From 1cdf806d21c3afdeb203c04705e8892d0f9d832f Mon Sep 17 00:00:00 2001
From: Niels Kaspers <153818647+nielskaspers@users.noreply.github.com>
Date: Sun, 15 Mar 2026 19:09:04 +0200
Subject: [PATCH] fix(sequential-thinking): use z.coerce for number and boolean
 params (#3533)
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fix(sequential-thinking): use z.coerce for number and safe preprocess for boolean params

Uses z.coerce.number() for number fields and a z.preprocess() helper for boolean fields to handle string-typed parameters from LLM clients. The preprocess approach correctly handles "false" → false, avoiding the z.coerce.boolean() footgun where Boolean("false") === true.

Fixes #3428
---
 src/sequentialthinking/index.ts | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/src/sequentialthinking/index.ts b/src/sequentialthinking/index.ts
index a3c34cca..217845bb 100644
--- a/src/sequentialthinking/index.ts
+++ b/src/sequentialthinking/index.ts
@@ -5,6 +5,16 @@ import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js"
 import { z } from "zod";
 import { SequentialThinkingServer } from './lib.js';
 
+/** Safe boolean coercion that correctly handles string "false" */
+const coercedBoolean = z.preprocess((val) => {
+  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());
+
 const server = new McpServer({
   name: "sequential-thinking-server",
   version: "0.2.0",
@@ -72,14 +82,14 @@ You should:
 11. Only set nextThoughtNeeded to false when truly done and a satisfactory answer is reached`,
     inputSchema: {
       thought: z.string().describe("Your current thinking step"),
-      nextThoughtNeeded: z.boolean().describe("Whether another thought step is needed"),
-      thoughtNumber: z.number().int().min(1).describe("Current thought number (numeric value, e.g., 1, 2, 3)"),
-      totalThoughts: z.number().int().min(1).describe("Estimated total thoughts needed (numeric value, e.g., 5, 10)"),
-      isRevision: z.boolean().optional().describe("Whether this revises previous thinking"),
-      revisesThought: z.number().int().min(1).optional().describe("Which thought is being reconsidered"),
-      branchFromThought: z.number().int().min(1).optional().describe("Branching point thought number"),
+      nextThoughtNeeded: coercedBoolean.describe("Whether another thought step is needed"),
+      thoughtNumber: z.coerce.number().int().min(1).describe("Current thought number (numeric value, e.g., 1, 2, 3)"),
+      totalThoughts: z.coerce.number().int().min(1).describe("Estimated total thoughts needed (numeric value, e.g., 5, 10)"),
+      isRevision: coercedBoolean.optional().describe("Whether this revises previous thinking"),
+      revisesThought: z.coerce.number().int().min(1).optional().describe("Which thought is being reconsidered"),
+      branchFromThought: z.coerce.number().int().min(1).optional().describe("Branching point thought number"),
       branchId: z.string().optional().describe("Branch identifier"),
-      needsMoreThoughts: z.boolean().optional().describe("If more thoughts are needed")
+      needsMoreThoughts: coercedBoolean.optional().describe("If more thoughts are needed")
     },
     annotations: {
       readOnlyHint: true,