diff --git a/internal/openrouter/client.go b/internal/openrouter/client.go
new file mode 100644
index 0000000..38332c3
--- /dev/null
+++ b/internal/openrouter/client.go
@@ -0,0 +1,151 @@
+// Package openrouter is a minimal client for the OpenRouter chat-completions
+// API, covering only what the LLM action backend needs: a single multimodal
+// (text + one image) request per step with strict json_schema structured
+// output. No streaming, tools, or other extras.
+package openrouter
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"time"
+)
+
+const defaultBaseURL = "https://openrouter.ai/api/v1"
+
+// requestTimeout bounds a single chat-completion round-trip. Vision + strict
+// structured output is slower than a plain text call, so this is generous; the
+// runner also passes a context the caller can cancel.
+const requestTimeout = 60 * time.Second
+
+// Client talks to the OpenRouter chat-completions endpoint.
+type Client struct {
+	httpClient *http.Client
+	apiKey     string
+	baseURL    string
+}
+
+// New builds a Client from the environment. OPENROUTER_API_KEY is required;
+// OPENROUTER_BASE_URL overrides the default endpoint (e.g. for tests).
+func New() (*Client, error) {
+	apiKey := os.Getenv("OPENROUTER_API_KEY")
+	if apiKey == "" {
+		return nil, errors.New("openrouter: OPENROUTER_API_KEY is not set")
+	}
+	baseURL := os.Getenv("OPENROUTER_BASE_URL")
+	if baseURL == "" {
+		baseURL = defaultBaseURL
+	}
+	return &Client{
+		httpClient: &http.Client{Timeout: requestTimeout},
+		apiKey:     apiKey,
+		baseURL:    baseURL,
+	}, nil
+}
+
+// Request is a chat-completions request body. Only the fields the action
+// backend sets are modeled.
+type Request struct {
+	Model          string          `json:"model"`
+	Messages       []Message       `json:"messages"`
+	ResponseFormat *ResponseFormat `json:"response_format,omitempty"`
+}
+
+// Message is one chat message. Content is always the array form (a list of
+// parts), which OpenRouter accepts for every role.
+type Message struct {
+	Role    string        `json:"role"`
+	Content []ContentPart `json:"content"`
+}
+
+// ContentPart is one piece of a message: either a text run or an image given
+// as a data URL.
+type ContentPart struct {
+	Type     string    `json:"type"`
+	Text     string    `json:"text,omitempty"`
+	ImageURL *ImageURL `json:"image_url,omitempty"`
+}
+
+// ImageURL carries an image as a (typically data:) URL.
+type ImageURL struct {
+	URL string `json:"url"`
+}
+
+// TextPart builds a text content part.
+func TextPart(text string) ContentPart {
+	return ContentPart{Type: "text", Text: text}
+}
+
+// ImagePart builds an image content part from a data URL.
+func ImagePart(dataURL string) ContentPart {
+	return ContentPart{Type: "image_url", ImageURL: &ImageURL{URL: dataURL}}
+}
+
+// ResponseFormat pins the model to a strict JSON schema.
+type ResponseFormat struct {
+	Type       string     `json:"type"`
+	JSONSchema JSONSchema `json:"json_schema"`
+}
+
+// JSONSchema is the strict structured-output schema.
+type JSONSchema struct {
+	Name   string         `json:"name"`
+	Strict bool           `json:"strict"`
+	Schema map[string]any `json:"schema"`
+}
+
+// Response is the slice of a chat-completions response we read.
+type Response struct {
+	Choices []Choice `json:"choices"`
+}
+
+// Choice is one completion choice.
+type Choice struct {
+	Message ResponseMessage `json:"message"`
+}
+
+// ResponseMessage carries the assistant's content. With json_schema output the
+// content is a JSON string matching the schema.
+type ResponseMessage struct {
+	Content string `json:"content"`
+}
+
+// ChatCompletion POSTs req to /chat/completions and decodes the response. A
+// non-2xx status is returned as an error carrying the response body.
+func (c *Client) ChatCompletion(ctx context.Context, req Request) (*Response, error) {
+	body, err := json.Marshal(req)
+	if err != nil {
+		return nil, fmt.Errorf("openrouter: marshal request: %w", err)
+	}
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, c.baseURL+"/chat/completions", bytes.NewReader(body))
+	if err != nil {
+		return nil, fmt.Errorf("openrouter: build request: %w", err)
+	}
+	httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
+	httpReq.Header.Set("Content-Type", "application/json")
+
+	resp, err := c.httpClient.Do(httpReq)
+	if err != nil {
+		return nil, fmt.Errorf("openrouter: request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	responseBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("openrouter: read response: %w", err)
+	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, fmt.Errorf("openrouter: status %d: %s", resp.StatusCode, string(responseBody))
+	}
+
+	var out Response
+	if err := json.Unmarshal(responseBody, &out); err != nil {
+		return nil, fmt.Errorf("openrouter: decode response: %w", err)
+	}
+	return &out, nil
+}