diff --git a/fuzz/completion_fuzzer.cpp b/fuzz/completion_fuzzer.cpp
new file mode 100644
index 0000000..4f4d12f
--- /dev/null
+++ b/fuzz/completion_fuzzer.cpp
@@ -0,0 +1,152 @@
+/*
+ * Fuzzer for the shell completion protocol of args.hxx.
+ *
+ * The hosted parse_args_fuzzer never registers a CompletionFlag, so the
+ * candidate-generation region of the parser was almost entirely unexercised
+ * (its early exit was the only covered part, per the 2026-10-15 coverage
+ * report).
+ *
+ * This harness registers a CompletionFlag alongside flags, value flags,
+ * mapped flags/positionals, positionals, and commands, then issues valid
+ * completion requests of the form
+ *
+ *     --completion <syntax> <cword> <prog> [words...]
+ *
+ * with mutated word indices (including malformed ones such as "1x", "-1",
+ * "+1", " 2", "2 "), partial options ("", "-", "--", "-f", "--f", "-m1"),
+ * separator/prefix overlaps ("--x", "---x", "--map="), mapped choices,
+ * terminator ("--"), and command names. The long separator is also varied,
+ * including values that overlap the "--" prefix, which historically drove
+ * an out-of-range substr in the bash completion handler
+ * (see test/completion_separator_prefix_overlap.cxx). Two requests are
+ * issued per input to exercise re-parse between completions.
+ */
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include <fuzzer/FuzzedDataProvider.h>
+
+#include <args.hxx>
+
+namespace
+{
+
+// Must be std::unordered_map: the Map* classes default their Map template
+// parameter to args::detail::unordered_map and std::map does not convert.
+const std::unordered_map<std::string, int> kChoices{{"alpha", 1}, {"beta", 2}};
+
+const char *const kSeparators[] = {"=", "-", ":", "==", "=-"};
+
+const char *const kCwords[] = {"1", "2", "3", "7", "0", "1x", "-1", "+1", " 2", "2 "};
+
+const char *const kWords[] = {
+    "", "-", "--", "-f", "--f", "--foo", "-b", "--bar", "--bar=", "-b=",
+    "-m", "--map=", "-m1", "--x", "---x", "alpha", "beta", "--",
+    "command1", "command2", "command3", "x",
+};
+
+template <typename T, size_t N>
+constexpr size_t ArraySize(T (&)[N])
+{
+    return N;
+}
+
+} // namespace
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+    FuzzedDataProvider fdp(data, size);
+
+    args::ArgumentParser parser("completion_fuzzer");
+    args::CompletionFlag completion(parser, {"completion"});
+
+    args::Flag foo(parser, "foo", "foo", {'f', "foo"});
+    args::ValueFlag<std::string> bar(parser, "bar", "bar", {'b', "bar"}, "abc");
+    args::ValueFlag<int> num(parser, "num", "num", {'n', "num"});
+    args::MapFlag<std::string, int> map(parser, "map", "map", {'m', "map"}, kChoices);
+    args::MapPositional<std::string, int> mpos(parser, "mpos", "mpos", kChoices);
+    args::Positional<std::string> pos(parser, "pos", "pos");
+
+    // Vary the long separator, including prefix-overlapping values.
+    try
+    {
+        parser.LongSeparator(kSeparators[fdp.ConsumeIntegralInRange<size_t>(0, ArraySize(kSeparators) - 1)]);
+    }
+    catch (...)
+    {
+    }
+
+    // Command subparsers, mirroring test/completion.cxx: arguments are
+    // created inside the coroutine and Parse() is called before any result
+    // is consumed; the objects stay alive for the duration of the parse.
+    args::Command command1(parser, "command1", "first command", [](args::Subparser &sp)
+    {
+        args::ValueFlag<std::string> cfoo(sp, "cfoo", "cfoo", {'f', "cfoo"}, "abc");
+        sp.Parse();
+        if (cfoo)
+        {
+            (void)*cfoo;
+        }
+    });
+
+    args::Command command2(parser, "command2", "second command", [](args::Subparser &sp)
+    {
+        sp.Parse();
+    });
+
+    for (int round = 0; round < 2; ++round)
+    {
+        std::vector<std::string> argv;
+        argv.reserve(8);
+        argv.emplace_back("--completion");
+        if (fdp.ConsumeBool())
+        {
+            argv.emplace_back("bash");
+        }
+        else
+        {
+            argv.emplace_back(fdp.ConsumeRandomLengthString(8));
+        }
+        if (fdp.ConsumeBool())
+        {
+            argv.emplace_back(kCwords[fdp.ConsumeIntegralInRange<size_t>(0, ArraySize(kCwords) - 1)]);
+        }
+        else
+        {
+            argv.emplace_back(fdp.ConsumeRandomLengthString(4));
+        }
+        argv.emplace_back("prog");
+
+        const size_t wordCount = fdp.ConsumeIntegralInRange<size_t>(0, 5);
+        for (size_t i = 0; i < wordCount; ++i)
+        {
+            if (fdp.ConsumeBool())
+            {
+                argv.emplace_back(kWords[fdp.ConsumeIntegralInRange<size_t>(0, ArraySize(kWords) - 1)]);
+            }
+            else
+            {
+                argv.emplace_back(fdp.ConsumeRandomLengthString(12));
+            }
+        }
+
+        try
+        {
+            parser.ParseArgs(argv);
+        }
+        catch (const args::Completion &)
+        {
+            // Normal completion control flow carrying the candidates.
+        }
+        catch (...)
+        {
+            // Malformed cword and other expected rejections.
+        }
+    }
+
+    return 0;
+}
diff --git a/fuzz/corpus/completion_fuzzer/bad-cword.seed b/fuzz/corpus/completion_fuzzer/bad-cword.seed
new file mode 100644
index 0000000..e398776
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/bad-cword.seed
@@ -0,0 +1 @@
+--completion bash 1x prog -
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/command.seed b/fuzz/corpus/completion_fuzzer/command.seed
new file mode 100644
index 0000000..da614f0
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/command.seed
@@ -0,0 +1 @@
+--completion bash 2 prog command1 
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/dash.seed b/fuzz/corpus/completion_fuzzer/dash.seed
new file mode 100644
index 0000000..1dc3c5f
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/dash.seed
@@ -0,0 +1 @@
+--completion bash 1 prog -
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/dashdash.seed b/fuzz/corpus/completion_fuzzer/dashdash.seed
new file mode 100644
index 0000000..32f6d3d
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/dashdash.seed
@@ -0,0 +1 @@
+--completion bash 1 prog --
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/map-join.seed b/fuzz/corpus/completion_fuzzer/map-join.seed
new file mode 100644
index 0000000..f6a35ed
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/map-join.seed
@@ -0,0 +1 @@
+--completion bash 1 prog --map=
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/map-sep.seed b/fuzz/corpus/completion_fuzzer/map-sep.seed
new file mode 100644
index 0000000..0b68718
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/map-sep.seed
@@ -0,0 +1 @@
+--completion bash 2 prog -m 
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/prefix-boundary.seed b/fuzz/corpus/completion_fuzzer/prefix-boundary.seed
new file mode 100644
index 0000000..27a9080
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/prefix-boundary.seed
@@ -0,0 +1 @@
+--completion bash 1 prog ---x
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/prefix-overlap.seed b/fuzz/corpus/completion_fuzzer/prefix-overlap.seed
new file mode 100644
index 0000000..b97ca46
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/prefix-overlap.seed
@@ -0,0 +1 @@
+--completion bash 1 prog --x
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/terminator.seed b/fuzz/corpus/completion_fuzzer/terminator.seed
new file mode 100644
index 0000000..a7efbe0
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/terminator.seed
@@ -0,0 +1 @@
+--completion bash 2 prog -- 
\ No newline at end of file
diff --git a/fuzz/corpus/completion_fuzzer/words.seed b/fuzz/corpus/completion_fuzzer/words.seed
new file mode 100644
index 0000000..d4ec732
--- /dev/null
+++ b/fuzz/corpus/completion_fuzzer/words.seed
@@ -0,0 +1 @@
+--completion zsh 3 prog -f --f al
\ No newline at end of file