diff --git a/bench/orchestrator-stats/2026-05-13-iter-str-concat.json b/bench/orchestrator-stats/2026-05-13-iter-str-concat.json
new file mode 100644
index 0000000..7f34613
--- /dev/null
+++ b/bench/orchestrator-stats/2026-05-13-iter-str-concat.json
@@ -0,0 +1,20 @@
+{
+  "iter_id": "str-concat",
+  "date": "2026-05-13",
+  "mode": "standard",
+  "outcome": "DONE",
+  "tasks_total": 7,
+  "tasks_completed": 7,
+  "reloops_per_task": {
+    "1": 0,
+    "2": 0,
+    "3": 0,
+    "4": 0,
+    "5": 0,
+    "6": 0,
+    "7": 0
+  },
+  "review_loops_spec": 0,
+  "review_loops_quality": 0,
+  "blocked_reason": null
+}
diff --git a/crates/ail/tests/snapshots/hello.ll b/crates/ail/tests/snapshots/hello.ll
index 21b7739..b6f4d02 100644
--- a/crates/ail/tests/snapshots/hello.ll
+++ b/crates/ail/tests/snapshots/hello.ll
@@ -14,6 +14,7 @@ declare ptr @ailang_int_to_str(i64)
 declare ptr @ailang_float_to_str(double)
 declare ptr @ailang_bool_to_str(i1)
 declare ptr @ailang_str_clone(ptr)
+declare ptr @ailang_str_concat(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_hello_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_hello_main_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/list.ll b/crates/ail/tests/snapshots/list.ll
index 91334a1..4d9ece9 100644
--- a/crates/ail/tests/snapshots/list.ll
+++ b/crates/ail/tests/snapshots/list.ll
@@ -14,6 +14,7 @@ declare ptr @ailang_int_to_str(i64)
 declare ptr @ailang_float_to_str(double)
 declare ptr @ailang_bool_to_str(i1)
 declare ptr @ailang_str_clone(ptr)
+declare ptr @ailang_str_concat(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_list_sum_list_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_list_sum_list_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/max3.ll b/crates/ail/tests/snapshots/max3.ll
index ad83a7a..b7be9f9 100644
--- a/crates/ail/tests/snapshots/max3.ll
+++ b/crates/ail/tests/snapshots/max3.ll
@@ -14,6 +14,7 @@ declare ptr @ailang_int_to_str(i64)
 declare ptr @ailang_float_to_str(double)
 declare ptr @ailang_bool_to_str(i1)
 declare ptr @ailang_str_clone(ptr)
+declare ptr @ailang_str_concat(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_max3_max_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_max3_max_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/sum.ll b/crates/ail/tests/snapshots/sum.ll
index 3f616b0..f2cfdf8 100644
--- a/crates/ail/tests/snapshots/sum.ll
+++ b/crates/ail/tests/snapshots/sum.ll
@@ -14,6 +14,7 @@ declare ptr @ailang_int_to_str(i64)
 declare ptr @ailang_float_to_str(double)
 declare ptr @ailang_bool_to_str(i1)
 declare ptr @ailang_str_clone(ptr)
+declare ptr @ailang_str_concat(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_sum_sum_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_sum_sum_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/ws_main.ll b/crates/ail/tests/snapshots/ws_main.ll
index 819e412..256f571 100644
--- a/crates/ail/tests/snapshots/ws_main.ll
+++ b/crates/ail/tests/snapshots/ws_main.ll
@@ -14,6 +14,7 @@ declare ptr @ailang_int_to_str(i64)
 declare ptr @ailang_float_to_str(double)
 declare ptr @ailang_bool_to_str(i1)
 declare ptr @ailang_str_clone(ptr)
+declare ptr @ailang_str_concat(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_ws_lib_add_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_ws_lib_add_adapter, ptr null }
diff --git a/crates/ail/tests/str_concat_e2e.rs b/crates/ail/tests/str_concat_e2e.rs
new file mode 100644
index 0000000..8e3f93b
--- /dev/null
+++ b/crates/ail/tests/str_concat_e2e.rs
@@ -0,0 +1,69 @@
+//! E2E pin for the `str_concat` heap-Str primitive shipped in iter
+//! `str-concat`. Asserts that `ail check` + `ail build` + run on
+//! `examples/show_user_adt_with_label.ail` produce stdout
+//! `Item 42\n` — i.e. that the Show body's
+//! `(app str_concat "Item " (app int_to_str n))` evaluates correctly
+//! and `print` emits the concatenated heap-Str.
+//!
+//! Without `str_concat` registered as a builtin (pre-iter state), the
+//! fixture fails `ail check` with `[unbound-var]: str_concat`. After
+//! the iter ships, both check and build succeed and the binary
+//! produces the expected stdout.
+
+use std::path::Path;
+use std::process::Command;
+
+fn ail_bin() -> &'static str {
+    env!("CARGO_BIN_EXE_ail")
+}
+
+#[test]
+fn str_concat_e2e_show_user_adt_with_label() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace
+        .join("examples")
+        .join("show_user_adt_with_label.ail");
+    assert!(src.exists(), "fixture missing: {}", src.display());
+
+    let check = Command::new(ail_bin())
+        .args(["check", src.to_str().unwrap()])
+        .output()
+        .expect("ail check failed to spawn");
+    assert_eq!(
+        check.status.code(),
+        Some(0),
+        "ail check must succeed; got stdout={} stderr={}",
+        String::from_utf8_lossy(&check.stdout),
+        String::from_utf8_lossy(&check.stderr)
+    );
+
+    let tmp = tempfile::tempdir().expect("tempdir");
+    let bin_path = tmp.path().join("a.out");
+    let build = Command::new(ail_bin())
+        .args([
+            "build",
+            src.to_str().unwrap(),
+            "-o",
+            bin_path.to_str().unwrap(),
+        ])
+        .output()
+        .expect("ail build failed to spawn");
+    assert_eq!(
+        build.status.code(),
+        Some(0),
+        "ail build must succeed; got stdout={} stderr={}",
+        String::from_utf8_lossy(&build.stdout),
+        String::from_utf8_lossy(&build.stderr)
+    );
+
+    let run = Command::new(&bin_path)
+        .output()
+        .expect("produced binary failed to spawn");
+    assert_eq!(run.status.code(), Some(0), "binary must exit 0");
+    let stdout = String::from_utf8_lossy(&run.stdout);
+    assert_eq!(
+        stdout, "Item 42\n",
+        "expected `Item 42\\n` from str_concat + int_to_str + print; got {stdout:?}"
+    );
+}
diff --git a/crates/ail/tests/unbound_in_instance_method_pin.rs b/crates/ail/tests/unbound_in_instance_method_pin.rs
index 4049286..38049d6 100644
--- a/crates/ail/tests/unbound_in_instance_method_pin.rs
+++ b/crates/ail/tests/unbound_in_instance_method_pin.rs
@@ -14,11 +14,11 @@
 //! Pre-fix observed behaviour (the bug):
 //! - `ail check` exits 0 with `ok (23 symbols across 2 modules)`.
 //! - `ail build` exits 1 with
-//!   `Error: monomorphise_workspace: unknown identifier: \`str_concat\``.
+//!   `Error: monomorphise_workspace: unknown identifier: \`format_label\``.
 //!
 //! Post-fix expected behaviour:
 //! - `ail check` exits 1 with an `[unbound-var]` error naming
-//!   `str_concat`. The mono pass never runs because check fails first.
+//!   `format_label`. The mono pass never runs because check fails first.
 //!
 //! Root cause (from debugger Phase 1-2):
 //! `crates/ailang-check/src/lib.rs::check_def` early-returns `Ok(())`
@@ -30,9 +30,9 @@
 //! method-body identifier graph.
 //!
 //! Fixture: `examples/bug_unbound_in_instance_method.ail`. The fixture
-//! uses `str_concat` (NOT a builtin) inside an instance-method lambda;
+//! uses `format_label` (NOT a builtin) inside an instance-method lambda;
 //! `int_to_str` IS a builtin and is correctly resolved. The fixture
-//! parses cleanly — the only error is the unbound `str_concat` in the
+//! parses cleanly — the only error is the unbound `format_label` in the
 //! method body.
 
 use std::path::Path;
@@ -43,13 +43,13 @@ fn ail_bin() -> &'static str {
 }
 
 /// RED today: `ail check` on `bug_unbound_in_instance_method.ail`
-/// must exit non-zero and emit `[unbound-var]` naming `str_concat`,
+/// must exit non-zero and emit `[unbound-var]` naming `format_label`,
 /// matching the diagnostic shape produced at fn-body level.
 ///
 /// Pre-fix, this test fails because `ail check` exits 0 and prints
 /// `ok (23 symbols across 2 modules)`.
 #[test]
-fn check_fires_unbound_var_for_str_concat_in_instance_method_body() {
+fn check_fires_unbound_var_for_format_label_in_instance_method_body() {
     let manifest_dir = env!("CARGO_MANIFEST_DIR");
     let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
     let src = workspace
@@ -81,8 +81,8 @@ fn check_fires_unbound_var_for_str_concat_in_instance_method_body() {
          got: {combined}"
     );
     assert!(
-        combined.contains("str_concat"),
-        "diagnostic must name the unbound identifier `str_concat`; \
+        combined.contains("format_label"),
+        "diagnostic must name the unbound identifier `format_label`; \
          got: {combined}"
     );
     assert!(
@@ -128,9 +128,9 @@ fn check_json_unbound_var_in_instance_method_body() {
         arr.iter().any(|d| {
             d.get("severity").and_then(|v| v.as_str()) == Some("error")
                 && d.get("code").and_then(|v| v.as_str()) == Some("unbound-var")
-                && d.to_string().contains("str_concat")
+                && d.to_string().contains("format_label")
         }),
         "expected an error diagnostic with code `unbound-var` naming \
-         `str_concat`; got: {stdout}"
+         `format_label`; got: {stdout}"
     );
 }
diff --git a/crates/ailang-check/src/builtins.rs b/crates/ailang-check/src/builtins.rs
index 67888c2..75d7cd0 100644
--- a/crates/ailang-check/src/builtins.rs
+++ b/crates/ailang-check/src/builtins.rs
@@ -230,6 +230,19 @@ pub fn install(env: &mut crate::Env) {
             ret_mode: ailang_core::ast::ParamMode::Own,
         },
     );
+    env.globals.insert(
+        "str_concat".into(),
+        Type::Fn {
+            params: vec![Type::str_(), Type::str_()],
+            ret: Box::new(Type::str_()),
+            effects: vec![],
+            param_modes: vec![
+                ailang_core::ast::ParamMode::Borrow,
+                ailang_core::ast::ParamMode::Borrow,
+            ],
+            ret_mode: ailang_core::ast::ParamMode::Own,
+        },
+    );
     env.globals.insert(
         "is_nan".into(),
         Type::Fn {
@@ -328,6 +341,7 @@ pub fn list() -> Vec<(&'static str, &'static str)> {
         ("int_to_str", "(Int) -> Str"),
         ("bool_to_str", "(Bool) -> Str"),
         ("str_clone", "(Str) -> Str"),
+        ("str_concat", "(Str, Str) -> Str"),
         ("is_nan", "(Float) -> Bool"),
         ("nan", "Float"),
         ("inf", "Float"),
@@ -513,6 +527,43 @@ mod tests {
         assert_eq!(ty, Type::str_(), "(str_clone \"hi\") must type as Str");
     }
 
+    #[test]
+    fn install_str_concat_signature() {
+        // Iter str-concat: `str_concat : (Str borrow, Str borrow) -> Str
+        // own`. Codegen lowers via the runtime C glue `ailang_str_concat`
+        // from `runtime/str.c`.
+        let mut env = Env::default();
+        install(&mut env);
+        let ty = env
+            .globals
+            .get("str_concat")
+            .expect("str_concat must be installed");
+        match ty {
+            Type::Fn {
+                params,
+                ret,
+                effects,
+                param_modes,
+                ret_mode,
+            } => {
+                assert_eq!(params.len(), 2);
+                assert!(matches!(params[0], Type::Con { ref name, .. } if name == "Str"));
+                assert!(matches!(params[1], Type::Con { ref name, .. } if name == "Str"));
+                assert!(matches!(**ret, Type::Con { ref name, .. } if name == "Str"));
+                assert!(effects.is_empty(), "str_concat must be effect-free");
+                assert_eq!(
+                    param_modes,
+                    &vec![
+                        ailang_core::ast::ParamMode::Borrow,
+                        ailang_core::ast::ParamMode::Borrow,
+                    ]
+                );
+                assert_eq!(*ret_mode, ailang_core::ast::ParamMode::Own);
+            }
+            other => panic!("expected Type::Fn; got {other:?}"),
+        }
+    }
+
     /// Iter 22-floats.3: `is_nan : (Float) -> Bool`. Codegen lowers to
     /// `fcmp uno double %x, %x` in iter 4 — typecheck only validates
     /// the signature here.
diff --git a/crates/ailang-codegen/src/lib.rs b/crates/ailang-codegen/src/lib.rs
index 6ea6f29..b3d2bb7 100644
--- a/crates/ailang-codegen/src/lib.rs
+++ b/crates/ailang-codegen/src/lib.rs
@@ -546,6 +546,7 @@ fn lower_workspace_inner(ws: &Workspace, alloc: AllocStrategy) -> Result<String>
     // strips when no caller exists.
     out.push_str("declare ptr @ailang_bool_to_str(i1)\n");
     out.push_str("declare ptr @ailang_str_clone(ptr)\n");
+    out.push_str("declare ptr @ailang_str_concat(ptr, ptr)\n");
     // Floats iter 4.4: saturating fp-to-int intrinsic for
     // float_to_int_truncate. NaN → 0, +Inf → i64::MAX, -Inf →
     // i64::MIN, finite-out-of-range saturates, finite-in-range
@@ -1975,6 +1976,26 @@ impl<'a> Emitter<'a> {
             ));
             return Ok((dst, "ptr".to_string()));
         }
+        if name == "str_concat" {
+            // Iter str-concat: lowers to the runtime C glue
+            // `ailang_str_concat(ptr, ptr) -> ptr` defined in
+            // `runtime/str.c`. Reads `len` from offset 0 of each
+            // source Str payload and allocates a fresh heap-Str
+            // slab sized for the combined bytes; works uniformly
+            // on static-Str and heap-Str inputs because the
+            // consumer ABI is identical. Common shape in Show
+            // bodies: `(app str_concat "label=" (app int_to_str x))`.
+            if args.len() != 2 {
+                return Err(CodegenError::Internal("str_concat arity".into()));
+            }
+            let (a, _) = self.lower_term(&args[0])?;
+            let (b, _) = self.lower_term(&args[1])?;
+            let dst = self.fresh_ssa();
+            self.body.push_str(&format!(
+                "  {dst} = call ptr @ailang_str_concat(ptr {a}, ptr {b})\n"
+            ));
+            return Ok((dst, "ptr".to_string()));
+        }
 
         // Cross-module call: exactly one dot in the name → resolve via import map.
         // Logic identical to the typechecker (see `synth` for `Term::Var`).
@@ -2190,6 +2211,7 @@ impl<'a> Emitter<'a> {
                 | "int_to_str"
                 | "bool_to_str"
                 | "str_clone"
+                | "str_concat"
         ) {
             return true;
         }
@@ -4291,4 +4313,51 @@ mod tests {
             "expected lowering of str_clone to call @ailang_str_clone; ir was:\n{ir}"
         );
     }
+
+    /// Iter str-concat: a `Term::App` calling `str_concat` lowers to
+    /// `call ptr @ailang_str_concat(ptr %a, ptr %b)`. Pins the new
+    /// builtin's extern declaration AND the lower_app arm together
+    /// (mirror of the str_clone IR pin above).
+    #[test]
+    fn str_concat_emits_call_to_ailang_str_concat() {
+        let m = Module {
+            schema: SCHEMA.into(),
+            name: "t".into(),
+            imports: vec![],
+            defs: vec![Def::Fn(FnDef {
+                name: "main".into(),
+                ty: Type::Fn {
+                    params: vec![],
+                    ret: Box::new(Type::unit()),
+                    effects: vec!["IO".into()],
+                    param_modes: vec![],
+                    ret_mode: ParamMode::Implicit,
+                },
+                params: vec![],
+                body: Term::Do {
+                    op: "io/print_str".into(),
+                    args: vec![Term::App {
+                        callee: Box::new(Term::Var { name: "str_concat".into() }),
+                        args: vec![
+                            Term::Lit { lit: Literal::Str { value: "x".into() } },
+                            Term::Lit { lit: Literal::Str { value: "y".into() } },
+                        ],
+                        tail: false,
+                    }],
+                    tail: false,
+                },
+                suppress: vec![],
+                doc: None,
+            })],
+        };
+        let ir = emit_ir(&m).unwrap();
+        assert!(
+            ir.contains("declare ptr @ailang_str_concat(ptr, ptr)"),
+            "expected extern declaration; ir was:\n{ir}"
+        );
+        assert!(
+            ir.contains("call ptr @ailang_str_concat(ptr "),
+            "expected lowering of str_concat to call @ailang_str_concat; ir was:\n{ir}"
+        );
+    }
 }
diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index f113f96..5bd6375 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -1991,6 +1991,36 @@ Routing through `print` replaces the ad-hoc `io/print_int|bool|float`
 idiom for new code; retiring the per-type effect-ops is queued as a
 P2 follow-up.
 
+### Heap-Str primitives
+
+The runtime ships a small family of operations that produce or
+transform heap-allocated `Str` values uniformly across static-Str
+and heap-Str inputs (the consumer ABI is identical between
+realisations — see §"Str ABI"). All take their input(s) by `borrow`
+and return an owned `Str`. Each is registered as a builtin in
+`crates/ailang-check/src/builtins.rs`, lowered inline in
+`crates/ailang-codegen/src/lib.rs::lower_app` to a `call ptr @ailang_<name>`,
+and backed by a `runtime/str.c` C helper.
+
+- `int_to_str : (borrow Int) -> Str` (iter 24.1) — decimal rendering
+  of an `Int`. Backs `Show Int` in the prelude.
+- `bool_to_str : (borrow Bool) -> Str` (iter 24.1) — `"true"`/`"false"`.
+  Backs `Show Bool` in the prelude.
+- `float_to_str : (borrow Float) -> Str` (iter 24.1, type-installed;
+  codegen ships in a future iter per roadmap). Will back `Show Float`.
+- `str_clone : (borrow Str) -> Str` (iter 24.1) — allocates a fresh
+  heap-Str copy of the input's bytes. Backs `Show Str` in the prelude.
+- `str_concat : (borrow Str, borrow Str) -> Str` (iter str-concat,
+  2026-05-13) — combines two `Str` values into a single owned `Str`.
+  General-purpose; commonly used in Show bodies for labelled output
+  (`(app str_concat "label=" (app int_to_str x))`).
+
+The four Show-backers above are not directly observable to the
+LLM-author writing a `Show <T>` instance — the prelude's instance
+bodies dispatch into them. `str_concat` IS directly observable
+because the LLM-author calls it explicitly when authoring an
+instance body that wants to combine fragments.
+
 Primitive output goes through `io/print_int` / `io/print_bool` /
 `io/print_str` directly.
 
diff --git a/docs/journals/2026-05-13-iter-str-concat.md b/docs/journals/2026-05-13-iter-str-concat.md
new file mode 100644
index 0000000..018782f
--- /dev/null
+++ b/docs/journals/2026-05-13-iter-str-concat.md
@@ -0,0 +1,149 @@
+# iter str-concat — heap-Str concatenation primitive in four-site lockstep
+
+**Date:** 2026-05-13
+**Started from:** 679572a92d594d94b915d09fd60e7ef7f7bb83bd
+**Status:** DONE
+**Tasks completed:** 7 of 7
+
+## Summary
+
+`str_concat : (borrow Str, borrow Str) -> Str` shipped as a heap-Str
+primitive in the four-site-lockstep pattern established by `str_clone`
+(iter 24.1). Closes fieldtest-form-a friction finding #4. The
+LLM-natural Show-body shape
+`(app str_concat "label=" (app int_to_str x))` now parses, checks,
+builds, and runs end-to-end via the new
+`examples/show_user_adt_with_label.ail` fixture (`Item 42\n`
+through `print . MkItem 42`).
+
+Net delta: +3 tests (559 baseline → 562 green): the E2E pin
+(`str_concat_e2e_show_user_adt_with_label`), the builtin-signature
+unit test (`install_str_concat_signature` asserting the arity-2
+Borrow-Borrow / Own-return shape directly on `env.globals`), and the
+codegen IR-pin (`str_concat_emits_call_to_ailang_str_concat`
+asserting both the extern declaration AND the call instruction in
+emitted IR). Five `crates/ail/tests/snapshots/*.ll` files were
+regenerated to absorb the new unconditional
+`declare ptr @ailang_str_concat(ptr, ptr)` line in the IR header
+preamble — same upkeep pattern as hs.4 (which regenerated the same
+5 snapshots for the unconditional `@ailang_int_to_str` /
+`@ailang_float_to_str` declares).
+
+Task 5 repaired the bug-fixture / pin-test collision: the
+`bugfix-instance-body-unbound-var` iter (commit 77f584a) used
+`str_concat` as the literal unbound name because that was the
+LLM-natural shape the fieldtester reached for; renamed to
+`format_label` (LLM-author-realistic helper name) to preserve the
+pin's intent (instance-method-body walked through unbound-var check)
+while substituting a name that will never become a builtin.
+
+## Per-task notes
+
+- iter str-concat.1: RED fixture `examples/show_user_adt_with_label.ail`
+  (single-ctor `Item Int` ADT + `Show Item` body producing labelled
+  output) + E2E pin `crates/ail/tests/str_concat_e2e.rs` (check +
+  build + run asserting `Item 42\n` stdout). Confirmed RED with
+  `[unbound-var] prelude.Show: unknown identifier: 'str_concat'`.
+- iter str-concat.2: `runtime/str.c` C helper `ailang_str_concat`
+  appended after `ailang_str_clone`; reads `len` from both source
+  payloads, `str_alloc(la+lb)`, memcpys both bytes, NUL-terminates.
+  Clean `clang -O2 -c` compile.
+- iter str-concat.3: `crates/ailang-check/src/builtins.rs` —
+  `env.globals.insert("str_concat", ...)` install entry after
+  str_clone block (arity-2, both Borrow, Own return); list() row
+  `("str_concat", "(Str, Str) -> Str")` after str_clone row;
+  `install_str_concat_signature` test reaches into `env.globals`
+  directly to match the arity-2 / param_modes / ret_mode discipline
+  (deeper than the sibling `synth_in_builtins_env`-based tests
+  because str_concat is the first builtin with arity-2 Borrow params).
+- iter str-concat.4: `crates/ailang-codegen/src/lib.rs` four-site lockstep:
+  extern declaration after `@ailang_str_clone`; lower_app arm after
+  str_clone arm (arity-2 check + two `lower_term` + `fresh_ssa` +
+  body push); `is_builtin_callable` list extended with `"str_concat"`;
+  IR-pin test mirrors the sibling `str_clone_emits_call_to_ailang_str_clone`
+  pattern (Module/FnDef/Term::Do scaffolding via `super::*` +
+  `SCHEMA`, not the plan's `ailang_surface::parse` structural sketch —
+  plan explicitly authorised this in Step 4 note). IR-pin test
+  asserts both the extern declare AND the call site. RED→GREEN flip
+  on the E2E pin happens at this task. Five IR snapshots regenerated
+  via `UPDATE_SNAPSHOTS=1` to absorb the new declare line (sole
+  diff verified at line 17 across all snapshots).
+- iter str-concat.5: bug-fixture / pin-test collision repaired —
+  `examples/bug_unbound_in_instance_method.ail` line 14 rename
+  `str_concat` → `format_label`; pin test
+  `crates/ail/tests/unbound_in_instance_method_pin.rs` `replace_all`
+  rename (one test name changed to
+  `check_fires_unbound_var_for_format_label_in_instance_method_body`).
+  Both pin tests pass; new fixture `ail check` reports
+  `ok (23 symbols across 2 modules)` — matches the plan's empirical
+  guess exactly.
+- iter str-concat.6: `docs/DESIGN.md` new §"Heap-Str primitives"
+  subsection between the milestone-24 paragraph and the
+  `Primitive output goes through ...` paragraph; lists all five
+  shipping primitives with their type, iter origin, and prelude
+  role, plus the user-visible vs prelude-internal distinction.
+- iter str-concat.7: `docs/roadmap.md` struck `[x] [feature] str_concat`
+  entry at end of P2 cluster (right before `## P3 — Ideas`). All
+  four cited fixtures (`show_user_adt_with_label`, `show_user_adt`,
+  `test_22c_user_class_e2e`, `cmp_max_smoke`) `ail check` clean.
+  `ail builtins | grep str_concat` returns `str_concat        (Str, Str) -> Str`.
+
+## Concerns
+
+- Task 3 (Step 5) test-count prediction was off: plan expected
+  `passed: 560 failed: 1` after registering the builtin but actual
+  was `passed: 558 failed: 3`. Reason: the two
+  `unbound_in_instance_method_pin` tests turn RED as soon as
+  `str_concat` is registered in the checker, NOT only at codegen.
+  The pins assert on `ail check`'s `[unbound-var]` diagnostic, which
+  depends only on the checker registry. Implementation is correct;
+  Task 5 fully repairs. No follow-up needed beyond noting the
+  prediction-vs-actual gap.
+- Task 4 (Step 7) test-count prediction was off: plan expected
+  `passed: 562 failed: 1` after codegen lands; actual after
+  IR-snapshot regen was `passed: 560 failed: 2`. The difference is
+  fully accounted for by the 2 still-RED pin tests (repaired in
+  T5) and by the IR-snapshot regen being out-of-band of the
+  per-task counters in the plan. Final state at T5 Step 5 is
+  exactly `passed: 562 failed: 0`, matching the plan's iter target.
+- IR snapshot regeneration was not explicitly scripted in the plan
+  for Task 4, but it follows the hs.4 precedent (which regenerated
+  the same 5 snapshots for the same reason: a new unconditional
+  declare line in the IR header). The sole diff in each snapshot
+  is the new `declare ptr @ailang_str_concat(ptr, ptr)` line on
+  IR line 17, immediately before the existing
+  `declare i64 @llvm.fptosi.sat.i64.f64(double)` declaration.
+
+## Known debt
+
+- (none)
+
+## Files touched
+
+Code:
+- `runtime/str.c` (M) — append `ailang_str_concat` helper
+- `crates/ailang-check/src/builtins.rs` (M) — install entry, list()
+  row, signature unit test
+- `crates/ailang-codegen/src/lib.rs` (M) — extern declare,
+  lower_app arm, is_builtin_callable list, IR-pin unit test
+- `crates/ail/tests/str_concat_e2e.rs` (new) — E2E pin
+- `crates/ail/tests/unbound_in_instance_method_pin.rs` (M) — rename
+  `str_concat` → `format_label` (4 sites + 1 test name)
+- `examples/show_user_adt_with_label.ail` (new) — Show body fixture
+- `examples/bug_unbound_in_instance_method.ail` (M) — rename
+  `str_concat` → `format_label` (1 site)
+
+IR snapshots regenerated (5; sole diff = new declare line):
+- `crates/ail/tests/snapshots/hello.ll`
+- `crates/ail/tests/snapshots/list.ll`
+- `crates/ail/tests/snapshots/max3.ll`
+- `crates/ail/tests/snapshots/sum.ll`
+- `crates/ail/tests/snapshots/ws_main.ll`
+
+Docs:
+- `docs/DESIGN.md` (M) — new §"Heap-Str primitives" subsection
+- `docs/roadmap.md` (M) — struck `[x] [feature] str_concat` line
+
+## Stats
+
+bench/orchestrator-stats/2026-05-13-iter-str-concat.json
diff --git a/docs/roadmap.md b/docs/roadmap.md
index 4cca72f..82920f2 100644
--- a/docs/roadmap.md
+++ b/docs/roadmap.md
@@ -328,6 +328,15 @@ context. Pick the next milestone from P1.)_
   other consumer survives.
   - context: JOURNAL 2026-05-11 ("Iteration ct.4") — milestone close
     follow-up; closed by iter ctt.1 — recon confirmed the rebuild is the load-bearing consumer of in-band DuplicateCtor (pinned by `crates/ailang-check/tests/duplicate_ctor_pin.rs`); the asymmetry with the mono side is intentional.
+- [x] **\[feature\]** `str_concat : (borrow Str, borrow Str) -> Str` —
+  heap-Str concatenation primitive. Shipped 2026-05-13 as iter
+  str-concat (closes fieldtest-form-a friction #4). Symmetric to the
+  iter 24.1 `str_clone` / `int_to_str` / `bool_to_str` plumbing:
+  runtime C helper (`ailang_str_concat`), `ailang-check` builtin
+  registration, `ailang-codegen` extern + `lower_app` arm, plus a
+  fresh `examples/show_user_adt_with_label.ail` corpus fixture
+  exercising the LLM-natural Show-body shape.
+  - context: per-iter journal 2026-05-13-iter-str-concat.md.
 
 ## P3 — Ideas
 
diff --git a/examples/bug_unbound_in_instance_method.ail b/examples/bug_unbound_in_instance_method.ail
index efe9a86..52ef154 100644
--- a/examples/bug_unbound_in_instance_method.ail
+++ b/examples/bug_unbound_in_instance_method.ail
@@ -11,7 +11,7 @@
         (lam
           (params (typed n (con Int)))
           (ret (con Str))
-          (body (app str_concat "n=" (app int_to_str n)))))))
+          (body (app format_label "n=" (app int_to_str n)))))))
   (fn main
     (type (fn-type (params) (ret (con Unit)) (effects IO)))
     (params)
diff --git a/examples/show_user_adt_with_label.ail b/examples/show_user_adt_with_label.ail
new file mode 100644
index 0000000..4387144
--- /dev/null
+++ b/examples/show_user_adt_with_label.ail
@@ -0,0 +1,13 @@
+(module show_user_adt_with_label
+  (data Item
+    (ctor MkItem (con Int)))
+  (instance
+    (class prelude.Show)
+    (type (con Item))
+    (method show
+      (body (lam (params (typed it (con Item))) (ret (con Str)) (body (match it
+        (case (pat-ctor MkItem n) (app str_concat "Item " (app int_to_str n)))))))))
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body (app print (term-ctor Item MkItem 42)))))
diff --git a/runtime/str.c b/runtime/str.c
index 93e3778..b9b064a 100644
--- a/runtime/str.c
+++ b/runtime/str.c
@@ -184,3 +184,30 @@ char *ailang_str_clone(const char *src) {
     payload[8 + len] = '\0';
     return payload;
 }
+
+/*
+ * `ailang_str_concat(a, b)` — heap-Str concatenation primitive
+ * shipped in iter str-concat (2026-05-13). Reads `len` headers from
+ * both source Str payloads (offset 0 of each), allocates a fresh
+ * heap-Str slab sized for the combined bytes via `str_alloc`,
+ * memcpys both payloads in order, NUL-terminates, returns the new
+ * payload pointer. Like `str_clone`, this works uniformly on static-
+ * Str and heap-Str inputs because the consumer ABI is identical
+ * between realisations (see DESIGN.md §"Heap-Str primitives").
+ *
+ * Used by Show bodies that want labelled output (e.g.
+ * `"Item " ++ int_to_str n`) and by any caller that needs to
+ * combine two existing Str values into a new owned Str.
+ *
+ * Returns the payload pointer of a fresh heap-Str slab. The rc
+ * header is initialised to 1 by `str_alloc`.
+ */
+char *ailang_str_concat(const char *a, const char *b) {
+    uint64_t la = *(const uint64_t *)a;
+    uint64_t lb = *(const uint64_t *)b;
+    char *payload = str_alloc(la + lb);
+    memcpy(payload + 8, a + 8, la);
+    memcpy(payload + 8 + la, b + 8, lb);
+    payload[8 + la + lb] = '\0';
+    return payload;
+}