diff --git a/design/src/cli.ts b/design/src/cli.ts
index d1d8eb3b..e73caca3 100644
--- a/design/src/cli.ts
+++ b/design/src/cli.ts
@@ -22,6 +22,7 @@ import { resolveApiKey, saveApiKey } from "./auth";
 import { extractDesignLanguage, updateDesignMd } from "./memory";
 import { diffMockups, verifyAgainstMockup } from "./diff";
 import { evolve } from "./evolve";
+import { generateDesignToCodePrompt } from "./design-to-code";
 
 function parseArgs(argv: string[]): { command: string; flags: Record<string, string | boolean> } {
   const args = argv.slice(2); // skip bun/node and script path
@@ -140,6 +141,20 @@ async function main(): Promise<void> {
       break;
     }
 
+    case "prompt": {
+      const promptImage = flags.image as string;
+      if (!promptImage) {
+        console.error("--image is required");
+        process.exit(1);
+      }
+      console.error(`Generating implementation prompt from ${promptImage}...`);
+      const proc2 = Bun.spawn(["git", "rev-parse", "--show-toplevel"]);
+      const root = (await new Response(proc2.stdout).text()).trim();
+      const d2c = await generateDesignToCodePrompt(promptImage, root || undefined);
+      console.log(JSON.stringify(d2c, null, 2));
+      break;
+    }
+
     case "setup":
       await runSetup();
       break;
@@ -152,6 +167,7 @@ async function main(): Promise<void> {
         outputDir: (flags["output-dir"] as string) || "/tmp/gstack-variants/",
         size: flags.size as string,
         quality: flags.quality as string,
+        viewports: flags.viewports as string,
       });
       break;
 
diff --git a/design/src/commands.ts b/design/src/commands.ts
index 6ff829cc..b077d3df 100644
--- a/design/src/commands.ts
+++ b/design/src/commands.ts
@@ -54,6 +54,11 @@ export const COMMANDS = new Map<string, {
     usage: "verify --mockup approved.png --screenshot live.png",
     flags: ["--mockup", "--screenshot", "--output"],
   }],
+  ["prompt", {
+    description: "Generate structured implementation prompt from approved mockup",
+    usage: "prompt --image approved.png",
+    flags: ["--image"],
+  }],
   ["extract", {
     description: "Extract design language from approved mockup into DESIGN.md",
     usage: "extract --image approved.png",
diff --git a/design/src/design-to-code.ts b/design/src/design-to-code.ts
new file mode 100644
index 00000000..358a6b4e
--- /dev/null
+++ b/design/src/design-to-code.ts
@@ -0,0 +1,88 @@
+/**
+ * Design-to-Code Prompt Generator.
+ * Extracts implementation instructions from an approved mockup via GPT-4o vision.
+ * Produces a structured prompt the agent can use to implement the design.
+ */
+
+import fs from "fs";
+import { requireApiKey } from "./auth";
+import { readDesignConstraints } from "./memory";
+
+export interface DesignToCodeResult {
+  implementationPrompt: string;
+  colors: string[];
+  typography: string[];
+  layout: string[];
+  components: string[];
+}
+
+/**
+ * Generate a structured implementation prompt from an approved mockup.
+ */
+export async function generateDesignToCodePrompt(
+  imagePath: string,
+  repoRoot?: string,
+): Promise<DesignToCodeResult> {
+  const apiKey = requireApiKey();
+  const imageData = fs.readFileSync(imagePath).toString("base64");
+
+  // Read DESIGN.md if available for additional context
+  const designConstraints = repoRoot ? readDesignConstraints(repoRoot) : null;
+
+  const controller = new AbortController();
+  const timeout = setTimeout(() => controller.abort(), 60_000);
+
+  try {
+    const contextBlock = designConstraints
+      ? `\n\nExisting DESIGN.md (use these as constraints):\n${designConstraints}`
+      : "";
+
+    const response = await fetch("https://api.openai.com/v1/chat/completions", {
+      method: "POST",
+      headers: {
+        "Authorization": `Bearer ${apiKey}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify({
+        model: "gpt-4o",
+        messages: [{
+          role: "user",
+          content: [
+            {
+              type: "image_url",
+              image_url: { url: `data:image/png;base64,${imageData}` },
+            },
+            {
+              type: "text",
+              text: `Analyze this approved UI mockup and generate a structured implementation prompt. Return valid JSON only:
+
+{
+  "implementationPrompt": "A detailed paragraph telling a developer exactly how to build this UI. Include specific CSS values, layout approach (flex/grid), component structure, and interaction behaviors. Reference the specific elements visible in the mockup.",
+  "colors": ["#hex - usage", ...],
+  "typography": ["role: family, size, weight", ...],
+  "layout": ["description of layout pattern", ...],
+  "components": ["component name - description", ...]
+}
+
+Be specific about every visual detail: exact hex colors, font sizes in px, spacing values, border-radius, shadows. The developer should be able to implement this without looking at the mockup again.${contextBlock}`,
+            },
+          ],
+        }],
+        max_tokens: 1000,
+        response_format: { type: "json_object" },
+      }),
+      signal: controller.signal,
+    });
+
+    if (!response.ok) {
+      const error = await response.text();
+      throw new Error(`API error (${response.status}): ${error.slice(0, 200)}`);
+    }
+
+    const data = await response.json() as any;
+    const content = data.choices?.[0]?.message?.content?.trim() || "";
+    return JSON.parse(content) as DesignToCodeResult;
+  } finally {
+    clearTimeout(timeout);
+  }
+}
diff --git a/design/src/variants.ts b/design/src/variants.ts
index 017fe564..e9d8ad77 100644
--- a/design/src/variants.ts
+++ b/design/src/variants.ts
@@ -16,6 +16,7 @@ export interface VariantsOptions {
   outputDir: string;
   size?: string;
   quality?: string;
+  viewports?: string; // "desktop,tablet,mobile" — generates at multiple sizes
 }
 
 const STYLE_VARIATIONS = [
@@ -109,12 +110,19 @@ export async function variants(options: VariantsOptions): Promise<void> {
     ? parseBrief(options.briefFile, true)
     : parseBrief(options.brief!, false);
 
-  const count = Math.min(options.count, 7); // Cap at 7 style variations
-  const size = options.size || "1536x1024";
   const quality = options.quality || "high";
 
   fs.mkdirSync(options.outputDir, { recursive: true });
 
+  // If viewports specified, generate responsive variants instead of style variants
+  if (options.viewports) {
+    await generateResponsiveVariants(apiKey, baseBrief, options.outputDir, options.viewports, quality);
+    return;
+  }
+
+  const count = Math.min(options.count, 7); // Cap at 7 style variations
+  const size = options.size || "1536x1024";
+
   console.error(`Generating ${count} variants...`);
   const startTime = Date.now();
 
@@ -171,3 +179,68 @@ export async function variants(options: VariantsOptions): Promise<void> {
     errors: failed,
   }, null, 2));
 }
+
+const VIEWPORT_CONFIGS: Record<string, { size: string; suffix: string; desc: string }> = {
+  desktop: { size: "1536x1024", suffix: "desktop", desc: "Desktop (1536x1024)" },
+  tablet: { size: "1024x1024", suffix: "tablet", desc: "Tablet (1024x1024)" },
+  mobile: { size: "1024x1536", suffix: "mobile", desc: "Mobile (1024x1536, portrait)" },
+};
+
+async function generateResponsiveVariants(
+  apiKey: string,
+  baseBrief: string,
+  outputDir: string,
+  viewports: string,
+  quality: string,
+): Promise<void> {
+  const viewportList = viewports.split(",").map(v => v.trim().toLowerCase());
+  const configs = viewportList.map(v => VIEWPORT_CONFIGS[v]).filter(Boolean);
+
+  if (configs.length === 0) {
+    console.error(`No valid viewports. Use: desktop, tablet, mobile`);
+    process.exit(1);
+  }
+
+  console.error(`Generating responsive variants: ${configs.map(c => c.desc).join(", ")}...`);
+  const startTime = Date.now();
+
+  const promises = configs.map((config, i) => {
+    const prompt = `${baseBrief}\n\nViewport: ${config.desc}. Adapt the layout for this screen size. ${
+      config.suffix === "mobile" ? "Use a single-column layout, larger touch targets, and mobile navigation patterns." :
+      config.suffix === "tablet" ? "Use a responsive layout that works for medium screens." :
+      ""
+    }`;
+    const outputPath = path.join(outputDir, `responsive-${config.suffix}.png`);
+    const delay = i * 1500;
+
+    return new Promise<{ path: string; success: boolean; error?: string }>(resolve =>
+      setTimeout(resolve, delay)
+    ).then(() => {
+      console.error(`  Starting ${config.desc}...`);
+      return generateVariant(apiKey, prompt, outputPath, config.size, quality);
+    });
+  });
+
+  const results = await Promise.allSettled(promises);
+  const elapsed = ((Date.now() - startTime) / 1000).toFixed(1);
+
+  const succeeded: string[] = [];
+  for (const result of results) {
+    if (result.status === "fulfilled" && result.value.success) {
+      const sz = fs.statSync(result.value.path).size;
+      console.error(`  ✓ ${path.basename(result.value.path)} (${(sz / 1024).toFixed(0)}KB)`);
+      succeeded.push(result.value.path);
+    } else {
+      const error = result.status === "fulfilled" ? result.value.error : (result.reason as Error).message;
+      console.error(`  ✗ ${error}`);
+    }
+  }
+
+  console.error(`\n${succeeded.length}/${configs.length} responsive variants generated (${elapsed}s)`);
+  console.log(JSON.stringify({
+    outputDir,
+    viewports: viewportList,
+    succeeded: succeeded.length,
+    paths: succeeded,
+  }, null, 2));
+}