diff --git a/.editorconfig b/.editorconfig
index 9dd1efb..500711b 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -8,7 +8,7 @@ indent_size = 4
 insert_final_newline = true
 trim_trailing_whitespace = true
 
-[*.{rs,hs,py}]
+[*.{rs,hs,py,js}]
 max_line_length = 100
 
 [*.md]
@@ -20,3 +20,6 @@ indent_size = 2
 
 [*.{yaml,yml,json}]
 indent_size = 2
+
+[*.{css,html}]
+indent_size = 2
diff --git a/AGENTS.md b/AGENTS.md
index ea29a6c..e3efb88 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -54,6 +54,9 @@ The shape today is:
 - `tools/exporter/`: Haskell tool that consumes hand-authored `.scenario.json` files in `tools/exporter/examples/` and emits the runner-IR JSON
   consumed by `crates/plan-runner`.
   See [`tools/exporter/README.md`](tools/exporter/README.md).
+- `tools/plan-viewer/`: static HTML viewer for `plan-runner` fixtures.
+  It evaluates a fixture in the browser and renders the plan DAG, per-node relations, input facts, and oracle comparison.
+  See [`tools/plan-viewer/README.md`](tools/plan-viewer/README.md).
 - `external/`: git submodules.
   `external/geolog` provides the Haskell query planner used by the exporter; `external/geomerge` is the Rust CRDT crate consumed by
   `storage::adapters::geomerge`.
diff --git a/tools/plan-viewer/README.md b/tools/plan-viewer/README.md
new file mode 100644
index 0000000..b1e2d51
--- /dev/null
+++ b/tools/plan-viewer/README.md
@@ -0,0 +1,37 @@
+# Plan Viewer
+
+A static HTML viewer for `plan-runner` fixtures.
+It renders the plan DAG, the input facts, and the relation computed at every plan node, so a fixture can be inspected without reading raw JSON.
+
+## Usage
+
+Open [`index.html`](index.html) in a browser, then drop a fixture from `crates/plan-runner/fixtures/` onto the page or pick one with the file input.
+
+When the repository is served over HTTP, a fixture can also be loaded through a query parameter:
+
+```sh
+python3 -m http.server 8000
+# then open:
+# http://localhost:8000/tools/plan-viewer/index.html?fixture=../../crates/plan-runner/fixtures/two_atom_join.json
+```
+
+## What It Shows
+
+- **Plan DAG**: one box per plan node, laid out left to right by join depth, with `L` and `R` labels on join inputs and the root node marked.
+  Each box shows the node's output columns and row count.
+- **Selected Node**: a plain-language description of the operator, its output columns, and the full relation computed at that node.
+  Wildcard columns (names starting with an underscore) are dimmed.
+- **Result Bindings**: the root relation projected to the fixture's `expected_bindings` columns, with a per-row check against the oracle and a list of any expected rows the plan did not produce.
+- **Input Facts**: one table per relation in the fixture's schema.
+
+The header badge reports whether the evaluated bindings match the fixture's `expected_bindings` as a multiset, mirroring `plan_runner::verify`.
+
+## Scope
+
+The viewer re-implements the scan, semijoin, and natural join semantics of `crates/query-ops` and the execution order of `crates/plan-runner` in JavaScript, for display only.
+The Rust crates and their tests remain the correctness oracle; if the two ever disagree, the Rust behavior wins and the viewer has a bug.
+
+Unsupported cases:
+
+- plan node actions other than `scan` and `join`
+- fixtures without a `query` and `schema` block
diff --git a/tools/plan-viewer/index.html b/tools/plan-viewer/index.html
new file mode 100644
index 0000000..de59322
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