# Add Stage Field

> Use when adding a new parameter or field to the Stage struct in stage/stage.go, to ensure all required locations are updated

- Skill: `prestodb/add-stage-field` (Agent Skill)
- Install (CLI): `npx skillmds@latest add prestodb/add-stage-field`
- Raw SKILL.md: https://api.skillmd.com/api/skills/prestodb/add-stage-field/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: prestodb (https://skillmd.com/u/prestodb)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/prestodb/add-stage-field

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# Add Stage Field

When adding a new field to the `Stage` struct, you must update multiple locations. Missing any causes silent bugs (fields not inherited, not merged, missing defaults).

## Checklist

Update these locations in order:

1. **`stage/stage.go` — `Stage` struct**: Add the field with a `json:"field_name,omitempty"` tag. Use pointer types (`*bool`, `*int`, `*string`) for optional fields that participate in inheritance (nil = "not set, inherit from parent").

2. **`stage/stage_utils.go` — `MergeWith()`**: Add merge logic. For pointer fields: `if other.Field != nil { s.Field = other.Field }`. For slices: `s.Field = append(s.Field, other.Field...)`. For maps: iterate and merge key-by-key (nil value = delete key).

3. **`stage/stage_utils.go` — `setDefaults()`**: Set a default value if the field needs one (e.g., `false` for bool flags, `0` for counts). Only needed for fields that must have a non-nil value before execution.

4. **`stage/stage_utils.go` — `propagateStates()`**: Propagate to child stages if the field should be inherited. Pattern: `if nextStage.Field == nil { nextStage.Field = s.Field }`. Skip this if the field is stage-local only.

5. **`stage/stage.go` — `newStreamInstance()`**: Copy the field if it's relevant to stream (concurrent) execution. Most execution-related fields should be copied here.

6. **Wiki: [Parameters](https://github.com/prestodb/pbench/wiki/Parameters)**: Document the new parameter with its JSON key, type, default, and description.

7. **Wiki: [Configuring PBench - Inherited Parameters](https://github.com/prestodb/pbench/wiki/Configuring-PBench#inherited-parameters-in-stage-files)**: Add to the inherited list if propagated in step 4.

## Verification

After implementation, grep to confirm the field name appears in all required locations:

```bash
grep -n 'FieldName' stage/stage.go stage/stage_utils.go
```

Ensure the count matches expectations (struct definition + MergeWith + setDefaults if needed + propagateStates if inherited + newStreamInstance if stream-relevant).