diff --git a/lib/minikanren/nafc.sx b/lib/minikanren/nafc.sx
new file mode 100644
index 00000000..181c84ac
--- /dev/null
+++ b/lib/minikanren/nafc.sx
@@ -0,0 +1,24 @@
+;; lib/minikanren/nafc.sx — Phase 5 piece C: negation as finite failure.
+;;
+;;   (nafc g)
+;;     succeeds (yields the input substitution) if g has zero answers
+;;     against that substitution; fails (mzero) if g has at least one.
+;;
+;; Caveat: `nafc` is unsound under the open-world assumption. It only
+;; makes sense for goals over fully-ground terms, or with the explicit
+;; understanding that adding more facts could flip the answer. Use
+;; `(project (...) ...)` to ensure the relevant vars are ground first.
+;;
+;; Caveat 2: stream-take forces g for at least one answer; if g is
+;; infinitely-ground (say, a divergent search over an unbound list),
+;; nafc itself will diverge. Standard miniKanren limitation.
+
+(define
+  nafc
+  (fn
+    (g)
+    (fn
+      (s)
+      (let
+        ((peek (stream-take 1 (g s))))
+        (if (empty? peek) (unit s) mzero)))))
diff --git a/lib/minikanren/tests/nafc.sx b/lib/minikanren/tests/nafc.sx
new file mode 100644
index 00000000..cec8eaa6
--- /dev/null
+++ b/lib/minikanren/tests/nafc.sx
@@ -0,0 +1,50 @@
+;; lib/minikanren/tests/nafc.sx — Phase 5 piece C tests for `nafc`.
+
+(mk-test
+  "nafc-failed-goal-succeeds"
+  (run* q (nafc (== 1 2)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-successful-goal-fails"
+  (run* q (nafc (== 1 1)))
+  (list))
+
+(mk-test
+  "nafc-double-negation"
+  (run* q (nafc (nafc (== 1 1))))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-with-conde-no-clauses-succeed"
+  (run*
+    q
+    (nafc
+      (conde ((== 1 2)) ((== 3 4)))))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-with-conde-some-clause-succeeds-fails"
+  (run*
+    q
+    (nafc
+      (conde ((== 1 1)) ((== 3 4)))))
+  (list))
+
+;; --- composing nafc with == as a guard ---
+
+(mk-test
+  "nafc-as-guard"
+  (run*
+    q
+    (fresh (x) (== x 5) (nafc (== x 99)) (== q x)))
+  (list 5))
+
+(mk-test
+  "nafc-guard-blocking"
+  (run*
+    q
+    (fresh (x) (== x 5) (nafc (== x 5)) (== q x)))
+  (list))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index e3de93d6..80984e36 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -128,7 +128,9 @@ Key semantic mappings:
 - [x] `conda` — soft-cut: first non-failing head wins; ALL of head's answers
       flow through rest-goals; later clauses not tried (`Phase 5 piece A`)
 - [x] `condu` — committed choice (Phase 2)
-- [ ] `nafc` — negation as finite failure with constraint
+- [x] `nafc` — negation as finite failure: `(nafc g)` yields the input subst
+      iff g has zero answers. Standard caveats apply (open-world unsoundness;
+      diverges if g is infinite). `Phase 5 piece C`.
 - [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`
 
 ### Phase 6 — arithmetic constraints CLP(FD)
@@ -157,6 +159,9 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Phase 5 piece C — nafc**: `lib/minikanren/nafc.sx`. Three-line
+  primitive: stream-take 1; if empty, `(unit s)`, else `mzero`. 7 tests including
+  double-negation and use as a guard. 201/201 cumulative.
 - **2026-05-07** — **Phase 5 piece B — project**: `lib/minikanren/project.sx` —
   defmacro that walks each named var, rebinds them, and runs the body's mk-conj.
   Demonstrated escape into host arithmetic / string ops (`(* n n)`, `(str s "!")`).