diff --git a/lib/go/conformance.sh b/lib/go/conformance.sh
index b978edf8..c7d1127a 100755
--- a/lib/go/conformance.sh
+++ b/lib/go/conformance.sh
@@ -29,6 +29,7 @@ trap "rm -f $TMPFILE $OUTFILE" EXIT
 SUITES=(
   "lex|go-test-pass|go-test-count"
   "parse|go-parse-test-pass|go-parse-test-count"
+  "types|go-types-test-pass|go-types-test-count"
 )
 
 cat > "$TMPFILE" <<'EPOCHS'
@@ -38,8 +39,10 @@ cat > "$TMPFILE" <<'EPOCHS'
 (load "lib/guest/pratt.sx")
 (load "lib/go/lex.sx")
 (load "lib/go/parse.sx")
+(load "lib/go/types.sx")
 (load "lib/go/tests/lex.sx")
 (load "lib/go/tests/parse.sx")
+(load "lib/go/tests/types.sx")
 EPOCHS
 
 idx=0
@@ -104,7 +107,6 @@ cat > lib/go/scoreboard.json <<JSON
   "total_pass": $TOTAL_PASS,
   "total": $TOTAL_COUNT,
   "suites": [$JSON_SUITES,
-    {"name":"types","pass":0,"total":0,"status":"pending"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
@@ -120,8 +122,7 @@ cat > lib/go/scoreboard.md <<MD
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
-$MD_ROWS| ⬜ | types | 0 | 0 |
-| ⬜ | eval | 0 | 0 |
+$MD_ROWS| ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |
diff --git a/lib/go/scoreboard.json b/lib/go/scoreboard.json
index 584ac961..5d229a9c 100644
--- a/lib/go/scoreboard.json
+++ b/lib/go/scoreboard.json
@@ -1,11 +1,11 @@
 {
   "language": "go",
-  "total_pass": 305,
-  "total": 305,
+  "total_pass": 317,
+  "total": 317,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
-    {"name":"types","pass":0,"total":0,"status":"pending"},
+    {"name":"types","pass":12,"total":12,"status":"ok"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
diff --git a/lib/go/scoreboard.md b/lib/go/scoreboard.md
index fb656217..c8ddf6d7 100644
--- a/lib/go/scoreboard.md
+++ b/lib/go/scoreboard.md
@@ -1,12 +1,12 @@
 # Go-on-SX Scoreboard
 
-**Total: 305 / 305 tests passing**
+**Total: 317 / 317 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
-| ⬜ | types | 0 | 0 |
+| ✅ | types | 12 | 12 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
diff --git a/lib/go/tests/types.sx b/lib/go/tests/types.sx
new file mode 100644
index 00000000..1c553fb3
--- /dev/null
+++ b/lib/go/tests/types.sx
@@ -0,0 +1,111 @@
+;; Go type-checker tests.
+
+(define go-types-test-count 0)
+(define go-types-test-pass 0)
+(define go-types-test-fails (list))
+
+(define
+  go-types-test
+  (fn
+    (name actual expected)
+    (set! go-types-test-count (+ go-types-test-count 1))
+    (if
+      (= actual expected)
+      (set! go-types-test-pass (+ go-types-test-pass 1))
+      (append! go-types-test-fails {:name name :expected expected :actual actual}))))
+
+;; Convenience: parse + synth in one step.
+(define gtsy (fn (ctx src) (go-synth ctx (go-parse src))))
+(define gtchk (fn (ctx src ty) (go-check ctx (go-parse src) ty)))
+
+;; ── context helpers ──────────────────────────────────────────────
+(go-types-test
+  "ctx: empty lookup returns nil"
+  (go-ctx-lookup go-ctx-empty "x")
+  nil)
+
+(go-types-test
+  "ctx: extend then lookup"
+  (go-ctx-lookup (go-ctx-extend go-ctx-empty "x" (list :ty-name "int")) "x")
+  (list :ty-name "int"))
+
+(go-types-test
+  "ctx: shadow via extend"
+  (go-ctx-lookup
+    (go-ctx-extend
+      (go-ctx-extend go-ctx-empty "x" (list :ty-name "int"))
+      "x"
+      (list :ty-name "string"))
+    "x")
+  (list :ty-name "string"))
+
+(go-types-test
+  "ctx: extend-field binds all names"
+  (let
+    ((ctx (go-ctx-extend-field go-ctx-empty (list :field (list "a" "b" "c") (list :ty-name "int")))))
+    (list
+      (go-ctx-lookup ctx "a")
+      (go-ctx-lookup ctx "b")
+      (go-ctx-lookup ctx "c")
+      (go-ctx-lookup ctx "d")))
+  (list
+    (list :ty-name "int")
+    (list :ty-name "int")
+    (list :ty-name "int")
+    nil))
+
+;; ── predeclared identifiers ──────────────────────────────────────
+(go-types-test
+  "predeclared: true"
+  (gtsy go-ctx-empty "true")
+  (list :ty-name "bool"))
+
+(go-types-test
+  "predeclared: false"
+  (gtsy go-ctx-empty "false")
+  (list :ty-name "bool"))
+
+(go-types-test
+  "predeclared: nil"
+  (gtsy go-ctx-empty "nil")
+  (list :ty-untyped-nil))
+
+;; ── synth: variable lookup ──────────────────────────────────────
+(go-types-test
+  "synth: bound variable returns its type"
+  (go-synth
+    (go-ctx-extend go-ctx-empty "x" (list :ty-name "int"))
+    (go-parse "x"))
+  (list :ty-name "int"))
+
+(go-types-test
+  "synth: unbound variable is a type error"
+  (go-synth go-ctx-empty (go-parse "ghost"))
+  (list :type-error :unbound "ghost"))
+
+;; ── check: structural type equality ─────────────────────────────
+(go-types-test
+  "check: ident vs declared type — matching"
+  (go-check
+    (go-ctx-extend go-ctx-empty "x" (list :ty-name "int"))
+    (go-parse "x")
+    (list :ty-name "int"))
+  :ok)
+
+(go-types-test
+  "check: ident vs declared type — mismatch"
+  (go-check
+    (go-ctx-extend go-ctx-empty "x" (list :ty-name "int"))
+    (go-parse "x")
+    (list :ty-name "string"))
+  (list :type-error :mismatch (list :ty-name "string") (list :ty-name "int")))
+
+(go-types-test
+  "check: unbound propagates the synth error"
+  (go-check go-ctx-empty (go-parse "ghost") (list :ty-name "int"))
+  (list :type-error :unbound "ghost"))
+
+;; ── report ──────────────────────────────────────────────────────
+(define
+  go-types-test-summary
+  (str "types " go-types-test-pass "/" go-types-test-count))
diff --git a/lib/go/types.sx b/lib/go/types.sx
new file mode 100644
index 00000000..24bd53fe
--- /dev/null
+++ b/lib/go/types.sx
@@ -0,0 +1,125 @@
+;; lib/go/types.sx — Go bidirectional type checker.
+;;
+;; Two judgments shape this file:
+;;
+;;   (go-synth CTX EXPR)            → TYPE-NODE  | (list :type-error TAG ...)
+;;     Given a context and an expression, produce a type.
+;;
+;;   (go-check CTX EXPR EXPECTED)   → :ok        | (list :type-error TAG ...)
+;;     Given a context, expression, and expected type, verify compatibility.
+;;
+;; The two judgments are mutually recursive. Synth produces types when the
+;; expression's shape determines them (variables, calls, literals).
+;; Check propagates types downward into expressions whose shape doesn't
+;; uniquely determine them (composite literals, untyped constants).
+;;
+;; Type representations reuse the parser's :ty-* AST nodes from
+;; lib/go/parse.sx — :ty-name, :ty-ptr, :ty-slice, :ty-array, :ty-map,
+;; :ty-chan, :ty-struct, :ty-interface, :ty-func, :ty-sel.
+;;
+;; Context: an association list of (NAME TYPE) bindings. Per-block scope
+;; via a fresh extension on entry.
+;;
+;; **Independent implementation.** lib/guest/static-types-bidirectional/
+;; does not exist yet; this work informs its eventual shape. Sister-plan
+;; design diary at plans/lib-guest-static-types-bidirectional.md tracks
+;; the chiselling insights as Phase 3 progresses.
+
+;; ── context ───────────────────────────────────────────────────────
+
+(define go-ctx-empty (list))
+
+(define
+  go-ctx-lookup
+  (fn
+    (ctx name)
+    (cond
+      (= (len ctx) 0)
+      nil
+      (= (first (first ctx)) name)
+      (nth (first ctx) 1)
+      :else (go-ctx-lookup (rest ctx) name))))
+
+(define go-ctx-extend (fn (ctx name type) (cons (list name type) ctx)))
+
+(define
+  go-ctx-extend-field
+  (fn
+    (ctx field)
+    (let
+      ((names (nth field 1)) (ty (nth field 2)))
+      (cond
+        (= (len names) 0)
+        ctx
+        :else (let
+          ((rest-ctx (go-ctx-extend ctx (first names) ty)))
+          (cond
+            (= (len names) 1)
+            rest-ctx
+            :else (go-ctx-extend-field rest-ctx (list :field (rest names) ty))))))))
+
+;; ── predeclared identifiers ──────────────────────────────────────
+
+(define
+  go-predeclared
+  (list
+    (list "true" (list :ty-name "bool"))
+    (list "false" (list :ty-name "bool"))
+    (list "nil" (list :ty-untyped-nil))))
+
+(define
+  go-predeclared-lookup
+  (fn
+    (name)
+    (cond
+      (= (len go-predeclared) 0)
+      nil
+      :else (go-ctx-lookup go-predeclared name))))
+
+;; ── type predicates ──────────────────────────────────────────────
+
+(define
+  go-type-error?
+  (fn
+    (x)
+    (and
+      (list? x)
+      (not (= (len x) 0))
+      (= (first x) :type-error))))
+
+(define go-type-equal? (fn (a b) (= a b)))
+
+;; ── synth ────────────────────────────────────────────────────────
+
+(define
+  go-synth
+  (fn
+    (ctx expr)
+    (cond
+      (and (list? expr) (= (first expr) :var))
+      (let
+        ((name (nth expr 1)))
+        (let
+          ((pre (go-predeclared-lookup name)))
+          (cond
+            (not (= pre nil))
+            pre
+            :else (let
+              ((t (go-ctx-lookup ctx name)))
+              (cond (= t nil) (list :type-error :unbound name) :else t)))))
+      :else (list :type-error :unsupported-synth expr))))
+
+;; ── check ────────────────────────────────────────────────────────
+
+(define
+  go-check
+  (fn
+    (ctx expr expected)
+    (let
+      ((got (go-synth ctx expr)))
+      (cond
+        (go-type-error? got)
+        got
+        (go-type-equal? got expected)
+        :ok :else
+        (list :type-error :mismatch expected got)))))
diff --git a/plans/go-on-sx.md b/plans/go-on-sx.md
index 3a0618cb..ea0efb23 100644
--- a/plans/go-on-sx.md
+++ b/plans/go-on-sx.md
@@ -214,22 +214,28 @@ Progress-log line → push `origin/loops/go`.
   deferred to a follow-up; it doesn't gate Phase 3.
 
 ### Phase 3 — Bidirectional type checker, MVP (`lib/go/types.sx`) ⬜
-- **Independent implementation.** Do NOT use lib/guest/static-types-
-  bidirectional/ — that kit doesn't exist yet and depends on this work
-  for its design. See `plans/lib-guest-static-types-bidirectional.md`.
-- Synth + check judgments. Context as a value (per-block scope).
-- Coverage MVP: declared-type variables, function signatures (params +
-  returns), call type-checking, simple composite types (slice, map, chan
-  element), interface satisfaction (structural match against method sets),
-  short variable declaration `:=` (synth from RHS).
-- **Untyped constants.** `42` has type `untyped int` until contextualised;
-  this is the canonical pitfall (see Gotchas below).
-- Defer: generics (Phase 7), full conversion rules.
-- Tests: positive (type-correct programs check) + negative (mismatched
-  types fail with informative errors carrying AST paths).
-- **Acceptance:** types/ suite at 60+ tests. Chisel note `shapes-static-
-  types-bidirectional` — append a paragraph to the sister plan's design
-  diary describing what synth/check shape emerged.
+- [x] Scaffold: `go-synth` / `go-check` skeletons; context-as-value
+      (`go-ctx-empty` / `-extend` / `-lookup` / `-extend-field`);
+      predeclared `true`/`false`/`nil`; structural type equality.
+- [ ] Literal kinds in AST (parser change: `(:literal KIND VALUE)`)
+      + literal synth (`:ty-untyped-int`/`-float`/`-string`/`-rune`).
+- [ ] Binary-op synth with untyped-constant flow (canonical pitfall:
+      `var x float64 = 42 / 7` must compute as untyped int / int = 6,
+      then convert to float64).
+- [ ] Var/const declaration checking (`var x T = expr`, `var x = expr`,
+      `const Pi = 3.14`).
+- [ ] Function declaration: extend ctx with params via `:field` group,
+      check body, verify return-list types match signature.
+- [ ] Call type-checking (synth callee, check args against param types,
+      synth result).
+- [ ] Composite type element checking (slice / map / chan).
+- [ ] Interface satisfaction (structural match against method sets).
+- [ ] Short variable declaration `:=` (synth RHS into LHS bindings).
+- Defer: generics (Phase 7), full conversion rules, type assertions,
+  type switches.
+- **Acceptance:** types/ suite at 60+ tests. Current: 12/12. Chisel note
+  `shapes-static-types-bidirectional` — sister-plan design diary is the
+  cross-language record.
 
 ### Phase 4 — Tree-walk evaluator (`lib/go/eval.sx`) ⬜
 - AST-walking interpreter over CEK. Each Go statement maps to one step
@@ -528,6 +534,16 @@ Minimal repro: see `lib/go/lex.sx#gl-oct-digit?` and `#gl-match-op`.
 
 _Newest first. Append one dated entry per commit._
 
+- 2026-05-27 — **Phase 3 scaffold.** First `lib/go/types.sx` cut: context
+  as an association list (`go-ctx-empty` + `-extend` + `-lookup`), a
+  load-bearing `go-ctx-extend-field` that consumes the `:field` binding-
+  group shape (validating the Phase 2 cross-deliverable observation),
+  predeclared `true`/`false`/`nil`, `go-synth` for identifier lookup,
+  `go-check` deferring to synth + structural equality. types suite
+  12/12, total 317/317. Literal kinds (untyped int/float/string/rune)
+  + binop synth + var-decl checking next. `[shapes-static-types-
+  bidirectional]` — sister-plan diary updated with the synth/check
+  Go-side surface as it emerges.
 - 2026-05-27 — **Phase 2 complete.** End-to-end multi-form file parsing.
   `go-parse` now returns single forms for backward compat (~169 tests
   unchanged) or `(list :file FORMS)` for multi-form input. Tests cover
diff --git a/plans/lib-guest-static-types-bidirectional.md b/plans/lib-guest-static-types-bidirectional.md
index fe27c0fc..c720bdd6 100644
--- a/plans/lib-guest-static-types-bidirectional.md
+++ b/plans/lib-guest-static-types-bidirectional.md
@@ -282,6 +282,36 @@ The kits compose; design accordingly.
 
 _Newest first. Append one dated entry per milestone landed._
 
+- 2026-05-27 — From Go-on-SX Phase 3 scaffold (`lib/go/types.sx` first
+  cut): the **independent synth/check shape** has landed. Two judgments,
+  both consuming a context-as-value:
+
+  ```
+  (go-synth CTX EXPR)             → TYPE-NODE | (:type-error TAG ...)
+  (go-check CTX EXPR EXPECTED)    → :ok       | (:type-error TAG ...)
+  ```
+
+  Context is an association list of `(NAME TYPE)` bindings; the
+  load-bearing extension primitive is `go-ctx-extend-field` which takes
+  a `(:field NAMES TYPE)` binding-group node and binds every NAME to
+  TYPE. This validates the earlier cross-deliverable observation: the
+  parser produces `:field` once, the type checker consumes it once,
+  same shape across struct fields / func params / var-decls.
+
+  **Design insight for the kit**: the synth/check pair is the canonical
+  judgment skeleton. `check` deferring to `synth + structural-equality`
+  is the v0 default that every consumer overrides for subtype-ish
+  relationships. The kit's `check` should accept a `subtype?` predicate
+  parameter so Go (untyped-constant flow), TS (variance), and Rust
+  (lifetime subtyping) each plug in their own variance discipline
+  without rewriting the whole judgment. The kit's `synth` stays uniform.
+
+  Error shape `(:type-error TAG ...)` with first-class tags
+  (`:unbound`, `:mismatch`, `:unsupported-synth`) gives consumers and
+  IDE tooling structured errors to dispatch on. Untyped-constant flow
+  and binop-synth — the canonical Go pitfall (`var x float64 = 42 / 7`)
+  — arrive next. Source: Go-on-SX commit landing `lib/go/types.sx`.
+
 - 2026-05-27 — From Go-on-SX Phase 2 (func decls landing): parser-side
   observation that's load-bearing for any bidirectional checker. Go's
   parser ended up with a single shape — `(list :field NAMES TYPE)` —