diff --git a/CLAUDE.md b/CLAUDE.md
index 32a2617..bbeb296 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -72,7 +72,7 @@ GIANT is a conditional generative surrogate for the Geant4 step function. It rep
 
 **Validation** (`giant/validate.py`): step-level marginal comparisons.
 
-**Analysis** (`giant/analysis/`, `giant analyze` CLI): a lean, streaming rollout-vs-reference plotting pipeline that compares one autoregressive `giant rollout` (for a given checkpoint) against a held-out miniCaloSim reference steps file, and produces publication-styled PDFs assembled into an HTML gallery. It exploits the fact that rollout output and a raw reference file share a world-frame physical column subset under identical names (`pre_*`/`post_*`/`edep`/`step_length`/`pdg`/`material`/`event_id`), so no ALR/local-frame decode is needed — everything is world-frame mm/MeV. Structure: `sources.py` (canonical LazyFrames + synthetic-termination-row filtering + the secondary view, which is `generation>0 & step_no==0` rollout tracks vs exploded `sec_*_list` reference columns), `reduce.py` (the streaming primitives — a single `hist1d` `group_by([group,bin]).len()` pass, per-event scalars, edep-weighted depth/transverse profiles, species share, leakage), `grouping.py`/`context.py` (fixed bin edges + energy-quantile/pdg/material group sets resolved once by `prep` into `shared.json`, so every compute job is one pass with no range scan), `catalog.py` (the declarative `PlotSpec` registry — marginals × {overall,energy,pdg,material}, per-event totals, shower profiles, species/leakage, secondaries), and `render.py` (the only module importing ETPlot's `plotstyle`/LaTeX; dispatches on `Reduced.kind`, writes PDFs + `metadata.yaml`). **Input is a `giant rollout` YAML sidecar** (`condor.py:load_rollout_yaml`): its `output`/`dataset` keys name the rollout parquet and the seed file (= the reference truth), and the rest of the YAML (checkpoint, geometry oracle, cutoffs) flows into each plot's gallery metadata. `prep` derives its own **run directory** next to the rollout parquet (`<...>/analysis_<id>/`) holding `shared.json`, `run_meta.json`, `reduced/`, `plots/`. **Compute/render split:** `giant analyze submit rollout.yaml` runs `prep` then submits one HTCondor job per plot (`compute-one --run-dir`, polars/numpy only — no LaTeX on workers), each writing a small `reduced/<id>.json`; the local `giant analyze render <run_dir>` turns those into the styled PDF/gallery tree. See `giant/analysis/__init__.py`.
+**Analysis** (`giant/analysis/`, `giant analyze` CLI): a lean, streaming rollout-vs-reference plotting pipeline that compares one autoregressive `giant rollout` (for a given checkpoint) against a held-out miniCaloSim reference steps file, and produces publication-styled PDFs assembled into an HTML gallery. It exploits the fact that rollout output and a raw reference file share a world-frame physical column subset under identical names (`pre_*`/`post_*`/`edep`/`step_length`/`pdg`/`material`/`event_id`), so no ALR/local-frame decode is needed — everything is world-frame mm/MeV. Structure: `sources.py` (canonical LazyFrames + synthetic-termination-row filtering + the secondary view, which is `generation>0 & step_no==0` rollout tracks vs exploded `sec_*_list` reference columns), `reduce.py` (the streaming primitives — a single `hist1d` `group_by([group,bin]).len()` pass, per-event scalars, edep-weighted depth/transverse profiles, species share, leakage), `grouping.py`/`context.py` (fixed bin edges + energy-quantile/pdg/material group sets resolved once by `prep` into `shared.json`, so every compute job is one pass with no range scan), `catalog.py` (the declarative `PlotSpec` registry — marginals × {overall,energy,pdg,material}, per-event totals, shower profiles, species/leakage, secondaries), and `render.py` (the only module importing ETPlot's `plotstyle`/LaTeX; dispatches on `Reduced.kind`, writes PDFs + `metadata.yaml`). **Input is a `giant rollout` YAML sidecar** (`condor.py:load_rollout_yaml`): its `output`/`dataset` keys name the rollout parquet and the seed file (= the reference truth), and the rest of the YAML (checkpoint, geometry oracle, cutoffs) flows into each plot's gallery metadata. `prep` derives its own **run directory** next to the rollout parquet (`<...>/analysis_<id>/`) holding `shared.json`, `run_meta.json`, `reduced_partial/`, `reduced/`, `plots/`. **Compute/merge/render split:** `giant analyze submit rollout.yaml --chunks N` runs `prep` (recording the run's chunk count `N` in `run_meta.json`) then submits one HTCondor job per (plot, chunk) pair (`compute-one --id --chunk --run-dir`, polars/numpy only — no LaTeX on workers), each streaming over an `event_id`-disjoint slice (`event_id % N == chunk`) and writing a small `reduced_partial/<id>__<chunk>.json`; every `PlotSpec` (`catalog.py`) splits into a `compute_partial`/`finalize` pair so a plot's chunks can be summed/concatenated back together correctly (`chunkable=False` specs — the router diagnostics, already bounded/subsampled — always run as a single chunk regardless of `N`). The local `giant analyze render <run_dir>` first joins every plot's chunk partials into `reduced/<id>.json` (`merge_all`, a no-op join when `N=1`), then turns those into the styled PDF/gallery tree. See `giant/analysis/__init__.py`.
 
 **Shower rollout** (`giant/rollout.py`, `giant rollout` CLI): autoregressively steps the two-stage model into a full shower — each primary post-step becomes the next pre-step, secondaries are pushed as new tracks, and per-step `material`/`layer_id` come from a `GeometryOracle` (`giant/geometry.py`, built via `dwarf build-geometry-oracle`) that learns position → (material, layer_id) from data and flags detector escape by nearest-neighbour distance. Tracks terminate on energy cutoff, per-track max steps, escape, or natural end; energy is deposited locally on every stop except escape (leakage), so showers conserve energy by construction.
 
diff --git a/giant/analysis/__init__.py b/giant/analysis/__init__.py
index a646f80..8af48cb 100644
--- a/giant/analysis/__init__.py
+++ b/giant/analysis/__init__.py
@@ -2,7 +2,8 @@
 
 Compares one autoregressive ``giant rollout`` against a held-out miniCaloSim
 reference file, producing publication-styled comparison plots generated in
-parallel on HTCondor (one job per plot, compute/render split).
+parallel on HTCondor (one job per plot x data chunk, compute/merge/render
+split).
 
 Only ``render`` (and the ``render`` CLI path) imports plotstyle/LaTeX; everything
 re-exported here is plotstyle-free so it runs on a compute worker. Import
@@ -17,11 +18,13 @@ from giant.analysis.condor import (
     compute_reduced,
     derive_run_dir,
     load_rollout_yaml,
+    merge_all,
+    merge_one,
     prep,
     write_submit,
 )
 from giant.analysis.context import Context, build_context
-from giant.analysis.reduced import Reduced
+from giant.analysis.reduced import Partial, Reduced
 from giant.analysis.sources import Side
 
 __all__ = [
@@ -34,10 +37,13 @@ __all__ = [
     "compute_reduced",
     "derive_run_dir",
     "load_rollout_yaml",
+    "merge_all",
+    "merge_one",
     "prep",
     "write_submit",
     "Context",
     "build_context",
+    "Partial",
     "Reduced",
     "Side",
 ]
diff --git a/giant/analysis/catalog.py b/giant/analysis/catalog.py
index a0e5bc0..fc665e3 100644
--- a/giant/analysis/catalog.py
+++ b/giant/analysis/catalog.py
@@ -2,9 +2,22 @@
 
 Each spec knows its stable ``id`` (used for the reduced-data filename, the PDF
 stem and the condor queue item), its gallery ``family`` (subdirectory), and a
-``compute(bundle) -> Reduced`` that runs the streaming reduction. Rendering lives
-in ``render.py`` and dispatches on ``Reduced.kind`` — the catalog itself never
-imports plotstyle, so ``compute-one`` jobs stay LaTeX-free.
+``compute_partial(bundle) -> dict`` / ``finalize(parts, ctx) -> Reduced`` pair
+that together run the streaming reduction. ``compute_partial`` runs once per
+``(plot, chunk)`` condor job against a ``Bundle`` whose four LazyFrames are
+already filtered to that chunk (see ``Bundle.open``'s ``chunk`` argument); it
+returns a small JSON-safe partial artifact — either a raw sum-mergeable count
+dict (histograms/species sums against fixed edges) or a raw per-event/
+per-secondary array to be concatenated (anything that derives its own edges or
+a mean/std from the full dataset). ``finalize`` merges the per-chunk partials
+(in chunk order) and does the actual histogramming/edge-selection/mean-std
+collapse, once, over the merged data — for ``n_chunks=1`` this reproduces
+exactly what a single unchunked pass would produce. Specs marked
+``chunkable=False`` (the router ones) always run as a single chunk regardless
+of the configured chunk count.
+
+Rendering lives in ``render.py`` and dispatches on ``Reduced.kind`` — the
+catalog itself never imports plotstyle, so ``compute-one`` jobs stay LaTeX-free.
 
 The registry is built by expanding parametric families (marginals over
 variable x grouping, secondaries, ...) into concrete specs.
@@ -12,7 +25,7 @@ variable x grouping, secondaries, ...) into concrete specs.
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import asdict, dataclass
 from typing import Callable
 
 import numpy as np
@@ -32,9 +45,11 @@ from giant.analysis.reduce import (
     event_scalars,
     hist1d,
     leakage_fraction,
+    profile_finalize,
+    profile_partial,
     species_share,
+    sum_merge,
     transverse_expr,
-    weighted_profile,
 )
 from giant.analysis.reduced import Reduced
 from giant.analysis.router_gating import (
@@ -58,9 +73,29 @@ class Bundle:
     checkpoint: str | None = None  # from the rollout YAML; router_gating only
 
     @classmethod
-    def open(cls, rollout, reference, ctx: Context, checkpoint=None) -> "Bundle":
+    def open(
+        cls,
+        rollout,
+        reference,
+        ctx: Context,
+        checkpoint=None,
+        chunk: tuple[int, int] | None = None,
+    ) -> "Bundle":
+        """Open both sides, optionally restricted to one event-disjoint chunk.
+
+        ``chunk = (chunk_index, n_chunks)`` filters both sides to
+        ``event_id % n_chunks == chunk_index`` *before* deriving the physical/
+        secondary views, so every downstream reduction (which is either
+        row-local or a ``group_by("event_id")``) sees a self-contained,
+        event-disjoint slice — no cross-chunk lookups are ever needed.
+        """
         r_all = open_side(rollout, Side.rollout)
         t_all = open_side(reference, Side.reference)
+        if chunk is not None:
+            idx, n = chunk
+            pred = pl.col("event_id") % n == idx
+            r_all = r_all.filter(pred)
+            t_all = t_all.filter(pred)
         return cls(
             ctx=ctx,
             r_all=r_all,
@@ -75,7 +110,26 @@ class Bundle:
 class PlotSpec:
     id: str
     family: str
-    compute: Callable[[Bundle], Reduced]
+    compute_partial: Callable[[Bundle], dict]
+    finalize: Callable[[list[dict], Context], Reduced]
+    chunkable: bool = True
+
+
+def _unchunkable(
+    compute: Callable[[Bundle], Reduced],
+) -> tuple[Callable[[Bundle], dict], Callable[[list[dict], Context], Reduced]]:
+    """Wrap a whole-dataset ``compute(bundle) -> Reduced`` as a trivial
+    ``(compute_partial, finalize)`` pair, for specs marked ``chunkable=False``
+    (which always run as a single chunk, so ``parts`` is always one element).
+    """
+
+    def partial(b: Bundle) -> dict:
+        return {"reduced": asdict(compute(b))}
+
+    def finalize(parts: list[dict], ctx: Context) -> Reduced:
+        return Reduced(**parts[0]["reduced"])
+
+    return partial, finalize
 
 
 # ---------------------------------------------------------------------------
@@ -90,6 +144,20 @@ def _counts(h: dict, key, nbins: int) -> list[int]:
     return h.get(key, np.zeros(nbins, dtype=np.int64)).astype(np.int64).tolist()
 
 
+def _partial_hist(
+    lf: pl.LazyFrame, value: pl.Expr, edges: np.ndarray, group: pl.Expr | None = None
+) -> dict[str, list[int]]:
+    """One chunk's raw ``hist1d`` result as a JSON-safe, sum-mergeable dict."""
+    nb = len(edges) - 1
+    h = hist1d(lf, value, edges, group=group)
+    return {str(k): _counts(h, k, nb) for k in h}
+
+
+def _finalize_counts(merged: dict[str, list], key, nbins: int) -> list[int]:
+    """One group's merged counts (zero-filled if the group never appeared)."""
+    return list(merged.get(str(key), [0] * nbins))
+
+
 def _np_hist_pair(
     r: np.ndarray, t: np.ndarray, nbins: int
 ) -> tuple[np.ndarray, np.ndarray, np.ndarray]:
@@ -132,12 +200,21 @@ def _marginal_edges(ctx: Context, var: str) -> np.ndarray:
 # ---------------------------------------------------------------------------
 
 
-def _marginal_overall(b: Bundle, var: str) -> Reduced:
-    label, expr = _var(var)
+def _marginal_overall_partial(b: Bundle, var: str) -> dict:
+    _, expr = _var(var)
     edges = _marginal_edges(b.ctx, var)
+    return {
+        "r": _partial_hist(b.r_phys, expr, edges),
+        "t": _partial_hist(b.t_phys, expr, edges),
+    }
+
+
+def _marginal_overall_finalize(parts: list[dict], ctx: Context, var: str) -> Reduced:
+    label, _ = _var(var)
+    edges = _marginal_edges(ctx, var)
     nb = len(edges) - 1
-    r = hist1d(b.r_phys, expr, edges)
-    t = hist1d(b.t_phys, expr, edges)
+    r = sum_merge([p["r"] for p in parts])
+    t = sum_merge([p["t"] for p in parts])
     return Reduced(
         id=f"marginal_{var}",
         family="marginals",
@@ -146,8 +223,8 @@ def _marginal_overall(b: Bundle, var: str) -> Reduced:
         xlabel=label,
         payload={
             "edges": edges.tolist(),
-            _ROLL: _counts(r, 0, nb),
-            _REF: _counts(t, 0, nb),
+            _ROLL: _finalize_counts(r, 0, nb),
+            _REF: _finalize_counts(t, 0, nb),
             "log_y": True,
         },
     )
@@ -160,31 +237,55 @@ def _energy_group_expr(lf: pl.LazyFrame, edges: np.ndarray) -> pl.Expr:
     )
 
 
-def _marginal_grouped(b: Bundle, var: str, axis: str) -> Reduced:
-    label, expr = _var(var)
+def _marginal_grouped_partial(b: Bundle, var: str, axis: str) -> dict:
+    _, expr = _var(var)
     edges = _marginal_edges(b.ctx, var)
-    nb = len(edges) - 1
-    groups: dict[str, dict] = {}
-
     if axis == "pdg":
         r = hist1d(b.r_phys, expr, edges, group=pl.col("pdg"))
         t = hist1d(b.t_phys, expr, edges, group=pl.col("pdg"))
-        for k in b.ctx.top_pdgs:
-            groups[pdg_label(k)] = {_ROLL: _counts(r, k, nb), _REF: _counts(t, k, nb)}
     elif axis == "material":
         r = hist1d(b.r_phys, expr, edges, group=pl.col("material"))
         t = hist1d(b.t_phys, expr, edges, group=pl.col("material"))
-        for m in b.ctx.materials:
-            groups[material_label(m)] = {
-                _ROLL: _counts(r, m, nb),
-                _REF: _counts(t, m, nb),
-            }
     else:  # energy
         e_edges = np.asarray(b.ctx.energy_edges)
         r = hist1d(b.r_phys, expr, edges, group=_energy_group_expr(b.r_phys, e_edges))
         t = hist1d(b.t_phys, expr, edges, group=_energy_group_expr(b.t_phys, e_edges))
+    nb = len(edges) - 1
+    return {
+        "r": {str(k): _counts(r, k, nb) for k in r},
+        "t": {str(k): _counts(t, k, nb) for k in t},
+    }
+
+
+def _marginal_grouped_finalize(
+    parts: list[dict], ctx: Context, var: str, axis: str
+) -> Reduced:
+    label, _ = _var(var)
+    edges = _marginal_edges(ctx, var)
+    nb = len(edges) - 1
+    r = sum_merge([p["r"] for p in parts])
+    t = sum_merge([p["t"] for p in parts])
+    groups: dict[str, dict] = {}
+
+    if axis == "pdg":
+        for k in ctx.top_pdgs:
+            groups[pdg_label(k)] = {
+                _ROLL: _finalize_counts(r, k, nb),
+                _REF: _finalize_counts(t, k, nb),
+            }
+    elif axis == "material":
+        for m in ctx.materials:
+            groups[material_label(m)] = {
+                _ROLL: _finalize_counts(r, m, nb),
+                _REF: _finalize_counts(t, m, nb),
+            }
+    else:  # energy
+        e_edges = np.asarray(ctx.energy_edges)
         for bi, lbl in enumerate(energy_bin_labels(e_edges)):
-            groups[lbl] = {_ROLL: _counts(r, bi, nb), _REF: _counts(t, bi, nb)}
+            groups[lbl] = {
+                _ROLL: _finalize_counts(r, bi, nb),
+                _REF: _finalize_counts(t, bi, nb),
+            }
 
     return Reduced(
         id=f"marginal_{var}_by_{axis}",
@@ -201,13 +302,19 @@ def _marginal_grouped(b: Bundle, var: str, axis: str) -> Reduced:
 # ---------------------------------------------------------------------------
 
 
-def _event_scalar(
-    b: Bundle, spec_id: str, title: str, xlabel: str, col: str, use_all: bool
-) -> Reduced:
+def _event_scalar_partial(b: Bundle, col: str, use_all: bool) -> dict:
     r_lf, t_lf = (b.r_all, b.t_all) if use_all else (b.r_phys, b.t_phys)
     r = event_scalars(r_lf)[col].to_numpy()
     t = event_scalars(t_lf)[col].to_numpy()
-    edges, rc, tc = _np_hist_pair(r, t, b.ctx.n_marginal_bins)
+    return {"r": r.tolist(), "t": t.tolist()}
+
+
+def _event_scalar_finalize(
+    parts: list[dict], ctx: Context, spec_id: str, title: str, xlabel: str
+) -> Reduced:
+    r = np.concatenate([np.asarray(p["r"], dtype=float) for p in parts])
+    t = np.concatenate([np.asarray(p["t"], dtype=float) for p in parts])
+    edges, rc, tc = _np_hist_pair(r, t, ctx.n_marginal_bins)
     return Reduced(
         id=spec_id,
         family="event",
@@ -223,18 +330,26 @@ def _event_scalar(
     )
 
 
-def _event_total_edep_by_energy(b: Bundle) -> Reduced:
-    e_edges = np.asarray(b.ctx.energy_edges)
+def _event_total_edep_by_energy_partial(b: Bundle) -> dict:
     r = event_scalars(b.r_all)
     t = event_scalars(b.t_all)
-    r_bin = np.clip(
-        np.digitize(r["incident_E"].to_numpy(), e_edges[1:-1]), 0, len(e_edges) - 2
-    )
-    t_bin = np.clip(
-        np.digitize(t["incident_E"].to_numpy(), e_edges[1:-1]), 0, len(e_edges) - 2
-    )
-    r_val, t_val = r["total_edep"].to_numpy(), t["total_edep"].to_numpy()
-    edges, _, _ = _np_hist_pair(r_val, t_val, b.ctx.n_marginal_bins)
+    return {
+        "r_incident": r["incident_E"].to_list(),
+        "r_edep": r["total_edep"].to_list(),
+        "t_incident": t["incident_E"].to_list(),
+        "t_edep": t["total_edep"].to_list(),
+    }
+
+
+def _event_total_edep_by_energy_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    e_edges = np.asarray(ctx.energy_edges)
+    r_inc = np.concatenate([np.asarray(p["r_incident"], dtype=float) for p in parts])
+    r_val = np.concatenate([np.asarray(p["r_edep"], dtype=float) for p in parts])
+    t_inc = np.concatenate([np.asarray(p["t_incident"], dtype=float) for p in parts])
+    t_val = np.concatenate([np.asarray(p["t_edep"], dtype=float) for p in parts])
+    r_bin = np.clip(np.digitize(r_inc, e_edges[1:-1]), 0, len(e_edges) - 2)
+    t_bin = np.clip(np.digitize(t_inc, e_edges[1:-1]), 0, len(e_edges) - 2)
+    edges, _, _ = _np_hist_pair(r_val, t_val, ctx.n_marginal_bins)
     groups: dict[str, dict] = {}
     for bi, lbl in enumerate(energy_bin_labels(e_edges)):
         rc = np.histogram(r_val[r_bin == bi], edges)[0]
@@ -258,15 +373,54 @@ def _event_total_edep_by_energy(b: Bundle) -> Reduced:
 # ---------------------------------------------------------------------------
 
 
-def _profile(
-    b: Bundle, spec_id: str, title: str, xlabel: str, coord_fn, edges_key: str
-) -> Reduced:
+def _profile_partial(b: Bundle, coord_fn, edges_key: str) -> dict:
     edges = np.asarray(getattr(b.ctx, edges_key))
-    r_ea, t_ea = entry_axis(b.r_all), entry_axis(b.t_all)
-    r_lf = attach_entry_axis(b.r_all, r_ea)
-    t_lf = attach_entry_axis(b.t_all, t_ea)
-    r_mean, r_std = weighted_profile(r_lf, coord_fn(), edges, pl.col("edep"))
-    t_mean, t_std = weighted_profile(t_lf, coord_fn(), edges, pl.col("edep"))
+    r_lf = attach_entry_axis(b.r_all, entry_axis(b.r_all))
+    t_lf = attach_entry_axis(b.t_all, entry_axis(b.t_all))
+    r_ids, r_mat = profile_partial(r_lf, coord_fn(), edges, pl.col("edep"))
+    t_ids, t_mat = profile_partial(t_lf, coord_fn(), edges, pl.col("edep"))
+    return {
+        "r_ids": r_ids.tolist(),
+        "r_mat": r_mat.tolist(),
+        "t_ids": t_ids.tolist(),
+        "t_mat": t_mat.tolist(),
+    }
+
+
+def _assert_event_disjoint(id_lists: list[list[int]], spec_id: str, side: str) -> None:
+    """Guard the chunking invariant profiles depend on: no event in two chunks.
+
+    A violation would silently double-count that event in the merged mean/RMS
+    with no other symptom, so this is worth a loud failure rather than a
+    quietly-wrong plot.
+    """
+    seen: set[int] = set()
+    for ids in id_lists:
+        overlap = seen & set(ids)
+        if overlap:
+            raise ValueError(
+                f"{spec_id} ({side}): event_id(s) {sorted(overlap)[:5]} appear "
+                "in more than one chunk — chunking must be event-disjoint"
+            )
+        seen.update(ids)
+
+
+def _profile_finalize(
+    parts: list[dict],
+    ctx: Context,
+    spec_id: str,
+    title: str,
+    xlabel: str,
+    edges_key: str,
+) -> Reduced:
+    edges = np.asarray(getattr(ctx, edges_key))
+    nb = len(edges) - 1
+    _assert_event_disjoint([p["r_ids"] for p in parts], spec_id, "rollout")
+    _assert_event_disjoint([p["t_ids"] for p in parts], spec_id, "reference")
+    r_mats = [np.asarray(p["r_mat"], dtype=float).reshape(-1, nb) for p in parts]
+    t_mats = [np.asarray(p["t_mat"], dtype=float).reshape(-1, nb) for p in parts]
+    r_mean, r_std = profile_finalize(r_mats)
+    t_mean, t_std = profile_finalize(t_mats)
     return Reduced(
         id=spec_id,
         family="shower",
@@ -289,14 +443,21 @@ def _profile(
 # ---------------------------------------------------------------------------
 
 
-def _species_share(b: Bundle) -> Reduced:
+def _species_share_partial(b: Bundle) -> dict:
     r = species_share(b.r_all)
     t = species_share(b.t_all)
-    r_map = dict(zip(r["pdg"].to_list(), r["total_edep"].to_list()))
-    t_map = dict(zip(t["pdg"].to_list(), t["total_edep"].to_list()))
+    return {
+        "r": {str(k): v for k, v in zip(r["pdg"].to_list(), r["total_edep"].to_list())},
+        "t": {str(k): v for k, v in zip(t["pdg"].to_list(), t["total_edep"].to_list())},
+    }
+
+
+def _species_share_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    r_map = sum_merge([p["r"] for p in parts])
+    t_map = sum_merge([p["t"] for p in parts])
     r_tot = sum(r_map.values()) or 1.0
     t_tot = sum(t_map.values()) or 1.0
-    labels = [pdg_label(k) for k in b.ctx.top_pdgs]
+    labels = [pdg_label(k) for k in ctx.top_pdgs]
     return Reduced(
         id="species_edep_share",
         family="species",
@@ -305,19 +466,22 @@ def _species_share(b: Bundle) -> Reduced:
         xlabel="species",
         payload={
             "labels": labels,
-            _ROLL: [r_map.get(k, 0.0) / r_tot for k in b.ctx.top_pdgs],
-            _REF: [t_map.get(k, 0.0) / t_tot for k in b.ctx.top_pdgs],
+            _ROLL: [r_map.get(str(k), 0.0) / r_tot for k in ctx.top_pdgs],
+            _REF: [t_map.get(str(k), 0.0) / t_tot for k in ctx.top_pdgs],
             "ylabel": "fraction of total deposited energy",
         },
     )
 
 
-def _leakage(b: Bundle) -> Reduced:
+def _leakage_partial(b: Bundle) -> dict:
     frac = leakage_fraction(b.r_all)
+    return {"frac": frac.tolist()}
+
+
+def _leakage_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    frac = np.concatenate([np.asarray(p["frac"], dtype=float) for p in parts])
     edges = np.linspace(
-        0.0,
-        max(float(frac.max()) if len(frac) else 1.0, 1e-3),
-        b.ctx.n_marginal_bins + 1,
+        0.0, max(float(frac.max()) if len(frac) else 1.0, 1e-3), ctx.n_marginal_bins + 1
     )
     counts = np.histogram(frac, edges)[0]
     return Reduced(
@@ -347,7 +511,7 @@ def _sec_frames(b: Bundle):
     )
 
 
-def _sec_count_per_event(b: Bundle) -> Reduced:
+def _sec_count_per_event_partial(b: Bundle) -> dict:
     r_sec, t_sec = _sec_frames(b)
     r = (
         r_sec.group_by("event_id")
@@ -361,9 +525,13 @@ def _sec_count_per_event(b: Bundle) -> Reduced:
         .collect(engine="streaming")["n"]
         .to_numpy()
     )
-    edges, rc, tc = _np_hist_pair(
-        r.astype(float), t.astype(float), min(b.ctx.n_marginal_bins, 40)
-    )
+    return {"r": r.tolist(), "t": t.tolist()}
+
+
+def _sec_count_per_event_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    r = np.concatenate([np.asarray(p["r"], dtype=float) for p in parts])
+    t = np.concatenate([np.asarray(p["t"], dtype=float) for p in parts])
+    edges, rc, tc = _np_hist_pair(r, t, min(ctx.n_marginal_bins, 40))
     return Reduced(
         id="sec_count_per_event",
         family="secondaries",
@@ -379,32 +547,21 @@ def _sec_count_per_event(b: Bundle) -> Reduced:
     )
 
 
-def _sec_count_per_species(b: Bundle) -> Reduced:
+def _counts_by_pdg(sec_lf: pl.LazyFrame) -> dict[str, int]:
+    df = sec_lf.group_by("pdg").agg(pl.len().alias("n")).collect(engine="streaming")
+    return {str(k): v for k, v in zip(df["pdg"].to_list(), df["n"].to_list())}
+
+
+def _sec_count_per_species_partial(b: Bundle) -> dict:
     r_sec, t_sec = _sec_frames(b)
-    r = dict(
-        zip(
-            *[
-                r_sec.group_by("pdg")
-                .agg(pl.len().alias("n"))
-                .collect(engine="streaming")[c]
-                .to_list()
-                for c in ("pdg", "n")
-            ]
-        )
-    )
-    t = dict(
-        zip(
-            *[
-                t_sec.group_by("pdg")
-                .agg(pl.len().alias("n"))
-                .collect(engine="streaming")[c]
-                .to_list()
-                for c in ("pdg", "n")
-            ]
-        )
-    )
+    return {"r": _counts_by_pdg(r_sec), "t": _counts_by_pdg(t_sec)}
+
+
+def _sec_count_per_species_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    r = sum_merge([p["r"] for p in parts])
+    t = sum_merge([p["t"] for p in parts])
     keys = sorted(set(r) | set(t), key=lambda k: -(r.get(k, 0) + t.get(k, 0)))[
-        : len(b.ctx.top_pdgs)
+        : len(ctx.top_pdgs)
     ]
     return Reduced(
         id="sec_count_per_species",
@@ -413,7 +570,7 @@ def _sec_count_per_species(b: Bundle) -> Reduced:
         title="Secondary count by species",
         xlabel="species",
         payload={
-            "labels": [pdg_label(k) for k in keys],
+            "labels": [pdg_label(int(k)) for k in keys],
             _ROLL: [float(r.get(k, 0)) for k in keys],
             _REF: [float(t.get(k, 0)) for k in keys],
             "ylabel": "secondary count",
@@ -421,12 +578,20 @@ def _sec_count_per_species(b: Bundle) -> Reduced:
     )
 
 
-def _sec_energy(b: Bundle) -> Reduced:
+def _sec_energy_partial(b: Bundle) -> dict:
     r_sec, t_sec = _sec_frames(b)
     edges = np.linspace(*b.ctx.sec_energy_range, b.ctx.n_sec_bins + 1)
-    r = hist1d(r_sec, pl.col("energy"), edges)
-    t = hist1d(t_sec, pl.col("energy"), edges)
+    return {
+        "r": _partial_hist(r_sec, pl.col("energy"), edges),
+        "t": _partial_hist(t_sec, pl.col("energy"), edges),
+    }
+
+
+def _sec_energy_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    edges = np.linspace(*ctx.sec_energy_range, ctx.n_sec_bins + 1)
     nb = len(edges) - 1
+    r = sum_merge([p["r"] for p in parts])
+    t = sum_merge([p["t"] for p in parts])
     return Reduced(
         id="sec_energy",
         family="secondaries",
@@ -435,27 +600,34 @@ def _sec_energy(b: Bundle) -> Reduced:
         xlabel="secondary energy [MeV]",
         payload={
             "edges": edges.tolist(),
-            _ROLL: _counts(r, 0, nb),
-            _REF: _counts(t, 0, nb),
+            _ROLL: _finalize_counts(r, 0, nb),
+            _REF: _finalize_counts(t, 0, nb),
             "log_y": True,
         },
     )
 
 
-def _sec_cos_angle(b: Bundle) -> Reduced:
+def _sec_cos_angle_partial(b: Bundle) -> dict:
     edges = np.linspace(-1.0, 1.0, b.ctx.n_sec_bins + 1)
-    nb = len(edges) - 1
     cos = (
         pl.col("sdx") * pl.col("axis_x")
         + pl.col("sdy") * pl.col("axis_y")
         + pl.col("sdz") * pl.col("axis_z")
     ).clip(-1.0, 1.0)
 
-    def _side(sec_lf: pl.LazyFrame, steps_lf: pl.LazyFrame) -> list[int]:
+    def _side(sec_lf: pl.LazyFrame, steps_lf: pl.LazyFrame) -> dict[str, list[int]]:
         ea = entry_axis(steps_lf)
-        return _counts(hist1d(attach_entry_axis(sec_lf, ea), cos, edges), 0, nb)
+        return _partial_hist(attach_entry_axis(sec_lf, ea), cos, edges)
 
     r_sec, t_sec = _sec_frames(b)
+    return {"r": _side(r_sec, b.r_phys), "t": _side(t_sec, b.t_all)}
+
+
+def _sec_cos_angle_finalize(parts: list[dict], ctx: Context) -> Reduced:
+    edges = np.linspace(-1.0, 1.0, ctx.n_sec_bins + 1)
+    nb = len(edges) - 1
+    r = sum_merge([p["r"] for p in parts])
+    t = sum_merge([p["t"] for p in parts])
     return Reduced(
         id="sec_cos_angle",
         family="secondaries",
@@ -464,13 +636,30 @@ def _sec_cos_angle(b: Bundle) -> Reduced:
         xlabel="cos of emission angle",
         payload={
             "edges": edges.tolist(),
-            _ROLL: _side(r_sec, b.r_phys),
-            _REF: _side(t_sec, b.t_all),
+            _ROLL: _finalize_counts(r, 0, nb),
+            _REF: _finalize_counts(t, 0, nb),
             "log_y": False,
         },
     )
 
 
+# ---------------------------------------------------------------------------
+# router diagnostics (not chunked — already bounded/subsampled)
+# ---------------------------------------------------------------------------
+
+_router_gating_partial, _router_gating_finalize = _unchunkable(
+    lambda b: compute_router_gating(b.checkpoint, b.r_phys, b.t_phys)
+)
+_router_share_pdg_partial, _router_share_pdg_finalize = _unchunkable(
+    lambda b: compute_router_share_by_pdg(
+        b.checkpoint, b.r_phys, b.t_phys, b.ctx.top_pdgs
+    )
+)
+_router_share_process_partial, _router_share_process_finalize = _unchunkable(
+    lambda b: compute_router_share_by_process(b.checkpoint, b.t_phys)
+)
+
+
 # ---------------------------------------------------------------------------
 # registry assembly
 # ---------------------------------------------------------------------------
@@ -486,7 +675,12 @@ def build_catalog() -> list[PlotSpec]:
     for var in MARGINAL_VARS:
         specs.append(
             PlotSpec(
-                f"marginal_{var}", "marginals", lambda b, v=var: _marginal_overall(b, v)
+                f"marginal_{var}",
+                "marginals",
+                compute_partial=lambda b, v=var: _marginal_overall_partial(b, v),
+                finalize=lambda parts, ctx, v=var: _marginal_overall_finalize(
+                    parts, ctx, v
+                ),
             )
         )
         for axis in GROUPING_AXES:
@@ -494,7 +688,12 @@ def build_catalog() -> list[PlotSpec]:
                 PlotSpec(
                     f"marginal_{var}_by_{axis}",
                     "marginals",
-                    lambda b, v=var, a=axis: _marginal_grouped(b, v, a),
+                    compute_partial=lambda b, v=var, a=axis: _marginal_grouped_partial(
+                        b, v, a
+                    ),
+                    finalize=lambda parts, ctx, v=var, a=axis: (
+                        _marginal_grouped_finalize(parts, ctx, v, a)
+                    ),
                 )
             )
 
@@ -502,86 +701,135 @@ def build_catalog() -> list[PlotSpec]:
         PlotSpec(
             "event_total_edep",
             "event",
-            lambda b: _event_scalar(
-                b,
+            compute_partial=lambda b: _event_scalar_partial(
+                b, "total_edep", use_all=True
+            ),
+            finalize=lambda parts, ctx: _event_scalar_finalize(
+                parts,
+                ctx,
                 "event_total_edep",
                 "Total deposited energy per event",
                 "total deposited energy [MeV]",
-                "total_edep",
-                use_all=True,
             ),
         ),
-        PlotSpec("event_total_edep_by_energy", "event", _event_total_edep_by_energy),
+        PlotSpec(
+            "event_total_edep_by_energy",
+            "event",
+            compute_partial=_event_total_edep_by_energy_partial,
+            finalize=_event_total_edep_by_energy_finalize,
+        ),
         PlotSpec(
             "event_mean_length",
             "event",
-            lambda b: _event_scalar(
-                b,
+            compute_partial=lambda b: _event_scalar_partial(
+                b, "mean_length", use_all=False
+            ),
+            finalize=lambda parts, ctx: _event_scalar_finalize(
+                parts,
+                ctx,
                 "event_mean_length",
                 "Mean step length per event",
                 "mean step length [mm]",
-                "mean_length",
-                use_all=False,
             ),
         ),
         PlotSpec(
             "event_n_steps",
             "event",
-            lambda b: _event_scalar(
-                b,
+            compute_partial=lambda b: _event_scalar_partial(
+                b, "n_steps", use_all=False
+            ),
+            finalize=lambda parts, ctx: _event_scalar_finalize(
+                parts,
+                ctx,
                 "event_n_steps",
                 "Number of steps per event",
                 "steps per event",
-                "n_steps",
-                use_all=False,
             ),
         ),
         PlotSpec(
             "shower_longitudinal",
             "shower",
-            lambda b: _profile(
-                b,
+            compute_partial=lambda b: _profile_partial(b, depth_expr, "depth_edges"),
+            finalize=lambda parts, ctx: _profile_finalize(
+                parts,
+                ctx,
                 "shower_longitudinal",
                 "Longitudinal shower profile",
                 "depth along shower axis [mm]",
-                depth_expr,
                 "depth_edges",
             ),
         ),
         PlotSpec(
             "shower_transverse",
             "shower",
-            lambda b: _profile(
-                b,
+            compute_partial=lambda b: _profile_partial(
+                b, transverse_expr, "transverse_edges"
+            ),
+            finalize=lambda parts, ctx: _profile_finalize(
+                parts,
+                ctx,
                 "shower_transverse",
                 "Transverse shower profile",
                 "radius from shower axis [mm]",
-                transverse_expr,
                 "transverse_edges",
             ),
         ),
-        PlotSpec("species_edep_share", "species", _species_share),
-        PlotSpec("leakage_fraction", "species", _leakage),
-        PlotSpec("sec_count_per_event", "secondaries", _sec_count_per_event),
-        PlotSpec("sec_count_per_species", "secondaries", _sec_count_per_species),
-        PlotSpec("sec_energy", "secondaries", _sec_energy),
-        PlotSpec("sec_cos_angle", "secondaries", _sec_cos_angle),
+        PlotSpec(
+            "species_edep_share",
+            "species",
+            compute_partial=_species_share_partial,
+            finalize=_species_share_finalize,
+        ),
+        PlotSpec(
+            "leakage_fraction",
+            "species",
+            compute_partial=_leakage_partial,
+            finalize=_leakage_finalize,
+        ),
+        PlotSpec(
+            "sec_count_per_event",
+            "secondaries",
+            compute_partial=_sec_count_per_event_partial,
+            finalize=_sec_count_per_event_finalize,
+        ),
+        PlotSpec(
+            "sec_count_per_species",
+            "secondaries",
+            compute_partial=_sec_count_per_species_partial,
+            finalize=_sec_count_per_species_finalize,
+        ),
+        PlotSpec(
+            "sec_energy",
+            "secondaries",
+            compute_partial=_sec_energy_partial,
+            finalize=_sec_energy_finalize,
+        ),
+        PlotSpec(
+            "sec_cos_angle",
+            "secondaries",
+            compute_partial=_sec_cos_angle_partial,
+            finalize=_sec_cos_angle_finalize,
+        ),
         PlotSpec(
             "router_gating",
             "model",
-            lambda b: compute_router_gating(b.checkpoint, b.r_phys, b.t_phys),
+            compute_partial=_router_gating_partial,
+            finalize=_router_gating_finalize,
+            chunkable=False,
         ),
         PlotSpec(
             "router_share_by_pdg",
             "model",
-            lambda b: compute_router_share_by_pdg(
-                b.checkpoint, b.r_phys, b.t_phys, b.ctx.top_pdgs
-            ),
+            compute_partial=_router_share_pdg_partial,
+            finalize=_router_share_pdg_finalize,
+            chunkable=False,
         ),
         PlotSpec(
             "router_share_by_process",
             "model",
-            lambda b: compute_router_share_by_process(b.checkpoint, b.t_phys),
+            compute_partial=_router_share_process_partial,
+            finalize=_router_share_process_finalize,
+            chunkable=False,
         ),
     ]
     return specs
diff --git a/giant/analysis/condor.py b/giant/analysis/condor.py
index b9ad621..606d93b 100644
--- a/giant/analysis/condor.py
+++ b/giant/analysis/condor.py
@@ -15,18 +15,25 @@ next to the rollout parquet, and lays everything out under it:
 
     <run_dir>/shared.json     fixed bin edges / group sets (prep)
     <run_dir>/run_meta.json   resolved rollout/reference paths + plot metadata
-    <run_dir>/reduced/<id>.json   one per compute job
+    <run_dir>/reduced_partial/<id>__<chunk>.json   one per (plot, chunk) job
+    <run_dir>/reduced/<id>.json   merged, per plot
     <run_dir>/plots/<family>/<id>.pdf   rendered locally
 
-Job model (one condor job per plot, compute/render split):
+Job model (one condor job per (plot, chunk), compute/merge/render split):
 
 1. ``prep`` runs once on the submit node — reads the YAML, resolves the shared
-   context from a subsample, writes ``shared.json`` + ``run_meta.json``.
-2. one job per catalog id runs ``giant analyze compute-one --run-dir`` on a
-   worker — a single streaming pass writing ``reduced/<id>.json`` (polars/numpy
-   only, no LaTeX).
-3. a final *local* ``giant analyze render`` turns those into the styled PDF +
-   gallery tree (that step imports plotstyle/LaTeX).
+   context from a subsample, writes ``shared.json`` + ``run_meta.json``
+   (including the run's configured ``n_chunks``).
+2. one job per catalog id x chunk index runs ``giant analyze compute-one
+   --run-dir`` on a worker — a single streaming pass over that
+   ``event_id``-disjoint chunk, writing ``reduced_partial/<id>__<chunk>.json``
+   (polars/numpy only, no LaTeX). Specs marked ``chunkable=False``
+   (``PlotSpec``, ``catalog.py``) always run as a single chunk.
+3. a *local* ``giant analyze render`` first merges every plot's chunk partials
+   (``merge_all`` — sums/concatenates them and re-derives any data-dependent
+   histogram edges or mean/std, per ``PlotSpec.finalize``) into
+   ``reduced/<id>.json``, then renders those into the styled PDF + gallery tree
+   (that step imports plotstyle/LaTeX).
 
 Files on ``/ceph`` or ``/work`` are reached via ``ProvidesETPResources``; no
 HTCondor file transfer of the multi-GB inputs.
@@ -42,6 +49,7 @@ import yaml
 
 from giant.analysis.catalog import Bundle, catalog_ids, get_spec
 from giant.analysis.context import Context, build_context
+from giant.analysis.reduced import Partial
 
 # Keys copied verbatim from a rollout YAML into each plot's gallery metadata.
 _PLOT_META_KEYS = (
@@ -104,6 +112,7 @@ class RunMeta:
     run_dir: str
     title: str
     plot_meta: dict
+    n_chunks: int = 1
 
     def save(self, path: str | Path) -> None:
         Path(path).write_text(json.dumps(self.__dict__, indent=2))
@@ -116,11 +125,16 @@ class RunMeta:
 def prep(
     rollout_yaml: str | Path,
     run_dir: str | Path | None = None,
+    n_chunks: int = 1,
     **ctx_kwargs,
 ) -> Path:
     """Read the rollout YAML, build the shared context, and lay out the run dir.
 
     Writes ``shared.json`` + ``run_meta.json`` and returns the run directory.
+    ``n_chunks`` is the run-level chunk count every ``compute-one``/``merge-one``
+    job reads back out of ``run_meta.json`` (via ``RunMeta.n_chunks``), so it is
+    resolved once here rather than re-passed (and risking disagreement) at every
+    later step.
     """
     y = load_rollout_yaml(rollout_yaml)
     run_path = derive_run_dir(y, run_dir)
@@ -137,12 +151,13 @@ def prep(
         run_dir=str(run_path),
         title=f"GIANT rollout analysis — {ckpt}",
         plot_meta=_plot_meta(y),
+        n_chunks=n_chunks,
     ).save(run_path / "run_meta.json")
     return run_path
 
 
 # ---------------------------------------------------------------------------
-# per-plot compute (what each condor job runs)
+# per-(plot, chunk) compute (what each condor job runs)
 # ---------------------------------------------------------------------------
 
 
@@ -153,18 +168,41 @@ def compute_reduced(
     shared: str | Path,
     out: str | Path,
     checkpoint: str | None = None,
+    chunk_index: int = 0,
+    n_chunks: int = 1,
 ) -> Path:
-    """Core: run one plot's reduction against explicit paths → ``Reduced`` JSON."""
+    """Core: run one (plot, chunk)'s partial reduction against explicit paths.
+
+    Writes a ``Partial`` JSON — the raw, not-yet-merged output of
+    ``PlotSpec.compute_partial`` — never a finished ``Reduced``; ``merge_one``
+    is what combines every chunk's ``Partial`` for a plot into the final
+    ``Reduced``. Specs with ``chunkable=False`` always run as a single chunk
+    regardless of ``n_chunks``.
+    """
     ctx = Context.load(shared)
-    bundle = Bundle.open(rollout, reference, ctx, checkpoint=checkpoint)
-    reduced = get_spec(spec_id).compute(bundle)
+    spec = get_spec(spec_id)
+    effective_n = n_chunks if spec.chunkable else 1
+    if not (0 <= chunk_index < effective_n):
+        raise ValueError(
+            f"{spec_id}: chunk_index={chunk_index} out of range for "
+            f"n_chunks={effective_n} (chunkable={spec.chunkable})"
+        )
+    bundle = Bundle.open(
+        rollout, reference, ctx, checkpoint=checkpoint, chunk=(chunk_index, effective_n)
+    )
+    partial = Partial(
+        id=spec_id,
+        family=spec.family,
+        chunk=chunk_index,
+        data=spec.compute_partial(bundle),
+    )
     out = Path(out)
-    reduced.save(out)
+    partial.save(out)
     return out
 
 
-def compute_one(spec_id: str, run_dir: str | Path) -> Path:
-    """Run one plot's reduction from a prepped run directory."""
+def compute_one(spec_id: str, run_dir: str | Path, chunk_index: int = 0) -> Path:
+    """Run one (plot, chunk)'s partial reduction from a prepped run directory."""
     run_path = Path(run_dir)
     meta = RunMeta.load(run_path / "run_meta.json")
     return compute_reduced(
@@ -172,11 +210,56 @@ def compute_one(spec_id: str, run_dir: str | Path) -> Path:
         meta.rollout,
         meta.reference,
         run_path / "shared.json",
-        run_path / "reduced" / f"{spec_id}.json",
+        run_path / "reduced_partial" / f"{spec_id}__{chunk_index}.json",
         checkpoint=meta.plot_meta.get("checkpoint"),
+        chunk_index=chunk_index,
+        n_chunks=meta.n_chunks,
     )
 
 
+# ---------------------------------------------------------------------------
+# per-plot merge (the join step ``render`` runs before rendering)
+# ---------------------------------------------------------------------------
+
+
+def merge_one(spec_id: str, run_dir: str | Path) -> Path:
+    """Merge every chunk's partial for one plot into the final ``Reduced`` JSON.
+
+    Fails loudly if fewer partials exist than the run's configured chunk count
+    for this plot — that is what catches an incomplete/failed condor job
+    instead of silently rendering a plot from partial data. Idempotent: safe
+    to call again (e.g. from ``render_run``) once all chunks are in.
+    """
+    run_path = Path(run_dir)
+    meta = RunMeta.load(run_path / "run_meta.json")
+    ctx = Context.load(run_path / "shared.json")
+    spec = get_spec(spec_id)
+    effective_n = meta.n_chunks if spec.chunkable else 1
+
+    partial_dir = run_path / "reduced_partial"
+    found = {
+        p.chunk: p
+        for p in (Partial.load(jf) for jf in partial_dir.glob(f"{spec_id}__*.json"))
+    }
+    missing = sorted(set(range(effective_n)) - set(found))
+    if missing:
+        raise FileNotFoundError(
+            f"{spec_id}: missing chunk partial(s) {missing} of {effective_n} "
+            f"under {partial_dir} — did every compute-one job finish?"
+        )
+
+    parts = [found[k].data for k in range(effective_n)]
+    reduced = spec.finalize(parts, ctx)
+    out = run_path / "reduced" / f"{spec_id}.json"
+    reduced.save(out)
+    return out
+
+
+def merge_all(run_dir: str | Path) -> list[Path]:
+    """Merge every catalog plot's chunk partials into ``reduced/<id>.json``."""
+    return [merge_one(spec_id, run_dir) for spec_id in catalog_ids()]
+
+
 # ---------------------------------------------------------------------------
 # submit description
 # ---------------------------------------------------------------------------
@@ -192,16 +275,17 @@ class SubmitConfig:
     request_cpus: int = 1
     request_walltime_s: int = 3600
     remote: bool = False  # +RemoteJob (grid I/O) vs ProvidesETPResources (local files)
+    n_chunks: int = 1  # per-plot data chunks; ignored for chunkable=False specs
 
 
 _WRAPPER = """#!/bin/bash
 set -euo pipefail
 cd {repo_dir}
-exec uv run giant analyze compute-one --id "$1" --run-dir {run_dir}
+exec uv run giant analyze compute-one --id "$1" --chunk "$2" --run-dir {run_dir}
 """
 
 
-def _submit_description(cfg: SubmitConfig, wrapper: Path, ids_file: Path) -> str:
+def _submit_description(cfg: SubmitConfig, wrapper: Path, jobs_file: Path) -> str:
     reqs_attrs = (
         "+RemoteJob = True\n"
         if cfg.remote
@@ -211,7 +295,7 @@ def _submit_description(cfg: SubmitConfig, wrapper: Path, ids_file: Path) -> str
         "universe = docker\n"
         f"docker_image = {cfg.docker_image}\n"
         f"executable = {wrapper}\n"
-        "arguments = $(plotid)\n"
+        "arguments = $(plotid) $(chunk)\n"
         "should_transfer_files = YES\n"
         "when_to_transfer_output = ON_EXIT\n"
         f"request_memory = {cfg.request_memory_mb}\n"
@@ -219,31 +303,41 @@ def _submit_description(cfg: SubmitConfig, wrapper: Path, ids_file: Path) -> str
         f"+RequestWalltime = {cfg.request_walltime_s}\n"
         f"accounting_group = {cfg.accounting_group}\n"
         f"{reqs_attrs}"
-        f"output = {cfg.run_dir}/logs/$(plotid).out\n"
-        f"error = {cfg.run_dir}/logs/$(plotid).err\n"
+        f"output = {cfg.run_dir}/logs/$(plotid)__$(chunk).out\n"
+        f"error = {cfg.run_dir}/logs/$(plotid)__$(chunk).err\n"
         f"log = {cfg.run_dir}/logs/condor.log\n"
-        f"queue plotid from {ids_file}\n"
+        f"queue plotid,chunk from {jobs_file}\n"
     )
 
 
 def write_submit(cfg: SubmitConfig, ids: list[str] | None = None) -> Path:
-    """Write the wrapper script, plot-id list, and HTCondor submit description.
+    """Write the wrapper script, (plot, chunk) job list, and HTCondor submit
+    description.
 
-    Returns the submit description path (``<run_dir>/analyze.sub``). Does not
-    submit — call ``condor_submit`` on the returned file.
+    Each catalog id gets ``cfg.n_chunks`` jobs, except ``chunkable=False``
+    specs (the router diagnostics), which always get exactly one regardless of
+    ``cfg.n_chunks``. Returns the submit description path
+    (``<run_dir>/analyze.sub``). Does not submit — call ``condor_submit`` on
+    the returned file.
     """
     ids = ids or catalog_ids()
     run_dir = cfg.run_dir
     (run_dir / "logs").mkdir(parents=True, exist_ok=True)
     (run_dir / "reduced").mkdir(parents=True, exist_ok=True)
+    (run_dir / "reduced_partial").mkdir(parents=True, exist_ok=True)
 
     wrapper = run_dir / "run_compute.sh"
     wrapper.write_text(_WRAPPER.format(repo_dir=cfg.repo_dir, run_dir=run_dir))
     wrapper.chmod(0o755)
 
-    ids_file = run_dir / "plotids.txt"
-    ids_file.write_text("\n".join(ids) + "\n")
+    jobs = [
+        (spec_id, chunk)
+        for spec_id in ids
+        for chunk in range(cfg.n_chunks if get_spec(spec_id).chunkable else 1)
+    ]
+    jobs_file = run_dir / "jobs.txt"
+    jobs_file.write_text("\n".join(f"{i},{k}" for i, k in jobs) + "\n")
 
     sub = run_dir / "analyze.sub"
-    sub.write_text(_submit_description(cfg, wrapper, ids_file))
+    sub.write_text(_submit_description(cfg, wrapper, jobs_file))
     return sub
diff --git a/giant/analysis/reduce.py b/giant/analysis/reduce.py
index 51bf035..15089f6 100644
--- a/giant/analysis/reduce.py
+++ b/giant/analysis/reduce.py
@@ -16,6 +16,8 @@ efficiency" section):
 
 from __future__ import annotations
 
+from typing import Any
+
 import numpy as np
 import polars as pl
 
@@ -58,6 +60,24 @@ def hist1d(
     return out
 
 
+def sum_merge(dicts: list[dict[str, Any]]) -> dict[str, Any]:
+    """Elementwise-sum a list of sum-mergeable count/total dicts (JSON-safe keys).
+
+    Used to merge chunked ``hist1d``/``species_share``-style partials, whose
+    values bin/group against edges or keys fixed by ``Context`` — a chunk's raw
+    count dict is exactly a partial sum, so merging is a plain elementwise sum
+    over the union of keys (a key absent from some chunk is all-zero there).
+    Values may be per-bin count lists or plain scalar totals; both round-trip
+    through ``np.asarray``/``.tolist()`` unchanged in shape.
+    """
+    out: dict[str, np.ndarray] = {}
+    for d in dicts:
+        for k, v in d.items():
+            arr = np.asarray(v)
+            out[k] = arr.copy() if k not in out else out[k] + arr
+    return {k: v.tolist() for k, v in out.items()}
+
+
 # ---------------------------------------------------------------------------
 # Per-event scalar observables (one bounded group_by pass)
 # ---------------------------------------------------------------------------
@@ -149,18 +169,19 @@ def transverse_expr() -> pl.Expr:
     return (tx**2 + ty**2 + tz**2).sqrt()
 
 
-def weighted_profile(
+def profile_partial(
     lf: pl.LazyFrame,
     coord: pl.Expr,
     edges: np.ndarray,
     weight: pl.Expr,
 ) -> tuple[np.ndarray, np.ndarray]:
-    """Event-averaged, ``weight``-summed profile of ``coord``, with an event-RMS band.
+    """One chunk's per-event x bin ``weight``-sum matrix: ``(event_ids, matrix)``.
 
-    One streaming ``group_by(event_id, bin)`` sums ``weight`` per (event, bin);
-    collapsed in numpy to the per-bin mean over events and its event-to-event std
-    (the band). ``coord``/``weight`` require the entry/axis columns attached.
-    Returns ``(mean, std)``, each length ``len(edges)-1``.
+    One streaming ``group_by(event_id, bin)`` sums ``weight`` per (event, bin).
+    A chunk's matrix rows are only the events present in that chunk, so chunks'
+    matrices stack cleanly with no cross-chunk lookup — this requires chunking
+    to be event-disjoint (every row of an event lands in one chunk).
+    ``coord``/``weight`` require the entry/axis columns attached.
     """
     lo, hi, nbins = float(edges[0]), float(edges[-1]), len(edges) - 1
     grid = (
@@ -177,7 +198,35 @@ def weighted_profile(
     uniq, inv = np.unique(ev, return_inverse=True)
     mat = np.zeros((len(uniq), nbins), dtype=np.float64)
     np.add.at(mat, (inv, grid["_b"].to_numpy()), grid["_ws"].to_numpy())
-    return mat.mean(axis=0), mat.std(axis=0)
+    return uniq, mat
+
+
+def profile_finalize(mats: list[np.ndarray]) -> tuple[np.ndarray, np.ndarray]:
+    """Collapse per-chunk per-event x bin matrices into the final mean/std profile.
+
+    Chunks are event-disjoint, so row-wise concatenation of their matrices
+    reconstructs the full per-event matrix; the mean/event-RMS collapse must
+    happen once over that full matrix — an average of per-chunk means/stds
+    would be wrong (chunks generally hold different numbers of events).
+    Returns ``(mean, std)``, each length ``nbins``.
+    """
+    full = np.concatenate(mats, axis=0)
+    return full.mean(axis=0), full.std(axis=0)
+
+
+def weighted_profile(
+    lf: pl.LazyFrame,
+    coord: pl.Expr,
+    edges: np.ndarray,
+    weight: pl.Expr,
+) -> tuple[np.ndarray, np.ndarray]:
+    """Single-pass profile of ``coord`` (mean +/- event-RMS band over events).
+
+    Convenience wrapper for the unchunked (whole-dataset) case; ``mean_std +
+    profile_partial`` is what a chunked compute/finalize split uses instead.
+    """
+    _, mat = profile_partial(lf, coord, edges, weight)
+    return profile_finalize([mat])
 
 
 # ---------------------------------------------------------------------------
diff --git a/giant/analysis/reduced.py b/giant/analysis/reduced.py
index 836c923..2188800 100644
--- a/giant/analysis/reduced.py
+++ b/giant/analysis/reduced.py
@@ -39,3 +39,27 @@ class Reduced:
     @classmethod
     def load(cls, path: str | Path) -> "Reduced":
         return cls(**json.loads(Path(path).read_text()))
+
+
+@dataclass
+class Partial:
+    """The raw, not-yet-finalized output of one ``(plot, chunk)`` compute job.
+
+    ``data`` holds whatever shape that plot's ``PlotSpec.compute_partial``
+    returns — a raw sum-mergeable count dict, or a raw per-event/per-secondary
+    array to be concatenated across chunks — never a finished histogram/profile.
+    ``PlotSpec.finalize`` is the only thing that knows how to interpret it.
+    """
+
+    id: str
+    family: str
+    chunk: int
+    data: dict
+
+    def save(self, path: str | Path) -> None:
+        Path(path).parent.mkdir(parents=True, exist_ok=True)
+        Path(path).write_text(json.dumps(asdict(self)))
+
+    @classmethod
+    def load(cls, path: str | Path) -> "Partial":
+        return cls(**json.loads(Path(path).read_text()))
diff --git a/giant/analysis/render.py b/giant/analysis/render.py
index 35cd3d6..76d2cd4 100644
--- a/giant/analysis/render.py
+++ b/giant/analysis/render.py
@@ -303,12 +303,16 @@ def render_all(
 def render_run(run_dir: str | Path, *, run_gallery: bool = False) -> list[Path]:
     """Render a prepped run directory: ``<run_dir>/reduced`` → ``<run_dir>/plots``.
 
-    Pulls the rollout provenance (checkpoint, paths, cutoffs) from
-    ``run_meta.json`` into every plot's gallery metadata.
+    First joins every plot's chunk partials (``reduced_partial/<id>__*.json``)
+    into ``reduced/<id>.json`` via ``merge_all`` — a no-op merge when the run
+    wasn't chunked (``n_chunks=1``) — then pulls the rollout provenance
+    (checkpoint, paths, cutoffs) from ``run_meta.json`` into every plot's
+    gallery metadata and renders.
     """
-    from giant.analysis.condor import RunMeta
+    from giant.analysis.condor import RunMeta, merge_all
 
     run_dir = Path(run_dir)
+    merge_all(run_dir)
     meta = RunMeta.load(run_dir / "run_meta.json")
     run_meta = {
         "title": meta.title,
diff --git a/giant/cli.py b/giant/cli.py
index e371120..1f9769f 100644
--- a/giant/cli.py
+++ b/giant/cli.py
@@ -1150,6 +1150,12 @@ def analyze_prep(
     n_energy_bins: Annotated[int, typer.Option("--energy-bins")] = 4,
     n_marginal_bins: Annotated[int, typer.Option("--bins")] = 50,
     top_k_pdg: Annotated[int, typer.Option("--top-pdg")] = 6,
+    chunks: Annotated[
+        int,
+        typer.Option(
+            "--chunks", help="Split each plot's data into this many event_id chunks"
+        ),
+    ] = 1,
 ) -> None:
     """Read the rollout YAML → shared.json + run_meta.json in the run directory."""
     from giant.analysis import prep
@@ -1157,6 +1163,7 @@ def analyze_prep(
     path = prep(
         rollout_yaml,
         run_dir,
+        n_chunks=chunks,
         n_energy_bins=n_energy_bins,
         n_marginal_bins=n_marginal_bins,
         top_k_pdg=top_k_pdg,
@@ -1172,11 +1179,34 @@ def analyze_compute_one(
     run_dir: Annotated[
         Path, typer.Option("--run-dir", help="Run directory from `analyze prep`")
     ],
+    chunk: Annotated[
+        int, typer.Option("--chunk", help="Chunk index (see `analyze prep --chunks`)")
+    ] = 0,
 ) -> None:
-    """Run one plot's streaming reduction (this is what each condor job runs)."""
+    """Run one (plot, chunk)'s streaming reduction (this is what each condor job runs)."""
     from giant.analysis import compute_one
 
-    path = compute_one(id, run_dir)
+    path = compute_one(id, run_dir, chunk_index=chunk)
+    typer.echo(f"wrote {path}")
+
+
+@analyze_app.command("merge-one")
+def analyze_merge_one(
+    id: Annotated[
+        str, typer.Option("--id", help="Catalog plot id (see `analyze list`)")
+    ],
+    run_dir: Annotated[
+        Path, typer.Option("--run-dir", help="Run directory from `analyze prep`")
+    ],
+) -> None:
+    """Merge one plot's chunk partials into its final reduced JSON.
+
+    Runs automatically as part of `analyze render`; useful standalone to
+    debug a specific plot without re-rendering everything.
+    """
+    from giant.analysis import merge_one
+
+    path = merge_one(id, run_dir)
     typer.echo(f"wrote {path}")
 
 
@@ -1224,16 +1254,23 @@ def analyze_submit(
         bool,
         typer.Option("--remote/--local", help="+RemoteJob vs ProvidesETPResources"),
     ] = False,
+    chunks: Annotated[
+        int,
+        typer.Option(
+            "--chunks",
+            help="Split each plot's data into this many event_id chunks/jobs",
+        ),
+    ] = 1,
     dry_run: Annotated[
         bool, typer.Option("--dry-run", help="Write files but don't condor_submit")
     ] = False,
 ) -> None:
-    """prep + write the HTCondor submit description (one job per plot), then submit."""
+    """prep + write the HTCondor submit description (one job per plot x chunk), then submit."""
     import subprocess
 
     from giant.analysis import SubmitConfig, prep, write_submit
 
-    path = prep(rollout_yaml, run_dir)
+    path = prep(rollout_yaml, run_dir, n_chunks=chunks)
     cfg = SubmitConfig(
         run_dir=path,
         accounting_group=accounting_group,
@@ -1241,6 +1278,7 @@ def analyze_submit(
         docker_image=docker_image,
         request_memory_mb=request_memory,
         remote=remote,
+        n_chunks=chunks,
     )
     sub = write_submit(cfg)
     typer.echo(f"run directory: {path}")
diff --git a/tests/test_catalog.py b/tests/test_catalog.py
index 90447d3..c8a47f0 100644
--- a/tests/test_catalog.py
+++ b/tests/test_catalog.py
@@ -2,21 +2,30 @@
 
 from __future__ import annotations
 
+import numpy as np
 import pytest
 
 from giant.analysis import build_catalog, catalog_ids, get_spec
-from giant.analysis.catalog import Bundle
-from giant.analysis.context import build_context
+from giant.analysis.catalog import Bundle, PlotSpec
+from giant.analysis.context import Context, build_context
 from tests.test_analysis_reduce import _reference_frame, _rollout_frame
 
 
-@pytest.fixture(scope="module")
-def bundle() -> Bundle:
+def _build_ctx() -> Context:
     r, t = _rollout_frame(), _reference_frame()
-    ctx = build_context(
+    return build_context(
         r, t, n_energy_bins=2, n_marginal_bins=10, top_k_pdg=3, sample_rows=1000
     )
-    return Bundle.open(r, t, ctx)
+
+
+@pytest.fixture(scope="module")
+def ctx() -> Context:
+    return _build_ctx()
+
+
+@pytest.fixture(scope="module")
+def bundle(ctx: Context) -> Bundle:
+    return Bundle.open(_rollout_frame(), _reference_frame(), ctx)
 
 
 def test_catalog_ids_unique_and_nonempty():
@@ -36,7 +45,7 @@ def test_get_spec_roundtrip_and_unknown():
 
 def test_every_spec_computes_valid_reduced(bundle: Bundle):
     for spec in build_catalog():
-        r = spec.compute(bundle)
+        r = spec.finalize([spec.compute_partial(bundle)], bundle.ctx)
         assert r.id == spec.id
         assert r.kind in {
             "overlay_hist",
@@ -81,3 +90,64 @@ def _validate_payload(r) -> None:
             for side in ("rollout", "reference"):
                 if side in p:
                     assert cat in p[side]
+
+
+# ---------------------------------------------------------------------------
+# chunked (compute_partial x N -> finalize) must match the unchunked (N=1) result
+# ---------------------------------------------------------------------------
+
+# One representative id per merge shape: sum-mergeable (marginal_edep,
+# sec_count_per_species via pdg-keyed sums), concat-then-finalize with
+# data-dependent edges (event_total_edep), concat-then-mean/std (shower_
+# longitudinal), concat-then-max-edge (leakage_fraction), pdg-keyed sum with a
+# ratio (species_edep_share), and a chunkable=False passthrough (router_gating).
+_CHUNK_EQUIVALENCE_IDS = [
+    "marginal_edep",
+    "species_edep_share",
+    "event_total_edep",
+    "shower_longitudinal",
+    "leakage_fraction",
+    "sec_count_per_species",
+    "router_gating",
+]
+
+
+def _assert_payload_close(a, b, path: str = "payload") -> None:
+    """Recursively compare two JSON-shaped payloads (float-tolerant)."""
+    assert type(a) is type(b), f"{path}: {type(a)} != {type(b)}"
+    if isinstance(a, dict):
+        assert set(a) == set(b), f"{path}: key mismatch {set(a)} != {set(b)}"
+        for k in a:
+            _assert_payload_close(a[k], b[k], f"{path}.{k}")
+    elif isinstance(a, list):
+        assert len(a) == len(b), f"{path}: length mismatch"
+        for i, (x, y) in enumerate(zip(a, b)):
+            _assert_payload_close(x, y, f"{path}[{i}]")
+    elif isinstance(a, float):
+        assert np.isclose(a, b, atol=1e-9), f"{path}: {a} != {b}"
+    else:
+        assert a == b, f"{path}: {a} != {b}"
+
+
+@pytest.mark.parametrize("spec_id", _CHUNK_EQUIVALENCE_IDS)
+def test_chunked_matches_unchunked(ctx: Context, spec_id: str):
+    """A plot computed over N event-disjoint chunks then merged must equal the
+    same plot computed in one unchunked pass — the core chunking correctness
+    guarantee (see the analysis-rollout-plots chunking plan)."""
+    spec: PlotSpec = get_spec(spec_id)
+    r, t = _rollout_frame(), _reference_frame()
+
+    unchunked_bundle = Bundle.open(r, t, ctx)
+    unchunked = spec.finalize([spec.compute_partial(unchunked_bundle)], ctx)
+
+    # 4 chunks over only 2 distinct event_ids also exercises empty chunks.
+    n_chunks = 4 if spec.chunkable else 1
+    parts = [
+        spec.compute_partial(Bundle.open(r, t, ctx, chunk=(k, n_chunks)))
+        for k in range(n_chunks)
+    ]
+    chunked = spec.finalize(parts, ctx)
+
+    assert chunked.id == unchunked.id
+    assert chunked.kind == unchunked.kind
+    _assert_payload_close(unchunked.payload, chunked.payload)
diff --git a/tests/test_condor.py b/tests/test_condor.py
index 1d6ff3e..6634bd8 100644
--- a/tests/test_condor.py
+++ b/tests/test_condor.py
@@ -16,11 +16,13 @@ from giant.analysis import (
     compute_reduced,
     derive_run_dir,
     load_rollout_yaml,
+    merge_one,
     prep,
     write_submit,
 )
+from giant.analysis.catalog import get_spec
 from giant.analysis.condor import Context
-from giant.analysis.reduced import Reduced
+from giant.analysis.reduced import Partial, Reduced
 from giant.constants import PREDICT_COORD_METADATA_KEY, ROLLOUT_COORD_VALUE
 from tests.test_analysis_reduce import _reference_frame, _rollout_frame
 
@@ -51,11 +53,14 @@ def _write_inputs(tmp_path: Path) -> Path:
     return yaml_path
 
 
-def _prep(rollout_yaml: Path, run_dir: str | Path | None = None) -> Path:
+def _prep(
+    rollout_yaml: Path, run_dir: str | Path | None = None, chunks: int = 1
+) -> Path:
     """``prep`` with small test-sized context bins/sampling."""
     return prep(
         rollout_yaml,
         run_dir,
+        n_chunks=chunks,
         n_energy_bins=2,
         n_marginal_bins=8,
         top_k_pdg=3,
@@ -87,15 +92,16 @@ def test_prep_lays_out_run_dir(tmp_path: Path):
     assert meta.reference.endswith("reference.parquet")
     assert meta.plot_meta["checkpoint"] == "/ckpt/best.pt"
     assert "best.pt" in meta.title
+    assert meta.n_chunks == 1
 
 
 def test_compute_one_from_run_dir(tmp_path: Path):
     run_dir = _prep(_write_inputs(tmp_path))
     out = compute_one("marginal_edep", run_dir)
-    assert out == run_dir / "reduced" / "marginal_edep.json"
-    reduced = Reduced.load(out)
-    assert reduced.id == "marginal_edep"
-    assert len(reduced.payload["rollout"]) == len(reduced.payload["edges"]) - 1
+    assert out == run_dir / "reduced_partial" / "marginal_edep__0.json"
+    partial = Partial.load(out)
+    assert partial.id == "marginal_edep" and partial.chunk == 0
+    assert "r" in partial.data and "t" in partial.data
 
 
 def test_compute_reduced_explicit_paths(tmp_path: Path):
@@ -108,7 +114,45 @@ def test_compute_reduced_explicit_paths(tmp_path: Path):
         run_dir / "shared.json",
         tmp_path / "r.json",
     )
-    assert Reduced.load(out).id == "marginal_step_length"
+    assert Partial.load(out).id == "marginal_step_length"
+
+
+def test_merge_one_produces_reduced(tmp_path: Path):
+    run_dir = _prep(_write_inputs(tmp_path))
+    compute_one("marginal_edep", run_dir)
+    out = merge_one("marginal_edep", run_dir)
+    assert out == run_dir / "reduced" / "marginal_edep.json"
+    reduced = Reduced.load(out)
+    assert reduced.id == "marginal_edep"
+    assert len(reduced.payload["rollout"]) == len(reduced.payload["edges"]) - 1
+
+
+def test_merge_one_fails_loudly_on_missing_chunk(tmp_path: Path):
+    run_dir = _prep(_write_inputs(tmp_path), chunks=2)
+    compute_one("marginal_edep", run_dir, chunk_index=0)  # chunk 1 never computed
+    with pytest.raises(FileNotFoundError, match="missing chunk"):
+        merge_one("marginal_edep", run_dir)
+
+
+def test_chunked_compute_and_merge_matches_unchunked(tmp_path: Path):
+    (tmp_path / "a").mkdir()
+    (tmp_path / "b").mkdir()
+    unchunked_dir = _prep(_write_inputs(tmp_path / "a"))
+    compute_one("marginal_step_length", unchunked_dir)
+    unchunked = Reduced.load(merge_one("marginal_step_length", unchunked_dir))
+
+    chunked_dir = _prep(_write_inputs(tmp_path / "b"), chunks=2)
+    for k in range(2):
+        compute_one("marginal_step_length", chunked_dir, chunk_index=k)
+    chunked = Reduced.load(merge_one("marginal_step_length", chunked_dir))
+
+    assert chunked.payload == unchunked.payload
+
+
+def test_compute_reduced_rejects_out_of_range_chunk(tmp_path: Path):
+    run_dir = _prep(_write_inputs(tmp_path))  # n_chunks=1 (default)
+    with pytest.raises(ValueError, match="out of range"):
+        compute_one("marginal_edep", run_dir, chunk_index=1)
 
 
 def test_write_submit_description(tmp_path: Path):
@@ -119,12 +163,15 @@ def test_write_submit_description(tmp_path: Path):
     assert "docker_image = mschnepf/slc7-condocker" in txt
     assert "requirements = TARGET.ProvidesETPResources" in txt
     assert "accounting_group = cms" in txt
-    assert "queue plotid from" in txt
-    assert (run_dir / "plotids.txt").read_text().split() == catalog_ids()
+    assert "queue plotid,chunk from" in txt
+    jobs = [line.split(",") for line in (run_dir / "jobs.txt").read_text().split()]
+    assert [i for i, _ in jobs] == catalog_ids()
+    assert all(k == "0" for _, k in jobs)  # n_chunks=1 default
     wrapper = run_dir / "run_compute.sh"
     assert wrapper.exists() and (wrapper.stat().st_mode & 0o111)
     body = wrapper.read_text()
-    assert "giant analyze compute-one --id" in body and "--run-dir" in body
+    assert "giant analyze compute-one --id" in body
+    assert "--chunk" in body and "--run-dir" in body
 
 
 def test_write_submit_remote_flag(tmp_path: Path):
@@ -135,3 +182,18 @@ def test_write_submit_remote_flag(tmp_path: Path):
     txt = write_submit(cfg).read_text()
     assert "+RemoteJob = True" in txt
     assert "ProvidesETPResources" not in txt
+
+
+def test_write_submit_chunks_respect_chunkable(tmp_path: Path):
+    assert get_spec("router_gating").chunkable is False
+    run_dir = _prep(_write_inputs(tmp_path))
+    cfg = SubmitConfig(
+        run_dir=run_dir, accounting_group="cms", repo_dir=tmp_path, n_chunks=4
+    )
+    write_submit(cfg)
+    jobs = [line.split(",") for line in (run_dir / "jobs.txt").read_text().split()]
+    counts: dict[str, int] = {}
+    for spec_id, _ in jobs:
+        counts[spec_id] = counts.get(spec_id, 0) + 1
+    assert counts["marginal_edep"] == 4
+    assert counts["router_gating"] == 1  # chunkable=False, ignores n_chunks