
 		}
 		wg.Done()
 	}()
-	if _, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}); err != nil {
+	if _, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil); err != nil {
 		t.Fatalf("after connecting: %v", err)
 	}
 	cc.Close()
 	wg.Wait()
-	if _, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}); !errors.Is(err, ErrConnectionClosed) {
+	if _, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil); !errors.Is(err, ErrConnectionClosed) {
 		t.Errorf("after disconnection, got error %v, want EOF", err)
 	}
 }
@@ -384,7 +384,7 @@ func TestCancellation(t *testing.T) {
 	defer sc.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
-	go sc.CallTool(ctx, "slow", map[string]any{})
+	go sc.CallTool(ctx, "slow", map[string]any{}, nil)
 	<-start
 	cancel()
 	select {
diff --git a/mcp/server.go b/mcp/server.go
index ce6ef81..18d4d09 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -204,7 +204,7 @@ func (s *Server) callTool(ctx context.Context, cc *ServerConnection, params *Cal
 	if !ok {
 		return nil, fmt.Errorf("%s: unknown tool %q", jsonrpc2.ErrInvalidParams, params.Name)
 	}
-	return tool.Handler(ctx, cc, params.Arguments)
+	return tool.Handler(ctx, cc, params)
 }
 
 func (s *Server) listResources(_ context.Context, _ *ServerConnection, params *ListResourcesParams) (*ListResourcesResult, error) {
diff --git a/mcp/server_example_test.go b/mcp/server_example_test.go
index 04042c1..b7ec788 100644
--- a/mcp/server_example_test.go
+++ b/mcp/server_example_test.go
@@ -39,7 +39,7 @@ func ExampleServer() {
 		log.Fatal(err)
 	}
 
-	res, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"})
+	res, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/mcp/sse_example_test.go b/mcp/sse_example_test.go
index eee822f..60bf3f4 100644
--- a/mcp/sse_example_test.go
+++ b/mcp/sse_example_test.go
@@ -40,7 +40,7 @@ func ExampleSSEHandler() {
 	}
 	defer client.Close()
 
-	res, err := client.CallTool(ctx, "add", map[string]any{"x": 1, "y": 2})
+	res, err := client.CallTool(ctx, "add", map[string]any{"x": 1, "y": 2}, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/mcp/sse_test.go b/mcp/sse_test.go
index 5cbd2cc..fc844a4 100644
--- a/mcp/sse_test.go
+++ b/mcp/sse_test.go
@@ -43,7 +43,7 @@ func TestSSEServer(t *testing.T) {
 				t.Fatal(err)
 			}
 			cc := <-conns
-			gotHi, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"})
+			gotHi, err := c.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
 			if err != nil {
 				t.Fatal(err)
 			}
diff --git a/mcp/tool.go b/mcp/tool.go
index 7a93c6c..2cc9a50 100644
--- a/mcp/tool.go
+++ b/mcp/tool.go
@@ -13,7 +13,7 @@ import (
 )
 
 // A ToolHandler handles a call to tools/call.
-type ToolHandler func(context.Context, *ServerConnection, map[string]json.RawMessage) (*CallToolResult, error)
+type ToolHandler func(context.Context, *ServerConnection, *CallToolParams) (*CallToolResult, error)
 
 // A Tool is a tool definition that is bound to a tool handler.
 type ServerTool struct {
@@ -39,10 +39,10 @@ func NewTool[TReq any](name, description string, handler func(context.Context, *
 	if err != nil {
 		panic(err)
 	}
-	wrapped := func(ctx context.Context, cc *ServerConnection, args map[string]json.RawMessage) (*CallToolResult, error) {
+	wrapped := func(ctx context.Context, cc *ServerConnection, params *CallToolParams) (*CallToolResult, error) {
 		// For simplicity, just marshal and unmarshal the arguments.
 		// This could be avoided in the future.
-		rawArgs, err := json.Marshal(args)
+		rawArgs, err := json.Marshal(params.Arguments)
 		if err != nil {
 			return nil, err
 		}