diff --git a/client/caller.go b/client/caller.go
new file mode 100644
index 0000000..b910bf1
--- /dev/null
+++ b/client/caller.go
@@ -0,0 +1,18 @@
+package client
+
+import (
+	"context"
+	"encoding/json"
+)
+
+// Caller invokes an MCP tool by name and returns its JSON result.
+type Caller interface {
+	Call(ctx context.Context, name string, args map[string]any) (json.RawMessage, error)
+}
+
+// Func adapts a function to Caller so tests can inject a stub.
+type Func func(ctx context.Context, name string, args map[string]any) (json.RawMessage, error)
+
+func (f Func) Call(ctx context.Context, name string, args map[string]any) (json.RawMessage, error) {
+	return f(ctx, name, args)
+}
diff --git a/client/enums.go b/client/enums.go
new file mode 100644
index 0000000..932f968
--- /dev/null
+++ b/client/enums.go
@@ -0,0 +1,41 @@
+package client
+
+// Side is an order side. Values are Robinhood wire strings.
+type Side string
+
+const (
+	Buy  Side = "buy"
+	Sell Side = "sell"
+)
+
+// OrderType is an order type. Values are Robinhood wire strings.
+type OrderType string
+
+const (
+	Market    OrderType = "market"
+	Limit     OrderType = "limit"
+	Stop      OrderType = "stop_market" // equity + options; crypto uses StopLoss
+	StopLimit OrderType = "stop_limit"
+	StopLoss  OrderType = "stop_loss" // crypto only
+)
+
+// TimeInForce is an order duration. Values are Robinhood wire strings.
+type TimeInForce string
+
+const (
+	GFD TimeInForce = "gfd"
+	GTC TimeInForce = "gtc"
+	GFW TimeInForce = "gfw" // crypto
+	GFM TimeInForce = "gfm" // crypto
+)
+
+// MarketHours is a trading-session window. Values are Robinhood wire strings.
+type MarketHours string
+
+const (
+	RegularHours              MarketHours = "regular_hours"
+	ExtendedHours             MarketHours = "extended_hours"
+	AllDayHours               MarketHours = "all_day_hours"
+	RegularCurbHours          MarketHours = "regular_curb_hours"
+	RegularCurbOvernightHours MarketHours = "regular_curb_overnight_hours"
+)
diff --git a/client/enums_test.go b/client/enums_test.go
new file mode 100644
index 0000000..95db7be
--- /dev/null
+++ b/client/enums_test.go
@@ -0,0 +1,17 @@
+package client_test
+
+import (
+	"testing"
+
+	"s1d3sw1ped/robinhood-agentic-mcp/client"
+)
+
+func TestEnums_wireValues(t *testing.T) {
+	t.Parallel()
+	if client.Buy != "buy" || client.Stop != "stop_market" || client.StopLoss != "stop_loss" {
+		t.Fatal("side/type")
+	}
+	if client.GFD != "gfd" || client.RegularHours != "regular_hours" {
+		t.Fatal("tif/hours")
+	}
+}
diff --git a/client/error.go b/client/error.go
new file mode 100644
index 0000000..a6d59a5
--- /dev/null
+++ b/client/error.go
@@ -0,0 +1,28 @@
+package client
+
+import "fmt"
+
+// ToolError is a failed MCP tool call or a parse of its result.
+type ToolError struct {
+	Name    string // MCP tool name
+	Message string
+	Err     error // transport/SDK/parse cause; may be nil
+}
+
+func (e *ToolError) Error() string {
+	return "mcp " + e.Name + ": " + e.Message
+}
+
+func (e *ToolError) Unwrap() error {
+	return e.Err
+}
+
+// ToolErrorf builds a ToolError. format may use %w so Unwrap works.
+func ToolErrorf(name, format string, args ...any) *ToolError {
+	err := fmt.Errorf(format, args...)
+	return &ToolError{
+		Name:    name,
+		Message: err.Error(),
+		Err:     err,
+	}
+}
diff --git a/client/error_test.go b/client/error_test.go
new file mode 100644
index 0000000..69f0b06
--- /dev/null
+++ b/client/error_test.go
@@ -0,0 +1,24 @@
+package client_test
+
+import (
+	"errors"
+	"testing"
+
+	"s1d3sw1ped/robinhood-agentic-mcp/client"
+)
+
+func TestToolError_formatAndAs(t *testing.T) {
+	t.Parallel()
+	inner := errors.New("boom")
+	err := client.ToolErrorf("get_accounts", "parse quotes: %w", inner)
+	if err.Error() != "mcp get_accounts: parse quotes: boom" {
+		t.Fatalf("%q", err.Error())
+	}
+	var te *client.ToolError
+	if !errors.As(err, &te) || te.Name != "get_accounts" {
+		t.Fatalf("%v", err)
+	}
+	if !errors.Is(err, inner) {
+		t.Fatal("unwrap")
+	}
+}