diff --git a/Sources/Common/SourceCode.swift b/Sources/Common/SourceCode.swift
index ee46294..55435d6 100644
--- a/Sources/Common/SourceCode.swift
+++ b/Sources/Common/SourceCode.swift
@@ -85,7 +85,7 @@ public struct SourceManager {
 
     let fm = FileManager()
     for path in self.paths {
-      let combined = path.pushing(file)
+      let combined = path.pushing(file).lexicallyNormalized()
       if fm.fileExists(atPath: combined.string) {
         return combined
       }
@@ -247,14 +247,24 @@ public struct SourceCode {
 }
 
 func do_preprocess(
-  _ file: URL, _ manager: SourceManager, _ starting: Int = 0
+  _ file: FilePath, _ manager: SourceManager, _ starting: Int = 0
 ) -> Result<(FileSourceLocation, String)> {
   // First (1) match group has the name of the include file.
   let re = /\#include[\s]*<([^\s>]*)>/
 
   do {
     var locations: [FileSourceLocation] = Array()
-    var contents = try String.init(contentsOf: file, encoding: String.defaultCStringEncoding)
+
+    // First, try to find the file.
+    guard let included_path = manager.firstExisting(file) else {
+      return .Error(
+        Error(
+          withMessage:
+            "Could not open \(file) for preprocessing"))
+    }
+
+    var contents = try String.init(
+      contentsOf: URL(filePath: included_path.string), encoding: String.defaultCStringEncoding)
     var changed = true
 
     while changed {
@@ -264,20 +274,9 @@ func do_preprocess(
         let before = contents[..<match.range.lowerBound]
         let after = contents[match.range.upperBound...]
 
-        // Determine whether there is a file with that name in the include path.
-        guard let included_path = manager.firstExisting(FilePath.init("\(match.1)")) else {
-          return .Error(
-            Error(
-              withMessage:
-                "Could not open \(match.1) for inclusion"))
-        }
-
-        // Try to make a url from the configured file.
-        let included_url = URL.init(filePath: included_path.string)
-
         // By calling ourselves recursively, the include being processed will
         // be _completely_ expanded (including any nested includes).
-        switch do_preprocess(included_url, manager, starting + before.count) {
+        switch do_preprocess(FilePath("\(match.1)"), manager, starting + before.count) {
         case .Ok((let location, let expanded)):
           // Recombine what was before and after the include being processed
           // with the expanded text.
@@ -298,7 +297,7 @@ func do_preprocess(
     return .Ok(
       (
         FileSourceLocation(
-          SourceLocation(starting..<(starting + contents.count)), FilePath(file.path()), locations),
+          SourceLocation(starting..<(starting + contents.count)), file, locations),
         contents
       ))
 
@@ -316,22 +315,7 @@ public struct SourceCodePreprocessor {
   }
 
   public func preprocess(_ file: FilePath) -> Result<SourceCode> {
-    // First, decide whether the given file exists at the path the user gave.
-    let fm = FileManager()
-    let file_to_open =
-      if !fm.fileExists(atPath: file.string) {
-        self.manager.firstExisting(file)
-      } else {
-        file
-      }
-
-    guard let file_to_open else {
-      return .Error(Error(withMessage: "Could not open \(file) for preprocessing"))
-    }
-
-    let url = URL.init(filePath: file_to_open.string)
-
-    switch do_preprocess(url, self.manager) {
+    switch do_preprocess(file, self.manager) {
     case .Ok((let location, let contents)):
       return .Ok(SourceCode(contents, self.manager, location))
     case .Error(let e): return .Error(e)
diff --git a/TestData/simple_cli_compilation_error.golden b/TestData/simple_cli_compilation_error.golden
index c847013..8751785 100644
--- a/TestData/simple_cli_compilation_error.golden
+++ b/TestData/simple_cli_compilation_error.golden
@@ -1,3 +1,3 @@
-Error: In /Users/hawkinsw/code/p4ce/TestData/Sources/action-parameters-wrong-order.p4, there was an error: 
+Error: In action-parameters-wrong-order.p4, there was an error: 
 ...ion a(inout bool aa, int ax, inout bool ay) {...
 All parameters with direction must precede directionless parameters
diff --git a/Tests/p4rseTests/SupportTests/SourceCode.swift b/Tests/p4rseTests/SupportTests/SourceCode.swift
index f8e66a8..95cf602 100644
--- a/Tests/p4rseTests/SupportTests/SourceCode.swift
+++ b/Tests/p4rseTests/SupportTests/SourceCode.swift
@@ -31,57 +31,49 @@ import TreeSitterP4
 @Test func test_preprocessor() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/simple.p4")
-  let expected_file = FilePath.init(FileManager().currentDirectoryPath + "/" + file.string)
-    .lexicallyNormalized()
+  let file = FilePath.init(stringLiteral: "simple.p4")
 
   let source = try! (#UseOkResult(prep.preprocess(file)))
   let program = try! #UseOkResult(Program.Compile(source.getSource()))
   #expect(#RequireOkResult((program.find_parser(withName: Identifier(name: "main_parser")))))
-  #expect(source.getLocations().getPath() == expected_file)
+  #expect(source.getLocations().getPath() == file)
 }
 
 @Test func test_preprocessor_search_for_file() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
   let file = FilePath.init(stringLiteral: "simple.p4")
-  let expected_file = FilePath.init(
-    FileManager().currentDirectoryPath + "/TestData/Sources/simple.p4")
 
   let source = try! (#UseOkResult(prep.preprocess(file)))
   let program = try! #UseOkResult(Program.Compile(source.getSource()))
   #expect(#RequireOkResult((program.find_parser(withName: Identifier(name: "main_parser")))))
-  #expect(source.getLocations().getPath() == expected_file)
+  #expect(source.getLocations().getPath() == file)
 }
 
 @Test func test_preprocessor_nested_includes() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/simple-split.p4")
-  let expected_file = FilePath.init(FileManager().currentDirectoryPath + "/" + file.string)
-    .lexicallyNormalized()
+  let file = FilePath.init(stringLiteral: "simple-split.p4")
 
   #expect(#RequireOkResult(prep.preprocess(file)))
 
   let source = try! (#UseOkResult(prep.preprocess(file)))
   let program = try! #UseOkResult(Program.Compile(source.getSource()))
   #expect(#RequireOkResult((program.find_parser(withName: Identifier(name: "main_parser")))))
-  #expect(source.getLocations().getPath() == expected_file)
+  #expect(source.getLocations().getPath() == file)
 }
 
 @Test func test_preprocessor_oneline_includes() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/simple-split-oneline.p4")
-  let expected_file = FilePath.init(FileManager().currentDirectoryPath + "/" + file.string)
-    .lexicallyNormalized()
+  let file = FilePath.init(stringLiteral: "simple-split-oneline.p4")
 
   #expect(#RequireOkResult(prep.preprocess(file)))
 
   let source = try! (#UseOkResult(prep.preprocess(file)))
   let program = try! #UseOkResult(Program.Compile(source.getSource()))
   #expect(#RequireOkResult((program.find_parser(withName: Identifier(name: "main_parser")))))
-  #expect(source.getLocations().getPath() == expected_file)
+  #expect(source.getLocations().getPath() == file)
 }
 
 @Test func test_preprocessor_missing_file() async throws {
@@ -99,11 +91,11 @@ import TreeSitterP4
 @Test func test_preprocessor_missing_included_file() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/simple-unfound.p4")
+  let file = FilePath.init(stringLiteral: "simple-unfound.p4")
 
   #expect(
     #RequireErrorResult(
-      Error(withMessage: "Could not open unfound.p4 for inclusion"), (prep.preprocess(file))))
+      Error(withMessage: "Could not open unfound.p4 for preprocessing"), (prep.preprocess(file))))
 
 }
 
@@ -131,7 +123,7 @@ import TreeSitterP4
 @Test func test_preprocessor_oneline_includes_locations() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/simple-split-oneline.p4")
+  let file = FilePath.init(stringLiteral: "simple-split-oneline.p4")
 
   #expect(#RequireOkResult(prep.preprocess(file)))
 
@@ -145,7 +137,7 @@ import TreeSitterP4
 @Test func test_preprocessor_nested_includes_locations() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/nested-split.p4")
+  let file = FilePath.init(stringLiteral: "nested-split.p4")
 
   #expect(#RequireOkResult(prep.preprocess(file)))
 
@@ -163,7 +155,7 @@ import TreeSitterP4
 @Test func test_preprocessor_nested_includes_annotated_source() async throws {
   let sm = SourceManager(["./TestData/Sources/"])
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/annotate.p4")
+  let file = FilePath.init(stringLiteral: "annotate.p4")
 
   let expected = """
     <struct Testing {
@@ -192,15 +184,13 @@ import TreeSitterP4
 @Test func test_preprocessor_nested_includes_get_file_location() async throws {
   let sm = SourceManager(["./TestData/Sources/"], FileManager())  // Add a FileManager to get absolute paths.
   let prep = SourceCodePreprocessor(sm)
-  let file = FilePath.init(stringLiteral: "./TestData/Sources/file-loc.p4")
+  let file = FilePath.init(stringLiteral: "file-loc.p4")
 
   let source = try! (#UseOkResult(prep.preprocess(file)))
 
-  let expected_file = FilePath.init(FileManager().currentDirectoryPath + "/" + file.string)
-    .lexicallyNormalized()
-  let expected_nested_file = sm.firstExisting("file-loc-parser.p4")!.lexicallyNormalized()
-  let expected_nested_nested_file = sm.firstExisting("file-loc-parser-state.p4")!
-    .lexicallyNormalized()
+  let expected_file = file
+  let expected_nested_file = FilePath(stringLiteral: "file-loc-parser.p4")
+  let expected_nested_nested_file = FilePath(stringLiteral: "file-loc-parser-state.p4")
 
   let found_file = try! #require(source.pathForLocation(0))
   let found_nested_file = try! #require(source.pathForLocation(55))