diff --git a/src/c_import.test.zig b/src/c_import.test.zig
new file mode 100644
index 0000000..e889b89
--- /dev/null
+++ b/src/c_import.test.zig
@@ -0,0 +1,68 @@
+const std = @import("std");
+const c_import = @import("c_import.zig");
+
+const SRC = "int f(void) { return 1; }";
+const HDR = "int f(void);";
+const VER = "19.1.7";
+
+const none: []const []const u8 = &.{};
+
+fn baseKey() u64 {
+    return c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+}
+
+test "cSourceCacheKey: stable when nothing changes" {
+    try std.testing.expectEqual(baseKey(), baseKey());
+}
+
+test "cSourceCacheKey: source bytes vary the key" {
+    const other = c_import.cSourceCacheKey("int f(void) { return 2; }", &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: declared header content varies the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{"int f(void); int g(void);"}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: defines vary the key (value and order)" {
+    const v2 = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=2"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != v2);
+
+    const ab = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{ "A=1", "B=1" }, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    const ba = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{ "B=1", "A=1" }, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(ab != ba);
+}
+
+test "cSourceCacheKey: flags vary the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O3"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: a define is not a flag (same string, different role)" {
+    const as_define = c_import.cSourceCacheKey(SRC, none, &.{"X"}, none, none, VER, null, null);
+    const as_flag = c_import.cSourceCacheKey(SRC, none, none, &.{"X"}, none, VER, null, null);
+    try std.testing.expect(as_define != as_flag);
+}
+
+test "cSourceCacheKey: include dirs vary the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"other"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: llvm version varies the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, "20.0.0", "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: triple and sysroot vary the key; absent is not empty" {
+    const other_triple = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "x86_64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other_triple);
+
+    const other_sysroot = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/ndk");
+    try std.testing.expect(baseKey() != other_sysroot);
+
+    const absent = c_import.cSourceCacheKey(SRC, none, none, none, none, VER, null, null);
+    const empty = c_import.cSourceCacheKey(SRC, none, none, none, none, VER, "", "");
+    try std.testing.expect(absent != empty);
+}
diff --git a/src/c_import.zig b/src/c_import.zig
index 82cf1d7..79319f1 100644
--- a/src/c_import.zig
+++ b/src/c_import.zig
@@ -33,6 +33,50 @@ pub const CImportInfo = struct {
     flags: []const []const u8,
 };
 
+/// Cache key for one compiled `#source` member. Everything that can
+/// change the produced object participates: the source bytes, the
+/// unit's declared `#include` headers BY CONTENT (editing a declared
+/// header invalidates), defines / flags / include dirs in declaration
+/// order, the LLVM version, and the cross-target (triple + sysroot).
+/// Section tags keep equal strings in different roles distinct (a
+/// define never aliases a flag, an absent triple never aliases an
+/// empty one). Transitive includes of the .c itself do NOT
+/// participate — the block's declared surface is the invalidation
+/// boundary.
+pub fn cSourceCacheKey(
+    source_bytes: []const u8,
+    header_bytes: []const []const u8,
+    defines: []const []const u8,
+    flags: []const []const u8,
+    include_dirs: []const []const u8,
+    llvm_version: []const u8,
+    triple: ?[]const u8,
+    sysroot: ?[]const u8,
+) u64 {
+    const Wyhash = std.hash.Wyhash;
+    var key = Wyhash.hash(0, "sx-c-import-v1");
+    key = Wyhash.hash(key, source_bytes);
+    key = Wyhash.hash(key, "\x01headers");
+    for (header_bytes) |hb| key = Wyhash.hash(key, hb);
+    key = Wyhash.hash(key, "\x01defines");
+    for (defines) |d| key = Wyhash.hash(key, d);
+    key = Wyhash.hash(key, "\x01flags");
+    for (flags) |f| key = Wyhash.hash(key, f);
+    key = Wyhash.hash(key, "\x01incdirs");
+    for (include_dirs) |inc| key = Wyhash.hash(key, inc);
+    key = Wyhash.hash(key, "\x01llvm");
+    key = Wyhash.hash(key, llvm_version);
+    if (triple) |t| {
+        key = Wyhash.hash(key, "\x01triple");
+        key = Wyhash.hash(key, t);
+    }
+    if (sysroot) |sr| {
+        key = Wyhash.hash(key, "\x01sysroot");
+        key = Wyhash.hash(key, sr);
+    }
+    return key;
+}
+
 /// Handle returned from loadCObjectsForJIT — caller must call unload() after JIT.
 pub const CImportHandle = struct {
     dylib_handle: ?*anyopaque = null,
diff --git a/src/root.zig b/src/root.zig
index 767d19f..a1f6774 100644
--- a/src/root.zig
+++ b/src/root.zig
@@ -16,6 +16,7 @@ pub const imports = @import("imports.zig");
 pub const imports_tests = @import("imports.test.zig");
 pub const core = @import("core.zig");
 pub const c_import = @import("c_import.zig");
+pub const c_import_tests = @import("c_import.test.zig");
 pub const ir = @import("ir/ir.zig");
 
 pub const lsp = struct {