diff --git a/src/libcmd/common-eval-args.hh b/src/libcmd/common-eval-args.hh
index 6359b2579..08a4b65e4 100644
--- a/src/libcmd/common-eval-args.hh
+++ b/src/libcmd/common-eval-args.hh
@@ -28,6 +28,26 @@ private:
     std::map<std::string, std::string> autoArgs;
 };
 
-SourcePath lookupFileArg(EvalState & state, std::string_view s);
+/** @brief Resolve an argument that is generally a file, but could be something that
+ * is easy to resolve to a file, like a <lookup path> or a tarball URL.
+ *
+ * In particular, this will resolve and fetch pseudo-URLs starting with
+ * @c channel:, flakerefs starting with @c flake:, and anything that
+ * @ref nix::fetchers::downloadTarball() can take.
+ *
+ * Non-absolute files are looked up relative to the current directory(?)
+ * FIXME: the process's current directory or EvalState's current directory?
+ *
+ * @param state The nix::EvalState to base settings, store, and nixPath from.
+ *
+ * @param fileArg The the path-ish to resolve.
+ *
+ * @return A nix::SourcePath to the resolved and fetched file.
+ *
+ * @exception nix::FileTransferError from nix::fetchers::downloadTarball(). Probably others.
+ *
+ * @exception nix::ThrownError for failed search path lookup. Probably others.
+ */
+SourcePath lookupFileArg(EvalState & state, std::string_view fileArg);
 
 }
diff --git a/tests/unit/libcmd/args.cc b/tests/unit/libcmd/args.cc
new file mode 100644
index 000000000..da7ad598e
--- /dev/null
+++ b/tests/unit/libcmd/args.cc
@@ -0,0 +1,57 @@
+#include <iostream>
+#include <memory>
+#include <string_view>
+
+#include <boost/core/demangle.hpp>
+#include <gtest/gtest.h>
+
+#include "common-eval-args.hh"
+#include "eval.hh"
+#include "filetransfer.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "util.hh"
+
+constexpr std::string_view INVALID_CHANNEL = "channel:example";
+constexpr std::string_view CHANNEL_URL = "https://nixos.org/channels/example/nixexprs.tar.xz";
+
+namespace nix
+{
+
+TEST(Arguments, lookupFileArg) {
+    initNix();
+    initGC();
+
+    std::string const unitDataPath = getEnv("_NIX_TEST_UNIT_DATA").value();
+    // Meson should be allowed to pass us a relative path here tbh.
+    auto const canonDataPath = CanonPath::fromCwd(unitDataPath);
+
+    std::string const searchPathElem = fmt("example=%s", unitDataPath);
+
+    SearchPath searchPath;
+    searchPath.elements.push_back(SearchPath::Elem::parse(searchPathElem));
+
+    auto store = openStore("dummy://");
+    auto statePtr = std::make_shared<EvalState>(searchPath, store, store);
+    auto & state = *statePtr;
+
+    SourcePath const foundUnitData = lookupFileArg(state, "<example>");
+    EXPECT_EQ(foundUnitData.path, canonDataPath);
+
+    // lookupFileArg should not resolve <search paths> if anything else is before or after it.
+    SourcePath const yepEvenSpaces = lookupFileArg(state, " <example>");
+    EXPECT_EQ(yepEvenSpaces.path, CanonPath::fromCwd(" <example>"));
+    EXPECT_EQ(lookupFileArg(state, "<example>/nixos").path, CanonPath::fromCwd("<example>/nixos"));
+
+    try {
+        lookupFileArg(state, INVALID_CHANNEL);
+    } catch (FileTransferError const & ex) {
+        std::string_view msg(ex.what());
+        EXPECT_NE(msg.find(CHANNEL_URL), msg.npos);
+    }
+
+    SourcePath const normalFile = lookupFileArg(state, unitDataPath);
+    EXPECT_EQ(normalFile.path, canonDataPath);
+}
+
+}
diff --git a/tests/unit/meson.build b/tests/unit/meson.build
index f5355cce8..c53677d48 100644
--- a/tests/unit/meson.build
+++ b/tests/unit/meson.build
@@ -212,3 +212,30 @@ test(
   protocol : 'gtest',
   verbose : true,
 )
+
+libcmd_tester = executable(
+  'liblixcmd-tests',
+  files('libcmd/args.cc'),
+  dependencies : [
+    liblixcmd,
+    liblixutil,
+    liblixmain,
+    liblixexpr,
+    liblixstore,
+    gtest,
+    boost,
+  ],
+)
+
+test(
+  'libcmd-eval-args',
+  libcmd_tester,
+  args : [tests_args, 'TestEvalArgs'],
+  env : {
+    # No special meaning here, it's just a file laying around that is unlikely to go anywhere
+    # any time soon.
+    '_NIX_TEST_UNIT_DATA': meson.project_source_root() / 'src/nix-env/buildenv.nix',
+  },
+  suite : 'check',
+  protocol : 'gtest',
+)