diff --git a/.changeset/improve-error-handling.md b/.changeset/improve-error-handling.md
new file mode 100644
index 0000000..dc72515
--- /dev/null
+++ b/.changeset/improve-error-handling.md
@@ -0,0 +1,5 @@
+---
+"@alchemy/cli": patch
+---
+
+Improve error messages for unknown commands, invalid networks, RPC failures, and access denied responses so users get actionable guidance instead of raw errors.
diff --git a/src/commands/block.ts b/src/commands/block.ts
index f6436a9..5fab6e8 100644
--- a/src/commands/block.ts
+++ b/src/commands/block.ts
@@ -41,6 +41,9 @@ Examples:
               "block must be a number, hex, or 'latest'",
             );
           }
+          if (num < 0) {
+            throw errInvalidArgs("Block number must be non-negative.");
+          }
           blockParam = `0x${num.toString(16)}`;
         }
 
diff --git a/src/index.ts b/src/index.ts
index d6a843f..990cb5b 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,5 @@
 import { Command, Help } from "commander";
-import { EXIT_CODES, errSetupRequired, exitWithError } from "./lib/errors.js";
+import { EXIT_CODES, errInvalidArgs, errSetupRequired, exitWithError } from "./lib/errors.js";
 import { setFlags, isJSONMode, quiet } from "./lib/output.js";
 import { formatCommanderError } from "./lib/error-format.js";
 import { load as loadConfig } from "./lib/config.js";
@@ -155,6 +155,7 @@ program
   .option("--debug", "Enable debug diagnostics")
   .option("--no-interactive", "Disable REPL and prompt-driven interactions")
   .addHelpCommand(false)
+  .allowExcessArguments(true)
   .exitOverride((err) => {
     if (
       err.code === "commander.help" ||
@@ -335,7 +336,18 @@ program
     }
     resetUpdateNoticeState();
   })
-  .action(async () => {
+  .action(async (_opts: unknown, cmd: Command) => {
+    // Commander routes here when no subcommand matches. If the user passed
+    // positional args (e.g. "alchemy abcd"), those are unknown commands.
+    const excessArgs = cmd.args;
+    if (excessArgs.length > 0) {
+      exitWithError(
+        errInvalidArgs(
+          `Unknown command '${excessArgs[0]}'. Run 'alchemy help' for available commands.`,
+        ),
+      );
+    }
+
     const cfg = loadConfig();
     if (!isSetupComplete(cfg) && !isInteractiveAllowed(program)) {
       throw errSetupRequired(getSetupStatus(cfg));
diff --git a/src/lib/admin-client.ts b/src/lib/admin-client.ts
index d7d7e3d..817c3dc 100644
--- a/src/lib/admin-client.ts
+++ b/src/lib/admin-client.ts
@@ -1,5 +1,6 @@
 import {
   errInvalidAccessKey,
+  errAccessDenied,
   errNotFound,
   errRateLimited,
   errAdminAPI,
@@ -128,7 +129,19 @@ export class AdminClient {
       ...(body !== undefined && { body: JSON.stringify(body) }),
     });
 
-    if (resp.status === 401 || resp.status === 403) throw errInvalidAccessKey();
+    if (resp.status === 401) throw errInvalidAccessKey();
+    if (resp.status === 403) {
+      const detail = await resp.text().catch(() => "");
+      // Try to extract a reason from the response body
+      let reason: string | undefined;
+      try {
+        const parsed = JSON.parse(detail);
+        reason = parsed?.message || parsed?.error?.message || parsed?.error || undefined;
+      } catch {
+        reason = detail || undefined;
+      }
+      throw errAccessDenied(typeof reason === "string" ? reason : undefined);
+    }
     if (resp.status === 404) {
       const text = await resp.text().catch(() => "");
       throw errNotFound(text || path);
diff --git a/src/lib/client-utils.ts b/src/lib/client-utils.ts
index aa9a5b4..999ad3a 100644
--- a/src/lib/client-utils.ts
+++ b/src/lib/client-utils.ts
@@ -1,4 +1,4 @@
-import { errInvalidArgs, errNetwork } from "./errors.js";
+import { CLIError, errInvalidArgs, errNetwork } from "./errors.js";
 import { timeout as globalTimeout } from "./output.js";
 
 export function isLocalhost(hostname: string): boolean {
@@ -50,6 +50,26 @@ export async function fetchWithTimeout(
     if (err instanceof DOMException && err.name === "TimeoutError") {
       throw errNetwork(`Request timed out after ${globalTimeout}ms`);
     }
-    throw errNetwork((err as Error).message);
+    const message = (err as Error).message ?? String(err);
+    // Node's fetch wraps DNS errors in a TypeError with the detail in .cause
+    const causeMessage = (err as { cause?: { message?: string } }).cause?.message ?? "";
+    const causeCode = (err as { cause?: { code?: string } }).cause?.code ?? "";
+    const fullErrorText = `${message} ${causeMessage} ${causeCode}`;
+    // Detect DNS resolution failures — typically caused by an invalid network slug
+    if (/ENOTFOUND|EAI_AGAIN|getaddrinfo/i.test(fullErrorText)) {
+      // Extract the hostname from the URL for a clearer error message
+      try {
+        const hostname = new URL(url).hostname;
+        const networkSlug = hostname.replace(/\.g\.alchemy\.com$/, "");
+        if (networkSlug !== hostname) {
+          throw errInvalidArgs(
+            `Unknown network '${networkSlug}'. Run 'alchemy network list' to see available networks.`,
+          );
+        }
+      } catch (innerErr) {
+        if (innerErr instanceof CLIError) throw innerErr;
+      }
+    }
+    throw errNetwork(message);
   }
 }
diff --git a/src/lib/client.ts b/src/lib/client.ts
index b52dae5..c2343a1 100644
--- a/src/lib/client.ts
+++ b/src/lib/client.ts
@@ -48,10 +48,14 @@ export class Client implements AlchemyClient {
     try {
       parsed = new URL(`https://${hostname}`);
     } catch {
-      throw errInvalidArgs(`Invalid network: ${network}`);
+      throw errInvalidArgs(
+        `Unknown network '${network}'. Run 'alchemy network list' to see available networks.`,
+      );
     }
     if (!parsed.hostname.endsWith(".g.alchemy.com")) {
-      throw errInvalidArgs(`Invalid network: ${network} — hostname must end with .g.alchemy.com`);
+      throw errInvalidArgs(
+        `Unknown network '${network}'. Run 'alchemy network list' to see available networks.`,
+      );
     }
   }
 
@@ -87,6 +91,18 @@ export class Client implements AlchemyClient {
     return errInvalidAPIKey(detail || undefined);
   }
 
+  private tryParseRPCError(text: string): CLIError | null {
+    try {
+      const parsed = JSON.parse(text);
+      if (parsed?.error?.code !== undefined && parsed?.error?.message !== undefined) {
+        return errRPC(parsed.error.code, parsed.error.message);
+      }
+    } catch {
+      // Not JSON — fall through
+    }
+    return null;
+  }
+
   private async doFetch(url: string, init: RequestInit): Promise<Response> {
     return fetchWithTimeout(url, init);
   }
@@ -116,6 +132,9 @@ export class Client implements AlchemyClient {
 
     if (!resp.ok) {
       const text = await resp.text().catch(() => "");
+      // Try to parse as JSON-RPC error before falling back to network error
+      const rpcError = this.tryParseRPCError(text);
+      if (rpcError) throw rpcError;
       throw errNetwork(`HTTP ${resp.status}: ${text}`);
     }
 
@@ -149,6 +168,8 @@ export class Client implements AlchemyClient {
 
     if (!resp.ok) {
       const text = await resp.text().catch(() => "");
+      const rpcError = this.tryParseRPCError(text);
+      if (rpcError) throw rpcError;
       throw errNetwork(`HTTP ${resp.status}: ${text}`);
     }
 
diff --git a/src/lib/errors.ts b/src/lib/errors.ts
index c1c402d..8011db6 100644
--- a/src/lib/errors.ts
+++ b/src/lib/errors.ts
@@ -130,8 +130,17 @@ export function errNetwork(detail: string): CLIError {
   );
 }
 
+const RPC_ERROR_HINTS: Record<number, string> = {
+  [-32700]: "Parse error. The request JSON is malformed.",
+  [-32600]: "Invalid request. Check the JSON-RPC request format.",
+  [-32601]: "Method not supported. Check the method name and ensure your plan supports it.",
+  [-32602]: "Invalid parameters. Check argument types and format.",
+  [-32603]: "Internal JSON-RPC error.",
+};
+
 export function errRPC(code: number, message: string): CLIError {
-  return new CLIError(ErrorCode.RPC_ERROR, `RPC error ${code}: ${message}`);
+  const hint = RPC_ERROR_HINTS[code];
+  return new CLIError(ErrorCode.RPC_ERROR, `RPC error ${code}: ${message}`, hint);
 }
 
 export function errInvalidArgs(detail: string): CLIError {
@@ -158,6 +167,17 @@ export function errInvalidAccessKey(): CLIError {
   );
 }
 
+export function errAccessDenied(detail?: string): CLIError {
+  const message = detail
+    ? `Access denied: ${detail}`
+    : "Access denied. Your access key may not have permission for this operation.";
+  return new CLIError(
+    ErrorCode.INVALID_ACCESS_KEY,
+    message,
+    "Check your account tier and feature access at https://dashboard.alchemy.com/",
+  );
+}
+
 export function errAppRequired(): CLIError {
   return new CLIError(
     ErrorCode.APP_REQUIRED,
diff --git a/tests/commands/block.test.ts b/tests/commands/block.test.ts
index b52d0b5..7d484cd 100644
--- a/tests/commands/block.test.ts
+++ b/tests/commands/block.test.ts
@@ -69,6 +69,40 @@ describe("block command", () => {
     expect(exitWithError).toHaveBeenCalledTimes(1);
   });
 
+  it("block -1 rejects negative block numbers", async () => {
+    const exitWithError = vi.fn();
+    vi.doMock("../../src/lib/resolve.js", () => ({
+      clientFromFlags: vi.fn(),
+    }));
+    vi.doMock("../../src/lib/output.js", () => ({
+      isJSONMode: () => true,
+      printJSON: vi.fn(),
+      verbose: false,
+    }));
+    vi.doMock("../../src/lib/ui.js", () => ({
+      bold: (s: string) => s,
+      dim: (s: string) => s,
+      withSpinner: vi.fn(),
+      printKeyValueBox: vi.fn(),
+      printSyntaxJSON: vi.fn(),
+    }));
+    vi.doMock("../../src/lib/block-format.js", () => ({
+      formatBlockTimestamp: vi.fn(),
+      formatHexQuantity: vi.fn(),
+      formatGasSummary: vi.fn(),
+    }));
+    vi.doMock("../../src/lib/errors.js", async () => ({ ...(await vi.importActual("../../src/lib/errors.js")), exitWithError }));
+
+    const { registerBlock } = await import("../../src/commands/block.js");
+    const program = new Command();
+    registerBlock(program);
+
+    await program.parseAsync(["node", "test", "block", "-1"], { from: "node" });
+    expect(exitWithError).toHaveBeenCalledTimes(1);
+    const err = exitWithError.mock.calls[0][0];
+    expect(err.message).toContain("non-negative");
+  });
+
   it("block latest prints JSON payload in json mode", async () => {
     const call = vi.fn().mockResolvedValue({ number: "0x10", hash: "0xhash" });
     const printJSON = vi.fn();
diff --git a/tests/lib/admin-client.test.ts b/tests/lib/admin-client.test.ts
index 290bc2e..e7eb973 100644
--- a/tests/lib/admin-client.test.ts
+++ b/tests/lib/admin-client.test.ts
@@ -335,7 +335,7 @@ describe("AdminClient", () => {
     }
   });
 
-  it("throws INVALID_ACCESS_KEY on 403", async () => {
+  it("throws INVALID_ACCESS_KEY on 403 with access denied message", async () => {
     const url = await createTestServer((_req, res) => {
       res.writeHead(403);
       res.end();
@@ -348,6 +348,24 @@ describe("AdminClient", () => {
     } catch (err) {
       expect(err).toBeInstanceOf(CLIError);
       expect((err as CLIError).code).toBe(ErrorCode.INVALID_ACCESS_KEY);
+      expect((err as CLIError).message).toContain("Access denied");
+    }
+  });
+
+  it("includes reason from 403 response body", async () => {
+    const url = await createTestServer((_req, res) => {
+      res.writeHead(403, { "Content-Type": "application/json" });
+      res.end(JSON.stringify({ message: "Feature not available on your current plan" }));
+    });
+
+    const client = new TestAdminClient(url);
+    try {
+      await client.listApps();
+      expect.fail("should have thrown");
+    } catch (err) {
+      expect(err).toBeInstanceOf(CLIError);
+      expect((err as CLIError).code).toBe(ErrorCode.INVALID_ACCESS_KEY);
+      expect((err as CLIError).message).toContain("Feature not available");
     }
   });
 
diff --git a/tests/lib/client.test.ts b/tests/lib/client.test.ts
index cf4607a..09979f1 100644
--- a/tests/lib/client.test.ts
+++ b/tests/lib/client.test.ts
@@ -150,6 +150,46 @@ describe("Client.call", () => {
     }
   });
 
+  it("throws RPC error when HTTP error body contains JSON-RPC error", async () => {
+    const url = await createTestServer((_req, res) => {
+      res.writeHead(400, { "Content-Type": "application/json" });
+      res.end(
+        JSON.stringify({
+          jsonrpc: "2.0",
+          error: { code: -32601, message: "The method fake_method does not exist" },
+          id: 1,
+        }),
+      );
+    });
+
+    const client = new TestClient(url);
+    try {
+      await client.call("fake_method");
+      expect.fail("should have thrown");
+    } catch (err) {
+      expect(err).toBeInstanceOf(CLIError);
+      expect((err as CLIError).code).toBe(ErrorCode.RPC_ERROR);
+      expect((err as CLIError).message).toContain("-32601");
+      expect((err as CLIError).hint).toBeDefined();
+    }
+  });
+
+  it("throws network error when HTTP error body is not JSON-RPC", async () => {
+    const url = await createTestServer((_req, res) => {
+      res.writeHead(500);
+      res.end("Internal Server Error");
+    });
+
+    const client = new TestClient(url);
+    try {
+      await client.call("eth_blockNumber");
+      expect.fail("should have thrown");
+    } catch (err) {
+      expect(err).toBeInstanceOf(CLIError);
+      expect((err as CLIError).code).toBe(ErrorCode.NETWORK_ERROR);
+    }
+  });
+
   it("throws on 429", async () => {
     const url = await createTestServer((_req, res) => {
       res.writeHead(429);
diff --git a/tests/lib/errors.test.ts b/tests/lib/errors.test.ts
index 0a69e18..43353c9 100644
--- a/tests/lib/errors.test.ts
+++ b/tests/lib/errors.test.ts
@@ -153,6 +153,24 @@ describe("convenience constructors", () => {
   }
 });
 
+describe("errRPC hints", () => {
+  it("includes hint for known RPC error codes", () => {
+    const err = errRPC(-32601, "Method not found");
+    expect(err.hint).toBeDefined();
+    expect(err.hint).toContain("Method not supported");
+  });
+
+  it("includes hint for invalid params", () => {
+    const err = errRPC(-32602, "Invalid params");
+    expect(err.hint).toContain("Invalid parameters");
+  });
+
+  it("has no hint for unknown RPC error codes", () => {
+    const err = errRPC(-32000, "Execution reverted");
+    expect(err.hint).toBeUndefined();
+  });
+});
+
 describe("EXIT_CODES", () => {
   it("has a mapping for every ErrorCode", () => {
     for (const code of Object.values(ErrorCode)) {