diff --git a/packages/opencode/src/mcp/index.ts b/packages/opencode/src/mcp/index.ts
index 585e339a23..6df9d4a20b 100644
--- a/packages/opencode/src/mcp/index.ts
+++ b/packages/opencode/src/mcp/index.ts
@@ -180,9 +180,15 @@ function localMcpEnvironment(command: string, environment?: Record<string, strin
       return [[key, value] as const]
     }),
   )
-  return {
+  const defaults = {
     ...inherited,
     ...(command === "opencode" ? { BUN_BE_BUN: "1" } : {}),
+  }
+  if (process.platform !== "win32" || !environment) return { ...defaults, ...environment }
+
+  const configured = new Set(Object.keys(environment).map((key) => key.toUpperCase()))
+  return {
+    ...Object.fromEntries(Object.entries(defaults).filter(([key]) => !configured.has(key.toUpperCase()))),
     ...environment,
   }
 }
diff --git a/packages/opencode/test/fixture/mcp-environment.ts b/packages/opencode/test/fixture/mcp-environment.js
similarity index 73%
rename from packages/opencode/test/fixture/mcp-environment.ts
rename to packages/opencode/test/fixture/mcp-environment.js
index f8a6e2f5c3..cd3a228440 100644
--- a/packages/opencode/test/fixture/mcp-environment.ts
+++ b/packages/opencode/test/fixture/mcp-environment.js
@@ -1,11 +1,12 @@
 import readline from "node:readline"
+import { writeFile } from "node:fs/promises"
 
-await Bun.write(process.env.MCP_ENV_OUTPUT!, JSON.stringify(process.env))
+await writeFile(process.env.MCP_ENV_OUTPUT, JSON.stringify(process.env))
 
 const lines = readline.createInterface({ input: process.stdin })
 lines.on("close", () => process.exit(0))
 lines.on("line", (line) => {
-  const request = JSON.parse(line) as { id?: number; method: string; params?: { protocolVersion?: string } }
+  const request = JSON.parse(line)
   if (request.method !== "initialize") return
   process.stdout.write(
     `${JSON.stringify({
diff --git a/packages/opencode/test/mcp/environment.test.ts b/packages/opencode/test/mcp/environment.test.ts
index 68fdffecfd..bb57967a64 100644
--- a/packages/opencode/test/mcp/environment.test.ts
+++ b/packages/opencode/test/mcp/environment.test.ts
@@ -6,54 +6,60 @@ import { TestInstance } from "../fixture/fixture"
 import { testEffect } from "../lib/effect"
 
 const it = testEffect(MCP.defaultLayer)
+const inherited = [
+  "APPDATA",
+  "HOME",
+  "LANG",
+  "LOCALAPPDATA",
+  "PATH",
+  "PATHEXT",
+  "SYSTEMROOT",
+  "TEMP",
+  "TMPDIR",
+  "USERPROFILE",
+] as const
 
 it.instance(
   "local subprocess receives only baseline and configured environment",
   () =>
     Effect.gen(function* () {
       const test = yield* TestInstance
-      const values = {
-        APPDATA: path.join(test.directory, "appdata"),
-        LC_TIME: "C",
-        OPENCODE_MCP_PARENT_SECRET: "parent-secret",
-        PATHEXT: ".EXE;.CMD",
-        SYSTEMROOT: path.join(test.directory, "windows"),
-        TMPDIR: test.directory,
-      }
-      const previous = Object.fromEntries(Object.keys(values).map((key) => [key, process.env[key]]))
-      Object.assign(process.env, values)
+      const previous = process.env.OPENCODE_MCP_PARENT_SECRET
+      process.env.OPENCODE_MCP_PARENT_SECRET = "parent-secret"
 
       yield* MCP.Service.use((mcp) =>
         Effect.gen(function* () {
           const output = path.join(test.directory, "environment.json")
           const result = yield* mcp.add("environment", {
             type: "local",
-            command: [process.execPath, path.join(import.meta.dir, "../fixture/mcp-environment.ts")],
+            command: [process.execPath, path.join(import.meta.dir, "../fixture/mcp-environment.js")],
             environment: {
               MCP_ENV_OUTPUT: output,
               MCP_EXPLICIT_TOKEN: "configured-token",
+              ...(process.platform === "win32" ? { Path: path.dirname(process.execPath) } : {}),
             },
           })
 
+          if (!("environment" in result.status)) throw new Error("Expected MCP status map")
           expect(result.status.environment).toEqual({ status: "connected" })
           const env = (yield* Effect.promise(() => Bun.file(output).json())) as Record<string, string>
           expect(env.OPENCODE_MCP_PARENT_SECRET).toBeUndefined()
           expect(env.MCP_EXPLICIT_TOKEN).toBe("configured-token")
-          expect(env.PATH).toBe(process.env.PATH!)
-          expect(env.HOME).toBe(process.env.HOME!)
-          expect(env.TMPDIR).toBe(values.TMPDIR)
-          expect(env.LC_TIME).toBe(values.LC_TIME)
-          expect(env.APPDATA).toBe(values.APPDATA)
-          expect(env.PATHEXT).toBe(values.PATHEXT)
-          expect(env.SYSTEMROOT).toBe(values.SYSTEMROOT)
+          inherited.forEach((key) => {
+            if (process.platform === "win32" && key === "PATH") return
+            if (process.env[key] !== undefined) expect(env[key]).toBe(process.env[key])
+          })
+          if (process.platform === "win32") {
+            expect(Object.entries(env).find(([key]) => key.toUpperCase() === "PATH")?.[1]).toBe(
+              path.dirname(process.execPath),
+            )
+          }
         }).pipe(Effect.ensuring(mcp.disconnect("environment").pipe(Effect.ignore))),
       ).pipe(
         Effect.ensuring(
           Effect.sync(() => {
-            Object.entries(previous).forEach(([key, value]) => {
-              if (value === undefined) delete process.env[key]
-              else process.env[key] = value
-            })
+            if (previous === undefined) delete process.env.OPENCODE_MCP_PARENT_SECRET
+            else process.env.OPENCODE_MCP_PARENT_SECRET = previous
           }),
         ),
       )