Flag Default Value

flag-default-value

Set the default value of a GrowthBook feature flag. The default value is what the SDK returns when no rules match a user — it's the flag's baseline state, not a targeting rule. Changes go through a draft revision and require publishing before they take effect.

All API calls go through the bundled helper: ${CLAUDE_PLUGIN_ROOT}/scripts/gb-call. It needs GB_API_KEY set in env or written to ~/.config/growthbook/.env by /growthbook:setup.

Required inputs

• Flag ID — kebab-case key. Use flag-search to resolve if the user gives a description.
• New default value — must match the flag's valueType. Always serialized as a string.

Workflow

1. Fetch the flag

gb-call GET /api/v2/features/

Capture valueType and current defaultValue. Show the user the current default so they can confirm the change.

2. Validate the new value

Pre-validate before sending — the API does not catch type mismatches at write time:

| valueType | Valid defaultValue | | --- | --- | | boolean | "true" or "false" (string, not JSON bool) | | number | Any string that parses as a number: "0", "3.14" | | json | Any valid JSON string: "{\"key\":\"value\"}" | | string | Any non-empty string |

If the user's value doesn't match, halt and ask them to fix it. Do not silently coerce.

3. Apply the change via a draft

echo '{"defaultValue":""}' \
  | gb-call PUT /api/v2/features//revisions/new/default-value -

The new magic version creates a fresh draft or layers onto an existing one atomically. Capture the returned version number.

4. Offer to publish

Ask: "Publish this default-value change now, or leave it as a draft?"

Changing the default value affects every user who matches no rules — this is potentially high-impact. Before publishing, surface the behavior change clearly:

> Changing the default from ` to will affect all users not matched by any rule. In that's currently rule(s) — users falling through all of them will now get instead of `.

Hand off to flag-publish for the publish step.

Guardrails

• Draft version threading. If a version number is already in context from a previous write skill in this session, use it explicitly (e.g. .../revisions/42/default-value) instead of new. This keeps all changes in the same draft. Fall back to new when starting fresh — it auto-creates or reuses the most recently updated open draft.
• defaultValue is always a string, regardless of valueType. The API rejects non-string values. "false" for boolean off, "0" for numeric zero, "{}" for an empty JSON object.
• Default value ≠ a force rule. The default value applies only when all rules fail to match. If the user wants to serve a specific value to targeted users, they want flag-targeting, not this skill.
• This is potentially high-traffic impact. The default value applies to every unmatched user in every enabled environment. Changing it from the "off" state (e.g., "false" → "true") can expose a feature to production traffic if there are no rules. Surface this before publishing.
• Changing default value for a boolean flag is equivalent to a global toggle when there are no targeting rules. Confirm the user understands this.
• Type mismatches surface at SDK evaluation time, not at write time. The v2 default-value endpoint accepts any string. A number flag with defaultValue: "not-a-number" won't error until clients evaluate it.

Endpoints used

• GET /api/v2/features/:id — fetch flag to get current defaultValue and valueType
• PUT /api/v2/features/:id/revisions/new/default-value (body: { "defaultValue": "" })

Handoffs

• flag-search — if the user gives a description instead of a flag ID
• flag-targeting — to add or edit rules that serve specific values to targeted users
• flag-cleanup — if the flag is no longer needed and should be archived
• flag-publish — to publish the draft, handle approval-required (400) and merge conflicts (409)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: growthbook
• Source: growthbook/skills
• License: MIT

Install and usage instructions live in the source repository linked above.