diff --git a/src/core/prompts/sections/__tests__/system-info.spec.ts b/src/core/prompts/sections/__tests__/system-info.spec.ts
index 7c3b53c426..421c3a23b6 100644
--- a/src/core/prompts/sections/__tests__/system-info.spec.ts
+++ b/src/core/prompts/sections/__tests__/system-info.spec.ts
@@ -10,6 +10,7 @@ vi.mock("../../../../utils/shell", () => ({
 }))
 
 import { getSystemInfoSection } from "../system-info"
+import { getShell } from "../../../../utils/shell"
 import osName from "os-name"
 
 const mockOsName = osName as unknown as ReturnType<typeof vi.fn>
@@ -101,4 +102,27 @@ describe("getSystemInfoSection", () => {
 		// sentinel string) breaks this exact join.
 		expect(result).toContain("default directory for all tool operations. When the user initially gives you a task")
 	})
+
+	describe("shell syntax", () => {
+		const cmdPath = "C:\\Windows\\System32\\cmd.exe"
+
+		it("states the dialect for cmd.exe when execute_command is available", () => {
+			vi.mocked(getShell).mockReturnValueOnce(cmdPath)
+
+			const result = getSystemInfoSection(mockCwd, policyFor())
+
+			expect(result).toContain(`Default Shell: ${cmdPath}
+Shell Syntax: Commands run through Windows cmd.exe`)
+		})
+
+		it("omits the dialect without execute_command", () => {
+			vi.mocked(getShell).mockReturnValueOnce(cmdPath)
+
+			expect(getSystemInfoSection(mockCwd, policyFor(false))).not.toContain("Shell Syntax")
+		})
+
+		it("omits the dialect for POSIX shells", () => {
+			expect(getSystemInfoSection(mockCwd, policyFor())).not.toContain("Shell Syntax")
+		})
+	})
 })
diff --git a/src/core/prompts/sections/system-info.ts b/src/core/prompts/sections/system-info.ts
index 003c555d75..6e7067bf7c 100644
--- a/src/core/prompts/sections/system-info.ts
+++ b/src/core/prompts/sections/system-info.ts
@@ -2,6 +2,7 @@ import os from "os"
 import osName from "os-name"
 
 import { getShell } from "../../../utils/shell"
+import { describeShellDialect } from "../../../utils/shellDialect"
 
 import type { EffectiveToolPolicy } from "../tools/effective-tool-policy"
 
@@ -33,12 +34,19 @@ export function getSystemInfoSection(cwd: string, policy: EffectiveToolPolicy):
 		? " New terminals will be created in the current workspace directory, however if you change directories in a terminal it will then have a different working directory; changing directories in a terminal does not modify the workspace directory, because you do not have access to change the workspace directory."
 		: ""
 
+	const shell = getShell()
+	const shellDialect = executeCommandAvailable ? describeShellDialect(shell) : undefined
+	const shellDialectLine = shellDialect
+		? `
+Shell Syntax: ${shellDialect}`
+		: ""
+
 	const details = `====
 
 SYSTEM INFORMATION
 
 Operating System: ${osInfo}
-Default Shell: ${getShell()}
+Default Shell: ${shell}${shellDialectLine}
 Home Directory: ${os.homedir().toPosix()}
 Current Workspace Directory: ${cwd.toPosix()}
 
diff --git a/src/utils/__tests__/shellDialect.spec.ts b/src/utils/__tests__/shellDialect.spec.ts
new file mode 100644
index 0000000000..c5db9ee722
--- /dev/null
+++ b/src/utils/__tests__/shellDialect.spec.ts
@@ -0,0 +1,23 @@
+import { describeShellDialect } from "../shellDialect"
+
+describe("describeShellDialect", () => {
+	it.each([
+		{ shell: "C:\\Windows\\System32\\cmd.exe", expected: "cmd.exe" },
+		{ shell: "C:\\Windows\\System32\\CMD.EXE", expected: "cmd.exe" },
+		{ shell: "C:\\Program Files\\PowerShell\\7\\pwsh.exe", expected: "PowerShell" },
+		{ shell: "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe", expected: "PowerShell" },
+	])("names the dialect for $shell", ({ shell, expected }) => {
+		expect(describeShellDialect(shell)).toContain(expected)
+	})
+
+	it("warns that ';' does not chain commands in cmd.exe", () => {
+		expect(describeShellDialect("C:\\Windows\\System32\\cmd.exe")).toContain("`;` is not a separator")
+	})
+
+	it.each(["/bin/bash", "/bin/zsh", "C:\\Program Files\\Git\\bin\\bash.exe", "C:\\Windows\\System32\\wsl.exe"])(
+		"adds nothing for POSIX shell %s",
+		(shell) => {
+			expect(describeShellDialect(shell)).toBeUndefined()
+		},
+	)
+})
diff --git a/src/utils/shellDialect.ts b/src/utils/shellDialect.ts
new file mode 100644
index 0000000000..048f62cd35
--- /dev/null
+++ b/src/utils/shellDialect.ts
@@ -0,0 +1,30 @@
+/**
+ * Describes the command syntax of Windows-native shells for the system prompt.
+ *
+ * Rules and habits written for bash silently do the wrong thing under these shells
+ * (e.g. `;` is not a command separator in cmd.exe), so the model is told which
+ * dialect it is writing for. Returns undefined for POSIX shells, where the shell
+ * path already names the dialect.
+ */
+export function describeShellDialect(shellPath: string): string | undefined {
+	// path.basename follows the host platform; split on both separators so Windows paths work anywhere.
+	const name = (shellPath.split(/[\\/]/).pop() ?? "").toLowerCase()
+
+	if (name === "cmd.exe" || name === "cmd") {
+		return (
+			"Commands run through Windows cmd.exe, not bash. Use cmd syntax: `&` or `&&` to chain commands " +
+			"(`;` is not a separator), `%VAR%` for variables, `set X=Y`, `%ERRORLEVEL%` for exit codes, " +
+			"and `dir`, `type`, `findstr`. POSIX tools may be missing or shadowed by other binaries on PATH."
+		)
+	}
+
+	if (name === "powershell.exe" || name === "pwsh.exe" || name === "powershell" || name === "pwsh") {
+		return (
+			"Commands run through PowerShell, not bash. Use PowerShell syntax: `;` to chain commands, " +
+			"`$env:VAR` for environment variables, `$LASTEXITCODE` for exit codes, and cmdlets such as " +
+			"`Get-ChildItem` and `Select-String`."
+		)
+	}
+
+	return undefined
+}