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PowerShell 5.1 has no && or || — agents loop on parser errors

versions · case

5.1hard-erroragentinteractivescriptfirst-partyrepro: verifiedpipeline-chain-ops
Affectspowershell 51
Fails asPARSERERROR
Mechanismpipeline chain ops
Safe fixlastexitcode gate

Symptom

Section titled “Symptom”

An AI agent (or a developer used to bash/pwsh 7) runs build.cmd && deploy.cmd in Windows PowerShell 5.1 and gets “The token ‘&&’ is not a valid statement separator in this version.” Agents told to “retry” replay the same line and loop forever on the identical parser error.

Repro

Section titled “Repro”
Terminal window
# Windows PowerShell 5.1
echo a && echo b
# ParserError: The token '&&' is not a valid statement separator in this version.
# pwsh 7: works (pipeline-chain operators were added in PowerShell 7).

Cause

Section titled “Cause”

Pipeline-chain operators &&/|| shipped in PowerShell 7. 5.1 treats them as parser errors. The trap compounds for agents: ; is NOT a substitute (it runs the next statement unconditionally, losing the failure gate), and bash habits like cd /d or heredocs also die in PS.

Workaround

Section titled “Workaround”
  • Gate conditionally: command1; if ($?) { command2 } — or check $LASTEXITCODE for native commands.
  • Agent system prompts targeting Windows hosts must ban &&/|| for 5.1 and map POSIX recovery commands (cat/ls/grep) to PS equivalents (Get-Content/Get-ChildItem/Select-String). The referenced fix ships exactly that guidance into an agent bridge.

Refs

Section titled “Refs”