diff --git a/internal/openrouter/client_test.go b/internal/openrouter/client_test.go
new file mode 100644
index 0000000..728ed5a
--- /dev/null
+++ b/internal/openrouter/client_test.go
@@ -0,0 +1,133 @@
+package openrouter
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestChatCompletionRequestShapeAndParse(t *testing.T) {
+	var captured map[string]any
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/chat/completions" {
+			t.Errorf("path = %q, want /chat/completions", r.URL.Path)
+		}
+		if got := r.Header.Get("Authorization"); got != "Bearer test-key" {
+			t.Errorf("Authorization = %q, want Bearer test-key", got)
+		}
+		body, _ := io.ReadAll(r.Body)
+		if err := json.Unmarshal(body, &captured); err != nil {
+			t.Fatalf("unmarshal request: %v", err)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"choices":[{"message":{"content":"{\"reasoning\":\"tap login\",\"ranked\":[2,0]}"}}]}`))
+	}))
+	defer server.Close()
+
+	t.Setenv("OPENROUTER_API_KEY", "test-key")
+	t.Setenv("OPENROUTER_BASE_URL", server.URL)
+	client, err := New()
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+
+	resp, err := client.ChatCompletion(context.Background(), Request{
+		Model: "vendor/model",
+		Messages: []Message{
+			{Role: "system", Content: []ContentPart{TextPart("system")}},
+			{Role: "user", Content: []ContentPart{
+				TextPart("candidates"),
+				ImagePart("data:image/png;base64,AAAA"),
+			}},
+		},
+		ResponseFormat: &ResponseFormat{
+			Type: "json_schema",
+			JSONSchema: JSONSchema{
+				Name:   "ranked_actions",
+				Strict: true,
+				Schema: map[string]any{"type": "object"},
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("ChatCompletion: %v", err)
+	}
+
+	// Request carried the model.
+	if captured["model"] != "vendor/model" {
+		t.Errorf("model = %v, want vendor/model", captured["model"])
+	}
+	// Request carried an image_url content part.
+	messages := captured["messages"].([]any)
+	user := messages[1].(map[string]any)
+	parts := user["content"].([]any)
+	foundImage := false
+	for _, part := range parts {
+		if part.(map[string]any)["type"] == "image_url" {
+			foundImage = true
+			image := part.(map[string]any)["image_url"].(map[string]any)
+			if !strings.HasPrefix(image["url"].(string), "data:image/png;base64,") {
+				t.Errorf("image url = %v, want data URL", image["url"])
+			}
+		}
+	}
+	if !foundImage {
+		t.Error("request carried no image_url content part")
+	}
+	// Request carried the strict json_schema response_format.
+	rf := captured["response_format"].(map[string]any)
+	if rf["type"] != "json_schema" {
+		t.Errorf("response_format.type = %v, want json_schema", rf["type"])
+	}
+	if schema := rf["json_schema"].(map[string]any); schema["strict"] != true {
+		t.Errorf("json_schema.strict = %v, want true", schema["strict"])
+	}
+
+	// Response parsed into the ranked-index content.
+	if len(resp.Choices) != 1 {
+		t.Fatalf("choices = %d, want 1", len(resp.Choices))
+	}
+	var content struct {
+		Reasoning string `json:"reasoning"`
+		Ranked    []int  `json:"ranked"`
+	}
+	if err := json.Unmarshal([]byte(resp.Choices[0].Message.Content), &content); err != nil {
+		t.Fatalf("unmarshal content: %v", err)
+	}
+	if content.Reasoning != "tap login" {
+		t.Errorf("reasoning = %q, want tap login", content.Reasoning)
+	}
+	if len(content.Ranked) != 2 || content.Ranked[0] != 2 || content.Ranked[1] != 0 {
+		t.Errorf("ranked = %v, want [2 0]", content.Ranked)
+	}
+}
+
+func TestNewRequiresAPIKey(t *testing.T) {
+	t.Setenv("OPENROUTER_API_KEY", "")
+	if _, err := New(); err == nil {
+		t.Fatal("expected error when OPENROUTER_API_KEY is unset")
+	}
+}
+
+func TestChatCompletionSurfacesHTTPError(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTooManyRequests)
+		_, _ = w.Write([]byte(`{"error":"rate limited"}`))
+	}))
+	defer server.Close()
+
+	t.Setenv("OPENROUTER_API_KEY", "test-key")
+	t.Setenv("OPENROUTER_BASE_URL", server.URL)
+	client, err := New()
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	_, err = client.ChatCompletion(context.Background(), Request{Model: "m"})
+	if err == nil || !strings.Contains(err.Error(), "429") {
+		t.Fatalf("expected 429 error, got %v", err)
+	}
+}