diff --git a/lfs.c b/lfs.c
index bf434976..3d96de0a 100644
--- a/lfs.c
+++ b/lfs.c
@@ -2232,6 +2232,7 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
         other_id_ = id_;
     } else if (tag == LFSR_TAG_SHRINK) {
         LFS_ASSERT(id < rbyd->weight);
+        LFS_ASSERT(lfsr_data_len(data) <= rbyd->weight);
         // noop?
         if (lfsr_data_len(data) == 0) {
             return 0;
@@ -3320,8 +3321,6 @@ static lfs_ssize_t lfsr_btree_get(lfs_t *lfs,
             buffer, size);
 }
 
-// TODO drop the root during merges
-// TODO inline?
 static int lfsr_btree_commit(lfs_t *lfs,
         lfsr_btree_t *btree, lfs_size_t id,
         lfsr_rbyd_t *rbyd, const struct lfsr_attr *attrs) {
@@ -3821,6 +3820,11 @@ static int lfsr_btree_commit(lfs_t *lfs,
             goto abort;
         }
 
+        // only child? can't merge
+        if (pweight == parent.weight) {
+            goto abort;
+        }
+
         // last child? try the left sibling
         lfs_ssize_t sid;
         lfs_ssize_t sdelta;
diff --git a/tests/test_btree.toml b/tests/test_btree.toml
index b781d4e1..0424a453 100644
--- a/tests/test_btree.toml
+++ b/tests/test_btree.toml
@@ -452,7 +452,6 @@ code = '''
             memmove(&sim[id+1], &sim[id], sim_size-id);
             sim[id] = alphas[i % 26];
             sim_size += 1;
-            printf("%c\n", alphas[i % 26]);
         }
 
         // check that btree matches sim
@@ -2068,10 +2067,282 @@ code = '''
     }
 '''
 
-#
-## [cases.test_btree_split]
-## [cases.test_btree_split_fuzz]
-#
-## [cases.test_btree_general_fuzz]
-#
-## TODO also test copy-on-write!
+
+# TODO [cases.test_btree_split]
+# TODO [cases.test_btree_split_fuzz]
+
+
+
+# Some more general fuzz testing
+[cases.test_btree_general_fuzz]
+defines.N = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]
+defines.ITER = 100
+in = 'lfs.c'
+code = '''
+    const char *alphas = "abcdefghijklmnopqrstuvwxyz";
+
+    // iterate through severals seeds that we can reproduce easily
+    for (uint32_t seed = 1; seed < ITER+1; seed++) {
+        // create lfs here since we need to reset each iteration, we're
+        // space constrained and we can't expect gc to work at this point
+        lfs_t lfs;
+        lfs_init(&lfs, cfg) => 0;
+        // create free lookahead
+        memset(lfs.free.buffer, 0, lfs.cfg->lookahead_size);
+        lfs.free.off = 0;
+        lfs.free.size = lfs_min(8*lfs.cfg->lookahead_size,
+                lfs.cfg->block_count);
+        lfs.free.i = 0;
+        lfs_alloc_ack(&lfs);
+
+        // create a btree
+        lfsr_btree_t btree = LFSR_BTREE_NULL;
+
+        // set up a simulation to compare against
+        //
+        // fun fact this is slower than our actual tree! unfun fact this is
+        // starting to be a problem...
+        char *sim = malloc(N);
+        lfs_size_t sim_size = 0;
+        memset(sim, 0, N);
+
+        uint32_t prng = seed;
+        for (lfs_size_t i = 0; i < (N-REMAINING); i++) {
+            // choose a pseudo-random op
+            uint8_t op = TEST_PRNG(&prng) % 3;
+            // choose a pseudo-random id
+            lfs_size_t id = TEST_PRNG(&prng) % (sim_size+1);
+
+            if (op == 0 || id == sim_size) {
+                // push to btree
+                lfsr_btree_push(&lfs, &btree, id,
+                        LFSR_TAG_INLINED, 1,
+                        &alphas[i % 26], 1) => 0;
+
+                // push to sim
+                memmove(&sim[id+1], &sim[id], sim_size-id);
+                sim[id] = alphas[i % 26];
+                sim_size += 1;
+
+            } else if (op == 1) {
+                // update btree
+                lfsr_btree_update(&lfs, &btree, id,
+                        LFSR_TAG_INLINED, 1,
+                        &alphas[i % 26], 1) => 0;
+
+                // update sim
+                sim[id] = alphas[i % 26];
+
+            } else {
+                // pop from btree
+                lfsr_btree_pop(&lfs, &btree, id) => 0;
+
+                // pop from sim
+                memmove(&sim[id], &sim[id+1], sim_size-(id+1));
+                sim_size -= 1;
+            }
+        }
+
+        // check that btree matches sim
+        printf("expd: [");
+        bool first = true;
+        for (lfs_size_t i = 0; i < sim_size; i++) {
+            if (!first) {
+                printf(", ");
+            }
+            first = false;
+            printf("%c", sim[i]);
+        }
+        printf("]\n");
+        printf("btree: 0x%x.%x 0x%x w%d\n",
+            btree.u.trunk.block,
+            btree.u.trunk.limit,
+            btree.tag,
+            btree.weight);
+        assert(btree.weight == sim_size);
+
+        uint8_t buffer[4];
+        lfsr_tag_t tag_;
+        lfs_size_t id_;
+        lfs_size_t weight_;
+        for (lfs_size_t i = 0; i < sim_size; i++) {
+            lfsr_btree_get(&lfs, &btree, i,
+                    &tag_, &id_, &weight_,
+                    buffer, 4) => 1;
+            assert(tag_ == LFSR_TAG_INLINED);
+            assert(id_ == i);
+            assert(weight_ == 1);
+            assert(memcmp(buffer, &sim[i], 1) == 0);
+        }
+
+        // and no extra elements
+        lfsr_btree_get(&lfs, &btree, sim_size,
+                &tag_, &id_, &weight_,
+                buffer, 4) => LFS_ERR_NOENT;
+
+        // clean up sim
+        free(sim);
+    }
+'''
+
+[cases.test_btree_general_sparse_fuzz]
+defines.N = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]
+defines.W = 5
+defines.ITER = 100
+in = 'lfs.c'
+code = '''
+    const char *alphas = "abcdefghijklmnopqrstuvwxyz";
+
+    // iterate through severals seeds that we can reproduce easily
+    for (uint32_t seed = 1; seed < ITER+1; seed++) {
+        // create lfs here since we need to reset each iteration, we're
+        // space constrained and we can't expect gc to work at this point
+        lfs_t lfs;
+        lfs_init(&lfs, cfg) => 0;
+        // create free lookahead
+        memset(lfs.free.buffer, 0, lfs.cfg->lookahead_size);
+        lfs.free.off = 0;
+        lfs.free.size = lfs_min(8*lfs.cfg->lookahead_size,
+                lfs.cfg->block_count);
+        lfs.free.i = 0;
+        lfs_alloc_ack(&lfs);
+
+        // create a btree
+        lfsr_btree_t btree = LFSR_BTREE_NULL;
+
+        // set up a simulation to compare against
+        //
+        // fun fact this is slower than our actual tree! unfun fact this is
+        // starting to be a problem...
+        char *sim = malloc(N);
+        lfs_size_t *sim_weights = malloc(N*sizeof(lfs_size_t));
+        lfs_size_t sim_size = 0;
+        memset(sim, 0, N);
+        memset(sim_weights, 0, N*sizeof(lfs_size_t));
+
+        uint32_t prng = seed;
+        for (lfs_size_t i = 0; i < (N-REMAINING); i++) {
+            // choose a pseudo-random op
+            uint8_t op = TEST_PRNG(&prng) % 3;
+            // choose a pseudo-random id
+            lfs_size_t id = TEST_PRNG(&prng) % (sim_size+1);
+            // choose a pseudo-random weight
+            lfs_size_t weight = 1 + (TEST_PRNG(&prng) % W);
+
+            // calculate actual id in btree space
+            lfs_size_t weighted_id = 0;
+            for (lfs_size_t j = 0; j < id; j++) {
+                weighted_id += sim_weights[j];
+            }
+
+            if (op == 0 || id == sim_size) {
+                // push to btree
+                lfsr_btree_push(&lfs, &btree, weighted_id,
+                        LFSR_TAG_INLINED, weight,
+                        &alphas[i % 26], 1) => 0;
+
+                // push to sim
+                memmove(&sim[id+1], &sim[id], sim_size-id);
+                memmove(&sim_weights[id+1], &sim_weights[id],
+                        (sim_size-id)*sizeof(lfs_size_t));
+                sim[id] = alphas[i % 26];
+                sim_weights[id] = weight;
+                sim_size += 1;
+
+            } else if (op == 1) {
+                // update btree
+                lfsr_btree_update(&lfs, &btree,
+                        weighted_id+sim_weights[id]-1, LFSR_TAG_INLINED, weight,
+                        &alphas[i % 26], 1) => 0;
+
+                // update sim
+                sim[id] = alphas[i % 26];
+                sim_weights[id] = weight;
+
+            } else {
+                // remove from btree
+                lfsr_btree_pop(&lfs, &btree,
+                        weighted_id+sim_weights[id]-1) => 0;
+
+                // remove from sim
+                memmove(&sim[id], &sim[id+1], sim_size-(id+1));
+                memmove(&sim_weights[id], &sim_weights[id+1],
+                        (sim_size-(id+1))*sizeof(lfs_size_t));
+                sim_size -= 1;
+            }
+        }
+
+        // check that btree matches sim
+        printf("expd: [");
+        bool first = true;
+        for (lfs_size_t i = 0; i < sim_size; i++) {
+            if (!first) {
+                printf(", ");
+            }
+            first = false;
+            printf("%c", sim[i]);
+        }
+        printf("]\n");
+        printf("btree: 0x%x.%x 0x%x w%d\n",
+            btree.u.trunk.block,
+            btree.u.trunk.limit,
+            btree.tag,
+            btree.weight);
+
+        lfs_size_t total_weight = 0;
+        for (lfs_size_t j = 0; j < sim_size; j++) {
+            total_weight += sim_weights[j];
+        }
+        assert(btree.weight == total_weight);
+
+        uint8_t buffer[4];
+        lfsr_tag_t tag_;
+        lfs_size_t id_;
+        lfs_size_t weight_;
+        for (lfs_size_t i = 0; i < sim_size; i++) {
+            // calculate actual id in btree space
+            lfs_size_t weighted_id = 0;
+            for (lfs_size_t j = 0; j < i; j++) {
+                weighted_id += sim_weights[j];
+            }
+
+            lfsr_btree_get(&lfs, &btree, weighted_id+sim_weights[i]-1,
+                    &tag_, &id_, &weight_,
+                    buffer, 4) => 1;
+            assert(tag_ == LFSR_TAG_INLINED);
+            assert(id_ == weighted_id+sim_weights[i]-1);
+            assert(weight_ == sim_weights[i]);
+            assert(memcmp(buffer, &sim[i], 1) == 0);
+        }
+
+        // and no extra elements
+        lfsr_btree_get(&lfs, &btree, total_weight,
+                &tag_, &id_, &weight_,
+                buffer, 4) => LFS_ERR_NOENT;
+
+        // also test that we can traverse the tree without prior knowledge
+        id_ = -1;
+        for (lfs_size_t i = 0; i < sim_size; i++) {
+            // calculate actual id in btree space
+            lfs_size_t weighted_id = 0;
+            for (lfs_size_t j = 0; j < i; j++) {
+                weighted_id += sim_weights[j];
+            }
+
+            lfsr_btree_get(&lfs, &btree, id_+1,
+                    &tag_, &id_, &weight_,
+                    buffer, 4) => 1;
+            assert(tag_ == LFSR_TAG_INLINED);
+            assert(id_ == weighted_id+sim_weights[i]-1);
+            assert(weight_ == sim_weights[i]);
+            assert(memcmp(buffer, &sim[i], 1) == 0);
+        }
+        lfsr_btree_get(&lfs, &btree, id_+1,
+                &tag_, &id_, &weight_,
+                buffer, 4) => LFS_ERR_NOENT;
+
+        // clean up sim
+        free(sim);
+    }
+'''
+