claude_design built-in MCP server always 404s: /v1/design/mcp uses login-OAT, not the designOauth token from /design login (v2.1.181)

Bug: built-in claude_design MCP server always fails with 404 — /v1/design/mcp is authenticated with the login-OAT (no user:design:* scopes) instead of the designOauth token created by /design login

Environment

• Claude Code: 2.1.181 (native install, linux-x64, commit 2b2bfc5e2472)
• Node: v24.3.0
• Account: Claude Max (subscription OAuth login)
• Feature gate: tengu_omelette_whisk = true → claude_design auto-registered (scope "dynamic"); env CLAUDE_CODE_ENABLE_DESIGN_MCP unset

Summary

The built-in first-party MCP server claude_design (https://api.anthropic.com/v1/design/mcp) never connects. /doctor reports:

claude_design: failed — Streamable HTTP error: Error POSTing to endpoint: 404 page not found

Running /design login succeeds and stores a valid, design-scoped token, but the connector still 404s on every relaunch. The login token the transport actually sends is the wrong one.

Steps to reproduce

1. Claude Max account with the design feature enabled (so claude_design is auto-injected as a dynamic first-party server).
2. Launch claude → /doctor → "1 MCP server not connected: claude_design … 404 page not found".
3. Run /design login, authorize in browser, complete the paste-code flow. It reports success; ~/.claude/.credentials.json gains a designOauth block with scopes user:design:read, user:design:write.
4. Relaunch claude → /doctor still shows claude_design failed with the same 404.

Root cause

• /v1/design/ is on FIRST_PARTY_MCP_PATH_PREFIXES, so the HTTP transport uses "login-OAT auto-attach" — it attaches the main claudeAiOauth access token.
• The main login token does not carry user:design:* scopes, and the main login flow does not request them (design scopes are a separate scope set).
• /design login stores the design-scoped token in a separate top-level designOauth credential block, which the transport never reads for this server.
• Net: the POST to /v1/design/mcp goes out with a token that lacks design scopes (effectively unauthenticated for this endpoint) → 404.

Evidence

1. Post-login MCP debug log (~/.cache/claude-cli-nodejs/<project>/mcp-logs-claude-design/*.jsonl), recorded after a successful /design login:

HTTP transport options: {"url":"https://api.anthropic.com/v1/design/mcp",
  "headers":{"User-Agent":"claude-code/2.1.181 (sdk-cli)","Accept-Encoding":"identity"},
  "hasAuthProvider":false,"timeoutMs":60000}
Testing basic HTTP connectivity to https://api.anthropic.com/v1/design/mcp
HTTP Connection failed after 192ms: Streamable HTTP error: Error POSTing to endpoint: 404 page not found (code: 404)

Note "hasAuthProvider":false — no auth provider was wired to the server.

2. Direct request with the designOauth access token (the one /design login created) → HTTP 200:

POST https://api.anthropic.com/v1/design/mcp   (Authorization: Bearer <designOauth.accessToken>)
{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-06-18",
  "capabilities":{"prompts":{},"tools":{"listChanged":true}},
  "serverInfo":{"name":"Claude Design","version":"0.1.0"}}}

3. Direct request with the main claudeAiOauth access token (what the CLI auto-attaches) → HTTP 404 "page not found".

4. Main login token scopes (after /design login, unchanged): user:file_upload, user:inference, user:mcp_servers, user:profile, user:sessions:claude_code — no user:design:*.
designOauth scopes: user:design:read, user:design:write.

5. The CLI binary ships a guard string acknowledging this exact inconsistency:

"A built-in first-party MCP server URL is not on the first-party allowlist
 (FIRST_PARTY_MCP_PATH_PREFIXES / isFirstPartyAnthropicHost) — login-OAT
 auto-attach and bare-name rendering would not fire. Update firstPartyBuiltins.ts
 or authState.ts so they agree."

Expected behavior

For /v1/design/ first-party URLs the transport should attach the designOauth token (the credential with user:design:* scopes produced by /design login) — OR /design login should mint/refresh the login-OAT to include user:design:* so the existing login-OAT auto-attach succeeds. Today the two halves disagree, so the connector can never authenticate.

Impact

Persistent "1 setup issue: MCP" on every launch for Claude Max users who have the design feature flag on. /design login appears to succeed but never clears the failure, and there is no user-side configuration that makes the built-in connector authenticate.

claude mcp add --scope user --transport http claude-design https://api.anthropic.com/v1/design/mcp