diff --git a/src/sequentialthinking/__tests__/input-schema.test.ts b/src/sequentialthinking/__tests__/input-schema.test.ts
new file mode 100644
index 0000000000..4ff7be663c
--- /dev/null
+++ b/src/sequentialthinking/__tests__/input-schema.test.ts
@@ -0,0 +1,66 @@
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { existsSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
+
+const packageRoot = path.join(path.dirname(fileURLToPath(import.meta.url)), '..');
+const distIndexPath = path.join(packageRoot, 'dist', 'index.js');
+
+// Regression coverage for #4651: nextThoughtNeeded must stay in the advertised
+// `required` array, and string coercion must keep accepting "True"/"FALSE"
+// while rejecting anything else. Runs against the built server so it checks
+// the schema the SDK actually emits, not the zod object.
+describe.skipIf(!existsSync(distIndexPath))('sequentialthinking input schema', () => {
+  let client: Client;
+
+  beforeAll(async () => {
+    const transport = new StdioClientTransport({
+      command: process.execPath,
+      args: [distIndexPath],
+      cwd: packageRoot,
+      stderr: 'pipe',
+    });
+    client = new Client({ name: 'input-schema-test', version: '0.0.0' });
+    await client.connect(transport);
+  });
+
+  afterAll(async () => {
+    await client?.close();
+  });
+
+  it('advertises nextThoughtNeeded as required', async () => {
+    const { tools } = await client.listTools();
+    const tool = tools.find(t => t.name === 'sequentialthinking');
+    expect(tool).toBeDefined();
+    expect(tool!.inputSchema.required).toEqual(
+      expect.arrayContaining(['thought', 'nextThoughtNeeded', 'thoughtNumber', 'totalThoughts'])
+    );
+  });
+
+  it('rejects a call that omits nextThoughtNeeded', async () => {
+    const result = await client.callTool({
+      name: 'sequentialthinking',
+      arguments: { thought: 't', thoughtNumber: 1, totalThoughts: 1 },
+    });
+    expect(result.isError).toBe(true);
+    expect(JSON.stringify(result.content)).toMatch(/nextThoughtNeeded/);
+  });
+
+  it.each(['True', 'FALSE', 'true', 'false'])('accepts the string %s', async (value) => {
+    const result = await client.callTool({
+      name: 'sequentialthinking',
+      arguments: { thought: 't', nextThoughtNeeded: value, thoughtNumber: 1, totalThoughts: 1 },
+    });
+    expect(result.isError).toBeFalsy();
+  });
+
+  it.each(['yes', '', '1'])('rejects the string %j', async (value) => {
+    const result = await client.callTool({
+      name: 'sequentialthinking',
+      arguments: { thought: 't', nextThoughtNeeded: value, thoughtNumber: 1, totalThoughts: 1 },
+    });
+    expect(result.isError).toBe(true);
+  });
+});
diff --git a/src/sequentialthinking/index.ts b/src/sequentialthinking/index.ts
index 54c42df199..1ae09d1db8 100644
--- a/src/sequentialthinking/index.ts
+++ b/src/sequentialthinking/index.ts
@@ -6,15 +6,15 @@ import { z } from "zod";
 import { SequentialThinkingServer } from './lib.js';
 import { SERVER_VERSION } from './version.js';
 
-/** Safe boolean coercion that correctly handles string "false" */
-const coercedBoolean = z.preprocess((val) => {
+/** Safe boolean coercion that correctly handles string "false". A union+transform,
+ * not z.preprocess (whose input type is `unknown`), so toJSONSchema keeps this required. */
+const coercedBoolean = z.union([z.boolean(), z.string()]).transform((val, ctx) => {
   if (typeof val === "boolean") return val;
-  if (typeof val === "string") {
-    if (val.toLowerCase() === "true") return true;
-    if (val.toLowerCase() === "false") return false;
-  }
-  return val;
-}, z.boolean());
+  if (val.toLowerCase() === "true") return true;
+  if (val.toLowerCase() === "false") return false;
+  ctx.addIssue({ code: "custom", message: `Expected boolean or "true"/"false" string, received "${val}"` });
+  return z.NEVER;
+});
 
 const server = new McpServer({
   name: "sequential-thinking-server",