diff --git a/crates/ail/src/main.rs b/crates/ail/src/main.rs
index cbd2c2c..99c9411 100644
--- a/crates/ail/src/main.rs
+++ b/crates/ail/src/main.rs
@@ -2032,6 +2032,13 @@ fn build_to(
         // load-time remains valid.
         registry: ws.registry.clone(),
     };
+    // Iter 22b.3: monomorphisation pass. After `lift_letrecs` and
+    // before codegen, every (class-method, concrete-type) call-site
+    // pair becomes a synthesised top-level fn; user call sites are
+    // rewritten to target those names. Class-free workspaces flow
+    // through bit-identically; see `ailang_check::monomorphise_workspace`.
+    let ws = ailang_check::monomorphise_workspace(&ws)
+        .map_err(|e| anyhow::anyhow!("monomorphise_workspace: {e}"))?;
     let ir = ailang_codegen::lower_workspace_with_alloc(&ws, alloc)?;
     let tmpdir = std::env::temp_dir().join(format!("ailang-{}", std::process::id()));
     std::fs::create_dir_all(&tmpdir)?;
diff --git a/crates/ail/tests/typeclass_22b3.rs b/crates/ail/tests/typeclass_22b3.rs
new file mode 100644
index 0000000..d5c0927
--- /dev/null
+++ b/crates/ail/tests/typeclass_22b3.rs
@@ -0,0 +1,49 @@
+//! Iter 22b.3: monomorphisation pass tests.
+//!
+//! Co-located with `typeclass_22b2.rs` so the typeclass-feature
+//! coverage is browsable in one directory. Tests use the same
+//! `examples_dir()` pattern — `cargo test`-cwd is the crate dir,
+//! the fixtures live in `<repo>/examples/`.
+
+use std::path::PathBuf;
+
+fn examples_dir() -> PathBuf {
+    // crates/ail/ → repo root → examples/
+    let mut d = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    d.pop();
+    d.pop();
+    d.join("examples")
+}
+
+#[test]
+fn monomorphise_workspace_is_identity_on_class_free_workspace() {
+    // Pick any class-free fixture; `examples/hello.ail.json` is
+    // the canonical class-free smoke test.
+    let entry = examples_dir().join("hello.ail.json");
+    let ws = ailang_core::load_workspace(&entry).expect("load");
+    let diags = ailang_check::check_workspace(&ws);
+    assert!(diags.is_empty(), "fixture must typecheck: {:?}", diags);
+
+    let ws = ailang_check::monomorphise_workspace(&ws)
+        .expect("mono on class-free workspace");
+
+    // Module set unchanged.
+    let before = ailang_core::load_workspace(&examples_dir().join("hello.ail.json"))
+        .expect("re-load");
+    assert_eq!(
+        ws.modules.keys().collect::<Vec<_>>(),
+        before.modules.keys().collect::<Vec<_>>(),
+        "module set must be identical"
+    );
+    // Per-module def list unchanged (compare by canonical-JSON hash).
+    for (mname, m) in &ws.modules {
+        let before_m = &before.modules[mname];
+        let h_after = ailang_core::module_hash(m);
+        let h_before = ailang_core::module_hash(before_m);
+        assert_eq!(
+            h_after, h_before,
+            "class-free module `{}` must hash-identical through mono",
+            mname
+        );
+    }
+}
diff --git a/crates/ailang-check/src/lib.rs b/crates/ailang-check/src/lib.rs
index ce5c3a3..669ba31 100644
--- a/crates/ailang-check/src/lib.rs
+++ b/crates/ailang-check/src/lib.rs
@@ -251,9 +251,11 @@ fn unify(a: &Type, b: &Type, subst: &mut Subst) -> Result<()> {
 pub mod builtins;
 pub mod diagnostic;
 pub mod lift;
+mod mono;
 
 pub use diagnostic::{Diagnostic, Severity};
 pub use lift::lift_letrecs;
+pub use mono::monomorphise_workspace;
 
 /// Internal error type produced by the typechecker.
 ///
diff --git a/crates/ailang-check/src/mono.rs b/crates/ailang-check/src/mono.rs
new file mode 100644
index 0000000..d4602ff
--- /dev/null
+++ b/crates/ailang-check/src/mono.rs
@@ -0,0 +1,69 @@
+//! Iter 22b.3: workspace monomorphisation pass.
+//!
+//! Slots into the build pipeline after [`crate::lift_letrecs`] and
+//! before `ailang_codegen::lower_workspace_with_alloc`. The pass
+//! consumes the workspace's typeclass [`Registry`] and the per-
+//! module class-method tables (already populated by
+//! [`crate::check_in_workspace`]) and produces a workspace with:
+//!
+//! 1. Synthesised [`Def::Fn`] entries — one per unique
+//!    `(class, method, type-hash)` triple observed at any
+//!    class-method call site — appended to the [`Registry`]'s
+//!    `defining_module` for that instance.
+//! 2. Rewritten call sites — every `Term::Var { name }` whose
+//!    `name` resolves through [`Env::class_methods`] is replaced
+//!    by the corresponding mono-symbol name (qualified with the
+//!    instance's `defining_module` iff that module differs from
+//!    the calling module).
+//!
+//! Pre-existing `Def::Class` and `Def::Instance` entries are
+//! preserved verbatim — codegen ignores them, but downstream
+//! tooling (e.g. `ail describe`) may still consult them.
+//!
+//! Class-free workspaces are byte-identical through the pass:
+//! the early-out check at the top of [`monomorphise_workspace`]
+//! returns the input clone untouched.
+//!
+//! ## Symbol-hashing invariant
+//!
+//! Synthesised FnDefs are post-typecheck artefacts. They do NOT
+//! enter `CheckedModule.symbols` (built from the original module
+//! at typecheck time and used by `ail diff` / `ail manifest`).
+//! Same convention as [`crate::lift_letrecs`].
+
+use ailang_core::ast::Def;
+use ailang_core::workspace::Workspace;
+use crate::Result;
+
+/// Iter 22b.3: workspace-wide monomorphisation pass entry. See the
+/// module-level doc for the architecture and contract.
+///
+/// Pre-condition: `ws` has been typechecked (`check_workspace(ws)`
+/// returned no errors) and lifted (`lift_letrecs` per module). The
+/// pass does not perform new type checking — it queries types via
+/// `synth` on already-typechecked bodies.
+pub fn monomorphise_workspace(ws: &Workspace) -> Result<Workspace> {
+    // Fast path — no class / instance defs anywhere → nothing to do.
+    // The pass also has no targets when there are class defs but no
+    // instance defs (no callable methods at concrete types), but
+    // the body walks would still be a wasted traversal; the
+    // class-free check is the cheap way to opt out.
+    if !workspace_has_typeclasses(ws) {
+        return Ok(ws.clone());
+    }
+
+    // Skeleton in Task 1: even with classes present, return the
+    // workspace clone unchanged. Subsequent tasks fill in collection,
+    // synthesis, and rewriting.
+    Ok(ws.clone())
+}
+
+/// Iter 22b.3: returns `true` iff any module in `ws` declares at
+/// least one [`Def::Class`] or [`Def::Instance`]. Cheap workspace
+/// scan; the early-out keeps class-free workspaces byte-identical
+/// through the pass.
+fn workspace_has_typeclasses(ws: &Workspace) -> bool {
+    ws.modules.values().any(|m| {
+        m.defs.iter().any(|d| matches!(d, Def::Class(_) | Def::Instance(_)))
+    })
+}