diff --git a/CLAUDE.md b/CLAUDE.md
index 006bd3b..539b8d9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -4,7 +4,7 @@
 
 OST MCP is the Model Context Protocol server for [OpenSourceTogether](https://opensource-together.com/). It lets developers discover and explore open-source projects directly from Claude Desktop, IDEs, and other MCP-compatible clients.
 
-It consumes the OST Linker REST API and exposes 7 MCP tools for project search, discovery, and similarity.
+It consumes the OST backend MCP gateway and exposes 7 MCP tools for project search, discovery, and similarity.
 
 ## Common Commands
 
@@ -27,11 +27,11 @@ npx vitest --watch             # Watch mode
 ## Architecture
 
 ```
-User (Claude Desktop/IDE) -> MCP Server (stdio) -> OSTClient (HTTP) -> OST Linker API
+User (Claude Desktop/IDE) -> MCP Server (stdio) -> OSTClient (HTTP) -> OST backend MCP gateway
 ```
 
 - `src/index.ts` — MCP server entry point, registers all tools
-- `src/client.ts` — HTTP client for the OST Linker REST API
+- `src/client.ts` — HTTP client for the OST backend MCP gateway
 - `src/tools/` — One file per MCP tool (or group)
 - `src/config.ts` — Reads `OST_API_URL` from env
 - `src/types.ts` — Shared TypeScript types
@@ -52,7 +52,8 @@ User (Claude Desktop/IDE) -> MCP Server (stdio) -> OSTClient (HTTP) -> OST Linke
 
 | Variable | Purpose | Default |
 |----------|---------|---------|
-| `OST_API_URL` | OST Linker API base URL | `https://api.opensource-together.com` |
+| `OST_API_KEY` | Personal Access Token from your OST account | **required** |
+| `OST_API_URL` | OST backend MCP gateway base URL | `https://api.opensource-together.com/v1/mcp` |
 
 ## Related Repos
 
@@ -70,9 +71,12 @@ Published as `@opensource-together/mcp` on npm. Users install via:
       "command": "npx",
       "args": ["@opensource-together/mcp"],
       "env": {
-        "OST_API_URL": "https://api.opensource-together.com"
+        "OST_API_KEY": "your-personal-access-token",
+        "OST_API_URL": "https://api.opensource-together.com/v1/mcp"
       }
     }
   }
 }
 ```
+
+> Generate your key at your OpenSource Together account settings.
diff --git a/README.md b/README.md
index 0c26822..edff8bd 100644
--- a/README.md
+++ b/README.md
@@ -13,12 +13,38 @@ Add this to your MCP client config:
   "mcpServers": {
     "ost": {
       "command": "npx",
-      "args": ["@opensource-together/mcp"]
+      "args": ["@opensource-together/mcp"],
+      "env": {
+        "OST_API_KEY": "your-personal-access-token"
+      }
     }
   }
 }
 ```
 
+> Generate your key at your OpenSource Together account settings.
+
+## Distribution
+
+Published as `@opensource-together/mcp` on npm. Example client config:
+
+```json
+{
+  "mcpServers": {
+    "ost": {
+      "command": "npx",
+      "args": ["@opensource-together/mcp"],
+      "env": {
+        "OST_API_KEY": "your-personal-access-token",
+        "OST_API_URL": "https://api.opensource-together.com/v1/mcp"
+      }
+    }
+  }
+}
+```
+
+> Generate your key at your OpenSource Together account settings.
+
 ## Tools
 
 | Tool | What it does |
@@ -33,7 +59,7 @@ Add this to your MCP client config:
 
 ## How it works
 
-Ask your agent > [@ost-mcp](https://github.com/opensource-together/ost-mcp) > [@ost-linker](https://github.com/opensource-together/ost-linker) > projects found
+Ask your agent > [@ost-mcp](https://github.com/opensource-together/ost-mcp) > [@ost-backend](https://github.com/opensource-together/ost-backend) > projects found
 
 - *"Find me React projects for e-commerce"*
 - *"What's trending in open source right now?"*
@@ -48,6 +74,13 @@ npm run build
 npm run dev
 ```
 
+## Environment Variables
+
+| Variable | Purpose | Default |
+|------|-------------|---------|
+| `OST_API_KEY` | Personal Access Token from your OST account | Required |
+| `OST_API_URL` | MCP gateway base URL | `https://api.opensource-together.com/v1/mcp` |
+
 ## License
 
 MIT
diff --git a/src/client.ts b/src/client.ts
index 2444075..cf24c5c 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -20,10 +20,27 @@ export class OSTClient {
     });
     if (!response.ok) {
       const body = await response.json().catch(() => ({}));
-      throw new Error(
-        (body as { detail?: string }).detail ||
-          `API error: ${response.status}`
-      );
+      const detail = (body as { detail?: string }).detail;
+
+      if (response.status === 401 || response.status === 403) {
+        throw new Error(
+          "Invalid or missing OST_API_KEY. Generate or regenerate a Personal Access Token in your OpenSource Together account settings."
+        );
+      }
+
+      if (response.status === 429) {
+        const retryAfter = response.headers.get("Retry-After");
+        const retryWindow = retryAfter ? `${retryAfter} seconds` : "shortly";
+        throw new Error(`Rate limit exceeded. Retry in ${retryWindow}.`);
+      }
+
+      if (response.status >= 500) {
+        throw new Error(
+          `OST backend temporarily unavailable (status ${response.status}). Please try again shortly.`
+        );
+      }
+
+      throw new Error(detail || `API error: ${response.status}`);
     }
     return response.json() as Promise<T>;
   }
diff --git a/src/config.ts b/src/config.ts
index 4b2e370..8cba816 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -2,9 +2,15 @@ export interface Config {
   apiUrl: string;
 }
 
-const DEFAULT_API_URL = "https://api.opensource-together.com";
+const DEFAULT_API_URL = "https://api.opensource-together.com/v1/mcp";
+const MISSING_API_KEY_ERROR =
+  "OST_API_KEY is required. Generate a Personal Access Token in your OpenSource Together account settings, then add OST_API_KEY to your MCP client config env block.";
 
 export function getConfig(): Config {
+  if (!process.env.OST_API_KEY) {
+    throw new Error(MISSING_API_KEY_ERROR);
+  }
+
   const rawUrl = process.env.OST_API_URL || DEFAULT_API_URL;
   const apiUrl = rawUrl.endsWith("/") ? rawUrl.slice(0, -1) : rawUrl;
 
diff --git a/tests/client.test.ts b/tests/client.test.ts
index 228c50b..8e14e1b 100644
--- a/tests/client.test.ts
+++ b/tests/client.test.ts
@@ -43,4 +43,43 @@ describe("OSTClient", () => {
 
     await expect(client.getProject("bad-id")).rejects.toThrow("Not found");
   });
+
+  it("maps 401 responses to a human-readable auth error", async () => {
+    global.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 401,
+      headers: new Headers(),
+      json: () => Promise.resolve({ detail: "Unauthorized" }),
+    });
+
+    await expect(client.getProject("bad-id")).rejects.toThrow(
+      "Invalid or missing OST_API_KEY. Generate or regenerate a Personal Access Token in your OpenSource Together account settings."
+    );
+  });
+
+  it("maps 429 responses to a retry-after message", async () => {
+    global.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 429,
+      headers: new Headers({ "Retry-After": "30" }),
+      json: () => Promise.resolve({ detail: "Too many requests" }),
+    });
+
+    await expect(client.getProject("slow-down")).rejects.toThrow(
+      "Rate limit exceeded. Retry in 30 seconds."
+    );
+  });
+
+  it("maps 503 responses to a temporary backend message", async () => {
+    global.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 503,
+      headers: new Headers(),
+      json: () => Promise.resolve({ detail: "Service unavailable" }),
+    });
+
+    await expect(client.getProject("server-down")).rejects.toThrow(
+      "OST backend temporarily unavailable (status 503). Please try again shortly."
+    );
+  });
 });
diff --git a/tests/config.test.ts b/tests/config.test.ts
index fafb82f..5ca1dd3 100644
--- a/tests/config.test.ts
+++ b/tests/config.test.ts
@@ -13,20 +13,31 @@ describe("getConfig", () => {
   });
 
   it("reads OST_API_URL from environment", () => {
+    process.env.OST_API_KEY = "test-key";
     process.env.OST_API_URL = "https://api.example.com";
     const config = getConfig();
     expect(config.apiUrl).toBe("https://api.example.com");
   });
 
   it("uses default URL when OST_API_URL is not set", () => {
+    process.env.OST_API_KEY = "test-key";
     delete process.env.OST_API_URL;
     const config = getConfig();
-    expect(config.apiUrl).toBe("https://api.opensource-together.com");
+    expect(config.apiUrl).toBe("https://api.opensource-together.com/v1/mcp");
   });
 
   it("strips trailing slash from URL", () => {
+    process.env.OST_API_KEY = "test-key";
     process.env.OST_API_URL = "https://api.example.com/";
     const config = getConfig();
     expect(config.apiUrl).toBe("https://api.example.com");
   });
+
+  it("throws when OST_API_KEY is not set", () => {
+    delete process.env.OST_API_KEY;
+
+    expect(() => getConfig()).toThrow(
+      "OST_API_KEY is required. Generate a Personal Access Token in your OpenSource Together account settings, then add OST_API_KEY to your MCP client config env block."
+    );
+  });
 });