diff --git a/clang/include/clang/Basic/DiagnosticOptions.def b/clang/include/clang/Basic/DiagnosticOptions.def
index ba8ce8f2e6d4f..11047a36b869d 100644
--- a/clang/include/clang/Basic/DiagnosticOptions.def
+++ b/clang/include/clang/Basic/DiagnosticOptions.def
@@ -101,6 +101,9 @@ DIAGOPT(ShowSafeBufferUsageSuggestions, 1, 0)
 // TO_UPSTREAM(BoundsSafety)
 DIAGOPT(BoundsSafetyAdoptionMode, 1, 0)
 
+// TO_UPSTREAM(CAS)
+DIAGOPT(FallbackRealFileSystemAbsolutePaths, 1, 0)
+
 #undef DIAGOPT
 #undef ENUM_DIAGOPT
 #undef VALUE_DIAGOPT
diff --git a/clang/lib/Frontend/CompileJobCache.cpp b/clang/lib/Frontend/CompileJobCache.cpp
index 82e7b56260f39..05d152ce9ad83 100644
--- a/clang/lib/Frontend/CompileJobCache.cpp
+++ b/clang/lib/Frontend/CompileJobCache.cpp
@@ -328,6 +328,9 @@ std::optional<int> CompileJobCache::initialize(CompilerInstance &Clang) {
     return Error::success();
   };
 
+  // Ensure path canonicalization in text diagnostics.
+  Invocation.getDiagnosticOpts().FallbackRealFileSystemAbsolutePaths = true;
+
   llvm::PrefixMapper PrefixMapper;
   llvm::SmallVector<llvm::MappedPrefix> Split;
   llvm::MappedPrefix::transformPairs(CacheOpts.PathPrefixMappings, Split);
@@ -638,6 +641,9 @@ Expected<std::optional<int>> CompileJobCache::replayCachedResult(
 
   assert(!Clang.getDiagnostics().hasErrorOccurred());
 
+  // Ensure path canonicalization in text diagnostics.
+  Clang.getDiagnosticOpts().FallbackRealFileSystemAbsolutePaths = true;
+
   std::optional<llvm::cas::CASID> MCOutputID;
   ObjectStoreCachingOutputs CachingOutputs(
       Clang, WorkingDir, std::move(PrefixMapper), WriteOutputAsCASID,
diff --git a/clang/lib/Frontend/TextDiagnostic.cpp b/clang/lib/Frontend/TextDiagnostic.cpp
index e6f592f9ca0c3..007c5167fdb44 100644
--- a/clang/lib/Frontend/TextDiagnostic.cpp
+++ b/clang/lib/Frontend/TextDiagnostic.cpp
@@ -16,7 +16,10 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/IOSandbox.h"
 #include "llvm/Support/Locale.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include <algorithm>
 #include <optional>
 
@@ -813,9 +816,7 @@ void TextDiagnostic::printDiagnosticMessage(raw_ostream &OS,
 }
 
 void TextDiagnostic::emitFilename(StringRef Filename, const SourceManager &SM) {
-#ifdef _WIN32
-  SmallString<4096> TmpFilename;
-#endif
+  SmallString<256> TmpFilename;
   if (DiagOpts.AbsolutePath) {
     auto File = SM.getFileManager().getOptionalFileRef(Filename);
     if (File) {
@@ -843,6 +844,18 @@ void TextDiagnostic::emitFilename(StringRef Filename, const SourceManager &SM) {
       Filename = SM.getFileManager().getCanonicalName(*File);
 #endif
     }
+
+    // TO_UPSTREAM(CAS)
+    if (DiagOpts.FallbackRealFileSystemAbsolutePaths && !Filename.empty() &&
+        !llvm::sys::path::is_absolute(Filename)) {
+      // When caching, the above may fail to canonicalize due to using the
+      // IncludeTreeFileSystem. In that case, we can safely fallback to the real
+      // filesytem since the text diagnostic output is not captured directly.
+      auto BypassSandbox = llvm::sys::sandbox::scopedDisable();
+      auto FS = llvm::vfs::getRealFileSystem();
+      if (FS->getRealPath(Filename, TmpFilename) == std::error_code())
+        Filename = TmpFilename;
+    }
   }
 
   OS << Filename;
diff --git a/clang/test/CAS/include-tree-fdiagnostics-absolute-paths.c b/clang/test/CAS/include-tree-fdiagnostics-absolute-paths.c
new file mode 100644
index 0000000000000..9a2911c112d4b
--- /dev/null
+++ b/clang/test/CAS/include-tree-fdiagnostics-absolute-paths.c
@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas %clang-cache \
+// RUN:   %clang subdir/tu.c -I. -fdiagnostics-absolute-paths -E \
+// RUN:   -Rcompile-job-cache > %t/stdout-miss 2> %t/stderr-miss
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=PP -input-file=%t/stdout-miss
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=DIAG -input-file=%t/stderr-miss
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=MISS -input-file=%t/stderr-miss
+
+// Again, but with a cache hit.
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas %clang-cache \
+// RUN:   %clang subdir/tu.c -I. -fdiagnostics-absolute-paths -E \
+// RUN:   -Rcompile-job-cache > %t/stdout-hit 2> %t/stderr-hit
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=PP -input-file=%t/stdout-hit
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=DIAG -input-file=%t/stderr-hit
+// RUN: FileCheck %s -DPREFIX=%/t -check-prefix=HIT -input-file=%t/stderr-hit
+
+// Preprocessor does not force absolute paths.
+// PP: # 1 "subdir/tu.c"
+// PP: # 1 "./h1.h"
+// PP: const char *filename = "./h1.h";
+
+// Diagnostics force absolute paths.
+// DIAG: [[PREFIX]]/subdir/tu.c:1:2: warning: "from tu"
+// DIAG: In file included from [[PREFIX]]/subdir/tu.c
+// DIAG: [[PREFIX]]/h1.h:1:2: warning: "from h1"
+
+// MISS: remark: compile job cache miss
+// HIT: remark: compile job cache hit
+
+//--- subdir/tu.c
+#warning "from tu"
+#include "h1.h"
+
+//--- h1.h
+#warning "from h1"
+const char *filename = __FILE__;
diff --git a/clang/test/CAS/libclang-replay-job.c b/clang/test/CAS/libclang-replay-job.c
index e81c07bd94add..1ba1ad6f7bf9c 100644
--- a/clang/test/CAS/libclang-replay-job.c
+++ b/clang/test/CAS/libclang-replay-job.c
@@ -78,6 +78,25 @@
 // RUN: diff -u %t/t1.d %t/a/b/rel.d
 // FIXME: Get clang's `-working-directory` to affect relative path for serialized diagnostics.
 
+// Check with -fdiagnostics-absolute-path
+// RUN: cd %t
+// RUN: c-index-test core -replay-cached-job -cas-path %t/cas @%t/cache-key \
+// RUN:   -fcas-plugin-path %llvmshlibdir/libCASPluginTest%pluginext \
+// RUN:   -fcas-plugin-option no-logging \
+// RUN:   -working-dir %t \
+// RUN: -- @%t/cc1.rsp \
+// RUN:   -fdiagnostics-absolute-paths \
+// RUN:   -Rcompile-job-cache-hit \
+// RUN:   -dependency-file %t/t3.d -o %t/output3.o 2> %t/output3.txt
+// RUN: FileCheck %s -input-file=%t/output2.txt -check-prefix=REL_DIAG
+// RUN: FileCheck %s -input-file=%t/output3.txt -check-prefix=ABS_DIAG
+// RUN: diff %t/output1.o %t/output3.o
+// RUN: diff -u %t/t1.d %t/t3.d
+// REL_DIAG: remark: compile job cache hit for
+// REL_DIAG: {{^}}main.c:1:2: warning:
+// ABS_DIAG: remark: compile job cache hit for
+// ABS_DIAG: libclang-replay-job.c.tmp{{.}}main.c:1:2: warning:
+
 // Use relative path to inputs and outputs.
 //--- cdb.json.template
 [{
diff --git a/clang/test/Modules/feature-availability.c b/clang/test/Modules/feature-availability.c
index e21522facc9d7..d9fdd1120d49a 100644
--- a/clang/test/Modules/feature-availability.c
+++ b/clang/test/Modules/feature-availability.c
@@ -1,3 +1,4 @@
+// RUN: rm -rf %t
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules %S/Inputs/feature-availability/module.modulemap -fmodule-name=Feature1 -emit-module -o %t/feature1.pcm
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules %S/Inputs/feature-availability/module.modulemap -fmodule-name=Feature2 -fmodule-file=%t/feature1.pcm -emit-module -o %t/feature2.pcm
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules -fmodule-file=%t/feature2.pcm -I %S/Inputs/feature-availability -emit-llvm -o - %s | FileCheck %s