diff --git a/Makefile b/Makefile
index 313f02ede..87cfc0699 100644
--- a/Makefile
+++ b/Makefile
@@ -41,6 +41,7 @@ makefiles += \
   tests/functional/ca/local.mk \
   tests/functional/dyn-drv/local.mk \
   tests/functional/test-libstoreconsumer/local.mk \
+  tests/functional/repl_characterization/local.mk \
   tests/functional/plugins/local.mk
 else
 makefiles += \
diff --git a/tests/functional/local.mk b/tests/functional/local.mk
index f94f514c9..af2a55e58 100644
--- a/tests/functional/local.mk
+++ b/tests/functional/local.mk
@@ -131,7 +131,7 @@ ifeq ($(HAVE_LIBCPUID), 1)
 endif
 
 ifeq ($(ENABLE_BUILD), yes)
-	nix_tests += test-libstoreconsumer.sh
+	nix_tests += test-libstoreconsumer.sh test-repl-characterization.sh
 
 	ifeq ($(BUILD_SHARED_LIBS), 1)
 		nix_tests += plugins.sh
@@ -142,6 +142,8 @@ $(d)/test-libstoreconsumer.sh.test $(d)/test-libstoreconsumer.sh.test-debug: \
   $(buildprefix)$(d)/test-libstoreconsumer/test-libstoreconsumer
 $(d)/plugins.sh.test $(d)/plugins.sh.test-debug: \
   $(buildprefix)$(d)/plugins/libplugintest.$(SO_EXT)
+$(d)/test-repl-characterization.sh.test $(d)/test-repl-characterization.sh.test-debug: \
+  $(buildprefix)$(d)/repl_characterization/test-repl-characterization
 
 install-tests += $(foreach x, $(nix_tests), $(d)/$(x))
 
diff --git a/tests/functional/repl_characterization/.gitignore b/tests/functional/repl_characterization/.gitignore
new file mode 100644
index 000000000..4c6412c2f
--- /dev/null
+++ b/tests/functional/repl_characterization/.gitignore
@@ -0,0 +1 @@
+test-repl-characterization
diff --git a/tests/functional/repl_characterization/data/basic.ast b/tests/functional/repl_characterization/data/basic.ast
new file mode 100644
index 000000000..d494b00aa
--- /dev/null
+++ b/tests/functional/repl_characterization/data/basic.ast
@@ -0,0 +1,11 @@
+Commentary "meow meow meow"
+Command "command"
+Output "output output one"
+Output ""
+Output ""
+Output "output output two"
+Commentary "meow meow"
+Command "command two"
+Output "output output output"
+Commentary "commentary"
+Output "output output output"
diff --git a/tests/functional/repl_characterization/data/basic.test b/tests/functional/repl_characterization/data/basic.test
new file mode 100644
index 000000000..d6b8427b4
--- /dev/null
+++ b/tests/functional/repl_characterization/data/basic.test
@@ -0,0 +1,11 @@
+meow meow meow
+  nix-repl> command
+  output output one
+
+
+  output output two
+meow meow
+  nix-repl> command two
+  output output output
+commentary
+  output output output
diff --git a/tests/functional/repl_characterization/local.mk b/tests/functional/repl_characterization/local.mk
new file mode 100644
index 000000000..aa0cf3615
--- /dev/null
+++ b/tests/functional/repl_characterization/local.mk
@@ -0,0 +1,15 @@
+programs += test-repl-characterization
+
+test-repl-characterization_DIR := $(d)
+
+# do not install
+test-repl-characterization_INSTALL_DIR :=
+
+test-repl-characterization_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+
+test-repl-characterization_CXXFLAGS += -I src/libutil -I tests/unit/libutil-support -DNIX_BIN_DIR="\"$(bindir)\""
+
+test-repl-characterization_LIBS = libutil libutil-test-support
+
+test-repl-characterization_LDFLAGS = $(THREAD_LDFLAGS) $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) $(LOWDOWN_LIBS) $(GTEST_LIBS)
diff --git a/tests/functional/repl_characterization/repl_characterization.cc b/tests/functional/repl_characterization/repl_characterization.cc
new file mode 100644
index 000000000..5b73e7a89
--- /dev/null
+++ b/tests/functional/repl_characterization/repl_characterization.cc
@@ -0,0 +1,42 @@
+#include <gtest/gtest.h>
+
+#include <string>
+#include <string_view>
+#include <optional>
+#include <unistd.h>
+
+#include "tests/characterization.hh"
+#include "tests/cli-literate-parser.hh"
+
+using namespace std::string_literals;
+
+namespace nix {
+
+static constexpr const char * REPL_PROMPT = "nix-repl> ";
+
+class ReplSessionTest : public CharacterizationTest
+{
+    Path unitTestData = getUnitTestData();
+
+public:
+    Path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData + "/" + testStem;
+    }
+};
+
+TEST_F(ReplSessionTest, parses)
+{
+    writeTest("basic.ast", [this]() {
+        const std::string content = readFile(goldenMaster("basic.test"));
+        auto parser = CLILiterateParser{REPL_PROMPT};
+        parser.feed(content);
+
+        std::ostringstream out{};
+        for (auto & bit : parser.syntax()) {
+            out << bit.print() << "\n";
+        }
+        return out.str();
+    });
+}
+};
diff --git a/tests/functional/test-repl-characterization.sh b/tests/functional/test-repl-characterization.sh
new file mode 100644
index 000000000..78dcab5fc
--- /dev/null
+++ b/tests/functional/test-repl-characterization.sh
@@ -0,0 +1,7 @@
+source common.sh
+
+# You might think that this can trivially be moved into a makefile, however,
+# there is various environmental initialization of this shell framework that
+# seems load bearing and seemingly prevents the tests working inside the
+# builder.
+_NIX_TEST_UNIT_DATA=$(pwd)/repl_characterization/data ./repl_characterization/test-repl-characterization
diff --git a/tests/unit/libutil-support/local.mk b/tests/unit/libutil-support/local.mk
index b4c8f2475..cfd88be99 100644
--- a/tests/unit/libutil-support/local.mk
+++ b/tests/unit/libutil-support/local.mk
@@ -10,6 +10,7 @@ libutil-test-support_SOURCES := $(wildcard $(d)/tests/*.cc)
 
 libutil-test-support_CXXFLAGS += $(libutil-tests_EXTRA_INCLUDES)
 
-libutil-test-support_LIBS = libutil
+# libexpr so we can steal their string printer from print.cc
+libutil-test-support_LIBS = libutil libexpr
 
 libutil-test-support_LDFLAGS := -pthread -lrapidcheck
diff --git a/tests/unit/libutil-support/tests/cli-literate-parser.cc b/tests/unit/libutil-support/tests/cli-literate-parser.cc
new file mode 100644
index 000000000..3b2345e8e
--- /dev/null
+++ b/tests/unit/libutil-support/tests/cli-literate-parser.cc
@@ -0,0 +1,174 @@
+#include "cli-literate-parser.hh"
+#include "libexpr/print.hh"
+#include "debug-char.hh"
+#include "types.hh"
+#include "util.hh"
+#include <iostream>
+#include <memory>
+#include <boost/algorithm/string/trim.hpp>
+
+using namespace std::string_literals;
+
+namespace nix {
+
+static constexpr const bool DEBUG_PARSER = false;
+
+constexpr auto CLILiterateParser::stateDebug(State const & s) -> const char *
+{
+    return std::visit(
+        overloaded{// clang-format off
+            [](Indent const&) -> const char * { return "indent"; },
+            [](Commentary const&) -> const char * { return "indent"; },
+            [](Prompt const&) -> const char * { return "prompt"; },
+            [](Command const&) -> const char * { return "command"; },
+            [](OutputLine const&) -> const char * { return "output_line"; }},
+        // clang-format on
+        s);
+}
+
+auto CLILiterateParser::Node::print() const -> std::string
+{
+    std::ostringstream s{};
+    switch (kind) {
+    case NodeKind::COMMENTARY:
+        s << "Commentary ";
+        break;
+    case NodeKind::COMMAND:
+        s << "Command ";
+        break;
+    case NodeKind::OUTPUT:
+        s << "Output ";
+        break;
+    }
+    printLiteralString(s, this->text);
+    return s.str();
+}
+
+void PrintTo(std::vector<CLILiterateParser::Node> const & nodes, std::ostream * os)
+{
+    for (auto & node : nodes) {
+        *os << node.print() << "\\n";
+    }
+}
+
+auto CLILiterateParser::parse(std::string prompt, std::string_view const & input, size_t indent) -> std::vector<Node>
+{
+    CLILiterateParser p{std::move(prompt), indent};
+    p.feed(input);
+    return std::move(p).intoSyntax();
+}
+
+auto CLILiterateParser::intoSyntax() && -> std::vector<Node>
+{
+    return std::move(this->syntax_);
+}
+
+CLILiterateParser::CLILiterateParser(std::string prompt, size_t indent)
+    : state_(indent == 0 ? State(Prompt{}) : State(Indent{}))
+    , prompt_(prompt)
+    , indent_(indent)
+    , lastWasOutput_(false)
+    , syntax_{}
+{
+    assert(!prompt.empty());
+}
+
+void CLILiterateParser::feed(char c)
+{
+    if constexpr (DEBUG_PARSER) {
+        std::cout << stateDebug(state_) << " " << DebugChar{c} << "\n";
+    }
+
+    if (c == '\n') {
+        onNewline();
+        return;
+    }
+
+    std::visit(
+        overloaded{
+            [&](Indent & s) {
+                if (c == ' ') {
+                    if (++s.pos >= indent_) {
+                        transition(Prompt{});
+                    }
+                } else {
+                    transition(Commentary{AccumulatingState{.lineAccumulator = std::string{c}}});
+                }
+            },
+            [&](Prompt & s) {
+                if (s.pos >= prompt_.length()) {
+                    transition(Command{AccumulatingState{.lineAccumulator = std::string{c}}});
+                    return;
+                } else if (c == prompt_[s.pos]) {
+                    // good prompt character
+                    ++s.pos;
+                } else {
+                    // didn't match the prompt, so it must have actually been output.
+                    s.lineAccumulator.push_back(c);
+                    transition(OutputLine{AccumulatingState{.lineAccumulator = std::move(s.lineAccumulator)}});
+                    return;
+                }
+                s.lineAccumulator.push_back(c);
+            },
+            [&](AccumulatingState & s) { s.lineAccumulator.push_back(c); }},
+        state_);
+}
+
+void CLILiterateParser::onNewline()
+{
+    State lastState = std::move(state_);
+    bool newLastWasOutput = false;
+
+    syntax_.push_back(std::visit(
+        overloaded{
+            [&](Indent & s) {
+                // XXX: technically this eats trailing spaces
+
+                // a newline following output is considered part of that output
+                if (lastWasOutput_) {
+                    newLastWasOutput = true;
+                    return Node::mkOutput("");
+                }
+                return Node::mkCommentary("");
+            },
+            [&](Commentary & s) { return Node::mkCommentary(std::move(s.lineAccumulator)); },
+            [&](Command & s) { return Node::mkCommand(std::move(s.lineAccumulator)); },
+            [&](OutputLine & s) {
+                newLastWasOutput = true;
+                return Node::mkOutput(std::move(s.lineAccumulator));
+            },
+            [&](Prompt & s) {
+                // INDENT followed by newline is also considered a blank output line
+                return Node::mkOutput(std::move(s.lineAccumulator));
+            }},
+        lastState));
+
+    transition(Indent{});
+    lastWasOutput_ = newLastWasOutput;
+}
+
+void CLILiterateParser::feed(std::string_view s)
+{
+    for (char ch : s) {
+        feed(ch);
+    }
+}
+
+void CLILiterateParser::transition(State new_state)
+{
+    // When we expect INDENT and we are parsing without indents, commentary
+    // cannot exist, so we want to transition directly into PROMPT before
+    // resuming normal processing.
+    if (Indent * i = std::get_if<Indent>(&new_state); i != nullptr && indent_ == 0) {
+        new_state = Prompt{AccumulatingState{}, i->pos};
+    }
+
+    state_ = new_state;
+}
+
+auto CLILiterateParser::syntax() const -> std::vector<Node> const &
+{
+    return syntax_;
+}
+
+};
diff --git a/tests/unit/libutil-support/tests/cli-literate-parser.hh b/tests/unit/libutil-support/tests/cli-literate-parser.hh
new file mode 100644
index 000000000..86a5bdd32
--- /dev/null
+++ b/tests/unit/libutil-support/tests/cli-literate-parser.hh
@@ -0,0 +1,127 @@
+#pragma once
+///@file
+
+#include <compare>
+#include <memory>
+#include <sstream>
+#include <variant>
+#include <vector>
+#include <string>
+
+namespace nix {
+/*
+ * A DFA parser for literate test cases for CLIs.
+ *
+ * FIXME: implement merging of these, so you can auto update cases that have
+ * comments.
+ *
+ * Format:
+ * COMMENTARY
+ * INDENT PROMPT COMMAND
+ * INDENT OUTPUT
+ *
+ * e.g.
+ * commentary commentary commentary
+ *   nix-repl> :t 1
+ *   an integer
+ *
+ * Yields:
+ * Commentary "commentary commentary commentary"
+ * Command ":t 1"
+ * Output "an integer"
+ *
+ * Note: one Output line is generated for each line of the sources, because
+ * this is effectively necessary to be able to align them in the future to
+ * auto-update tests.
+ */
+class CLILiterateParser
+{
+public:
+
+    enum class NodeKind {
+        COMMENTARY,
+        COMMAND,
+        OUTPUT,
+    };
+
+    struct Node
+    {
+        NodeKind kind;
+        std::string text;
+        std::strong_ordering operator<=>(Node const &) const = default;
+
+        static Node mkCommentary(std::string text)
+        {
+            return Node{.kind = NodeKind::COMMENTARY, .text = text};
+        }
+
+        static Node mkCommand(std::string text)
+        {
+            return Node{.kind = NodeKind::COMMAND, .text = text};
+        }
+
+        static Node mkOutput(std::string text)
+        {
+            return Node{.kind = NodeKind::OUTPUT, .text = text};
+        }
+
+        auto print() const -> std::string;
+    };
+
+    CLILiterateParser(std::string prompt, size_t indent = 2);
+
+    auto syntax() const -> std::vector<Node> const &;
+
+    /** Feeds a character into the parser */
+    void feed(char c);
+
+    /** Feeds a string into the parser */
+    void feed(std::string_view s);
+
+    /** Parses an input in a non-streaming fashion */
+    static auto parse(std::string prompt, std::string_view const & input, size_t indent = 2) -> std::vector<Node>;
+
+    /** Consumes a CLILiterateParser and gives you the syntax out of it */
+    auto intoSyntax() && -> std::vector<Node>;
+
+private:
+
+    struct AccumulatingState
+    {
+        std::string lineAccumulator;
+    };
+    struct Indent
+    {
+        size_t pos = 0;
+    };
+    struct Commentary : public AccumulatingState
+    {};
+    struct Prompt : AccumulatingState
+    {
+        size_t pos = 0;
+    };
+    struct Command : public AccumulatingState
+    {};
+    struct OutputLine : public AccumulatingState
+    {};
+
+    using State = std::variant<Indent, Commentary, Prompt, Command, OutputLine>;
+    State state_;
+
+    constexpr static auto stateDebug(State const&) -> const char *;
+
+    const std::string prompt_;
+    const size_t indent_;
+
+    /** Last line was output, so we consider a blank to be part of the output */
+    bool lastWasOutput_;
+
+    std::vector<Node> syntax_;
+
+    void transition(State newState);
+    void onNewline();
+};
+
+// Override gtest printing for lists of nodes
+void PrintTo(std::vector<CLILiterateParser::Node> const & nodes, std::ostream * os);
+};
diff --git a/tests/unit/libutil-support/tests/debug-char.hh b/tests/unit/libutil-support/tests/debug-char.hh
new file mode 100644
index 000000000..765d8553f
--- /dev/null
+++ b/tests/unit/libutil-support/tests/debug-char.hh
@@ -0,0 +1,24 @@
+///@file
+#include <ostream>
+#include <boost/io/ios_state.hpp>
+
+namespace nix {
+
+struct DebugChar
+{
+    char c;
+};
+
+inline std::ostream & operator<<(std::ostream & s, DebugChar c)
+{
+    boost::io::ios_flags_saver _ifs(s);
+
+    if (isprint(c.c)) {
+        s << static_cast<char>(c.c);
+    } else {
+        s << std::hex << "0x" << (static_cast<unsigned int>(c.c) & 0xff);
+    }
+    return s;
+}
+
+}