diff --git a/crates/kaish-kernel/src/kernel.rs b/crates/kaish-kernel/src/kernel.rs
index 59fe5fe1..e5cbb24d 100644
--- a/crates/kaish-kernel/src/kernel.rs
+++ b/crates/kaish-kernel/src/kernel.rs
@@ -3305,7 +3305,7 @@ impl Kernel {
                 let result = match self.eval_arithmetic_async(expr_str).await {
                     Ok(n) if n != 0 => ExecResult::success(""),
                     Ok(_) => ExecResult::failure(1, ""),
-                    Err(e) => ExecResult::failure(2, e.to_string()).into_fault(),
+                    Err(e) => ExecResult::failure(2, format!("{e:#}")).into_fault(),
                 };
                 self.update_last_result(&result).await;
                 if !result.ok() {
diff --git a/crates/kaish-kernel/src/scheduler/pipeline.rs b/crates/kaish-kernel/src/scheduler/pipeline.rs
index 03a182d9..d511fe3d 100644
--- a/crates/kaish-kernel/src/scheduler/pipeline.rs
+++ b/crates/kaish-kernel/src/scheduler/pipeline.rs
@@ -256,7 +256,7 @@ async fn eval_redirect_target(
     let value = dispatcher
         .eval_expr(expr, ctx)
         .await
-        .map_err(|e| e.to_string())?;
+        .map_err(|e| format!("{e:#}"))?;
     // Decision D: a bare collection can't be a redirect target either — same
     // process-boundary guard as external argv (see `structured_boundary_error`).
     if let Some(msg) = crate::interpreter::structured_boundary_error("a redirect target", &value) {
diff --git a/crates/kaish-kernel/src/tools/builtin/timeout.rs b/crates/kaish-kernel/src/tools/builtin/timeout.rs
index e1568996..ff1ce535 100644
--- a/crates/kaish-kernel/src/tools/builtin/timeout.rs
+++ b/crates/kaish-kernel/src/tools/builtin/timeout.rs
@@ -174,7 +174,7 @@ impl Tool for Timeout {
                 }
                 result
             }
-            Err(e) => ExecResult::failure(1, format!("timeout: {}", e)),
+            Err(e) => ExecResult::failure(1, format!("timeout: {e:#}")),
         }
     }
 }
diff --git a/crates/kaish-kernel/tests/error_cause_chain_tests.rs b/crates/kaish-kernel/tests/error_cause_chain_tests.rs
new file mode 100644
index 00000000..7b55588d
--- /dev/null
+++ b/crates/kaish-kernel/tests/error_cause_chain_tests.rs
@@ -0,0 +1,94 @@
+//! An `anyhow::Error` chain built with `.context(...)` shows only its
+//! outermost frame under `{}`/`.to_string()`; `{:#}` walks the whole chain.
+//! Several sites folded a chained fault into `ExecResult.err` via
+//! `.to_string()`, permanently baking in the terse form and discarding the
+//! real cause forever (unlike `Kernel::execute`'s own `KernelError`, which
+//! keeps the `anyhow::Error` alive so a caller can still choose `{:#}` later
+//! — see `kernel_error_tests.rs`).
+//!
+//! Each test below reaches a fault two `.context()`/`anyhow!()` frames deep
+//! — a nested `$(...)` around `x=$((1/0))` — and pins that the innermost
+//! cause (`divides by zero`) survives to `ExecResult.err`.
+
+#![allow(clippy::unwrap_used, clippy::expect_used)]
+
+use kaish_kernel::{Kernel, KernelConfig};
+
+fn kernel() -> Kernel {
+    Kernel::new(KernelConfig::isolated().with_skip_validation(true)).expect("failed to create kernel")
+}
+
+/// `eval_redirect_target` (`scheduler/pipeline.rs`) ran a redirect target's
+/// `$(...)` through the full async evaluator and folded a fault via
+/// `.map_err(|e| e.to_string())`. A redirect target that is itself a command
+/// substitution containing a failing assignment carries a two-frame chain
+/// ("failed to evaluate assignment" wrapping "arithmetic error: ... divides
+/// by zero"); `to_string()` showed only the outer frame.
+#[tokio::test]
+async fn redirect_target_fault_keeps_its_cause_chain() {
+    let kernel = kernel();
+    let result = kernel
+        .execute("echo hi > $(x=$((1/0)))")
+        .await
+        .expect("redirect evaluation fails into an ExecResult, not a KernelError");
+
+    assert!(!result.ok(), "a failing redirect target must not succeed: {result:?}");
+    assert!(
+        result.err.contains("divides by zero"),
+        "the redirect target's real cause must survive: {:?}",
+        result.err
+    );
+}
+
+/// `timeout`'s dispatch-error arm (`tools/builtin/timeout.rs`) formatted the
+/// re-dispatched command's `anyhow::Error` with `{}` instead of `{:#}`. A
+/// user tool body that faults through a chain of `.context()` calls (calling
+/// it re-dispatches through the full statement executor) lost every frame
+/// but the outermost.
+#[tokio::test]
+async fn timeout_dispatch_fault_keeps_its_cause_chain() {
+    let kernel = kernel().into_arc();
+    let script = "function boom { x=$((1/0)) }\ntimeout 5 boom";
+    let result = kernel.execute(script).await.expect("timeout's fault path returns an ExecResult");
+
+    assert!(!result.ok(), "a faulting tool body under timeout must not succeed: {result:?}");
+    assert!(
+        result.err.contains("divides by zero"),
+        "the tool body's real cause must survive through timeout: {:?}",
+        result.err
+    );
+}
+
+/// `Stmt::Arith`'s fault arm (`kernel.rs`) is the sibling of `Stmt::Test`'s
+/// (already fixed to use `{:#}`) but still folded its `anyhow::Error` with
+/// `.to_string()`. A bare `(( $(...) ))` whose command substitution contains
+/// a failing assignment carries the same two-frame chain as the redirect
+/// case above.
+#[tokio::test]
+async fn bare_arith_statement_fault_keeps_its_cause_chain() {
+    let kernel = kernel();
+    let result = kernel
+        .execute("(( $(x=$((1/0))) ))")
+        .await
+        .expect("a bare (( )) fault returns an ExecResult, not a KernelError");
+
+    assert!(!result.ok(), "a faulting (( )) command substitution must not succeed: {result:?}");
+    assert!(
+        result.err.contains("divides by zero"),
+        "the arithmetic statement's real cause must survive: {:?}",
+        result.err
+    );
+}
+
+/// The plain, single-frame case must be unaffected by the `{:#}` fix: a bare
+/// `(( 1/0 ))` has no outer `.context()` to add a second frame, so its
+/// message is unchanged (this pins that the fix does not start showing
+/// duplicate or unexpected text on the common case).
+#[tokio::test]
+async fn bare_arith_statement_simple_fault_is_unchanged() {
+    let kernel = kernel();
+    let result = kernel.execute("(( 1/0 ))").await.expect("a bare (( )) fault returns an ExecResult");
+
+    assert!(!result.ok());
+    assert_eq!(result.err.trim_end(), "arithmetic error: `1 / 0` divides by zero");
+}