diff --git a/docs/superpowers/plans/2026-05-18-pyg-uv-flat-index.md b/docs/superpowers/plans/2026-05-18-pyg-uv-flat-index.md
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+# PyG uv Flat Index Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Make generated uv-based projects with PyTorch Geometric resolve PyG wheel pages correctly by emitting a flat uv index and binding PyG extension packages to that explicit source.
+
+**Architecture:** Keep the current template structure intact and change only the generated uv configuration for PyG: mark the wheel page as a flat index and map PyG extension packages to that explicit index. Validate by regenerating projects that enable PyG and rerunning dependency resolution.
+
+**Tech Stack:** Copier, Jinja2, uv, PyTorch Geometric
+
+---
+
+### Task 1: Add flat index metadata and source mappings for PyG uv packages
+
+**Files:**
+- Modify: `pyproject.toml.jinja`
+- Test: generated `pyproject.toml` in a temporary copied project
+
+- [ ] **Step 1: Write the failing test**
+
+Use the existing reproduction as the failing test: generate a project with uv + PyG enabled and run `uv lock`, expecting failure before the template change.
+
+```bash
+uvx copier copy --trust --defaults \
+  --data project_name="tmp-pyg-flat" \
+  --data package_name="tmp_pyg_flat" \
+  --data python_version="3.12" \
+  --data package_manager="uv" \
+  --data pytorch_cuda_preset="pytorch-2.8.0-cuda-12.6" \
+  --data use_pytorch_geometric=true \
+  --data use_lightning=true \
+  --data use_hydra=true \
+  --data logger_choice="none" \
+  . /tmp/tmp-pyg-flat
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Run:
+
+```bash
+rtk uv lock --project /tmp/tmp-pyg-flat
+```
+
+Expected: dependency resolution fails while probing the PyG wheel page or while resolving PyG extension wheels, often by falling back to a source build for `torch-cluster`.
+
+- [ ] **Step 3: Write minimal implementation**
+
+Update the PyG-specific uv configuration in `pyproject.toml.jinja`.
+
+```toml
+[[tool.uv.index]]
+name = "pytorch-geometric"
+url = "https://data.pyg.org/whl/torch-{{ pytorch_version }}+cu{{ cuda_version | replace('.', '') }}.html"
+format = "flat"
+explicit = true
+
+[tool.uv.sources]
+torch-scatter = { index = "pytorch-geometric" }
+torch-sparse = { index = "pytorch-geometric" }
+torch-cluster = { index = "pytorch-geometric" }
+```
+
+- [ ] **Step 4: Run test to verify it passes**
+
+Regenerate the same project, then run:
+
+```bash
+rtk uv lock --project /tmp/tmp-pyg-flat
+```
+
+Expected: `uv lock` succeeds and resolves PyG dependencies.
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add pyproject.toml.jinja
+git commit -m "fix: mark PyG uv source as flat index"
+```
+
+### Task 2: Sanity-check generated configuration
+
+**Files:**
+- Modify: none
+- Test: generated `/tmp/tmp-pyg-flat/pyproject.toml`
+
+- [ ] **Step 1: Write the failing test**
+
+Check that the generated file lacks the required flat format and source mappings before the change.
+
+```bash
+grep -n 'format = "flat"' /tmp/tmp-pyg-flat/pyproject.toml
+grep -n '\[tool\.uv\.sources\]\|torch-scatter\|torch-sparse\|torch-cluster' /tmp/tmp-pyg-flat/pyproject.toml
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Expected: no matching lines before the template fix.
+
+- [ ] **Step 3: Write minimal implementation**
+
+No additional implementation beyond Task 1.
+
+- [ ] **Step 4: Run test to verify it passes**
+
+Regenerate the project and run:
+
+```bash
+grep -n 'format = "flat"' /tmp/tmp-pyg-flat/pyproject.toml
+grep -n '\[tool\.uv\.sources\]\|torch-scatter\|torch-sparse\|torch-cluster' /tmp/tmp-pyg-flat/pyproject.toml
+```
+
+Expected: the generated PyG uv configuration contains `format = "flat"` and the three source mappings.
+
+- [ ] **Step 5: Commit**
+
+No additional commit beyond Task 1 unless follow-up changes are needed.
diff --git a/docs/superpowers/specs/2026-05-18-pyg-uv-flat-index-design.md b/docs/superpowers/specs/2026-05-18-pyg-uv-flat-index-design.md
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+++ b/docs/superpowers/specs/2026-05-18-pyg-uv-flat-index-design.md
@@ -0,0 +1,36 @@
+# PyG uv Flat Index Design
+
+## Goal
+Fix generated uv-based projects that enable PyTorch Geometric so `uv lock` can resolve PyG extension wheels from `data.pyg.org` without failing on index probing or falling back to source builds.
+
+## Problem
+The template currently emits a PyG-specific `[[tool.uv.index]]` entry pointing at a `data.pyg.org` HTML wheel listing, but it does not mark that source as a flat index and it does not bind PyG extension packages to that explicit index. `uv` treats normal indexes as Simple API-style repositories unless told otherwise, and indexes marked `explicit = true` are only used for packages explicitly mapped to them. As a result, generated projects can fail during `uv lock` with 403 errors while probing the PyG URL or by attempting to build packages like `torch-cluster` from source instead of resolving wheels.
+
+## Chosen Approach
+Update the generated uv configuration for PyG by adding `format = "flat"` to the `pytorch-geometric` `[[tool.uv.index]]` block in `pyproject.toml.jinja` and add `[tool.uv.sources]` mappings for `torch-scatter`, `torch-sparse`, and `torch-cluster` so those packages resolve from the explicit PyG index.
+
+## Why This Approach
+- It matches `uv`'s documented handling of find-links style package pages.
+- It matches a known-good working configuration already validated in another project.
+- It is the smallest possible template change that addresses the root cause.
+- It preserves the current package-selection model and does not require changing Copier questions or post-generation installation steps.
+
+## Scope
+### In scope
+- Add `format = "flat"` to the generated PyG uv index block.
+- Add `[tool.uv.sources]` mappings for `torch-scatter`, `torch-sparse`, and `torch-cluster`.
+- Regenerate uv + PyG projects and verify `uv lock` succeeds.
+
+### Out of scope
+- Changing available PyTorch/CUDA presets.
+- Reworking Pixi behavior.
+- Replacing the uv index approach with custom install commands.
+
+## Testing
+1. Generate projects with `package_manager=uv` and `use_pytorch_geometric=true`.
+2. Confirm the generated `pyproject.toml` includes both `format = "flat"` and `[tool.uv.sources]` mappings for the PyG extension packages.
+3. Run `uv lock` in the generated projects.
+4. Confirm dependency resolution succeeds for PyG packages such as `torch-cluster` without source build fallback.
+
+## Risks
+The only meaningful risk is that some PyG wheel pages may still have upstream availability gaps for specific torch/CUDA combinations. That would be a separate compatibility issue, not a misconfiguration of `uv` index format.
diff --git a/pyproject.toml.jinja b/pyproject.toml.jinja
index 8378aab..4145ac4 100755
--- a/pyproject.toml.jinja
+++ b/pyproject.toml.jinja
@@ -98,7 +98,13 @@ explicit = true
 [[tool.uv.index]]
 name = "pytorch-geometric"
 url = "https://data.pyg.org/whl/torch-{{ pytorch_version }}+cu{{ cuda_version | replace('.', '') }}.html"
+format = "flat"
 explicit = true
+
+[tool.uv.sources]
+torch-scatter = { index = "pytorch-geometric" }
+torch-sparse = { index = "pytorch-geometric" }
+torch-cluster = { index = "pytorch-geometric" }
 {% endif -%}
 
 # ============================================================================