diff --git a/crates/ail/tests/print_str_no_auto_newline_e2e.rs b/crates/ail/tests/print_str_no_auto_newline_e2e.rs
new file mode 100644
index 0000000..c80677a
--- /dev/null
+++ b/crates/ail/tests/print_str_no_auto_newline_e2e.rs
@@ -0,0 +1,87 @@
+//! RED tests for Gitea #29: `io/print_str` must print exactly the
+//! bytes of its argument with no implicit trailing newline.
+//!
+//! Today the codegen at `crates/ailang-codegen/src/lib.rs:2734-2754`
+//! lowers `(do io/print_str s)` to a C `@puts(ptr)` call. `puts`
+//! writes the C string PLUS a trailing newline, so `io/print_str` is
+//! de-facto `println_str`. That is undocumented in the design ledger
+//! and was empirically caught in the cross-model-authoring naming-A/B
+//! run on 2026-05-21: Qwen3-Coder repeatedly wrote
+//! `(do io/print_str "text\n")` with explicit `\n`, expecting the
+//! bytes to be printed literally — and got doubled newlines, failing
+//! every t3_main_prints stdout match across all three cohorts.
+//!
+//! These tests will turn GREEN once the codegen swaps the `@puts`
+//! call for `fputs(s, stdout)` (Option 1 from Gitea #29). Until
+//! then they pin the symptom: two single-character prints emit
+//! `"ab"`, not `"a\nb\n"`; one print of `"x\n"` emits `"x\n"`, not
+//! `"x\n\n"`.
+//!
+//! NOTE: Unlike `show_print_e2e.rs` these assertions compare RAW
+//! stdout — no `.trim()`. The whole point of #29 is the byte-level
+//! property; trimming would mask both bugs.
+
+use std::path::PathBuf;
+use std::process::Command;
+
+fn fixture_path(name: &str) -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("../../examples")
+        .join(name)
+}
+
+fn build_and_run_raw(fixture: &str) -> String {
+    let src = fixture_path(fixture);
+    let out = std::env::temp_dir().join(format!("ail_{}.bin", fixture.replace('.', "_")));
+
+    let build = Command::new(env!("CARGO_BIN_EXE_ail"))
+        .args(["build", src.to_str().unwrap(), "-o", out.to_str().unwrap()])
+        .output()
+        .expect("ail build");
+    assert!(
+        build.status.success(),
+        "ail build failed for {fixture}:\nstdout: {}\nstderr: {}",
+        String::from_utf8_lossy(&build.stdout),
+        String::from_utf8_lossy(&build.stderr),
+    );
+
+    let run = Command::new(&out).output().expect("run binary");
+    assert!(
+        run.status.success(),
+        "binary exited non-zero:\nstdout: {}\nstderr: {}",
+        String::from_utf8_lossy(&run.stdout),
+        String::from_utf8_lossy(&run.stderr),
+    );
+    // Raw stdout — no trimming. The bug is at the byte level.
+    String::from_utf8(run.stdout).expect("stdout is valid UTF-8")
+}
+
+/// Property: two consecutive `(do io/print_str ...)` calls with
+/// single-character no-newline arguments produce exactly the
+/// concatenation of those characters on stdout — no separator, no
+/// trailing newline.
+///
+/// Fixture body: `(seq (do io/print_str "a") (do io/print_str "b"))`.
+///
+/// Expected (post-fix): `"ab"`.
+/// Today (pre-fix):     `"a\nb\n"` — `@puts` appends a newline per call.
+#[test]
+fn io_print_str_does_not_append_auto_newline() {
+    let stdout = build_and_run_raw("print_str_no_auto_newline.ail");
+    assert_eq!(stdout, "ab", "got: {stdout:?}");
+}
+
+/// Property: a single `(do io/print_str "x\n")` emits exactly one
+/// newline — the one the author embedded. The codegen must not
+/// double it.
+///
+/// Fixture body: `(do io/print_str "x\n")`.
+///
+/// Expected (post-fix): `"x\n"`.
+/// Today (pre-fix):     `"x\n\n"` — embedded `\n` plus auto-newline
+/// from `@puts`.
+#[test]
+fn io_print_str_does_not_double_embedded_newline() {
+    let stdout = build_and_run_raw("print_str_embedded_newline_no_doubling.ail");
+    assert_eq!(stdout, "x\n", "got: {stdout:?}");
+}
diff --git a/examples/print_str_embedded_newline_no_doubling.ail b/examples/print_str_embedded_newline_no_doubling.ail
new file mode 100644
index 0000000..66a29de
--- /dev/null
+++ b/examples/print_str_embedded_newline_no_doubling.ail
@@ -0,0 +1,6 @@
+(module print_str_embedded_newline_no_doubling
+  (fn main
+    (doc "RED fixture for Gitea #29 (companion to print_str_no_auto_newline.ail): when the author embeds a single `\\n` in the argument, exactly one newline must reach stdout — not two. Today the codegen lowers io/print_str to `@puts(ptr)` which appends its own newline, so this program emits \"x\\n\\n\" (two newlines) instead of the author-intended \"x\\n\". Pins the no-doubling property as a separate axis from the no-auto-newline property: even authors who want a newline must not get a doubled one.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body (do io/print_str "x\n"))))
diff --git a/examples/print_str_no_auto_newline.ail b/examples/print_str_no_auto_newline.ail
new file mode 100644
index 0000000..95c8ced
--- /dev/null
+++ b/examples/print_str_no_auto_newline.ail
@@ -0,0 +1,6 @@
+(module print_str_no_auto_newline
+  (fn main
+    (doc "RED fixture for Gitea #29: `io/print_str` must print exactly the bytes of its argument with NO auto-newline. Today the codegen lowers this op to a C `@puts(ptr)` call, which writes the string PLUS a trailing newline — so two consecutive prints of single-character strings emit \"a\\nb\\n\" instead of the expected \"ab\". This is the de-facto-println behaviour LLM authors (Qwen3-Coder, naming-A/B run 2026-05-21) trip over when they write explicit `\\n` and get doubled newlines. Property protected once the codegen swaps `@puts` for `fputs(s, stdout)`: io/print_str is a byte-faithful print, the author chooses whether to embed a newline.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body (seq (do io/print_str "a") (do io/print_str "b")))))