diff --git a/packages/cli-json-cas/src/cli.test.ts b/packages/cli-json-cas/src/cli.test.ts
index 90146d5..508ebf7 100644
--- a/packages/cli-json-cas/src/cli.test.ts
+++ b/packages/cli-json-cas/src/cli.test.ts
@@ -23,6 +23,25 @@ async function runCli(
   return { stdout, stderr, exitCode };
 }
 
+async function runCliWithStdin(
+  args: string[],
+  storePath: string,
+  stdin: string,
+): Promise<{ stdout: string; stderr: string; exitCode: number }> {
+  const finalArgs = ["bun", entrypoint, "--store", storePath, ...args];
+  const proc = Bun.spawn(finalArgs, {
+    stdout: "pipe",
+    stderr: "pipe",
+    stdin: "pipe",
+  });
+  proc.stdin.write(stdin);
+  proc.stdin.end();
+  const exitCode = await proc.exited;
+  const stdout = await new Response(proc.stdout).text();
+  const stderr = await new Response(proc.stderr).text();
+  return { stdout, stderr, exitCode };
+}
+
 describe("ucas command alias", () => {
   test("T1: ucas bin entry exists in package.json", async () => {
     const pkg = await Bun.file(pkgPath).json();
@@ -287,13 +306,14 @@ describe("ucas render command", () => {
     const tmpStore = mkdtempSync(join(tmpdir(), "json-cas-test-"));
     try {
       await runCli(["init"], tmpStore);
-      const { exitCode, stdout } = await runCli(
+      const { exitCode, stderr } = await runCli(
         ["render", "ZZZZZZZZZZZZZ"],
         tmpStore,
       );
-      // Missing hash renders as cas: reference
-      expect(exitCode).toBe(0);
-      expect(stdout).toContain("cas:ZZZZZZZZZZZZZ");
+      // Missing hash should exit with error
+      expect(exitCode).not.toBe(0);
+      expect(stderr).toContain("Node not found");
+      expect(stderr).toContain("ZZZZZZZZZZZZZ");
     } finally {
       rmSync(tmpStore, { recursive: true, force: true });
     }
@@ -596,4 +616,105 @@ describe("Suite 6: CLI Integration with Templates", () => {
       rmSync(tmpStore, { recursive: true, force: true });
     }
   });
+
+  test("R8: render with non-existent hash exits with error", async () => {
+    const tmpStore = mkdtempSync(join(tmpdir(), "json-cas-test-"));
+    try {
+      await runCli(["init"], tmpStore);
+      const { exitCode, stderr, stdout } = await runCli(
+        ["render", "AAAAAAAAAAAAA"],
+        tmpStore,
+      );
+      expect(exitCode).not.toBe(0);
+      expect(stderr).toContain("Node not found");
+      expect(stderr).toContain("AAAAAAAAAAAAA");
+      expect(stdout).toBe("");
+    } finally {
+      rmSync(tmpStore, { recursive: true, force: true });
+    }
+  });
+
+  test("R9: render with valid hash exits successfully", async () => {
+    const tmpStore = mkdtempSync(join(tmpdir(), "json-cas-test-"));
+    try {
+      await runCli(["init"], tmpStore);
+
+      // Get @string type hash
+      const { stdout: typesJson } = await runCli(["types"], tmpStore);
+      const types = JSON.parse(typesJson);
+      const stringType = types["@string"];
+
+      // Create and store a simple string node
+      const nodeFile = join(tmpStore, "test.json");
+      writeFileSync(nodeFile, JSON.stringify("hello world"));
+      const { stdout: nodeHash } = await runCli(
+        ["put", stringType, nodeFile],
+        tmpStore,
+      );
+
+      // Render the valid hash
+      const { exitCode, stdout, stderr } = await runCli(
+        ["render", nodeHash.trim()],
+        tmpStore,
+      );
+
+      expect(exitCode).toBe(0);
+      expect(stdout).toContain("hello world");
+      expect(stderr).toBe("");
+      expect(stdout).not.toContain("Error");
+      expect(stdout).not.toContain("Node not found");
+    } finally {
+      rmSync(tmpStore, { recursive: true, force: true });
+    }
+  });
+
+  test("R10: render --pipe with valid envelope succeeds", async () => {
+    const tmpStore = mkdtempSync(join(tmpdir(), "json-cas-test-"));
+    try {
+      await runCli(["init"], tmpStore);
+
+      // Get @string type hash
+      const { stdout: typesJson } = await runCli(["types"], tmpStore);
+      const types = JSON.parse(typesJson);
+      const stringType = types["@string"];
+
+      // Create envelope and pipe to render
+      const envelope = JSON.stringify({ type: stringType, value: "test" });
+      const { exitCode, stdout, stderr } = await runCliWithStdin(
+        ["render", "--pipe"],
+        tmpStore,
+        envelope,
+      );
+
+      expect(exitCode).toBe(0);
+      expect(stdout).toContain("test");
+      expect(stderr).toBe("");
+    } finally {
+      rmSync(tmpStore, { recursive: true, force: true });
+    }
+  });
+
+  test("R11: render --pipe with invalid type hash still renders", async () => {
+    const tmpStore = mkdtempSync(join(tmpdir(), "json-cas-test-"));
+    try {
+      await runCli(["init"], tmpStore);
+
+      // Use invalid type hash in envelope
+      const envelope = JSON.stringify({
+        type: "ZZZZZZZZZZZZZ",
+        value: "test",
+      });
+      const { exitCode, stdout, stderr } = await runCliWithStdin(
+        ["render", "--pipe"],
+        tmpStore,
+        envelope,
+      );
+
+      expect(exitCode).toBe(0);
+      expect(stdout).toContain("test");
+      expect(stderr).toBe("");
+    } finally {
+      rmSync(tmpStore, { recursive: true, force: true });
+    }
+  });
 });
diff --git a/packages/cli-json-cas/src/index.ts b/packages/cli-json-cas/src/index.ts
index 51bdf4d..2f0bd20 100644
--- a/packages/cli-json-cas/src/index.ts
+++ b/packages/cli-json-cas/src/index.ts
@@ -486,6 +486,9 @@ async function cmdRender(args: string[]): Promise<void> {
       process.stdout.write(output);
     }
   } catch (error) {
+    if (error instanceof CasNodeNotFoundError) {
+      die(`Error: Node not found: ${error.hash}`);
+    }
     if (error instanceof Error) {
       die(error.message);
     }
diff --git a/packages/json-cas/src/render.test.ts b/packages/json-cas/src/render.test.ts
index eaf629b..bc06d85 100644
--- a/packages/json-cas/src/render.test.ts
+++ b/packages/json-cas/src/render.test.ts
@@ -1,9 +1,10 @@
 import { describe, expect, test } from "bun:test";
 import { bootstrap } from "./bootstrap.js";
-import { render, renderDirect } from "./render.js";
+import { render, renderAsync, renderDirect } from "./render.js";
 import { putSchema } from "./schema.js";
 import { createMemoryStore } from "./store.js";
 import type { Hash } from "./types.js";
+import { CasNodeNotFoundError } from "./variable-store.js";
 
 describe("Suite 1: Basic Rendering (No Nesting)", () => {
   test("1.1 Render Simple Primitives", async () => {
@@ -65,13 +66,14 @@ describe("Suite 1: Basic Rendering (No Nesting)", () => {
     expect(output.trim()).toBe(`cas:${hash}`);
   });
 
-  test("1.5 Render Non-existent Hash", () => {
+  test("1.5 Render Non-existent Hash Throws Error", () => {
     const store = createMemoryStore();
     const fakeHash = "ZZZZZZZZZZZZZ" as Hash;
 
-    // Non-existent node renders as cas: reference
-    const output = render(store, fakeHash);
-    expect(output.trim()).toBe(`cas:${fakeHash}`);
+    // Non-existent root node should throw
+    expect(() => render(store, fakeHash)).toThrow(CasNodeNotFoundError);
+    expect(() => render(store, fakeHash)).toThrow("Node not found");
+    expect(() => render(store, fakeHash)).toThrow(fakeHash);
   });
 });
 
@@ -1055,3 +1057,93 @@ describe("Suite 9: renderDirect (in-memory rendering)", () => {
     expect(output).toContain("key: val");
   });
 });
+
+describe("Suite 10: Missing Root Hash Error Handling (Issue #53)", () => {
+  test("10.1 renderAsync() throws CasNodeNotFoundError for missing root hash", async () => {
+    const store = createMemoryStore();
+    await bootstrap(store);
+    const fakeHash = "AAAAAAAAAAAAA" as Hash;
+
+    await expect(renderAsync(store, fakeHash)).rejects.toThrow(
+      CasNodeNotFoundError,
+    );
+    await expect(renderAsync(store, fakeHash)).rejects.toThrow("Node not found");
+    await expect(renderAsync(store, fakeHash)).rejects.toThrow(fakeHash);
+  });
+
+  test("10.2 render() throws CasNodeNotFoundError for missing root hash", () => {
+    const store = createMemoryStore();
+    const fakeHash = "ZZZZZZZZZZZZZ" as Hash;
+
+    expect(() => render(store, fakeHash)).toThrow(CasNodeNotFoundError);
+    expect(() => render(store, fakeHash)).toThrow("Node not found");
+    expect(() => render(store, fakeHash)).toThrow(fakeHash);
+  });
+
+  test("10.3 renderDirect() does NOT throw for non-existent type hash", () => {
+    const store = createMemoryStore();
+    const fakeTypeHash = "0000000000000" as Hash;
+    const output = renderDirect(fakeTypeHash, { key: "value" }, store, null);
+
+    expect(output).toContain("key: value");
+  });
+
+  test("10.4 Missing nested node renders as cas: reference (no error)", async () => {
+    const store = createMemoryStore();
+    await bootstrap(store);
+
+    const parentSchema = await putSchema(store, {
+      type: "object",
+      properties: {
+        title: { type: "string" },
+        child: { type: "string", format: "cas_ref" },
+      },
+    });
+
+    const fakeChildHash = "ZZZZZZZZZZZZZ" as Hash;
+    const parentHash = await store.put(parentSchema, {
+      title: "root",
+      child: fakeChildHash,
+    });
+
+    const output = render(store, parentHash);
+
+    expect(output).toContain("title: root");
+    expect(output).toContain(`cas:${fakeChildHash}`);
+  });
+
+  test("10.5 Resolution below epsilon renders as cas: reference (no error)", async () => {
+    const store = createMemoryStore();
+    await bootstrap(store);
+
+    const nodeSchema = await putSchema(store, {
+      type: "object",
+      properties: {
+        level: { type: "number" },
+        next: {
+          anyOf: [{ type: "string", format: "cas_ref" }, { type: "null" }],
+        },
+      },
+    });
+
+    // Create 3-level chain
+    let currentHash: Hash | null = null;
+    for (let i = 2; i >= 0; i--) {
+      currentHash = await store.put(nodeSchema, {
+        level: i,
+        next: currentHash,
+      });
+    }
+
+    const output = render(store, currentHash as Hash, {
+      resolution: 1.0,
+      decay: 0.1,
+      epsilon: 0.5,
+    });
+
+    // Level 0 should be expanded (resolution = 1.0 > 0.5)
+    expect(output).toContain("level: 0");
+    // Level 1+ should be cas: references (0.1, 0.01 < 0.5)
+    expect(output).toContain("cas:");
+  });
+});
diff --git a/packages/json-cas/src/render.ts b/packages/json-cas/src/render.ts
index 85d163c..d935e74 100644
--- a/packages/json-cas/src/render.ts
+++ b/packages/json-cas/src/render.ts
@@ -1,6 +1,7 @@
 import { renderWithTemplate } from "./liquid-render.js";
 import { collectRefs, getSchema, putSchema, refs } from "./schema.js";
 import type { Hash, Store } from "./types.js";
+import { CasNodeNotFoundError } from "./variable-store.js";
 import type { VariableStore } from "./variable-store.js";
 
 export type RenderOptions = {
@@ -55,6 +56,11 @@ export function render(
 ): string {
   const { resolution, decay, epsilon } = validateAndExtractOptions(options);
 
+  // Check if root node exists
+  if (store.get(hash) === null) {
+    throw new CasNodeNotFoundError(hash);
+  }
+
   const visited = new Set<Hash>();
   return renderNode(store, hash, resolution, decay, epsilon, visited);
 }
@@ -70,6 +76,12 @@ export async function renderAsync(
   options?: RenderOptions,
 ): Promise<string> {
   const { resolution, decay, epsilon } = validateAndExtractOptions(options);
+
+  // Check if root node exists
+  if (store.get(hash) === null) {
+    throw new CasNodeNotFoundError(hash);
+  }
+
   const varStore = options?.varStore;
 
   // If varStore provided, try template rendering first
diff --git a/packages/json-cas/src/variable-store.ts b/packages/json-cas/src/variable-store.ts
index 2f1808b..cd421e3 100644
--- a/packages/json-cas/src/variable-store.ts
+++ b/packages/json-cas/src/variable-store.ts
@@ -36,8 +36,11 @@ export class SchemaMismatchError extends Error {
 }
 
 export class CasNodeNotFoundError extends Error {
-  constructor(hash: string) {
-    super(`CAS node not found: ${hash}`);
+  constructor(
+    public readonly hash: string,
+    message?: string,
+  ) {
+    super(message ?? `Node not found: ${hash}`);
     this.name = "CasNodeNotFoundError";
   }
 }