diff --git a/src/features/skill-mcp-manager/env-cleaner.test.ts b/src/features/skill-mcp-manager/env-cleaner.test.ts
new file mode 100644
index 0000000..1e0df07
--- /dev/null
+++ b/src/features/skill-mcp-manager/env-cleaner.test.ts
@@ -0,0 +1,201 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test"
+import { createCleanMcpEnvironment, EXCLUDED_ENV_PATTERNS } from "./env-cleaner"
+
+describe("createCleanMcpEnvironment", () => {
+  // Store original env to restore after tests
+  const originalEnv = { ...process.env }
+
+  afterEach(() => {
+    // Restore original environment
+    for (const key of Object.keys(process.env)) {
+      if (!(key in originalEnv)) {
+        delete process.env[key]
+      }
+    }
+    for (const [key, value] of Object.entries(originalEnv)) {
+      process.env[key] = value
+    }
+  })
+
+  describe("NPM_CONFIG_* filtering", () => {
+    it("filters out uppercase NPM_CONFIG_* variables", () => {
+      // #given
+      process.env.NPM_CONFIG_REGISTRY = "https://private.registry.com"
+      process.env.NPM_CONFIG_CACHE = "/some/cache/path"
+      process.env.NPM_CONFIG_PREFIX = "/some/prefix"
+      process.env.PATH = "/usr/bin"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.NPM_CONFIG_REGISTRY).toBeUndefined()
+      expect(cleanEnv.NPM_CONFIG_CACHE).toBeUndefined()
+      expect(cleanEnv.NPM_CONFIG_PREFIX).toBeUndefined()
+      expect(cleanEnv.PATH).toBe("/usr/bin")
+    })
+
+    it("filters out lowercase npm_config_* variables", () => {
+      // #given
+      process.env.npm_config_registry = "https://private.registry.com"
+      process.env.npm_config_cache = "/some/cache/path"
+      process.env.npm_config_https_proxy = "http://proxy:8080"
+      process.env.npm_config_proxy = "http://proxy:8080"
+      process.env.HOME = "/home/user"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.npm_config_registry).toBeUndefined()
+      expect(cleanEnv.npm_config_cache).toBeUndefined()
+      expect(cleanEnv.npm_config_https_proxy).toBeUndefined()
+      expect(cleanEnv.npm_config_proxy).toBeUndefined()
+      expect(cleanEnv.HOME).toBe("/home/user")
+    })
+  })
+
+  describe("YARN_* filtering", () => {
+    it("filters out YARN_* variables", () => {
+      // #given
+      process.env.YARN_CACHE_FOLDER = "/yarn/cache"
+      process.env.YARN_ENABLE_IMMUTABLE_INSTALLS = "true"
+      process.env.YARN_REGISTRY = "https://yarn.registry.com"
+      process.env.NODE_ENV = "production"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.YARN_CACHE_FOLDER).toBeUndefined()
+      expect(cleanEnv.YARN_ENABLE_IMMUTABLE_INSTALLS).toBeUndefined()
+      expect(cleanEnv.YARN_REGISTRY).toBeUndefined()
+      expect(cleanEnv.NODE_ENV).toBe("production")
+    })
+  })
+
+  describe("PNPM_* filtering", () => {
+    it("filters out PNPM_* variables", () => {
+      // #given
+      process.env.PNPM_HOME = "/pnpm/home"
+      process.env.PNPM_STORE_DIR = "/pnpm/store"
+      process.env.USER = "testuser"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.PNPM_HOME).toBeUndefined()
+      expect(cleanEnv.PNPM_STORE_DIR).toBeUndefined()
+      expect(cleanEnv.USER).toBe("testuser")
+    })
+  })
+
+  describe("NO_UPDATE_NOTIFIER filtering", () => {
+    it("filters out NO_UPDATE_NOTIFIER variable", () => {
+      // #given
+      process.env.NO_UPDATE_NOTIFIER = "1"
+      process.env.SHELL = "/bin/bash"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.NO_UPDATE_NOTIFIER).toBeUndefined()
+      expect(cleanEnv.SHELL).toBe("/bin/bash")
+    })
+  })
+
+  describe("custom environment overlay", () => {
+    it("merges custom env on top of clean process.env", () => {
+      // #given
+      process.env.PATH = "/usr/bin"
+      process.env.NPM_CONFIG_REGISTRY = "https://private.registry.com"
+      const customEnv = {
+        MCP_API_KEY: "secret-key",
+        CUSTOM_VAR: "custom-value",
+      }
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment(customEnv)
+
+      // #then
+      expect(cleanEnv.PATH).toBe("/usr/bin")
+      expect(cleanEnv.NPM_CONFIG_REGISTRY).toBeUndefined()
+      expect(cleanEnv.MCP_API_KEY).toBe("secret-key")
+      expect(cleanEnv.CUSTOM_VAR).toBe("custom-value")
+    })
+
+    it("custom env can override process.env values", () => {
+      // #given
+      process.env.NODE_ENV = "development"
+      const customEnv = {
+        NODE_ENV: "production",
+      }
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment(customEnv)
+
+      // #then
+      expect(cleanEnv.NODE_ENV).toBe("production")
+    })
+  })
+
+  describe("undefined value handling", () => {
+    it("skips undefined values from process.env", () => {
+      // #given - process.env can have undefined values in TypeScript
+      const envWithUndefined = { ...process.env, UNDEFINED_VAR: undefined }
+      Object.assign(process.env, envWithUndefined)
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then - should not throw and should not include undefined values
+      expect(cleanEnv.UNDEFINED_VAR).toBeUndefined()
+      expect(Object.values(cleanEnv).every((v) => v !== undefined)).toBe(true)
+    })
+  })
+
+  describe("mixed case handling", () => {
+    it("filters both uppercase and lowercase npm config variants", () => {
+      // #given - pnpm/yarn can set both cases simultaneously
+      process.env.NPM_CONFIG_CACHE = "/uppercase/cache"
+      process.env.npm_config_cache = "/lowercase/cache"
+      process.env.NPM_CONFIG_REGISTRY = "https://uppercase.registry.com"
+      process.env.npm_config_registry = "https://lowercase.registry.com"
+
+      // #when
+      const cleanEnv = createCleanMcpEnvironment()
+
+      // #then
+      expect(cleanEnv.NPM_CONFIG_CACHE).toBeUndefined()
+      expect(cleanEnv.npm_config_cache).toBeUndefined()
+      expect(cleanEnv.NPM_CONFIG_REGISTRY).toBeUndefined()
+      expect(cleanEnv.npm_config_registry).toBeUndefined()
+    })
+  })
+})
+
+describe("EXCLUDED_ENV_PATTERNS", () => {
+  it("contains patterns for npm, yarn, and pnpm configs", () => {
+    // #given / #when / #then
+    expect(EXCLUDED_ENV_PATTERNS.length).toBeGreaterThanOrEqual(4)
+
+    // Test that patterns match expected strings
+    const testCases = [
+      { pattern: "NPM_CONFIG_REGISTRY", shouldMatch: true },
+      { pattern: "npm_config_registry", shouldMatch: true },
+      { pattern: "YARN_CACHE_FOLDER", shouldMatch: true },
+      { pattern: "PNPM_HOME", shouldMatch: true },
+      { pattern: "NO_UPDATE_NOTIFIER", shouldMatch: true },
+      { pattern: "PATH", shouldMatch: false },
+      { pattern: "HOME", shouldMatch: false },
+      { pattern: "NODE_ENV", shouldMatch: false },
+    ]
+
+    for (const { pattern, shouldMatch } of testCases) {
+      const matches = EXCLUDED_ENV_PATTERNS.some((regex: RegExp) => regex.test(pattern))
+      expect(matches).toBe(shouldMatch)
+    }
+  })
+})
diff --git a/src/features/skill-mcp-manager/env-cleaner.ts b/src/features/skill-mcp-manager/env-cleaner.ts
new file mode 100644
index 0000000..9a3faba
--- /dev/null
+++ b/src/features/skill-mcp-manager/env-cleaner.ts
@@ -0,0 +1,27 @@
+// Filters npm/pnpm/yarn config env vars that break MCP servers in pnpm projects (#456)
+export const EXCLUDED_ENV_PATTERNS: RegExp[] = [
+  /^NPM_CONFIG_/i,
+  /^npm_config_/,
+  /^YARN_/,
+  /^PNPM_/,
+  /^NO_UPDATE_NOTIFIER$/,
+]
+
+export function createCleanMcpEnvironment(
+  customEnv: Record<string, string> = {}
+): Record<string, string> {
+  const cleanEnv: Record<string, string> = {}
+
+  for (const [key, value] of Object.entries(process.env)) {
+    if (value === undefined) continue
+
+    const shouldExclude = EXCLUDED_ENV_PATTERNS.some((pattern) => pattern.test(key))
+    if (!shouldExclude) {
+      cleanEnv[key] = value
+    }
+  }
+
+  Object.assign(cleanEnv, customEnv)
+
+  return cleanEnv
+}
diff --git a/src/features/skill-mcp-manager/manager.ts b/src/features/skill-mcp-manager/manager.ts
index a43418f..089e186 100644
--- a/src/features/skill-mcp-manager/manager.ts
+++ b/src/features/skill-mcp-manager/manager.ts
@@ -3,6 +3,7 @@ import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js"
 import type { Tool, Resource, Prompt } from "@modelcontextprotocol/sdk/types.js"
 import type { ClaudeCodeMcpServer } from "../claude-code-mcp-loader/types"
 import { expandEnvVarsInObject } from "../claude-code-mcp-loader/env-expander"
+import { createCleanMcpEnvironment } from "./env-cleaner"
 import type { SkillMcpClientInfo, SkillMcpServerContext } from "./types"
 
 interface ManagedClient {
@@ -115,15 +116,7 @@ export class SkillMcpManager {
     const command = config.command
     const args = config.args || []
 
-    // Always inherit parent process environment
-    const mergedEnv: Record<string, string> = {}
-    for (const [key, value] of Object.entries(process.env)) {
-      if (value !== undefined) mergedEnv[key] = value
-    }
-    // Overlay with skill-specific env vars if present
-    if (config.env) {
-      Object.assign(mergedEnv, config.env)
-    }
+    const mergedEnv = createCleanMcpEnvironment(config.env)
 
     this.registerProcessCleanup()