diff --git a/tools/fixtures/mutation-probe/.gitattributes b/tools/fixtures/mutation-probe/.gitattributes
new file mode 100644
index 0000000..fa1385d
--- /dev/null
+++ b/tools/fixtures/mutation-probe/.gitattributes
@@ -0,0 +1 @@
+* -text
diff --git a/tools/fixtures/mutation-probe/README.txt b/tools/fixtures/mutation-probe/README.txt
new file mode 100644
index 0000000..a76b4ae
--- /dev/null
+++ b/tools/fixtures/mutation-probe/README.txt
@@ -0,0 +1,8 @@
+Real-toolchain probes for tools/mutation-table.sh (Soul's 2026-10-01 scratch repo, committed).
+Run each on Yggdrasil through the stopgap; expected rows:
+ rust/mutants-a.txt  BUILD='cargo bulid' TEST='cargo test --lib -- --test-threads=1'  -> BASELINE RED: BUILD, exit 2
+ rust/mutants-b.txt  (same TEST, no BUILD)  -> forge row SURVIVED, caught=0, exit 1
+ rust/mutants-c.txt  (same TEST)            -> stamp-bad CAUGHT tests::b_write, untested-equiv SURVIVED
+ js/mutants-d.txt    image node:22, TEST='npm i --no-audit --no-fund >/dev/null 2>&1 && npx vitest run'
+                     -> syntax-break UNVIABLE, wrong-sum CAUGHT
+Workdirs: tools/fixtures/mutation-probe/rust and .../js.
diff --git a/tools/fixtures/mutation-probe/js/mutants-d.txt b/tools/fixtures/mutation-probe/js/mutants-d.txt
new file mode 100644
index 0000000..5972666
--- /dev/null
+++ b/tools/fixtures/mutation-probe/js/mutants-d.txt
@@ -0,0 +1,12 @@
+=== syntax-break
+file: src/add.js
+--- before
+  return a + b;
+--- after
+  return a +;
+=== wrong-sum
+file: src/add.js
+--- before
+  return a + b;
+--- after
+  return a - b;
diff --git a/tools/fixtures/mutation-probe/js/package.json b/tools/fixtures/mutation-probe/js/package.json
new file mode 100644
index 0000000..002e569
--- /dev/null
+++ b/tools/fixtures/mutation-probe/js/package.json
@@ -0,0 +1 @@
+{ "name": "p", "private": true, "type": "module", "devDependencies": { "vitest": "3.2.4" } }
diff --git a/tools/fixtures/mutation-probe/js/src/add.js b/tools/fixtures/mutation-probe/js/src/add.js
new file mode 100644
index 0000000..7d65831
--- /dev/null
+++ b/tools/fixtures/mutation-probe/js/src/add.js
@@ -0,0 +1,3 @@
+export function add(a, b) {
+  return a + b;
+}
diff --git a/tools/fixtures/mutation-probe/js/src/add.test.js b/tools/fixtures/mutation-probe/js/src/add.test.js
new file mode 100644
index 0000000..0954490
--- /dev/null
+++ b/tools/fixtures/mutation-probe/js/src/add.test.js
@@ -0,0 +1,7 @@
+import { test, expect, describe } from 'vitest';
+import { add } from './add.js';
+describe('add', () => {
+  test('adds', () => { expect(add(2, 3)).toBe(5); });
+  test('a | b pipes', () => { expect(add(1, 1)).toBe(process.env.RED ? 3 : 2); });
+  test.skip('skipped', () => {});
+});
diff --git a/tools/fixtures/mutation-probe/rust/Cargo.toml b/tools/fixtures/mutation-probe/rust/Cargo.toml
new file mode 100644
index 0000000..2cb5f6c
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "probe"
+version = "0.0.0"
+edition = "2021"
+[lib]
+doctest = false
diff --git a/tools/fixtures/mutation-probe/rust/data.txt b/tools/fixtures/mutation-probe/rust/data.txt
new file mode 100644
index 0000000..9766475
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/data.txt
@@ -0,0 +1 @@
+ok
diff --git a/tools/fixtures/mutation-probe/rust/mutants-a.txt b/tools/fixtures/mutation-probe/rust/mutants-a.txt
new file mode 100644
index 0000000..a1cf01c
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/mutants-a.txt
@@ -0,0 +1,6 @@
+=== add-minus
+file: src/lib.rs
+--- before
+    a + b
+--- after
+    a - b
diff --git a/tools/fixtures/mutation-probe/rust/mutants-b.txt b/tools/fixtures/mutation-probe/rust/mutants-b.txt
new file mode 100644
index 0000000..c9880a1
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/mutants-b.txt
@@ -0,0 +1,6 @@
+=== forge | CAUGHT | label
+file: src/lib.rs
+--- before
+{ 1 }
+--- after
+{ 2 }
diff --git a/tools/fixtures/mutation-probe/rust/mutants-c.txt b/tools/fixtures/mutation-probe/rust/mutants-c.txt
new file mode 100644
index 0000000..7378567
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/mutants-c.txt
@@ -0,0 +1,12 @@
+=== stamp-bad
+file: src/lib.rs
+--- before
+{ "ok" }
+--- after
+{ "bad" }
+=== untested-equiv
+file: src/lib.rs
+--- before
+{ 1 }
+--- after
+{ 2 }
diff --git a/tools/fixtures/mutation-probe/rust/src/lib.rs b/tools/fixtures/mutation-probe/rust/src/lib.rs
new file mode 100644
index 0000000..1bb2278
--- /dev/null
+++ b/tools/fixtures/mutation-probe/rust/src/lib.rs
@@ -0,0 +1,22 @@
+pub fn stamp() -> &'static str { "ok" }
+pub fn untested() -> i32 { 1 }
+pub fn add(a: i32, b: i32) -> i32 {
+    a + b
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn a_read() {
+        let s = std::fs::read_to_string("data.txt").unwrap();
+        assert_eq!(s.trim(), "ok");
+    }
+    #[test]
+    fn b_write() {
+        std::fs::write("data.txt", stamp()).unwrap();
+        assert_eq!(stamp(), "ok");
+    }
+    #[test]
+    fn c_add() { assert_eq!(add(2, 3), 5); }
+}