Core: Point the tools CLI at a private composed Storybook's own MCP

[kasperpeulen]

What I did

Linear: SB-2125

Since #36428, storybook tools docs lists composed refs. A private ref, such as a private Chromatic project, answers 401, and the CLI has no credentials. The agent got a dead end:

# Tetra (private)
id: tetra

error: Failed to get component manifest: Failed to fetch manifest: 401 Unauthorized

The decision is that the CLI does not get its own login. Private and hosted Storybooks are read over MCP. This is a regression: the deprecated storybook ai CLI already gave the right answer for this case. This PR brings that notice back, with the same text:

# Tetra (private)
id: tetra

This composed Storybook is private and cannot be read through the local Storybook MCP proxy.

Use this source's own MCP endpoint instead:
https://main--69b825e208f8a441cf8ff5fe.chromatic.com/mcp

History: why this is a regression

1. May 2026, Handle private composed sources as own-MCP notices mcp#242 ("Handle private composed sources as own-MCP notices"). storybook ai reached the dev server's /mcp through @storybook/mcp-proxy, which marked its requests with X-Storybook-MCP-Proxy: true. For those requests addon-mcp skipped the OAuth challenge and threw RequiresOwnMcpError for a private ref. That rendered as the notice above.

2. July 2026, product decision (#sb-skills-and-plugins). The CLI is for the local Storybook. Remote and private Storybooks go over MCP: that Storybook's own /mcp, or the Chromatic MCP. So the CLI should never get credentials, only point the agent at the right MCP.

3. August 2026, the move to shared core toolsets (Skills M4: Run addon-mcp and @storybook/mcp on the shared core toolsets #35677 / Skills M4: Run addon-mcp and @storybook/mcp on the shared core toolsets #35858) and to storybook tools (Skills M5a: CLI: Add the public storybook tools command derived at runtime from the OSA toolsets #35850, CLI plugins: Delegate to storybook skills / storybook tools and align the evals #35957). The proxy bypass did not survive:

   • On next, once a ref requires auth, addon-mcp's requireAuth answers 401 to every unauthenticated request. The proxy header no longer bypasses it.
   • X-Storybook-MCP-Proxy is read only for telemetry.
   • RequiresOwnMcpError, its formatter and the notice rendering all survived in core, but nothing threw the error any more.

   The exact commit is inferred from the PR scope, not bisected.

4. September 2026, Core: List composed refs in the tools CLI docs toolset #36428 (SB-2122). storybook tools docs started listing composed refs. A private ref's 401 then surfaced as the bare error above. That made the gap visible on the plugin path, whose skills route every docs lookup through the CLI.

This PR throws the existing error again at the one place the CLI reads refs, so the existing rendering takes over.

Changes:

• sourceUrlManifestProvider throws RequiresOwnMcpError on a 401. This provider is used only by the composition that common-preset registers for the tools CLI, in both local and attached mode. docs list already turns that error into the notice section.
• docs show and docs show-story treat the error as an answer, the same way @storybook/mcp already does. They print the notice and return ok, so the CLI exits 0, does not report a crash to telemetry, and --json carries the text in a new notice field.
• RequiresOwnMcpError.message is now the notice text itself, so every place that prints the error shows the same notice.

The CLI treats a 401 on the ref's components manifest as the signal. addon-mcp also probes the ref's /mcp. A non-Storybook auth proxy in front of a ref would therefore point the agent at a /mcp that may not exist. That is acceptable: such a ref cannot be read without credentials either way.

MCP behaviour is unchanged:

• addon-mcp still handles private refs with OAuth and its own manifest provider.
• @storybook/mcp still returns the same non-error notice when an embedder's provider throws RequiresOwnMcpError. Its existing tests pass unchanged.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

Caution

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

1. yarn && yarn nx compile react-vite
2. Create a throwaway project under code/, for example code/.private-ref/:
   • .storybook/main.ts:

     export default {
       framework: '@storybook/react-vite',
       stories: ['../src/*.stories.tsx'],
       refs: {
         tetra: { title: 'Tetra (private)', url: 'https://main--69b825e208f8a441cf8ff5fe.chromatic.com' },
       },
     };

   • src/Button.tsx and src/Button.stories.tsx with any small component and story.
   • The ref is a private Chromatic project. Without a login it answers 401 with WWW-Authenticate.
3. cd code && node core/dist/bin/dispatcher.js tools --no-attach --config-dir .private-ref/.storybook docs list. The Tetra (private) section shows the own-MCP notice with …chromatic.com/mcp, and the Local section still lists Button.
4. node core/dist/bin/dispatcher.js tools --no-attach --config-dir .private-ref/.storybook docs show --id anything --storybookId tetra prints the same notice and exits with code 0. Add --json to see it in the notice field.
5. Delete code/.private-ref/ afterwards. It is not gitignored.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Declare whether manual QA will be needed for this PR during the next release, through qa:needed or qa:skip

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

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Walkthrough

When a source manifest fetch returns HTTP 401, the provider raises RequiresOwnMcpError for that source. Documentation show and showStory handlers return the formatted MCP endpoint notice as a successful result when source access raises this error.

Changes

Private source MCP notice

Layer / File(s)	Summary
Handle unauthorized source manifests code/core/src/shared/open-service/toolsets/docs/access-provider.ts, code/core/src/shared/open-service/toolsets/docs/sources.ts, code/core/src/shared/open-service/toolsets/docs/access-provider.test.ts	For HTTP 401 responses, the manifest provider raises RequiresOwnMcpError with the source. The error message uses formatRequiresOwnMcpNotice, and a test checks that the listing notice points to the source’s /mcp endpoint.
Return notices from documentation show handlers code/core/src/shared/open-service/toolsets/docs/definition.ts, code/core/src/shared/open-service/toolsets/docs/definition.test.ts	The show and showStory handlers convert RequiresOwnMcpError into successful results with a notice. Output types, resolution, rendering, and tests support this behavior.

Sequence Diagram(s)

sequenceDiagram
  participant ShowHandler as show or showStory handler
  participant SourceLookup as resolveFromSource
  participant SourceAccess as source access method
  ShowHandler->>SourceLookup: resolve selected source
  SourceLookup->>SourceAccess: list or resolve source
  SourceAccess-->>SourceLookup: RequiresOwnMcpError
  SourceLookup-->>ShowHandler: notice message
  ShowHandler-->>ShowHandler: render successful notice result

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Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to cb433

Private composed Storybook refs that return 401 now produce a notice pointing to their own MCP endpoint, in both docs show and show-story. The only open item is a minor test-style concern, so the change is low risk to merge.

Security Architecture Review

Security architecture risk: 🔵 Low · up to cb433

Private-source access remains denied. The new response provides endpoint guidance rather than logging in, forwarding credentials, or connecting automatically. No introduced security weakness was established, but authentication at the advertised endpoint and behavior of external consumers were not fully assessed.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The assessed exposure change is endpoint guidance reaching existing documentation-tool callers for configured composed sources. The inspected handlers and MCP adapter do not turn that guidance into additional network access or credential authority. A downstream agent may independently act on the guidance; that behavior was not verified here.

Trust Boundaries and Controls

• observed — The source URL provider ignores the incoming request and adds no authentication headers in the changed fetch path. Returning a successful notice does not bypass the remote source's HTTP 401. Hosted access still receives request context through its existing per-request construction.
• observed — Manifest URL construction retains its existing concatenation behavior rather than resolving remote manifest references as arbitrary URLs. The PR changes handling after a response is received, not the fetch destination construction. This counterevidence does not establish the security of every configured URL or redirect.

Resilience and Maintainability Implications

• observed — Listing outcomes remain isolated by source, and the two top-level manifest fetches settle before their results are classified. The new handler catch returns a notice without recording an authenticated state or scheduling recovery work. Other exceptions are rethrown; repeated provider calls fetch anew rather than reusing a newly introduced authorization cache.

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Review comments at
@code/core/src/shared/open-service/toolsets/docs/access-provider.test.ts:
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