diff --git a/depend b/depend
index 8b764bd060c49c..c3252dd87e821b 100644
--- a/depend
+++ b/depend
@@ -11694,6 +11694,7 @@ prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/arena.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/ast.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/buffer.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/compiler/align.h
+prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/compiler/assume.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/compiler/exported.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/compiler/flex_array.h
 prism/diagnostic.$(OBJEXT): $(top_srcdir)/prism/compiler/force_inline.h
diff --git a/lib/prism/prism.gemspec b/lib/prism/prism.gemspec
index aac056b3f889e8..c18d68f4d8a5f9 100644
--- a/lib/prism/prism.gemspec
+++ b/lib/prism/prism.gemspec
@@ -48,6 +48,7 @@ Gem::Specification.new do |spec|
     "include/prism.h",
     "include/prism/compiler/accel.h",
     "include/prism/compiler/align.h",
+    "include/prism/compiler/assume.h",
     "include/prism/compiler/exported.h",
     "include/prism/compiler/fallthrough.h",
     "include/prism/compiler/filesystem.h",
diff --git a/prism/compiler/assume.h b/prism/compiler/assume.h
new file mode 100644
index 00000000000000..500e93e73cdc4d
--- /dev/null
+++ b/prism/compiler/assume.h
@@ -0,0 +1,25 @@
+/**
+ * @file compiler/assume.h
+ */
+#ifndef PRISM_COMPILER_ASSUME_H
+#define PRISM_COMPILER_ASSUME_H
+
+/**
+ * Tell the compiler that the given expression can be assumed to be true. Unlike
+ * assert, this emits no runtime check — it only feeds the optimizer's value
+ * range analysis so it can prune impossible paths. Use it to communicate an
+ * invariant the caller guarantees but that the compiler cannot otherwise prove.
+ */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L /* C23 or later */
+    #define PRISM_ASSUME(expr_) [[assume(expr_)]]
+#elif defined(__clang__)
+    #define PRISM_ASSUME(expr_) __builtin_assume(expr_)
+#elif defined(_MSC_VER)
+    #define PRISM_ASSUME(expr_) __assume(expr_)
+#elif defined(__GNUC__)
+    #define PRISM_ASSUME(expr_) ((expr_) ? (void) 0 : __builtin_unreachable())
+#else
+    #define PRISM_ASSUME(expr_) ((void) 0)
+#endif
+
+#endif
diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index 0dea7328694cf3..0683978084da04 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -1,5 +1,6 @@
 #include "prism/internal/diagnostic.h"
 
+#include "prism/compiler/assume.h"
 #include "prism/compiler/inline.h"
 
 #include "prism/internal/allocator.h"
@@ -446,6 +447,7 @@ pm_diagnostic_id_name(pm_diagnostic_id_t diag_id) {
 static PRISM_INLINE const char *
 pm_diagnostic_id_message(pm_diagnostic_id_t diag_id) {
     assert(diag_id < PM_DIAGNOSTIC_ID_MAX);
+    PRISM_ASSUME(diag_id < PM_DIAGNOSTIC_ID_MAX);
 
     const char *message = diagnostic_messages[diag_id].message;
     assert(message);
@@ -456,6 +458,7 @@ pm_diagnostic_id_message(pm_diagnostic_id_t diag_id) {
 static PRISM_INLINE uint8_t
 pm_diagnostic_id_level(pm_diagnostic_id_t diag_id) {
     assert(diag_id < PM_DIAGNOSTIC_ID_MAX);
+    PRISM_ASSUME(diag_id < PM_DIAGNOSTIC_ID_MAX);
 
     return (uint8_t) diagnostic_messages[diag_id].level;
 }