Features | Fern Documentation

Early access

The CLI generator is in early access. Reach out to get started.

Generated CLIs ship with a common set of runtime flags and environment variables: output formatting, pagination, dry-run previewing, TLS and proxy configuration, exit codes, and structured logging. APIs with templated server URLs also expose per-variable flags and environment variables.

Output formatting

Use the --format flag to control how responses are displayed.

Format	Flag	Description
JSON	`--format json`	Default. Pretty-printed JSON.
Table	`--format table`	Columnar table. Nested objects flatten to `parent.child` column names.
YAML	`--format yaml`	YAML representation of the response.
CSV	`--format csv`	Comma-separated values, suitable for piping into spreadsheet tools.

$ # Display plants in a table
$ contoso plants list --format table
$ 
$ # Export orders as CSV
$ contoso orders list --format csv > orders.csv

Dry-run mode

Pass --dry-run to validate arguments and preview the HTTP request without sending it. The CLI prints the method, URL, headers, and body it would send, then exits.

$ contoso plants get --params '{"plantId": "abc"}' --dry-run

Pagination

For endpoints annotated with x-fern-pagination, the CLI auto-paginates when the --page-all flag is set.

Flag	Description	Default
`--page-all`	Fetch every page and emit one JSON line per page (NDJSON).	Off
`--page-limit <N>`	Maximum number of pages to fetch.	10
`--page-delay <MS>`	Delay in milliseconds between page requests.	100

$ # Fetch all plants, one JSON line per page
$ contoso plants list --page-all
$ 
$ # Limit to 5 pages with a 200 ms delay
$ contoso plants list --page-all --page-limit 5 --page-delay 200

Paginated output works with all output formats. For table and CSV formats, headers are only emitted on the first page so the output concatenates cleanly.

Passing parameters

Flag	Purpose
`--params <JSON>`	URL path and query parameters as a JSON object.
`--json <JSON>`	Request body for POST, PUT, and PATCH methods.

$ # Path + query parameters
$ contoso plants get --params '{"plantId": "abc"}'
$ 
$ # Request body
$ contoso plants create \
>   --json '{"name": "Monstera", "species": "Monstera deliciosa", "sunlight": "indirect"}'

Server URL variables

For APIs with templated server URLs (such as https://api.example.com/stores/{store_hash}/v3), the CLI automatically exposes each template variable as a CLI flag and environment variable.

$ # Pass the variable as a flag
$ bigcommerce --store-hash abc123 v3 customers list
$ 
$ # Or set it via an environment variable
$ export BIGCOMMERCE_STORE_HASH=abc123
$ bigcommerce v3 customers list

File uploads and downloads

For endpoints with format: binary request bodies, pass a file path as the --file argument. For binary responses, use --output <PATH> to save the response body to a file.

Exit codes

Code	Meaning	Example cause
`0`	Success	Command completed normally.
`1`	API error	Server returned a 4xx/5xx response.
`2`	Auth error	Credentials missing or invalid.
`3`	Validation error	Bad arguments, unknown command, or invalid JSON.
`4`	Discovery error	Couldn’t load API schema.
`5`	Internal error	Unexpected failure.

All errors are emitted as structured JSON on stderr, making them easy to parse in scripts and CI pipelines.

TLS, proxies, and CA bundles

Every generated CLI honors environment variables for TLS and proxy configuration at runtime. Variables are scoped by binary name — <NAME> is the CLI’s binary name uppercased with hyphens mapped to underscores (for example, CONTOSO).

Variable	Effect
`<NAME>_CA_BUNDLE`	Path to a PEM file appended to the default trust roots.
`<NAME>_INSECURE=1`	Disable TLS verification. Logs a warning. Not for production use.
`<NAME>_PROXY`	HTTP/HTTPS proxy URL, overriding `HTTPS_PROXY` / `HTTP_PROXY`.
`<NAME>_NO_PROXY`	Comma-separated proxy bypass list scoped to this CLI.
`<NAME>_TIMEOUT_SECS`	Total request timeout. None by default.
`<NAME>_CONNECT_TIMEOUT_SECS`	Connection-establishment timeout.

Standard environment variables (HTTPS_PROXY, HTTP_PROXY, NO_PROXY, SSL_CERT_FILE) are honored when the scoped overrides are absent.

Common scenarios

Behind a MITM proxy (Proxyman, Charles, mitmproxy):

$ export SSL_CERT_FILE=~/path/to/proxyman-ca.pem
$ export HTTPS_PROXY=http://127.0.0.1:9090
$ contoso plants list

Corporate network with a custom root CA:

$ export CONTOSO_CA_BUNDLE=/etc/ssl/corp-roots.pem
$ contoso plants list

Structured logging

Logging is off by default. Set <NAME>_LOG to a tracing filter to emit structured logs to stderr. Set <NAME>_LOG_FILE to a directory path to write daily rotated JSON log files.

$ CONTOSO_LOG=debug contoso plants list