fix(mcp): expand aliased enum values in react-docgen-typescript output

[keidy9]

Summary

Fixes a serializer bug in parseComponentDocLike where a prop typed as a named alias of a string-literal union (e.g. variant?: ButtonVariant) renders as the alias name instead of the resolved members.

When react-docgen-typescript is configured with shouldExtractLiteralValuesFromEnum: true, it produces:

type: {
  name: "enum",
  raw: "ButtonVariant",            // alias name
  value: [{value: '"primary"'}, {value: '"neutral"'}, ...]   // resolved literals
}

The previous code did prop.type?.raw ?? prop.type?.name and never read type.value, so consumers received the alias name with no member list. The bug doesn't surface for inline unions (variant?: "primary" | "secondary") because type.raw already holds the literals — which is exactly the shape used in packages/mcp/fixtures/button.fixture.json, so the existing tests passed.

The companion react-docgen path (serializeTsType) handles aliased unions correctly by walking elements; only the RDT path was broken.

Change

• New serializeRdtType helper: when type.name === "enum" and type.value is a non-empty array, join its members. Otherwise fall back to type.raw ?? type.name (preserving prior behavior for non-enum types and for enums with no value array).
• Renamed the existing "prefers type.raw over type.name for enums" test to "expands enum members for inline string-literal unions" — the snapshot is unchanged because for inline unions the joined members equal the raw string.
• Added a regression test covering the aliased-union case (type.raw === "ButtonVariant", type.value populated) — fails on main, passes here.
• Added a fallback test for an enum with type.raw but no type.value, asserting the alias name is still returned in that degraded shape.

Why

Reported by a user whose Button design-system component uses type ButtonVariant = "primary" | "neutral" | "danger" | "dangerSecondary" | "muted" | "link" | "custom". Storybook's Controls panel correctly populates the select dropdown from __docgenInfo.type.value, so the data is present in the bundle — only the MCP's prop renderer was dropping it. After this patch, the get-documentation output enumerates the variants the same way it does for inline unions like iconPosition?: "left" | "right".

Test plan

• pnpm vitest --project=@storybook/mcp run — 163/163 pass
• New tests cover: inline union (snapshot unchanged), aliased union (regression), enum with raw but no value (fallback)
• Reviewer: confirm no other call site reads type.raw directly and would break with the now-expanded form

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: be1b897

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@storybook/mcp	Patch
@storybook/addon-mcp	Patch

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[keidy9]

Hi @JReinhold, would you mind reviewing please?

[pkg-pr-new]

npm i https://pkg.pr.new/storybookjs/mcp/@storybook/addon-mcp@222

npm i https://pkg.pr.new/storybookjs/mcp/@storybook/mcp@222

commit: a56859f

[Copilot]

Pull request overview

Fixes @storybook/mcp’s react-docgen-typescript (RDT) prop type serialization so string-literal unions referenced via named aliases (e.g. variant?: ButtonVariant) render expanded union members instead of only the alias name, aligning MCP output with what Storybook Controls already derives from __docgenInfo.

Changes:

• Add serializeRdtType to expand enum-named RDT types using type.value when present, with fallback to prior raw/name behavior.
• Update and extend unit tests to cover inline unions, aliased unions (regression), and the degraded “no value array” fallback case.
• Add a changeset for the user-facing patch release.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/mcp/src/utils/parse-react-docgen.ts	Introduces serializeRdtType and routes RDT parsing through it to expand aliased enum/union members.
packages/mcp/src/utils/parse-react-docgen.test.ts	Renames the existing enum test and adds regression + fallback coverage for aliased unions and missing type.value.
.changeset/fix-rdt-aliased-enum-values.md	Declares a patch release and documents the behavior change for consumers.

[keidy9]

Hi @kasperpeulen, could you review please?