diff --git a/README.md b/README.md
index 2cff7ad..9bcb32d 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,15 @@ uv pip install -e ".[gpu-cu13]"
 
 The app auto-detects GPU availability at runtime and falls back to CPU if anything goes wrong — no configuration needed. The CPU sklearn path is auto-accelerated by [scikit-learn-intelex](https://github.com/uxlfoundation/scikit-learn-intelex)[^1]. You can also manually select backends (`cuML`, `sklearn`) in the sidebar.
 
+### Reproducibility
+
+To get reproducible projections and clusters, enable **Use fixed seed** in the sidebar and pin the backend instead of `auto`: the GPU backend is `cuML`, the CPU backend is `sklearn` (auto-accelerated by `scikit-learn-intelex` on x86 CPUs). With a seed and a pinned backend, results are identical across app restarts on both backends, with one exception: cuML t-SNE, which never reproduces exactly.
+
+- **PCA** is a deterministic decomposition, with no stochastic optimization involved. cuML PCA uses a full eigendecomposition and always returns the same result, seed or no seed. sklearn can auto-select a randomized SVD solver, so the app passes the seed to make it reproducible.
+- **UMAP** and **KMeans** reproduce exactly on both backends when a seed is set. (Seeded UMAP trades some speed for determinism.)
+- **t-SNE** reproduces on `sklearn` when a seed is set. cuML's implementation is highly parallelized and documented as [not completely deterministic between runs, even with the same `random_state`](https://docs.rapids.ai/api/cuml/stable/api/generated/cuml.manifold.tsne/) (see [rapidsai/cuml#2980](https://github.com/rapidsai/cuml/issues/2980)). Select `sklearn` when t-SNE results need to be reproducible.
+- `auto` chooses a backend from data size and hardware, so the same seed can run different algorithms on different machines. Exact coordinates may also differ across library versions and hardware; a seed guarantees repeatability within one environment, not across environments.
+
 ## Usage
 
 ### Standalone Apps
diff --git a/shared/utils/clustering.py b/shared/utils/clustering.py
index 4f85e83..4de3857 100644
--- a/shared/utils/clustering.py
+++ b/shared/utils/clustering.py
@@ -199,7 +199,9 @@ def _reduce_dim_sklearn(embeddings: np.ndarray, method: str, seed: Optional[int]
     effective_workers = -1 if n_workers > 1 else n_workers
 
     if method.upper() == "PCA":
-        reducer = PCA(n_components=2)
+        # Pass random_state so the randomized SVD solver (auto-selected for
+        # large inputs) is reproducible when a seed is set; None keeps it random.
+        reducer = PCA(n_components=2, random_state=seed)
     elif method.upper() == "TSNE":
         # Adjust perplexity to be valid for the sample size
         n_samples = embeddings.shape[0]
@@ -244,6 +246,8 @@ def _reduce_dim_cuml(embeddings: np.ndarray, method: str, seed: Optional[int], n
 
         if method.upper() == "PCA":
             from cuml.decomposition import PCA as cuPCA
+            # cuML PCA takes no random_state and needs none: its full-SVD solver
+            # is deterministic, so results are already reproducible run-to-run.
             reducer = cuPCA(n_components=2)
         elif method.upper() == "TSNE":
             from cuml.manifold import TSNE as cuTSNE