diff --git a/Sources/SKCore/BuildServerBuildSystem.swift b/Sources/SKCore/BuildServerBuildSystem.swift
index dcf189b2..fba894d7 100644
--- a/Sources/SKCore/BuildServerBuildSystem.swift
+++ b/Sources/SKCore/BuildServerBuildSystem.swift
@@ -221,6 +221,26 @@ extension BuildServerBuildSystem: BuildSystem {
   }
 
   public func filesDidChange(_ events: [FileEvent]) {}
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    guard let fileUrl = uri.fileURL else {
+      return .unhandled
+    }
+
+    // FIXME: We should not make any assumptions about which files the build server can handle.
+    // Instead we should query the build server which files it can handle (#492).
+    let path = AbsolutePath(fileUrl.path)
+    if projectRoot.isAncestorOfOrEqual(to: path) {
+      return .handled
+    }
+
+    let realpath = resolveSymlinks(path)
+    if realpath != path, projectRoot.isAncestorOfOrEqual(to: realpath) {
+      return .handled
+    }
+
+    return .unhandled
+  }
 }
 
 private func loadBuildServerConfig(path: AbsolutePath, fileSystem: FileSystem) throws -> BuildServerConfig {
diff --git a/Sources/SKCore/BuildSystem.swift b/Sources/SKCore/BuildSystem.swift
index e0725f1b..094d368c 100644
--- a/Sources/SKCore/BuildSystem.swift
+++ b/Sources/SKCore/BuildSystem.swift
@@ -14,6 +14,18 @@ import BuildServerProtocol
 import LanguageServerProtocol
 import TSCBasic
 
+/// Defines how well a `BuildSystem` can handle a file with a given URI.
+public enum FileHandlingCapability: Comparable {
+  /// The build system can't handle the file at all
+  case unhandled
+
+  /// The build system has fallback build settings for the file
+  case fallback
+
+  /// The build system knows how to handle the file
+  case handled
+}
+
 /// Provider of FileBuildSettings and other build-related information.
 ///
 /// The primary role of the build system is to answer queries for
@@ -62,6 +74,8 @@ public protocol BuildSystem: AnyObject {
 
   /// Called when files in the project change.
   func filesDidChange(_ events: [FileEvent])
+
+  func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability
 }
 
 public let buildTargetsNotSupported =
diff --git a/Sources/SKCore/BuildSystemManager.swift b/Sources/SKCore/BuildSystemManager.swift
index 5f53a50e..84bc2d70 100644
--- a/Sources/SKCore/BuildSystemManager.swift
+++ b/Sources/SKCore/BuildSystemManager.swift
@@ -348,6 +348,10 @@ extension BuildSystemManager: BuildSystem {
       }
     }
   }
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    return max(buildSystem?.fileHandlingCapability(for: uri) ?? .unhandled, fallbackBuildSystem?.fileHandlingCapability(for: uri) ?? .unhandled)
+  }
 }
 
 extension BuildSystemManager: BuildSystemDelegate {
diff --git a/Sources/SKCore/CompilationDatabaseBuildSystem.swift b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
index dcd7b8cf..1bb46fc6 100644
--- a/Sources/SKCore/CompilationDatabaseBuildSystem.swift
+++ b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
@@ -187,6 +187,19 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
       }
     }
   }
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    guard let fileUrl = uri.fileURL else {
+      return .unhandled
+    }
+    return queue.sync {
+      if database(for: fileUrl) != nil {
+        return .handled
+      } else {
+        return .unhandled
+      }
+    }
+  }
 }
 
 extension CompilationDatabaseBuildSystem {
diff --git a/Sources/SKCore/FallbackBuildSystem.swift b/Sources/SKCore/FallbackBuildSystem.swift
index 8faf8d3e..ceb6cade 100644
--- a/Sources/SKCore/FallbackBuildSystem.swift
+++ b/Sources/SKCore/FallbackBuildSystem.swift
@@ -100,4 +100,8 @@ public final class FallbackBuildSystem: BuildSystem {
   }
 
   public func filesDidChange(_ events: [FileEvent]) {}
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    return .fallback
+  }
 }
diff --git a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
index 6b083a4a..eb00ce45 100644
--- a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
+++ b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
@@ -361,6 +361,19 @@ extension SwiftPMWorkspace: SKCore.BuildSystem {
       }
     }
   }
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    guard let fileUrl = uri.fileURL else {
+      return .unhandled
+    }
+    return self.queue.sync {
+      if targetDescription(for: AbsolutePath(fileUrl.path)) != nil {
+        return .handled
+      } else {
+        return .unhandled
+      }
+    }
+  }
 }
 
 extension SwiftPMWorkspace {
diff --git a/Sources/SourceKitLSP/SourceKitServer.swift b/Sources/SourceKitLSP/SourceKitServer.swift
index 718d14d3..84f2db6a 100644
--- a/Sources/SourceKitLSP/SourceKitServer.swift
+++ b/Sources/SourceKitLSP/SourceKitServer.swift
@@ -49,11 +49,7 @@ public final class SourceKitServer: LanguageServer {
     return documentManager
   }
 
-  private var workspaces: [Workspace] = [] {
-    didSet {
-      assert(workspaces.count <= 1, "Multiple workspaces aren't supported yet")
-    }
-  }
+  private var workspaces: [Workspace] = []
 
   // **Public for testing**
   public var _workspaces: [Workspace] {
@@ -81,7 +77,22 @@ public final class SourceKitServer: LanguageServer {
   }
 
   public func workspaceForDocument(uri: DocumentURI) -> Workspace? {
-    return workspaces.first
+    if workspaces.count == 1 {
+      // Special handling: If there is only one workspace, open all files in it.
+      // This retains the behavior of SourceKit-LSP before it supported multiple workspaces.
+      return workspaces.first
+    }
+
+    // Pick the workspace with the best FileHandlingCapability for this file.
+    // If there is a tie, use the workspace that occurred first in the list.
+    var bestWorkspace: (workspace: Workspace?, fileHandlingCapability: FileHandlingCapability) = (nil, .unhandled)
+    for workspace in workspaces {
+      let fileHandlingCapability = workspace.buildSystemManager.fileHandlingCapability(for: uri)
+      if fileHandlingCapability > bestWorkspace.fileHandlingCapability {
+        bestWorkspace = (workspace, fileHandlingCapability)
+      }
+    }
+    return bestWorkspace.workspace
   }
 
   public override func _registerBuiltinHandlers() {
diff --git a/Tests/SKCoreTests/BuildSystemManagerTests.swift b/Tests/SKCoreTests/BuildSystemManagerTests.swift
index ea1d56ab..5ad31b49 100644
--- a/Tests/SKCoreTests/BuildSystemManagerTests.swift
+++ b/Tests/SKCoreTests/BuildSystemManagerTests.swift
@@ -505,6 +505,14 @@ class ManualBuildSystem: BuildSystem {
   }
 
   func filesDidChange(_ events: [FileEvent]) {}
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    if map[uri] != nil {
+      return .handled
+    } else {
+      return .unhandled
+    }
+  }
 }
 
 /// A `BuildSystemDelegate` setup for testing.
diff --git a/Tests/SourceKitLSPTests/BuildSystemTests.swift b/Tests/SourceKitLSPTests/BuildSystemTests.swift
index 9ae9ccdb..fbaadc83 100644
--- a/Tests/SourceKitLSPTests/BuildSystemTests.swift
+++ b/Tests/SourceKitLSPTests/BuildSystemTests.swift
@@ -67,6 +67,14 @@ final class TestBuildSystem: BuildSystem {
   }
 
   func filesDidChange(_ events: [FileEvent]) {}
+
+  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
+    if buildSettingsByFile[uri] != nil {
+      return .handled
+    } else {
+      return .unhandled
+    }
+  }
 }
 
 final class BuildSystemTests: XCTestCase {