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+# Discover a loaded blueprint's sweepable axes — Implementation Plan
+
+> **Parent spec:** `docs/specs/0096-discover-a-loaded-blueprints-sweepable-axes.md`
+>
+> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to
+> run this plan. Steps use `- [ ]` checkboxes for tracking.
+
+**Goal:** Add `aura sweep <blueprint.json> --list-axes` — a read-only query
+sub-mode that lists a loaded blueprint's open sweepable knobs (one
+`<name>:<kind>` per line, exactly as `--axis` names them) then exits.
+
+**Architecture:** The axis names `--axis` accepts are produced by the sweep's
+harness-wrapping (`wrap_stage1r(loaded_signal)` nests the signal composite, so
+`collect_params` prefixes the names — `stage1_signal.fast.length`, not the raw
+`fast.length`). So the query lands on the SWEEP verb and reuses the SAME probe
+the sweep resolves against, via a new shared `blueprint_axis_probe` helper that
+also subsumes the two existing inline copies of that probe
+(`blueprint_sweep_family`, `blueprint_mc_family`) — one source of truth for the
+wrapped namespace. All changes are in `crates/aura-cli`; engine/registry
+untouched (invariants 1/8/9).
+
+**Tech Stack:** `crates/aura-cli/src/main.rs` (the dispatch, the arg grammar,
+the two new fns, the probe refactor), `crates/aura-cli/tests/cli_run.rs`
+(E2Es). Read-only references: `Composite::param_space` /`collect_params`
+(`crates/aura-engine/src/blueprint.rs`), `ParamSpec`/`ScalarKind`
+(`crates/aura-core`).
+
+---
+
+**Files this plan creates or modifies:**
+
+- Modify: `crates/aura-cli/src/main.rs` — new `blueprint_axis_probe` +
+  `list_blueprint_axes` fns; `SweepBlueprintArgs.list_axes` field;
+  `parse_sweep_blueprint_args` `--list-axes` recognition + standalone-guard;
+  `["sweep", ..]` dispatch short-circuit; refactor `blueprint_sweep_family`
+  (~:3186-3191) and `blueprint_mc_family` (~:3235-3240) to the shared probe.
+- Test: `crates/aura-cli/src/main.rs` (`#[cfg(test)] mod tests`, ~:4193) —
+  unit `blueprint_axis_probe` + unit `parse_sweep_blueprint_args` list-axes.
+- Test: `crates/aura-cli/tests/cli_run.rs` — 4 new E2Es beside the existing
+  sweep block (~:3269-3610).
+
+Line numbers are the current-tree anchors from recon; they may drift ±a few —
+match on the surrounding code, not the number.
+
+---
+
+## Task 1: Shared probe helper `blueprint_axis_probe` (single-source the wrapped namespace)
+
+**Files:**
+- Modify: `crates/aura-cli/src/main.rs` (new fn ~:3166; refactor ~:3186-3191 and ~:3235-3240)
+- Test: `crates/aura-cli/src/main.rs` (`mod tests`)
+
+- [ ] **Step 1: Write the failing unit test**
+
+Add to the `#[cfg(test)] mod tests` block (near the existing
+`blueprint_sweep_member_equals_single_run_and_shares_topology_hash`). It reads
+the two committed fixtures via `include_str!` (path relative to `src/main.rs`):
+
+```rust
+#[test]
+fn blueprint_axis_probe_lists_prefixed_open_knobs() {
+    // The open fixture's two SMA lengths are the sweepable knobs; the probe
+    // wraps the signal (name "stage1_signal") so the names are prefixed —
+    // exactly what `--axis` binds.
+    let open = include_str!("../tests/fixtures/stage1_signal_open.json");
+    let space = blueprint_axis_probe(open).param_space();
+    let names: Vec<&str> = space.iter().map(|p| p.name.as_str()).collect();
+    assert_eq!(names, ["stage1_signal.fast.length", "stage1_signal.slow.length"]);
+    assert!(space.iter().all(|p| matches!(p.kind, ScalarKind::I64)));
+
+    // A closed blueprint (both lengths bound) has no open axes.
+    let closed = include_str!("../tests/fixtures/stage1_signal.json");
+    assert!(blueprint_axis_probe(closed).param_space().is_empty());
+}
+```
+
+- [ ] **Step 2: Run the test to verify it fails (RED — unresolved name)**
+
+Run: `cargo test -p aura-cli --bin aura blueprint_axis_probe`
+Expected: FAIL to compile — `cannot find function 'blueprint_axis_probe' in this scope`.
+
+- [ ] **Step 3: Add the `blueprint_axis_probe` helper**
+
+Insert just above `fn blueprint_sweep_family` (~:3166):
+
+```rust
+/// The exact wrapped probe the loaded-blueprint sweep resolves its axes
+/// against: the loaded signal wrapped in the stage1-r scaffolding (stop bound,
+/// reduce, no cost), taps discarded. `param_space()` on it is the axis
+/// namespace `--axis` binds; `.axis()` consumes it to seed a sweep. Single
+/// source for the sweep terminal, the MC closed-check, AND `--list-axes`, so
+/// the listed names track the swept names by construction (incl. across #159's
+/// harness retirement). The reload is infallible under the SAME
+/// dispatch-boundary contract the callers already rely on: the doc is
+/// `blueprint_from_json`-validated at the `["sweep", ..]` / `["mc", ..]`
+/// boundary before this runs, so a malformed doc has already exited 2 and
+/// never reaches the `.expect`.
+fn blueprint_axis_probe(doc: &str) -> Composite {
+    let signal = blueprint_from_json(doc, &|t| std_vocabulary(t))
+        .expect("doc parse-validated at the dispatch boundary; reload is infallible");
+    let (tx_eq, _) = mpsc::channel();
+    let (tx_ex, _) = mpsc::channel();
+    let (tx_r, _) = mpsc::channel();
+    let (tx_req, _) = mpsc::channel();
+    wrap_stage1r(signal, tx_eq, tx_ex, tx_r, tx_req, false, true, None)
+}
+```
+
+- [ ] **Step 4: Subsume the `blueprint_sweep_family` inline probe**
+
+In `blueprint_sweep_family`, replace the four `let (tx_*, _) = mpsc::channel();`
+lines plus `let probe = wrap_stage1r(reload(doc), tx_eq, tx_ex, tx_r, tx_req,
+false, true, None);` (~:3186-3190) with a single call. The following
+`let space = probe.param_space();` and the later `probe.axis(...)` stay
+unchanged (the helper returns the `Composite`, so both the borrow and the
+consume still work):
+
+```rust
+    // a single throwaway floated build, only to resolve param_space (borrow) then seed
+    // the named axes (move) — its taps are never drained. The wrapped probe is the one
+    // `blueprint_axis_probe` single-sources (identical `false, true, None` wrap).
+    let probe = blueprint_axis_probe(doc);
+    let space = probe.param_space();
+```
+
+- [ ] **Step 5: Subsume the `blueprint_mc_family` inline probe**
+
+In `blueprint_mc_family`, replace the four `let (tx_*, _) = mpsc::channel();`
+lines plus `let space = wrap_stage1r(reload(doc), tx_eq, tx_ex, tx_r, tx_req,
+false, true, None).param_space();` (~:3235-3240) with:
+
+```rust
+    // probe the wrapped param_space (the same probe the sweep resolves against);
+    // MC needs it empty. `blueprint_axis_probe` is the single source of that wrap.
+    let space = blueprint_axis_probe(doc).param_space();
+```
+
+- [ ] **Step 6: Verify the new test passes and the refactor is behaviour-preserving**
+
+Run: `cargo test -p aura-cli --bin aura blueprint_axis_probe`
+Expected: PASS (1 test).
+
+Run: `cargo test -p aura-cli --bin aura blueprint_mc_family`
+Expected: PASS (the existing `blueprint_mc_family_seeds_differ` and
+`blueprint_mc_family_rejects_an_open_blueprint` — the closed-check still works,
+proving the MC probe refactor preserved `param_space`).
+
+Run: `cargo test -p aura-cli --bin aura blueprint_sweep_member_equals_single_run_and_shares_topology_hash`
+Expected: PASS (the sweep probe refactor preserved axis resolution).
+
+Run: `cargo build --workspace`
+Expected: `Finished` — 0 errors.
+
+---
+
+## Task 2: The `--list-axes` surface (arg field, parse grammar, list fn, dispatch)
+
+**Files:**
+- Modify: `crates/aura-cli/src/main.rs` (`SweepBlueprintArgs` ~:3907; `parse_sweep_blueprint_args` ~:3917-3945; new `list_blueprint_axes` fn; dispatch ~:4112-4118)
+- Test: `crates/aura-cli/src/main.rs` (`mod tests`)
+
+- [ ] **Step 1: Write the failing unit test**
+
+Add beside the existing `parse_sweep_blueprint_args_grammar` test:
+
+```rust
+#[test]
+fn parse_sweep_blueprint_args_list_axes_is_standalone() {
+    let a = parse_sweep_blueprint_args(&["--list-axes"]).expect("--list-axes alone parses");
+    assert!(a.list_axes);
+    assert!(a.axes.is_empty());
+    // mutually exclusive with a real sweep, in either order:
+    assert!(parse_sweep_blueprint_args(&["--list-axes", "--axis", "stage1_signal.fast.length=2,3"]).is_err());
+    assert!(parse_sweep_blueprint_args(&["--axis", "stage1_signal.fast.length=2,3", "--list-axes"]).is_err());
+    // a bare sweep (no --list-axes) still requires >= 1 axis and reports list_axes=false:
+    let b = parse_sweep_blueprint_args(&["--axis", "stage1_signal.fast.length=2,3"]).expect("bare sweep parses");
+    assert!(!b.list_axes);
+}
+```
+
+- [ ] **Step 2: Run the test to verify it fails (RED — no `list_axes` field)**
+
+Run: `cargo test -p aura-cli --bin aura parse_sweep_blueprint_args`
+Expected: FAIL to compile — `no field 'list_axes' on type 'SweepBlueprintArgs'`.
+
+- [ ] **Step 3: Add the `list_axes` field to `SweepBlueprintArgs`**
+
+At ~:3907:
+
+```rust
+struct SweepBlueprintArgs {
+    axes: Vec<(String, Vec<Scalar>)>,
+    name: String,
+    persist: bool,
+    data: DataChoice,
+    list_axes: bool,
+}
+```
+
+- [ ] **Step 4: Recognize `--list-axes` (valueless) + standalone-guard in `parse_sweep_blueprint_args`**
+
+`--list-axes` carries no value, but the loop consumes a value per flag
+(`t.split_first()`), so handle it *before* that split. Add a `list_axes` local,
+a valueless arm at the top of the loop, and a post-loop standalone branch.
+The standalone check is `rest.len() != 1` — `parse` receives only the flags
+tail, so a lone `--list-axes` means the tail is exactly one token.
+
+Replace the body from `let mut tail = rest;` through the final `Ok(...)`
+(~:3922-3944) with:
+
+```rust
+    let mut list_axes = false;
+    let mut tail = rest;
+    while let Some((flag, t)) = tail.split_first() {
+        if *flag == "--list-axes" {
+            list_axes = true;
+            tail = t;
+            continue;
+        }
+        let (value, t) = t.split_first().ok_or_else(usage)?;
+        if data.accept(flag, value, &usage)? {
+            tail = t;
+            continue;
+        }
+        match *flag {
+            "--axis" => {
+                let (n, csv) = value.split_once('=').ok_or_else(usage)?;
+                if n.is_empty() || axes.iter().any(|(a, _)| a == n) { return Err(usage()); } // empty / duplicate axis
+                let vals = parse_scalar_csv(csv).ok_or_else(usage)?;
+                axes.push((n.to_string(), vals));
+            }
+            "--name" if name.is_none() => name = Some(((*value).to_string(), false)),
+            "--trace" if name.is_none() => name = Some(((*value).to_string(), true)),
+            _ => return Err(usage()),
+        }
+        tail = t;
+    }
+    if list_axes {
+        // A query, not a sweep: it must stand alone (the flags tail is only itself).
+        if rest.len() != 1 {
+            return Err("--list-axes lists axes and takes no other flags".to_string());
+        }
+        return Ok(SweepBlueprintArgs {
+            axes,
+            name: "sweep".to_string(),
+            persist: false,
+            data: data.finish(&usage)?,
+            list_axes: true,
+        });
+    }
+    if axes.is_empty() { return Err(usage()); } // a sweep needs at least one axis
+    let (name, persist) = name.unwrap_or_else(|| ("sweep".to_string(), false));
+    Ok(SweepBlueprintArgs { axes, name, persist, data: data.finish(&usage)?, list_axes: false })
+```
+
+- [ ] **Step 5: Add the `list_blueprint_axes` fn**
+
+Insert next to `blueprint_axis_probe` (above `blueprint_sweep_family`):
+
+```rust
+/// `aura sweep <blueprint.json> --list-axes`: one `<name>:<kind>` line per open
+/// sweepable knob, in `param_space()` order; a closed blueprint prints nothing.
+/// Names are exactly what `--axis` binds (same probe as the sweep terminal).
+fn list_blueprint_axes(doc: &str) {
+    for p in blueprint_axis_probe(doc).param_space() {
+        println!("{}:{:?}", p.name, p.kind); // ScalarKind Debug -> I64/F64/Bool/Timestamp
+    }
+}
+```
+
+- [ ] **Step 6: Thread the dispatch destructure + short-circuit**
+
+In the `["sweep", rest @ ..]` blueprint branch (~:4112-4118), extend the
+destructure with `list_axes` and short-circuit to the listing before the run:
+
+```rust
+                let SweepBlueprintArgs { axes, name, persist, data, list_axes } =
+                    parse_sweep_blueprint_args(&rest[1..]).unwrap_or_else(|msg| {
+                        eprintln!("aura: {msg}");
+                        std::process::exit(2);
+                    });
+                if list_axes {
+                    list_blueprint_axes(&doc);
+                    return;
+                }
+                run_blueprint_sweep(&doc, &axes, &name, persist, DataSource::from_choice(data));
+                return;
+```
+
+- [ ] **Step 7: Verify the unit test passes and the workspace builds**
+
+Run: `cargo test -p aura-cli --bin aura parse_sweep_blueprint_args`
+Expected: PASS (the existing `parse_sweep_blueprint_args_grammar` +
+`parse_sweep_blueprint_args_list_axes_is_standalone`).
+
+Run: `cargo build --workspace`
+Expected: `Finished` — 0 errors (the `SweepBlueprintArgs` field addition is
+threaded at both its construction sites — the two `Ok(SweepBlueprintArgs {…})`
+in `parse_sweep_blueprint_args` and the dispatch destructure — so the build is
+whole).
+
+---
+
+## Task 3: E2E coverage (the four user-facing behaviours)
+
+**Files:**
+- Test: `crates/aura-cli/tests/cli_run.rs` (new tests beside the sweep block ~:3269-3610)
+
+- [ ] **Step 1: Write the four E2Es**
+
+Use the sweep block's fixture-path convention — an absolute
+`env!("CARGO_MANIFEST_DIR")` path (cwd-independent; `--list-axes` writes nothing
+to disk, so no `temp_cwd` is needed). The malformed case mirrors
+`aura_run_rejects_an_unknown_node_blueprint`.
+
+```rust
+#[test]
+fn aura_sweep_list_axes_prints_prefixed_names() {
+    let bp = format!("{}/tests/fixtures/stage1_signal_open.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(env!("CARGO_BIN_EXE_aura"))
+        .args(["sweep", &bp, "--list-axes"])
+        .output()
+        .expect("spawn aura sweep --list-axes");
+    assert_eq!(out.status.code(), Some(0));
+    let stdout = String::from_utf8(out.stdout).expect("utf-8");
+    assert_eq!(stdout, "stage1_signal.fast.length:I64\nstage1_signal.slow.length:I64\n");
+}
+
+#[test]
+fn aura_sweep_list_axes_closed_is_empty() {
+    let bp = format!("{}/tests/fixtures/stage1_signal.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(env!("CARGO_BIN_EXE_aura"))
+        .args(["sweep", &bp, "--list-axes"])
+        .output()
+        .expect("spawn aura sweep --list-axes (closed)");
+    assert_eq!(out.status.code(), Some(0));
+    assert!(out.stdout.is_empty(), "a closed blueprint has no open axes");
+}
+
+#[test]
+fn aura_sweep_list_axes_rejects_other_flags() {
+    let bp = format!("{}/tests/fixtures/stage1_signal_open.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(env!("CARGO_BIN_EXE_aura"))
+        .args(["sweep", &bp, "--list-axes", "--axis", "stage1_signal.fast.length=2,3"])
+        .output()
+        .expect("spawn aura sweep --list-axes + --axis");
+    assert_eq!(out.status.code(), Some(2));
+    let stderr = String::from_utf8(out.stderr).expect("utf-8");
+    assert!(stderr.contains("--list-axes lists axes"), "stderr was: {stderr}");
+}
+
+#[test]
+fn aura_sweep_list_axes_rejects_malformed_blueprint() {
+    let bp = format!("{}/tests/fixtures/unknown_node.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(env!("CARGO_BIN_EXE_aura"))
+        .args(["sweep", &bp, "--list-axes"])
+        .output()
+        .expect("spawn aura sweep --list-axes (malformed)");
+    assert_eq!(out.status.code(), Some(2), "malformed blueprint must be rejected, not panic");
+    let stderr = String::from_utf8(out.stderr).expect("utf-8");
+    assert!(stderr.contains("UnknownNodeType"), "stderr was: {stderr}");
+    assert!(!stderr.contains("panicked"), "must reject cleanly, not panic: {stderr}");
+}
+```
+
+- [ ] **Step 2: Run the four E2Es**
+
+Run: `cargo test -p aura-cli --test cli_run aura_sweep_list_axes`
+Expected: PASS — 4 tests (`aura_sweep_list_axes_prints_prefixed_names`,
+`..._closed_is_empty`, `..._rejects_other_flags`, `..._rejects_malformed_blueprint`).
+The single filter `aura_sweep_list_axes` matches exactly these 4 named tests.
+
+- [ ] **Step 3: Full-suite + lint gate**
+
+Run: `cargo test --workspace`
+Expected: PASS — no regressions across the workspace.
+
+Run: `cargo clippy --workspace --all-targets -- -D warnings`
+Expected: `Finished` — 0 warnings.
+
+---
+
+## Self-review (planner Step 5)
+
+1. **Spec coverage:** §Goal/§Architecture → Tasks 1-2 (probe on the sweep verb,
+   prefixed names); §Concrete code shapes → Tasks 1-2 (all four shown shapes:
+   `blueprint_axis_probe`, `list_blueprint_axes`, `SweepBlueprintArgs.list_axes`,
+   dispatch); §Error handling → Task 2 Step 4 (standalone guard) + Task 3 Step 1
+   (malformed + closed cases); §Testing strategy (all four deliverable tests) →
+   Task 1 unit, Task 2 unit, Task 3 E2Es; §Acceptance criteria 1-6 → covered by
+   the E2Es + the build/clippy gate (Task 3 Step 3). ✓
+2. **Placeholder scan:** no TBD/TODO/"similar to"/"add appropriate". ✓
+3. **Type/name consistency:** `blueprint_axis_probe`, `list_blueprint_axes`,
+   `SweepBlueprintArgs.list_axes`, `parse_sweep_blueprint_args`,
+   `blueprint_sweep_family`, `blueprint_mc_family`, `wrap_stage1r`, `ScalarKind`
+   consistent across tasks and match the current tree. ✓
+4. **Step granularity:** each step is one edit or one command. ✓
+5. **No commit steps.** ✓
+6. **Pin/replacement contiguity:** the E2E asserts `stdout ==
+   "stage1_signal.fast.length:I64\nstage1_signal.slow.length:I64\n"` and
+   `stderr.contains("--list-axes lists axes")` / `"UnknownNodeType"` — each pinned
+   substring appears contiguously in the corresponding impl (`println!` format
+   and the `Err("--list-axes lists axes and takes no other flags")` literal /
+   the existing engine `UnknownNodeType` error). ✓
+7. **Compile-gate vs. deferred-caller:** the `SweepBlueprintArgs.list_axes`
+   field addition (Task 2 Step 3) breaks exactly two sites — the two
+   `Ok(SweepBlueprintArgs {…})` returns (threaded in Step 4) and the dispatch
+   destructure (threaded in Step 6) — ALL inside Task 2, before its Step 7 build
+   gate. No caller deferred past a build gate. ✓
+8. **Verification filters resolve:** `blueprint_axis_probe` (new, Task 1 Step 1),
+   `blueprint_mc_family` (existing `blueprint_mc_family_seeds_differ` +
+   `_rejects_an_open_blueprint`), `blueprint_sweep_member_equals_single_run_and_shares_topology_hash`
+   (existing), `parse_sweep_blueprint_args` (existing `_grammar` + new
+   `_list_axes_is_standalone`), `aura_sweep_list_axes` (the 4 new E2Es, shared
+   prefix) — every filter matches ≥1 named test in the post-task tree. ✓