eventsource maps its `bun` export condition to the ESM build, and Bun resolves
`bun` before `require`. So a CJS require("eventsource") under Bun receives an
async ESM module and throws "require() async module ... is unsupported". The
package ships a real CJS build behind `require`, but Bun never reaches it.
Two CJS consumers in our graph hit this: the MCP SDK's own dist/cjs/client/sse.js,
and @ag-ui/mcp-apps-middleware, which requires that SDK path unconditionally at
module load. It surfaced as an intermittent failure of the runtime bun
integration job -- intermittent because it is a load-order race, where the run
only passes if the ESM graph happens to evaluate eventsource first.
Dropping the `bun` key makes Bun fall through to `import` for ESM consumers
(same file as before) and `require` for CJS consumers (the CJS build they need).
Takes the bun integration test from 5/20 to 20/20 locally. A version bump is not
an alternative: eventsource 4.1.0 still ships the same mapping.
Jordan Ritter
6a8dc4ec50