What causes "this feature is not supported" in direnv/direnv?

Use `direnv export json` to get JSON output; only real shells support hook

this feature is not supported

Error message

this feature is not supported

What it means

The JSON 'shell' is a serialization backend (pretty-printed JSON export) for editors and external tools, not an interactive shell, so it cannot generate a hook script. Hook() on jsonShell always returns this error.

Source

Thrown at internal/cmd/shell_json.go:16

package cmd

import (
	"encoding/json"
	"errors"
)

// jsonShell is not a real shell
type jsonShell struct{}

// JSON is not really a shell but it fits. Useful to add support to editor and
// other external tools that understand JSON as a format.
var JSON Shell = jsonShell{}

func (sh jsonShell) Hook() (string, error) {
	return "", errors.New("this feature is not supported")
}

func (sh jsonShell) Export(e ShellExport) (string, error) {
	out, err := json.MarshalIndent(e, "", "  ")
	if err != nil {
		return "", err
	}
	return string(out), nil
}

func (sh jsonShell) Dump(env Env) (string, error) {
	out, err := json.MarshalIndent(env, "", "  ")
	if err != nil {
		return "", err
	}
	return string(out), nil
}

View on GitHub (pinned to b00e451f54)

Solutions

Use `direnv export json` to get JSON output; only real shells support hook
Hook a real shell: `eval "$(direnv hook bash)"` etc.
In tooling, guard: only call Hook for shells that support it (bash, zsh, fish, tcsh, elvish, etc.)

Example fix

// before
eval "$(direnv hook json)"
// after
eval "$(direnv export json)"

Defensive patterns

Strategy: type-guard

Validate before calling

if [ "$1" = "json" ]; then direnv export json; else eval "$(direnv hook "$1")"; fi

Type guard

func supportsHook(s cmd.Shell) bool {
    switch s.(type) {
    case cmd.JSON, cmd.GzEnv, cmd.Systemd, cmd.Vim:
        return false
    }
    return true
}

Prevention

Use `direnv export json` for JSON output, never `direnv hook json`
Treat JSON as an export format for tools, not a shell
Filter pseudo-shells before invoking Hook in generic tooling

When it happens

Trigger: Calling Hook() on direnv's JSON shell, e.g. `direnv hook json` or code that selects the json shell and invokes Hook.

Common situations: Users mistake `direnv export json` for `direnv hook json`; editor integrations that consume JSON export accidentally wire up hook; generic tooling calls Hook on every registered shell.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of direnv/direnv@b00e451f54 (2026-09-05). Data as JSON: /api/errors/bb5c71f2ddce3890. Report an issue: GitHub.