diff --git a/c/Makefile b/c/Makefile
index ccf0a3c..7fb9b02 100644
--- a/c/Makefile
+++ b/c/Makefile
@@ -166,7 +166,7 @@ else
 PYTHON    ?= python3
 endif
 CUDA_OBJ  =
-TEST_BINS = tests/test_json$(EXE) tests/test_st$(EXE) tests/test_tier$(EXE) tests/test_grammar$(EXE) tests/test_schema_gbnf$(EXE) tests/test_decode_batch$(EXE) tests/test_idot$(EXE) tests/test_kv_alloc$(EXE) tests/test_i4_acc512$(EXE) tests/test_compat_direct$(EXE)
+TEST_BINS = tests/test_json$(EXE) tests/test_st$(EXE) tests/test_tier$(EXE) tests/test_grammar$(EXE) tests/test_schema_gbnf$(EXE) tests/test_decode_batch$(EXE) tests/test_idot$(EXE) tests/test_i4_grouped$(EXE) tests/test_kv_alloc$(EXE) tests/test_i4_acc512$(EXE) tests/test_compat_direct$(EXE)
 ifneq (,$(LINUX))
 TEST_BINS += tests/test_uring$(EXE)
 endif
@@ -297,6 +297,9 @@ tests/test_decode_batch$(EXE): tests/test_decode_batch.c decode_batch.h
 tests/test_idot$(EXE): tests/test_idot.c glm.c st.h uring.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
 	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
 
+tests/test_i4_grouped$(EXE): tests/test_i4_grouped.c glm.c st.h uring.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
+	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+
 tests/test_kv_alloc$(EXE): tests/test_kv_alloc.c glm.c st.h json.h tok.h tok_unicode.h compat.h grammar.h tier.h
 	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
 
diff --git a/c/tests/test_i4_grouped.c b/c/tests/test_i4_grouped.c
new file mode 100644
index 0000000..928d3ac
--- /dev/null
+++ b/c/tests/test_i4_grouped.c
@@ -0,0 +1,126 @@
+/* Exactness test for the grouped-int4 kernel (fmt=4, one f32 scale per `gs`
+ * elements along I) against a plain-C reference that dequantizes and multiplies
+ * in double.
+ *
+ * Why this test exists: matmul_i4_grouped is the REFERENCE the CUDA fmt=4 path
+ * (#298) is expected to reproduce, and it had no test of its own. Debugging a
+ * backend against an unverified oracle means two moving targets. Anyone porting
+ * fmt=4 to a new backend can now diff against a kernel that is known exact here.
+ *
+ * Covers: I a clean multiple of gs, I with a partial last group (the `glen`
+ * clamp), odd I (the nibble tail), gs larger than I (single group), and the
+ * nibble edges 0 and 15 (which decode to -8 and +7 — an offset encoding, NOT
+ * two's complement; getting this backwards is silent and looks like noise).
+ *
+ * FP note: the kernel sums each group in f32 (AVX2 accumulator + scalar tail)
+ * while the reference sums in double, so we compare against a relative epsilon
+ * rather than bit-exactly. The tolerance is tight enough that a wrong scale
+ * index, a wrong group boundary or a swapped nibble cannot hide under it —
+ * those are O(1) relative errors, not O(1e-6). */
+#define main coli_glm_main_unused
+#include "../glm.c"
+#undef main
+
+static uint32_t rng_state=0xC0FFEEu;
+static uint32_t xr(void){ rng_state^=rng_state<<13; rng_state^=rng_state>>17; rng_state^=rng_state<<5; return rng_state; }
+static float frand(void){ return (float)((int)(xr()%2001)-1000)/1000.0f; }
+
+/* Reference: dequantize nibble -> (v-8)*scale[group], accumulate in double.
+ * Deliberately the dumbest possible expression of the format. */
+static void ref_grouped(double *y, double *mag, const float *x, const uint8_t *q4,
+                        const float *scale, int S, int I, int O, int gs){
+    int rb=(I+1)/2, ng=(I+gs-1)/gs;
+    for(int o=0;o<O;o++){
+        const uint8_t *w=q4+(int64_t)o*rb;
+        const float *scl=scale+(int64_t)o*ng;
+        for(int s=0;s<S;s++){
+            const float *xs=x+(int64_t)s*I; double a=0, m=0;
+            for(int i=0;i<I;i++){
+                uint8_t byte=w[i>>1];
+                int nib=(i&1)?(int)(byte>>4):(int)(byte&0xF);
+                double term=(double)xs[i] * (double)(nib-8) * (double)scl[i/gs];
+                a += term; m += fabs(term);
+            }
+            y[(int64_t)s*O+o]=a;
+            /* Sum of |terms|: the scale the f32 rounding error actually lives on.
+             * Comparing against |result| instead would flag pure cancellation --
+             * a dot product of signed terms can land near zero, and then a 1e-6
+             * absolute error reads as a 1e-3 relative one. That is the accumulator's
+             * precision, not a kernel defect. A wrong scale index or group boundary
+             * shifts the result by a fraction OF THE TERMS, so it is caught here. */
+            mag[(int64_t)s*O+o]=m;
+        }
+    }
+}
+
+static int check(const char *name, int S, int I, int O, int gs, int fill_edges){
+    int rb=(I+1)/2, ng=(I+gs-1)/gs;
+    uint8_t *q4=malloc((size_t)O*rb);
+    float *scale=malloc((size_t)O*ng*sizeof(float));
+    float *x=malloc((size_t)S*I*sizeof(float));
+    float *y=malloc((size_t)S*O*sizeof(float));
+    double *yr=malloc((size_t)S*O*sizeof(double));
+    double *ym=malloc((size_t)S*O*sizeof(double));
+    if(!q4||!scale||!x||!y||!yr||!ym){ fprintf(stderr,"%s: OOM\n",name); return 1; }
+
+    for(size_t i=0;i<(size_t)O*rb;i++) q4[i]=(uint8_t)(xr()&0xFF);
+    if(fill_edges){
+        /* nibble extremes: 0x0F -> +7, 0x00 -> -8. A two's-complement misread
+         * turns 15 into -1 instead of +7 and the error is data-dependent noise. */
+        for(size_t i=0;i<(size_t)O*rb && i<64;i++) q4[i]=(i&1)?0x00:0xFF;
+    }
+    /* scales span a few orders of magnitude: a wrong group index shows up big */
+    for(int i=0;i<O*ng;i++) scale[i]=(0.001f+(float)(xr()%1000)/1000.0f)*((xr()&1)?1.f:-1.f);
+    for(int i=0;i<S*I;i++) x[i]=frand();
+
+    matmul_i4_grouped(y,x,q4,scale,S,I,O,gs);
+    ref_grouped(yr,ym,x,q4,scale,S,I,O,gs);
+
+    int bad=0; double worst=0;
+    for(int i=0;i<S*O;i++){
+        double d=fabs((double)y[i]-yr[i]);
+        double rel = ym[i]>1e-30 ? d/ym[i] : d;   /* error relative to the summed magnitude */
+        if(rel>worst) worst=rel;
+        if(rel>1e-6){
+            if(bad<3) fprintf(stderr,"%s: [%d] got %.9g want %.9g (|terms| %.3g, rel %.3g)\n",
+                              name,i,(double)y[i],yr[i],ym[i],rel);
+            bad++;
+        }
+    }
+    free(q4);free(scale);free(x);free(y);free(yr);free(ym);
+    if(bad){ fprintf(stderr,"%s: FAIL (%d/%d mismatched, worst rel %.3g)\n",name,bad,S*O,worst); return 1; }
+    printf("  %-42s ok (S=%d I=%d O=%d gs=%d ng=%d, worst rel %.2g)\n",name,S,I,O,gs,ng,worst);
+    return 0;
+}
+
+int main(void){
+    int fail=0;
+    printf("test_i4_grouped: matmul_i4_grouped vs plain-C dequant reference\n");
+
+    /* the shape the g64 checkpoints actually use */
+    fail|=check("gs=64, I multiple of gs",            2, 512, 8, 64, 0);
+    fail|=check("gs=64, single row single token",     1, 128, 1, 64, 0);
+    fail|=check("gs=64, nibble edges (0x00/0xFF)",    1, 256, 4, 64, 1);
+
+    /* partial last group: glen clamp, the classic off-by-one */
+    fail|=check("gs=64, partial last group (I=200)",  2, 200, 4, 64, 0);
+    fail|=check("gs=64, I just over a group (I=65)",  1,  65, 3, 64, 0);
+    fail|=check("gs=64, I one under a group (I=63)",  1,  63, 3, 64, 0);
+
+    /* odd I: the scalar nibble tail (i+1 == I) */
+    fail|=check("gs=64, odd I (I=201)",               2, 201, 4, 64, 0);
+    fail|=check("gs=16, odd I (I=33)",                1,  33, 2, 16, 0);
+
+    /* gs > I: everything in one group */
+    fail|=check("gs=128 > I=64 (single group)",       1,  64, 4, 128, 0);
+
+    /* the other documented group size */
+    fail|=check("gs=128, I multiple of gs",           2, 512, 4, 128, 0);
+
+    /* batch: S>1 exercises the per-s inner loop against a shared scale row */
+    fail|=check("gs=64, batch S=8",                   8, 320, 6, 64, 0);
+
+    if(fail){ printf("test_i4_grouped: FAIL\n"); return 1; }
+    printf("test_i4_grouped: ok\n");
+    return 0;
+}