`HTTPClientTransport` fails to build on Windows: `#if !os(Linux)` guards no longer match the `EventSource` platform condition

[ArmanS201]

Summary

On Windows, building any target that depends on MCP fails with:

error: no such module 'EventSource'
Sources/MCP/Base/Transports/HTTPClientTransport.swift:5:12

The source guards in HTTPClientTransport.swift and the EventSource dependency condition in Package.swift disagree about Windows, so the platform takes the Apple code path without the Apple dependency being linked.

Cause

Package.swift links EventSource on Apple platforms only:

.product(name: "EventSource", package: "eventsource",
         condition: .when(platforms: [.macOS, .iOS, .tvOS, .visionOS, .watchOS, .macCatalyst]))

HTTPClientTransport.swift still gates its usage on Linux instead:

#if !os(Linux)
    import EventSource
#endif

On Windows, !os(Linux) evaluates to true, so the import is compiled, but the product was never linked. The mirrored #if os(Linux) blocks that provide the fallback SSE path evaluate to false, so Windows gets neither path.

There are 5 such guards, all in Sources/MCP/Base/Transports/HTTPClientTransport.swift (lines 4, 269, 307, 470, 566).

Likely regression

This looks like an unintended consequence of #143 (merged 2025-08-10), which moved the platform logic out of Package.swift into an SPM .when(platforms:) condition. That PR changed Package.swift only; the #if !os(Linux) guards in the source were left as they were.

Before that change both sides keyed off the same !os(Linux) condition, so they could not drift. After it, Package.swift says "Apple only" while the source says "not Linux", and Windows sits in the gap.

Reproduction

Any package depending on MCP, built on Windows with Swift 6.3.2. Observed on windows-latest (GitHub Actions), swift-sdk 0.12.1:

swift build -c release

Suggested fix

Key both sides off availability rather than a platform denylist:

#if canImport(EventSource)
    import EventSource
#endif

and invert the paired #if os(Linux) blocks to #if !canImport(EventSource). That keeps Linux behaviour identical and makes Windows take the same fallback path Linux already takes, without enumerating platforms in two places that can drift again.

Note

Linux is unaffected: its guards are consistent on both sides. This is Windows-only.