diff --git a/core/clippy.toml b/core/clippy.toml
new file mode 100644
index 0000000..b9246ee
--- /dev/null
+++ b/core/clippy.toml
@@ -0,0 +1,24 @@
+# Paths the deterministic core must never use. Clippy resolves imports, so
+# aliases and grouped imports such as `use std::{fs, env}` are caught too.
+# The text search in core/tests/forbidden_apis.rs covers the rest.
+
+disallowed-types = [
+    { path = "std::collections::HashMap", reason = "iteration order differs between runs; use BTreeMap or Vec" },
+    { path = "std::collections::HashSet", reason = "iteration order differs between runs; use BTreeSet or Vec" },
+    { path = "std::hash::RandomState", reason = "seeded differently in every run" },
+    { path = "std::hash::DefaultHasher", reason = "seeded differently in every run" },
+    { path = "std::time::Instant", reason = "the core must not read the clock" },
+    { path = "std::time::SystemTime", reason = "the core must not read the clock" },
+]
+
+disallowed-methods = [
+    { path = "std::env::var", reason = "the core must not depend on the environment" },
+    { path = "std::env::args", reason = "the core must not depend on the environment" },
+    { path = "std::thread::spawn", reason = "the core must stay single-threaded" },
+    { path = "std::fs::read", reason = "the core must not touch files" },
+    { path = "std::fs::read_to_string", reason = "the core must not touch files" },
+    { path = "std::fs::write", reason = "the core must not touch files" },
+    { path = "std::fs::File::open", reason = "the core must not touch files" },
+    { path = "std::fs::File::create", reason = "the core must not touch files" },
+    { path = "std::process::exit", reason = "the core must not end the process" },
+]
diff --git a/core/tests/forbidden_apis.rs b/core/tests/forbidden_apis.rs
index 8913c42..3f56d59 100644
--- a/core/tests/forbidden_apis.rs
+++ b/core/tests/forbidden_apis.rs
@@ -1,5 +1,13 @@
 //! Guards the lockstep rules: the core must not use anything whose result
 //! can differ between machines or between runs.
+//!
+//! Clippy checks the same rules on resolved paths (see `core/clippy.toml`),
+//! which also catches aliases and grouped imports. This text search adds
+//! what Clippy cannot express, such as floating point types and global
+//! mutable state, and it runs even when Clippy does not.
+
+// Tests may read files. The rules of core/clippy.toml apply to core/src.
+#![allow(clippy::disallowed_methods)]
 
 use std::fs;
 use std::path::{Path, PathBuf};
@@ -29,9 +37,33 @@ const FORBIDDEN: &[(&str, &str)] = &[
         "std::net",
         "the core must not open connections; the shell does that",
     ),
+    (
+        "std::io",
+        "the core must not read or write streams; the shell does that",
+    ),
+    ("std::process", "the core must not start or end processes"),
+    ("static mut", "global mutable state can differ between runs"),
+    (
+        "thread_local",
+        "global mutable state can differ between runs",
+    ),
+    ("Atomic", "global mutable state can differ between runs"),
     ("unsafe", "the core forbids unsafe code"),
 ];
 
+/// Every forbidden term in `text`, as (line number, term, reason).
+fn violations(text: &str) -> Vec<(usize, &'static str, &'static str)> {
+    let mut found = Vec::new();
+    for (index, line) in text.lines().enumerate() {
+        for &(needle, reason) in FORBIDDEN {
+            if line.contains(needle) {
+                found.push((index + 1, needle, reason));
+            }
+        }
+    }
+    found
+}
+
 fn rust_files(dir: &Path, out: &mut Vec<PathBuf>) {
     for entry in fs::read_dir(dir).expect("source directory is readable") {
         let path = entry.expect("directory entry is readable").path();
@@ -58,16 +90,8 @@ fn core_source_uses_no_forbidden_apis() {
     let mut problems = Vec::new();
     for file in &files {
         let text = fs::read_to_string(file).expect("source file is readable");
-        for (number, line) in text.lines().enumerate() {
-            for (needle, reason) in FORBIDDEN {
-                if line.contains(needle) {
-                    problems.push(format!(
-                        "{}:{}: `{needle}`: {reason}",
-                        file.display(),
-                        number + 1
-                    ));
-                }
-            }
+        for (line, needle, reason) in violations(&text) {
+            problems.push(format!("{}:{line}: `{needle}`: {reason}", file.display()));
         }
     }
     assert!(
@@ -76,3 +100,23 @@ fn core_source_uses_no_forbidden_apis() {
         problems.join("\n")
     );
 }
+
+#[test]
+fn every_forbidden_term_is_detected() {
+    for &(needle, _) in FORBIDDEN {
+        let text = format!("pub fn fine() {{}}\nlet x = {needle};\n");
+        let found = violations(&text);
+        assert!(
+            found
+                .iter()
+                .any(|&(line, term, _)| line == 2 && term == needle),
+            "`{needle}` was not detected: {found:?}"
+        );
+    }
+}
+
+#[test]
+fn clean_code_has_no_violations() {
+    let text = "pub fn add(a: u32, b: u32) -> u32 {\n    a.wrapping_add(b)\n}\n";
+    assert!(violations(text).is_empty());
+}
diff --git a/core/tests/no_dependencies.rs b/core/tests/no_dependencies.rs
new file mode 100644
index 0000000..3d45a7b
--- /dev/null
+++ b/core/tests/no_dependencies.rs
@@ -0,0 +1,67 @@
+//! The core builds from the standard library alone. A foreign crate could
+//! bring floating point, hash maps or threads in through the back door.
+
+// Tests may read files. The rules of core/clippy.toml apply to core/src.
+#![allow(clippy::disallowed_methods)]
+
+use std::fs;
+use std::path::Path;
+
+/// Every manifest section that may add a dependency, with the lines it
+/// declares. Development dependencies are allowed, because they never reach
+/// the game.
+fn dependency_sections(manifest: &str) -> Vec<(String, Vec<String>)> {
+    let mut sections: Vec<(String, Vec<String>)> = Vec::new();
+    let mut inside = false;
+    for line in manifest.lines() {
+        let line = line.trim();
+        if line.starts_with('[') {
+            inside = line.contains("dependencies") && !line.contains("dev-dependencies");
+            if inside {
+                sections.push((line.to_owned(), Vec::new()));
+            }
+            continue;
+        }
+        if inside && !line.is_empty() && !line.starts_with('#') {
+            if let Some((_, entries)) = sections.last_mut() {
+                entries.push(line.to_owned());
+            }
+        }
+    }
+    sections
+}
+
+#[test]
+fn the_core_has_no_dependencies() {
+    let path = Path::new(env!("CARGO_MANIFEST_DIR")).join("Cargo.toml");
+    let manifest = fs::read_to_string(&path).expect("the manifest is readable");
+    for (header, entries) in dependency_sections(&manifest) {
+        assert!(
+            !header.contains("dependencies."),
+            "the core declares a dependency table: {header}"
+        );
+        assert!(
+            entries.is_empty(),
+            "{header} of the core must stay empty, found: {entries:?}"
+        );
+    }
+}
+
+#[test]
+fn dependency_sections_are_found_with_their_entries() {
+    let manifest = "[package]\nname = \"x\"\n\n[dependencies]\n# a comment\nserde = \"1\"\n\n\
+                    [dev-dependencies]\nproptest = \"1\"\n\n[dependencies.rand]\nversion = \"0.8\"\n";
+    assert_eq!(
+        dependency_sections(manifest),
+        vec![
+            (
+                "[dependencies]".to_owned(),
+                vec!["serde = \"1\"".to_owned()]
+            ),
+            (
+                "[dependencies.rand]".to_owned(),
+                vec!["version = \"0.8\"".to_owned()]
+            ),
+        ]
+    );
+}