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v2 tsdown fix - #1840
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felixweinberger
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Apr 1, 2026
@claude review |
@modelcontextprotocol/client@modelcontextprotocol/server@modelcontextprotocol/express@modelcontextprotocol/fastify@modelcontextprotocol/hono@modelcontextprotocol/nodecommit: |
🦋 Changeset detectedLatest commit: fbdd299 The changes in this PR will be included in the next version bump. This PR includes changesets to release 6 packages
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felixweinberger
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verified the exports are fixed by this locally.
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LGTM — the core/public path mapping fix correctly restores the missing type exports, and failOnWarn: 'ci-only' is a sensible guard against silent DTS failures going forward.
Extended reasoning...
Overview
This PR modifies six tsdown.config.ts build config files (server, client, and four middleware packages). The substantive fix adds a @modelcontextprotocol/core/public subpath entry to the dts.compilerOptions.paths override in server and client, restoring ~270 type re-exports that were previously silently dropped. The secondary change adds failOnWarn: 'ci-only' to all six configs.
Security risks
None. These are build-time TypeScript compiler configuration files with no runtime security surface.
Level of scrutiny
Low. Changes are confined to build configuration, not runtime logic. The root cause is well-explained (paths override replaces rather than merges with tsconfig paths, so the /public subpath was unmapped), and the fix is the minimal correct addition to address it. The failOnWarn addition is mechanical and clearly beneficial.
Other factors
A pre-existing bug in the four middleware configs (wrong relative depth in path mappings — ../server/ instead of ../../server/) is flagged by the inline comment but was not introduced by this PR. Per the bug report, those paths are currently dead no-ops that TypeScript silently ignores, so they do not regress functionality. The primary fix has been validated by the author with measurable output (39 → 310 exports). All tests and typechecks pass.
| import { defineConfig } from 'tsdown'; | ||
| export default defineConfig({ | ||
| failOnWarn: 'ci-only', | ||
| // 1. Entry Points | ||
| // Directly matches package.json include/exclude globs | ||
| entry: ['src/index.ts'], |
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🟣 Pre-existing bug: all four middleware tsdown configs have incorrect relative paths in dts.compilerOptions.paths that resolve to non-existent directories. In packages/middleware/node/tsdown.config.ts, ../core/src/index.ts (with baseUrl: ".") resolves to packages/middleware/core/src/index.ts which does not exist — the correct path is ../../core/src/index.ts. Similarly, express/fastify/hono use ../server/src/index.ts which resolves to the non-existent packages/middleware/server/src/index.ts instead of packages/server/src/index.ts (../../server/src/index.ts). This PR did not introduce the bug but touches all four files by adding failOnWarn; the path mappings are currently dead no-ops since TypeScript silently falls back to node_modules resolution.
Extended reasoning...
What the bug is and how it manifests
All four middleware tsdown configs (packages/middleware/node, packages/middleware/express, packages/middleware/fastify, packages/middleware/hono) contain path mappings in dts.compilerOptions.paths that point to non-existent locations. The paths use a single ../ prefix, but these packages sit two levels deep in the monorepo (packages/middleware/<pkg>/), so they require ../../ to reach sibling top-level packages.
The specific code path that triggers it
In packages/middleware/node/tsdown.config.ts, baseUrl: "." resolves to packages/middleware/node/. Therefore ../core/src/index.ts resolves to packages/middleware/core/src/index.ts. No such directory exists — core lives at packages/core/. The correct relative path is ../../core/src/index.ts. The same depth error affects express, fastify, and hono: ../server/src/index.ts resolves to packages/middleware/server/src/index.ts (non-existent) instead of packages/server/src/index.ts (requires ../../server/src/index.ts).
Why existing code does not prevent it
For contrast, packages/server/tsdown.config.ts correctly uses ../core/src/index.ts because it lives at packages/server/ — only one level deep — so ../core correctly resolves to packages/core/. The middleware packages copied this pattern without adjusting for their extra directory nesting. TypeScript does not emit a warning or error when a paths mapping points to a non-existent file; it silently skips that alternative and falls back to standard node_modules resolution. This is why builds currently succeed and failOnWarn: ci-only (added by this PR) will not surface the issue.
Impact
The path mappings are dead no-ops. The DTS bundler resolves @modelcontextprotocol/core and @modelcontextprotocol/server from installed node_modules rather than from local source. In a workspace with workspace:* protocol this keeps versions in sync, so type output is currently correct. However, this is exactly the class of bug this PR is trying to fix for server and client — using installed package types instead of local source can cause stale or incomplete type exports if the installed package has not been rebuilt after source changes.
How to fix it
Change ../core/src/index.ts → ../../core/src/index.ts in packages/middleware/node/tsdown.config.ts, and change ../server/src/index.ts → ../../server/src/index.ts in packages/middleware/express/tsdown.config.ts, packages/middleware/fastify/tsdown.config.ts, and packages/middleware/hono/tsdown.config.ts.
Step-by-step proof
- Config file location:
packages/middleware/node/tsdown.config.ts baseUrl: "."→ resolves to directorypackages/middleware/node/- Path entry:
@modelcontextprotocol/core → ["../core/src/index.ts"] - Resolution:
packages/middleware/node/+../core/src/index.ts=packages/middleware/core/src/index.ts packages/middleware/core/does not exist in the repository- TypeScript silently falls back;
@modelcontextprotocol/coreis resolved fromnode_modulesinstead - Correct path:
packages/middleware/node/+../../core/src/index.ts=packages/core/src/index.ts✓
The refutation notes this overlaps with bug_002; both describe the same underlying pattern, but bug_003 covers all four middleware packages comprehensively and the fix is concrete and actionable regardless of deduplication.
fix(build): restore missing
core/publictype re-exports in built packagesMotivation and Context
The tsdown DTS bundler was silently dropping all ~270 type re-exports from
@modelcontextprotocol/core/public(includingTool,CallToolResult,Resource,ServerCapabilities, etc.) in the builtdist/index.d.mtsof both@modelcontextprotocol/serverand@modelcontextprotocol/client.Root cause: The tsdown
dts.compilerOptions.pathsoverride inserver/tsdown.config.tsandclient/tsdown.config.tsonly mapped@modelcontextprotocol/corebut not@modelcontextprotocol/core/public. Since this override replaces (not merges with) the tsconfig paths, the DTS resolver couldn't resolve the/publicsubpath import, causingexport * from '@modelcontextprotocol/core/public'to be silently dropped. The build emitted a warning ("Failed to emit declaration file") but continued, producing packages missing most of their public API types.How Has This Been Tested?
pnpm build:all— builds cleanly with no warnings (previously emitted "Failed to emit declaration file" warning)Tooland othercore/publictypes appear indist/index.d.mtsexport list for both server and client (went from 39 to 310 exports in the server package)pnpm check:all— typecheck + lint passpnpm test:all— all tests passBreaking Changes
None. This is a bug fix — it restores types that were intended to be exported but were missing from the built output.
Types of changes
Checklist
Additional context
Two changes per config:
Bug fix — Added the missing
@modelcontextprotocol/core/publicpath mapping todts.compilerOptions.pathsinserver/tsdown.config.tsandclient/tsdown.config.ts.Prevention — Added
failOnWarn: 'ci-only'to all 6 tsdown configs (server, client, and 4 middleware packages) so that DTS warnings like this will fail CI builds in the future instead of silently producing incomplete packages.