diff --git a/crates/ailang-check/src/lib.rs b/crates/ailang-check/src/lib.rs
index 9324cd0..c34dad5 100644
--- a/crates/ailang-check/src/lib.rs
+++ b/crates/ailang-check/src/lib.rs
@@ -38,6 +38,7 @@ use indexmap::IndexMap;
 use std::collections::{BTreeMap, BTreeSet};
 
 mod linearity;
+mod pre_desugar_validation;
 mod reuse_shape;
 mod suppress_filter;
 pub mod uniqueness;
@@ -696,6 +697,30 @@ pub fn check_module(m: &Module) -> Vec<Diagnostic> {
 /// Module iteration order is deterministic: entry first, then the rest in
 /// BTreeMap order, so output ordering is stable.
 pub fn check_workspace(ws: &Workspace) -> Vec<Diagnostic> {
+    // Floats milestone post-fieldtest B1: hard-reject
+    // `Pattern::Lit { Literal::Float }` before desugar rewrites lit
+    // arms into `(== scrutinee lit)`. See `pre_desugar_validation`
+    // for the full rationale. Walked per-def so the diagnostic
+    // carries the offending def's name; modules are visited in
+    // BTreeMap order, defs in declaration order. All hits in this
+    // pass are accumulated and returned together (consistent with
+    // pass-2 multi-diagnose); if any are present, later passes do
+    // not run, since a violating Float-lit pattern would otherwise
+    // be silently rewritten by desugar before typecheck.
+    {
+        let mut pre_desugar_errors: Vec<Diagnostic> = Vec::new();
+        for m in ws.modules.values() {
+            for def in &m.defs {
+                if let Err(e) = pre_desugar_validation::reject_float_patterns_in_def(def) {
+                    let wrapped = CheckError::Def(def.name().to_string(), Box::new(e));
+                    pre_desugar_errors.push(wrapped.to_diagnostic());
+                }
+            }
+        }
+        if !pre_desugar_errors.is_empty() {
+            return pre_desugar_errors;
+        }
+    }
     // Iter 16a: desugar every module of the workspace before any check
     // logic runs. The rewrite is pure and per-module; we rebuild a
     // workspace shell around the desugared modules (paths and entry
@@ -800,6 +825,13 @@ pub struct CheckedModule {
 /// kept for legacy callers (snapshot tests, the `manifest` codepath
 /// that needs the hash-to-type mapping).
 pub fn check(m: &Module) -> Result<CheckedModule> {
+    // Floats milestone post-fieldtest B1: hard-reject
+    // `Pattern::Lit { Literal::Float }` before desugar runs, because
+    // `desugar::build_eq` rewrites lit-pattern arms (Float included)
+    // into `(== scrutinee lit)` and would otherwise hide the
+    // unreachable Float-pattern arm in `type_check_pattern`. See
+    // `pre_desugar_validation` for the full rationale.
+    pre_desugar_validation::reject_float_patterns_in_module(m)?;
     // Iter 16a: desugar nested ctor patterns before constructing the
     // trivial workspace. See `check_module` for the rationale. The
     // returned `CheckedModule.symbols` content hashes are derived from
diff --git a/crates/ailang-check/src/pre_desugar_validation.rs b/crates/ailang-check/src/pre_desugar_validation.rs
new file mode 100644
index 0000000..49c7cb1
--- /dev/null
+++ b/crates/ailang-check/src/pre_desugar_validation.rs
@@ -0,0 +1,140 @@
+//! Floats milestone post-fieldtest B1: pre-desugar walker that hard-
+//! rejects `Pattern::Lit { lit: Literal::Float { .. } }`.
+//!
+//! Why a separate pre-desugar pass:
+//! [`ailang_core::desugar::build_eq`] rewrites `Pattern::Lit` arms
+//! (Int / Bool / Str / **Float**) into `Term::App { callee: ==, .. }`
+//! before typecheck runs. As a consequence the in-typechecker
+//! Float-pattern-reject arm in [`crate::type_check_pattern`] is
+//! unreachable for any input that flows through the public check
+//! entry points (`check`, `check_module`, `check_workspace`), all of
+//! which call `desugar_module` first.
+//!
+//! This walker runs *before* desugar and short-circuits on the first
+//! `Pattern::Lit { lit: Literal::Float }` it finds, returning the
+//! bare [`CheckError::FloatPatternNotAllowed`]. Callers that build a
+//! diagnostic (the multi-diagnose entry points) wrap the bare error
+//! in [`CheckError::Def`] themselves so the diagnostic carries the
+//! offending def's name. The in-typechecker arm at
+//! [`crate::type_check_pattern`] remains as defence-in-depth for
+//! direct `synth` callers (the `reject_float_pattern_in_match` unit
+//! test in `crate::builtins` exercises that path).
+
+use ailang_core::ast::*;
+
+use crate::CheckError;
+
+/// Walks every `Term::Match` arm in `m` and returns
+/// `Err(CheckError::FloatPatternNotAllowed)` on the first
+/// `Pattern::Lit { lit: Literal::Float }` encountered. Defs are
+/// visited in declaration order; the walk bails on the first hit.
+///
+/// The returned error is bare (not wrapped in [`CheckError::Def`]).
+/// The single-error entry point [`crate::check`] propagates it as-is
+/// so callers asserting `matches!(err, CheckError::FloatPatternNotAllowed)`
+/// stay clean. Multi-diagnose entry points should call
+/// [`reject_float_patterns_in_def`] per def to attach the def name.
+pub(crate) fn reject_float_patterns_in_module(m: &Module) -> Result<(), CheckError> {
+    for def in &m.defs {
+        reject_float_patterns_in_def(def)?;
+    }
+    Ok(())
+}
+
+/// Per-def variant of [`reject_float_patterns_in_module`]. Returns
+/// the bare [`CheckError::FloatPatternNotAllowed`] on first hit;
+/// callers wrap with [`CheckError::Def`] themselves to keep the
+/// def-name attribution on the diagnostic.
+pub(crate) fn reject_float_patterns_in_def(def: &Def) -> Result<(), CheckError> {
+    match def {
+        Def::Fn(f) => walk_term(&f.body),
+        Def::Const(c) => walk_term(&c.value),
+        Def::Type(_) => Ok(()),
+        Def::Class(c) => {
+            for method in &c.methods {
+                if let Some(default) = &method.default {
+                    walk_term(default)?;
+                }
+            }
+            Ok(())
+        }
+        Def::Instance(i) => {
+            for method in &i.methods {
+                walk_term(&method.body)?;
+            }
+            Ok(())
+        }
+    }
+}
+
+fn walk_term(t: &Term) -> Result<(), CheckError> {
+    match t {
+        Term::Lit { .. } | Term::Var { .. } => Ok(()),
+        Term::App { callee, args, .. } => {
+            walk_term(callee)?;
+            for a in args {
+                walk_term(a)?;
+            }
+            Ok(())
+        }
+        Term::Let { value, body, .. } => {
+            walk_term(value)?;
+            walk_term(body)
+        }
+        Term::LetRec { body, in_term, .. } => {
+            walk_term(body)?;
+            walk_term(in_term)
+        }
+        Term::If { cond, then, else_ } => {
+            walk_term(cond)?;
+            walk_term(then)?;
+            walk_term(else_)
+        }
+        Term::Do { args, .. } => {
+            for a in args {
+                walk_term(a)?;
+            }
+            Ok(())
+        }
+        Term::Ctor { args, .. } => {
+            for a in args {
+                walk_term(a)?;
+            }
+            Ok(())
+        }
+        Term::Match { scrutinee, arms } => {
+            walk_term(scrutinee)?;
+            for arm in arms {
+                walk_pattern(&arm.pat)?;
+                walk_term(&arm.body)?;
+            }
+            Ok(())
+        }
+        Term::Lam { body, .. } => walk_term(body),
+        Term::Seq { lhs, rhs } => {
+            walk_term(lhs)?;
+            walk_term(rhs)
+        }
+        Term::Clone { value } => walk_term(value),
+        Term::ReuseAs { source, body } => {
+            walk_term(source)?;
+            walk_term(body)
+        }
+    }
+}
+
+fn walk_pattern(p: &Pattern) -> Result<(), CheckError> {
+    match p {
+        Pattern::Wild | Pattern::Var { .. } => Ok(()),
+        Pattern::Lit {
+            lit: Literal::Float { .. },
+        } => Err(CheckError::FloatPatternNotAllowed),
+        Pattern::Lit { .. } => Ok(()),
+        Pattern::Ctor { fields, .. } => {
+            for sub in fields {
+                walk_pattern(sub)?;
+            }
+            Ok(())
+        }
+    }
+}