{"record":{"id":"819c493b74042a25","repo":"hashicorp/nomad","slug":"failed-to-generate-command-ui","errorCode":null,"errorMessage":"failed to generate command UI","messagePattern":"failed to generate command UI","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"command/ui/writer_ui.go","lineNumber":67,"sourceCode":"\t\t\twUI.writer = u.OutputWriter\n\t\t\twUI.errorWriter = u.ErrorWriter\n\t\t\twUI.baseUi = u\n\t\t\tdone = true\n\t\tcase *cli.BasicUi:\n\t\t\twUI.reader = u.Reader\n\t\t\twUI.writer = u.Writer\n\t\t\twUI.errorWriter = u.ErrorWriter\n\t\t\twUI.baseUi = u\n\t\t\tdone = true\n\t\tcase *cli.ColoredUi:\n\t\t\tui = u.Ui\n\t\tdefault:\n\t\t\treturn nil, fmt.Errorf(\"writer ui: unsupported Ui type: %T\", ui)\n\t\t}\n\t}\n\n\tif !done {\n\t\treturn nil, errors.New(\"failed to generate command UI\")\n\t}\n\n\treturn &wUI, nil\n}\n\nfunc (w *WriterUI) InputReader() io.Reader  { return w.reader }\nfunc (w *WriterUI) OutputWriter() io.Writer { return w.writer }\nfunc (w *WriterUI) ErrorWriter() io.Writer  { return w.errorWriter }\n\nfunc (w *WriterUI) Output(message string) { w.Ui.Output(message) }\nfunc (w *WriterUI) Info(message string)   { w.Ui.Info(message) }\nfunc (w *WriterUI) Warn(message string)   { w.Ui.Warn(message) }\nfunc (w *WriterUI) Error(message string)  { w.Ui.Error(message) }\n\nfunc (w *WriterUI) Ask(query string) (string, error)       { return w.Ui.Ask(query) }\nfunc (w *WriterUI) AskSecret(query string) (string, error) { return w.Ui.AskSecret(query) }\n","sourceCodeStart":49,"sourceCodeEnd":84,"githubUrl":"https://github.com/hashicorp/nomad/blob/482b49bf1aec006f089bcfc7e632d8f6ac303e5e/command/ui/writer_ui.go#L49-L84","documentation":"NewWriterUI walks a cli.Ui chain (ColoredUi -> BasicUi/MockUi) to extract raw reader/writer streams. If the loop exits without resolving a supported base UI (because the chain ends in a nil Ui or was empty), it throws this sentinel error. It means no *cli.BasicUi or *cli.MockUi was found at the bottom of the UI chain.","triggerScenarios":"Passing nil to NewWriterUI; passing a Ui chain whose base is not *cli.BasicUi or *cli.MockUi (e.g. a custom cli.Ui implementation or a wrapped Ui other than cli.ColoredUi).","commonSituations":"Custom UI wrappers injected into the Vault command framework that don't unwrap to BasicUi; tests constructing a WriterUI with a MockUi nested incorrectly or no UI at all; running commands in environments where the UI was replaced by a non-standard implementation.","solutions":["Ensure the UI chain terminates in a *cli.BasicUi (production) or *cli.MockUi (tests)","If using ColoredUi, verify it wraps a BasicUi via its .Ui field","Check that the cli.Ui passed to NewWriterUI is non-nil","Wrap any custom Ui implementations so they expose an underlying BasicUi"],"exampleFix":"// before\nwui, err := ui.NewWriterUI(myCustomUi{})\n// after\nwui, err := ui.NewWriterUI(&cli.ColoredUi{Ui: &cli.BasicUi{Reader: os.Stdin, Writer: os.Stdout, ErrorWriter: os.Stderr}})","handlingStrategy":"type-guard","validationCode":"func canWrapWriterUI(u cli.Ui) bool {\n\tswitch v := u.(type) {\n\tcase *cli.BasicUi, *cli.MockUi:\n\t\treturn true\n\tcase *cli.ColoredUi:\n\t\treturn canWrapWriterUI(v.Ui)\n\tdefault:\n\t\treturn false\n\t}\n}\nif !canWrapWriterUI(myUi) { /* fix UI construction before calling NewWriterUI */ }","typeGuard":"func isWriterUICompatible(u cli.Ui) bool {\n\tfor u != nil {\n\t\tswitch t := u.(type) {\n\t\tcase *cli.BasicUi, *cli.MockUi:\n\t\t\treturn true\n\t\tcase *cli.ColoredUi:\n\t\t\tu = t.Ui\n\t\tdefault:\n\t\t\treturn false\n\t\t}\n\t}\n\treturn false\n}","tryCatchPattern":"wUI, err := ui.NewWriterUI(u)\nif err != nil {\n\tif err.Error() == \"failed to generate command UI\" {\n\t\t// fall back to the plain cli.Ui instead of raw stream access\n\t}\n\treturn err\n}","preventionTips":["Always build UIs as (ColoredUi ->)? BasicUi chains; never pass bare custom Ui implementations","In tests, use cli.NewMockUi() as the base","Assert the UI is non-nil before calling NewWriterUI","Document custom Ui wrappers must expose a BasicUi underneath"],"tags":["go","cli","ui"],"backgroundTag":"unsupported-ui-type","analyzedSha":"482b49bf1aec006f089bcfc7e632d8f6ac303e5e","analyzedAt":"2026-09-04T07:54:14.808Z","contentChangedAt":"2026-09-04T07:54:14.808Z","schemaVersion":2},"datasetVersion":"2026-09-08T10:18:20.063Z"}