diff --git a/api_request_builder.go b/api_request_builder.go
new file mode 100644
index 0000000..8abceab
--- /dev/null
+++ b/api_request_builder.go
@@ -0,0 +1,124 @@
+package mujica
+
+import (
+	"fmt"
+	"log/slog"
+	"net/url"
+	"slices"
+
+	"github.com/egoist/mygo/ui"
+
+	"mujicaui/icons"
+	"mujicaui/theme"
+)
+
+// ApiRequest is an HTTP request being built; the caller sends it.
+type ApiRequest struct {
+	// Method is GET when empty.
+	Method  string
+	URL     string
+	Params  []KeyValue
+	Headers []KeyValue
+	Body    string
+}
+
+// ApiRequestBuilderOptions configure an ApiRequestBuilder.
+type ApiRequestBuilderOptions struct {
+	// Sending shows the request in flight and blocks another send.
+	Sending  bool
+	Disabled bool
+}
+
+// ApiRequestResult reports this frame's events.
+type ApiRequestResult struct {
+	Element *ui.Element
+	// Send asks the caller to send the request.
+	Send    bool
+	Changed bool
+}
+
+type apiRequestState struct{ tab int }
+
+var apiMethods = []string{"GET", "POST", "PUT", "PATCH", "DELETE", "HEAD", "OPTIONS"}
+
+// ApiRequestBuilder edits method, URL, query parameters, headers and body, and reports a send.
+func ApiRequestBuilder(c *ui.Context, req *ApiRequest, opts ApiRequestBuilderOptions) ApiRequestResult {
+	if req == nil {
+		panic("mujica: ApiRequestBuilder needs a request")
+	}
+	if req.Method == "" {
+		req.Method = "GET"
+	}
+	if !slices.Contains(apiMethods, req.Method) {
+		panic("mujica: ApiRequestBuilder method " + req.Method + " is not offered")
+	}
+	u := Density(c).Unit()
+	var r ApiRequestResult
+	r.Element = ui.Column(c).Shrink(0).Gap(u * 3).Disabled(opts.Disabled)
+	st := ui.Local(r.Element, "mujica.apirequest", func() apiRequestState { return apiRequestState{} })
+	locked := r.Element.IsDisabled()
+	full, urlErr := apiRequestURL(*req)
+	r.Element.Children(func() {
+		ui.Row(c).Gap(u * 2).AlignItems(ui.Start).Children(func() {
+			methods := make([]SelectOption[string], len(apiMethods))
+			for i, m := range apiMethods {
+				methods[i] = SelectOption[string]{Value: m, Label: m}
+			}
+			r.Changed = Select(c, &req.Method, methods, SelectOptions{Label: msg(c, "api_request_builder.method"), Disabled: locked}).Changed()
+			shownErr := ""
+			if req.URL != "" && urlErr != nil {
+				shownErr = urlErr.Error()
+			}
+			in := TextInput(c, &req.URL, TextInputOptions{Placeholder: "https://", Label: msg(c, "api_request_builder.url"), Disabled: locked, Error: shownErr})
+			in.Element.Grow(1)
+			r.Changed = r.Changed || in.Changed()
+			ready := urlErr == nil && !opts.Sending && !locked
+			send := Button(c, msg(c, "api_request_builder.send"), ButtonOptions{Icon: icons.Must("upload"), Loading: opts.Sending, Disabled: locked || urlErr != nil})
+			r.Send = ready && (send.Clicked() || in.Submitted())
+		})
+		if urlErr == nil {
+			codeMono(c, req.Method+" "+full).TextColor(Tokens(c).TextMuted)
+		}
+		tabs := []Tab{
+			{Label: fmt.Sprintf(msg(c, "api_request_builder.params"), len(keyValueEnabled(req.Params))), Panel: func() {
+				r.Changed = KeyValueInput(c, &req.Params, KeyValueInputOptions{Label: msg(c, "api_request_builder.params_label"), Disabled: locked}).Changed() || r.Changed
+			}},
+			{Label: fmt.Sprintf(msg(c, "api_request_builder.headers"), len(keyValueEnabled(req.Headers))), Panel: func() {
+				r.Changed = KeyValueInput(c, &req.Headers, KeyValueInputOptions{Label: msg(c, "api_request_builder.headers_label"), Disabled: locked}).Changed() || r.Changed
+			}},
+			{Label: msg(c, "api_request_builder.body"), Panel: func() {
+				if req.Method == "GET" || req.Method == "HEAD" {
+					agentCaption(c, msg(c, "api_request_builder.no_body"))
+				}
+				body := TextArea(c, &req.Body, TextAreaOptions{Label: msg(c, "api_request_builder.body"), MinHeight: 120, MaxHeight: 320, Disabled: locked})
+				body.Input.Font(theme.MonoFont)
+				r.Changed = r.Changed || body.Changed()
+			}},
+		}
+		Tabs(c, &st.tab, tabs, TabsOptions{Label: msg(c, "api_request_builder.parts")})
+	})
+	if r.Send {
+		slog.Debug("mujica: api_request_builder send", "method", req.Method) // never the URL: it may hold a token
+	}
+	return r
+}
+
+// apiRequestURL joins the URL with its enabled query parameters, or says why it is invalid.
+func apiRequestURL(req ApiRequest) (string, error) {
+	if req.URL == "" {
+		return "", fmt.Errorf("empty URL")
+	}
+	u, err := url.Parse(req.URL)
+	if err != nil {
+		return "", err
+	}
+	if (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
+		return "", fmt.Errorf("the URL needs http:// or https:// and a host")
+	}
+	q := u.Query()
+	for _, kv := range keyValueEnabled(req.Params) {
+		q.Add(kv.Key, kv.Value)
+	}
+	u.RawQuery = q.Encode()
+	return u.String(), nil
+}
diff --git a/api_request_builder_test.go b/api_request_builder_test.go
new file mode 100644
index 0000000..151a7c5
--- /dev/null
+++ b/api_request_builder_test.go
@@ -0,0 +1,93 @@
+package mujica
+
+import (
+	"testing"
+
+	"github.com/egoist/mygo/ui"
+)
+
+func apiTester(req *ApiRequest, opts *ApiRequestBuilderOptions, sends *int) *ui.Tester {
+	return ui.NewTester(func(c *ui.Context) {
+		Use(c, Settings{})
+		ui.Column(c).Padding(8).Children(func() {
+			if ApiRequestBuilder(c, req, *opts).Send {
+				*sends++
+			}
+		})
+	}, 760, 520)
+}
+
+func TestApiRequestBuilderBuildAndSend(t *testing.T) {
+	var req ApiRequest
+	opts := ApiRequestBuilderOptions{}
+	sends := 0
+	tt := apiTester(&req, &opts, &sends)
+	if req.Method != "GET" {
+		t.Fatalf("default method %q", req.Method)
+	}
+	_ = tt.Click("Send") // disabled without a URL
+	_ = tt.Click("Request URL")
+	tt.Type("ftp://court")
+	if !tt.HasText("the URL needs http:// or https:// and a host") || sends != 0 {
+		t.Fatalf("invalid URL: %q", tt.Texts())
+	}
+	req.URL = ""
+	tt.Frame()
+	_ = tt.Click("Request URL")
+	tt.Type("https://court.example/guests")
+	_ = tt.Click("Add row")
+	_ = tt.Click("Row 1 Key")
+	tt.Type("page")
+	tt.Key(0, ui.KeyTab)
+	tt.Type("2")
+	if !tt.HasText("GET https://court.example/guests?page=2") || !tt.HasText("Params (1)") {
+		t.Fatalf("preview: %q", tt.Texts())
+	}
+	_ = tt.Click("Request URL")
+	tt.Key(0, ui.KeyEnter)
+	_ = tt.Click("Send")
+	if sends != 2 {
+		t.Fatalf("sends %d", sends)
+	}
+	_ = tt.Click("Method")
+	_ = tt.Click("POST")
+	_ = tt.Click("Body")
+	if req.Method != "POST" || tt.HasText("GET and HEAD requests usually carry no body.") {
+		t.Fatalf("method %q", req.Method)
+	}
+}
+
+func TestApiRequestBuilderSendingAndDisabled(t *testing.T) {
+	req := ApiRequest{URL: "https://court.example"}
+	opts := ApiRequestBuilderOptions{Sending: true}
+	sends := 0
+	tt := apiTester(&req, &opts, &sends)
+	_ = tt.Click("Request URL")
+	tt.Key(0, ui.KeyEnter)
+	if sends != 0 {
+		t.Fatal("sent while sending")
+	}
+	opts = ApiRequestBuilderOptions{Disabled: true}
+	tt.Frame()
+	_ = tt.Click("Send")
+	_ = tt.Click("Request URL")
+	tt.Type("x")
+	if sends != 0 || req.URL != "https://court.example" {
+		t.Fatalf("disabled: %q sends %d", req.URL, sends)
+	}
+	defer func() {
+		if recover() == nil {
+			t.Fatal("unknown method did not panic")
+		}
+	}()
+	apiTester(&ApiRequest{Method: "BREW"}, &ApiRequestBuilderOptions{}, &sends)
+}
+
+func TestApiRequestBuilderSnapshot(t *testing.T) {
+	snapshot(t, "api_request_builder", 720, 330, func(c *ui.Context) {
+		req := ApiRequest{Method: "POST", URL: "https://court.example/guests",
+			Params:  []KeyValue{{Key: "notify", Value: "true", Enabled: true}},
+			Headers: []KeyValue{{Key: "Accept", Value: "application/json", Enabled: true}}}
+		ui.Column(c).Padding(16).Children(func() { ApiRequestBuilder(c, &req, ApiRequestBuilderOptions{}) })
+	})
+}
diff --git a/messages_devtools.go b/messages_devtools.go
index 57d55fd..1b8a9e1 100644
--- a/messages_devtools.go
+++ b/messages_devtools.go
@@ -95,6 +95,17 @@ func init() {
 		"schema_tree.column":   {"Column", "列"},
 		"schema_tree.index":    {"Index", "索引"},
 
+		"api_request_builder.method":        {"Method", "方法"},
+		"api_request_builder.url":           {"Request URL", "请求地址"},
+		"api_request_builder.send":          {"Send", "发送"},
+		"api_request_builder.params":        {"Params (%d)", "参数（%d）"},
+		"api_request_builder.params_label":  {"Query parameters", "查询参数"},
+		"api_request_builder.headers":       {"Headers (%d)", "请求头（%d）"},
+		"api_request_builder.headers_label": {"Request headers", "请求头"},
+		"api_request_builder.body":          {"Body", "正文"},
+		"api_request_builder.no_body":       {"GET and HEAD requests usually carry no body.", "GET 与 HEAD 请求通常不带正文。"},
+		"api_request_builder.parts":         {"Request parts", "请求组成"},
+
 		"alert_list.empty": {"No alerts", "没有告警"},
 	})
 }
diff --git a/testdata/snapshots/darwin/api_request_builder-dark.png b/testdata/snapshots/darwin/api_request_builder-dark.png
new file mode 100644
index 0000000..9639561
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diff --git a/testdata/snapshots/darwin/api_request_builder-light.png b/testdata/snapshots/darwin/api_request_builder-light.png
new file mode 100644
index 0000000..89bc6c3
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