Design, build, review, or test command-line interfaces that work well for humans, scripts, agents, pipes, and CI. Use when defining command trees, arguments and flags, stdout/stderr contracts, human and structured output, exit codes, prompts, configuration precedence, destructive-action safeguards, TTY behavior, completion, or CLI compatibility. Also use when a TUI needs a stable non-interactive command surface. Language-agnostic core with optional TypeScript implementation references.
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Design, build, review, or test command-line interfaces that work well for humans, scripts, agents, pipes, and CI. Use when defining command trees, arguments and flags, stdout/stderr contracts, human and structured output, exit codes, prompts, configuration precedence, destructive-action safeguards, TTY behavior, completion, or CLI compatibility. Also use when a TUI needs a stable non-interactive command surface. Language-agnostic core with optional TypeScript implementation references.
This skill covers language-agnostic CLI design principles. Apply them proportionately to the tool's users, compatibility promises, platforms, and existing conventions.
Read repository instructions and inspect the existing command surface before changing it. Existing flags, output schemas, exit codes, shell completions, and automation consumers are public contracts even when they were not designed deliberately.
TypeScript implementation patterns are in the resources/ directory. Load them on demand when building a CLI in TypeScript:
Resource
Load when...
output-architecture.md
Implementing Result types, entry point wiring, formatters, logger, JSON envelope schemas
Adding a scriptable CLI beside a TUI or other interactive interface
Core Principle
stdout is for DATA only — the product the user asked for.stderr is for EVERYTHING ELSE — diagnostics, progress, spinners, warnings, errors.
This separation is what makes mycli --json | jq ... work. One spinner character on stdout breaks every downstream pipe.
"Whatever software you're building, you can be absolutely certain that people will use it in ways you didn't anticipate. Your software will become a part in a larger system — your only choice is over whether it will be a well-behaved part." — clig.dev
The Unix Stream Contract
Content
Stream
Why
Primary output (data, results, JSON)
stdout
Pipeable, buffered for throughput
Progress bars, spinners, status
stderr
Not data — must not corrupt pipes
Warnings, errors, diagnostics
stderr
Visible to user even when stdout is piped
Debug/verbose output
stderr
Diagnostic, never data
Buffering behavior:
stdout: line-buffered when connected to a TTY, block-buffered when piped (~2x faster than stderr)
stderr: unbuffered — every write is a syscall (immediate but expensive)
Check each stream independently — stdout being piped does not mean stderr is piped
When stdout is piped, the user doesn't want your status messages in their data. All non-data output must go to stderr.
For a deep dive on buffering behavior and performance implications, see resources/stream-contracts.md.
Keep Handlers Pure
The practical rule: functions that do the work should return data, not write to stdout. The CLI entry point handles all I/O.
Entry point (CLI main) Your logic (handlers)
───────────────────── ─────────────────────
parse args (input) → structured result
detect format (json/plain/human) no printing to stdout
call handler no writing to stderr
format the result no calling exit
write to correct stream just returns data
set exit code
This isn't an architecture mandate — it's just clean function design. The benefits are concrete:
Testable without subprocess spawning — call the handler, assert on the returned value
Format flexibility for free — same data renders as JSON, plain text, or coloured tables by swapping one function
Reusable — the same handler works from a CLI, MCP server, HTTP API, or programmatic import
For simple CLIs where the "handler" is just calling a library, this separation already exists naturally — your library returns data, your CLI formats it. No extra layers needed.
If a project already uses hexagonal architecture, the CLI entry point commonly acts as a driving adapter and delegates to application behavior. Do not introduce that architecture merely to obtain testable handlers.
For TypeScript implementation patterns (Result types, entry point wiring, formatters, logger interfaces), see resources/output-architecture.md.
Format Flag Contract
Choose the smallest output contract the CLI needs. A common hierarchy is:
Default: Human-Readable
Colors, tables, formatted text
Progress bars and spinners on stderr
Output tailored for terminal width
May change between versions — this is not a contract
--plain: Grep/Awk-Friendly
One record per line, no formatting, no colors
Stable when documented for scripts; otherwise direct automation to structured output
Flat table rows, no borders, no grouped sections
Enables: mycli list --plain | grep error | wc -l
"Encourage your users to use --plain or --json in scripts to keep output stable." — clig.dev
--json: Structured Data
stdout contains ONLY valid JSON — no spinners, no color, no progress
stderr continues normally — human diagnostics still visible
Define and test a machine-readable error contract as well as success output
Treat documented fields and shapes as versioned public API
--json implies non-interactive regardless of TTY
Choose one canonical flag, such as --format json; keep --json only as a documented alias when useful. A consistent envelope is one valid design:
A direct resource or array can be a better JSON success shape when it composes more naturally. Whichever shape is chosen, specify where machine-readable failures are written. A practical default is one structured error object on stderr with a non-zero exit code; an established envelope-on-stdout contract is also valid if diagnostics remain separate.
NDJSON for Streaming
For large datasets, use NDJSON (one JSON object per \n):
Each line is independently parseable
Include a type field per record for multiplexing events
Final line can be a summary record
Enables: mycli run --format ndjson | while read -r line; do ...; done
For NDJSON specification details, see resources/stream-contracts.md.
Exit Codes
Code
Meaning
When
0
Success
Operation completed as expected
1
Domain failure
Tool-specific failure (e.g. quality threshold not met)
2
Invalid usage
Bad flags, missing required args, validation error
75
Temporary failure (optional)
Network timeout, service unavailable — retry may help
78
Configuration error (optional)
Invalid config file, missing required config
130
SIGINT
User pressed Ctrl-C (128 + 2)
143
SIGTERM
Process terminated (128 + 15)
Rules:
Non-zero exit code MUST have a stderr explanation
Document exit codes in --help
Never use codes above 125 for application errors (reserved for signals: 128 + signal number)
Start with portable 0, 1, and 2; add codes such as 75 and 78 only when consumers benefit and the supported platforms interpret them consistently
Map non-zero codes to the most important failure modes for your tool
TTY Detection
Check priority order (first match wins):
Priority
Condition
Effect
1
--format json or --json flag
Non-interactive, no color, no animation
2
--no-color flag
Disable color (output may still be interactive)
3
NO_COLOR env (non-empty)
Disable color
4
Project-supported force-color setting
Enable color even when not a TTY, without overriding an explicit no-color choice
5
TERM=dumb
Disable color and animations
6
CI=true
No interactive prompts
7
stdout is not a TTY (!isatty(stdout))
Plain output, no animations on stdout
8
Default
Full interactive with colors
Check stdout and stderr independently. When stdout is piped but stderr is a TTY, you can still show spinners on stderr while keeping stdout clean for the pipe consumer.
Optionally support MYCLI_NO_COLOR for app-specific color override.
Input Design
Flags Over Arguments
1 positional arg: acceptable (the "main thing")
2 positional args: reasonable when their order is conventional and memorable; otherwise consider flags
3+ positional args: usually hard to discover unless they are a same-type variadic list
Familiar idioms such as cp source dest and project-established command grammar are stronger evidence than a numeric rule
Flags are self-documenting, order-independent, and future-proof.
# Bad — which is source, which is destination?
mycli copy myapp backup
# Good — explicit
mycli copy --from myapp --to backup
Standard Flags
Always provide long forms. Short flags only for the most common operations.
Flag
Meaning
-h, --help
Show help (this should only mean help)
--version
Print version to stdout
-q, --quiet
Suppress non-essential output
-v, --verbose
More detail in human output
-d, --debug
Diagnostic output to stderr
-f, --force
Skip confirmation prompts
-n, --dry-run
Show what would happen without doing it
--json
Structured JSON output
--plain
Stable, grep-friendly plain text
--no-color
Disable color output
--no-input
Disable all prompts/interactivity
-o, --output
Output file
--fields
Select output columns
Prompts and Interactivity
All prompts MUST be bypassable via flags for scriptability
Confirmation → --yes or --force
Selection → --type=value
Text input → --name=value
Passwords → --password-file=path or stdin pipe
If stdin is not a TTY, never prompt — fail with a clear error or use defaults
Secrets: never via flag values (leak to ps output and shell history). Prefer, in order: OS keychain or a 0600 credential file (~/.config/mycli/credentials), then stdin (mycli login --with-token < token.txt), then env vars only where the platform injects them (CI secret stores) — env leaks to child processes and crash reports, so never make it the primary documented path
Scale confirmation to severity: mild → y/N prompt with --yes bypass; moderate → prompt plus suggest --dry-run first; severe/irreversible (delete a database, overwrite production) → require typing the resource name to confirm
Conventions
Support -- to stop flag parsing: mycli run -- --flag-for-child-process
Support - for stdin/stdout file arguments: curl ... | mycli process -
Accept both --flag=value and --flag value
If stdin is expected but is an interactive terminal, display help immediately (don't hang like cat)
What went wrong — context: which resource, operation, input
How to fix it — exact command or action the user should take
Reference — docs URL or mycli help <topic> (optional)
Human Mode
Error: CONFIG_MISSING — Configuration file not found
No configuration file found at ./mycli.config.ts or ~/.config/mycli/config.ts
Fix: Run `mycli init` to create a default configuration file
Docs: https://mycli.dev/docs/configuration
Put the most important information last (the eye is drawn to the end)
Use red sparingly and intentionally
Suggest corrections for typos ("Did you mean 'deploy'?")
Group similar errors under one header — don't repeat 50 similar-looking lines
Write debug logs to a file, not the terminal (unless --debug)
Structured Mode
Errors are structured too — not just success responses:
{"ok":false,"error":{"code":"CONFIG_MISSING","message":"No configuration file found at ./mycli.config.ts","fix":"Run `mycli init` to create a default configuration file","transient":false}}
The transient boolean tells retry logic whether the failure may be temporary. Emit the object on the documented machine-error stream; do not print a human error and JSON error into the same stream.
State Changes and Transparency
Confirm state changes — say what changed and show (or point to) the resulting state; traditionally-silent commands look broken to humans
Make current state easy to see — a status-style command for anything with complex state (the git status pattern)
Make hidden actions explicit — if you read/write files not passed as arguments or talk to remote servers, say so (stderr in human mode)
Page long output through $PAGER (e.g. less -FIRX) — only when stdout is a TTY, never when piped
Robustness
Validate input early — fail before any side effects, with a clear message
Responsiveness beats silence — benchmark startup and show progress on stderr for perceptibly long interactive work; do not emit progress for fast or piped operations
Timeouts on all network operations — configurable, with a documented transient-failure signal when retrying is safe
Recoverable by re-run — after a transient failure, re-running the command should resume or be safely idempotent
Crash-only design — exit fast on failure, defer cleanup to the next run; add timeouts to cleanup so Ctrl-C never hangs (second Ctrl-C skips cleanup) — see resources/stream-contracts.md
Expect misuse — script wrapping, bad connections, concurrent instances, environments you never tested
Composability Patterns
Design for real-world pipes: filtering with jq, streaming NDJSON line-by-line, feeding stdin, chaining commands through emitted identifiers, selecting columns with --fields, and fanning out with xargs -P.
See resources/composability.md for the worked shell examples covering each of these patterns.
Key patterns:
Create commands output identifiers so subsequent commands can chain
List commands support --fields for column selection (reduces output size, critical for agent efficiency)
--quiet for CI scripts that only care about the exit code
NDJSON for streaming large datasets without buffering everything in memory
--dry-run with --json outputs planned changes as structured data
CLI and TUI Composition
When a product has both a CLI and a TUI:
Keep domain operations behind one application layer; the CLI and TUI are separate presentation adapters.
Make every important TUI mutation available through a documented non-interactive command unless the action is inherently visual or continuous.
Do not make hidden --json behavior the only automation path. Expose stable commands and structured output directly.
Let the TUI launch explicitly through a command such as <tool> tui when the default invocation must remain safe for scripts. Preserve an established bare-command TUI when compatibility requires it.
Share authentication, configuration, validation, and error codes without sharing terminal rendering code.
Verify parity for permissions, validation, dry runs, destructive confirmations, and result semantics across both interfaces.
Use the tui-expert skill alongside this one for state, layout, keyboard, rendering, resizing, and terminal accessibility concerns.
Subcommand Design
Choose a grammar that matches user intent. Resource-oriented CLIs often use noun verb (mycli config set); action-oriented tools often use verb noun (git clone repo). Do not mix grammars arbitrarily.
Be consistent across all subcommands — same flag names for same things
No ambiguous pairs (update vs upgrade is confusing)
No catch-all subcommands (you can never add subcommands with conflicting names)
No arbitrary abbreviations — aliases must be explicit and stable
With no args: show help, status, a safe default, or an established TUI. Never block on a surprise prompt when stdin is not interactive.
Help
mycli --help — top-level help
mycli help <subcommand> — subcommand help
mycli <subcommand> --help — same as above
If run with missing required args, show concise help + 1-2 examples + "use --help for more"
Examples are the most-read section — lead with them
Include flag types, defaults, and allowed values for finite sets
Include a support/bug-report link in top-level help; pre-populate issue URLs with diagnostics where possible
Suggest the likely command on obvious typos ("Did you mean 'deploy'?") — ask, never auto-execute
Output Stability Contract
Stdout is a public API. Breaking changes to stdout format are breaking changes to the CLI.
Change
Impact
Adding new optional JSON fields
Safe (additive)
Adding new subcommands
Safe
Adding new flags with preserving defaults
Safe
Removing or renaming flags
Breaking
Removing or renaming JSON fields
Breaking
Changing exit codes
Breaking
Changing default behavior
Breaking
Changing human-readable output
Usually OK (not a contract)
When in doubt, add alongside — don't modify. Deprecate with stderr warnings before removing — and once you can detect that users have migrated, stop warning.
Naming, Distribution, Telemetry
Name: short, memorable, lowercase, easy to type; not so generic it collides with existing commands
Distribution: prefer a single binary; language-ecosystem tools (npm, pip) may reasonably assume their interpreter. Make uninstalling easy and documented
Telemetry: never collect usage/crash data without explicit consent — opt-in, stating what, why, and retention. Instrumented web docs or download counts are usually enough
Anti-Patterns
#
Anti-Pattern
Why It's Wrong
1
Mixing data and diagnostics on stdout
Breaks every pipe: mycli list | jq . fails if warnings are on stdout
Agents can't type 'y'. Every prompt needs --yes/--force. Non-TTY without bypass = hang
4
Ambiguous success behavior
Mutating human workflows often need confirmation; Unix-style checks and filters may intentionally succeed silently. Document the rule and offer quiet output where useful
5
Designing for humans OR machines, not both
Detect context (TTY vs pipe), adapt automatically
6
Output that doesn't guide the next action
Every output is a signpost: success = next command, failure = fix command
7
Breaking existing CLI contracts
Flag names, exit codes, output shape are contracts. Add alongside, never modify
8
console.log anywhere except the CLI adapter
Handlers must return data; only the presentation layer writes to streams
9
Handlers that exit the process directly
Let the entry point decide. Handlers return errors as data
10
Non-zero exit without stderr explanation
Scripts need both the code and the reason
11
Verbose default output
A single test run can generate 419KB. Support --fields, --quiet, --json
Verification Checklist
After designing or reviewing a CLI:
stdout has ONLY data; stderr has everything else
Commands that return data or plans expose an appropriate stable machine-readable format
Exit codes are semantic and documented in --help
Every prompt has a --yes/--force/--flag bypass
Errors include: code, message, fix suggestion
--dry-run available for mutating commands
Progress/spinners go to stderr, never stdout
NO_COLOR, TERM=dumb, and --no-color respected
Piped output contains zero ANSI escape codes
Success output includes next-action guidance
Existing flags, exit codes, output fields never removed or renamed
JSON schema is versioned (additions safe, removals breaking)
Secrets never via flags; keychain/credential-file/stdin preferred, env only when platform-injected (CI)
Severe destructive actions require typed confirmation (resource name), not just y/N
Network operations have configurable timeouts and a documented transient-failure signal
Startup and time-to-feedback have been measured against an explicit project budget
Ctrl-C exits fast with bounded cleanup
--help includes 2-3 realistic examples
Documented plain output is grep-parseable when the CLI promises a plain-text automation contract
CLI and TUI operations share semantics without coupling their rendering layers
Quick Reference
Stream routing, exit codes, and standard flags are tabled in the body — see "The Unix Stream Contract", "Exit Codes", and "Standard Flags" above.
Format Hierarchy
Default (TTY) → colors, tables, formatted text
--plain → one record per line, stable, grep-friendly
--json → structured JSON, versioned schema
--format ndjson → streaming, one JSON object per line
Configuration Order
flags > env vars > project config > user config > defaults
