diff --git a/crates/bashkit-python/src/lib.rs b/crates/bashkit-python/src/lib.rs
index c49a8405..81abb30c 100644
--- a/crates/bashkit-python/src/lib.rs
+++ b/crates/bashkit-python/src/lib.rs
@@ -233,41 +233,47 @@ impl PyBash {
     }
 
     /// Execute commands synchronously (blocking).
-    fn execute_sync(&self, commands: String) -> PyResult<ExecResult> {
+    /// Releases GIL before blocking on tokio to prevent deadlock with callbacks.
+    fn execute_sync(&self, py: Python<'_>, commands: String) -> PyResult<ExecResult> {
         let inner = self.inner.clone();
         let rt = tokio::runtime::Runtime::new()
             .map_err(|e| PyRuntimeError::new_err(format!("Failed to create runtime: {}", e)))?;
 
-        rt.block_on(async move {
-            let mut bash = inner.lock().await;
-            match bash.exec(&commands).await {
-                Ok(result) => Ok(ExecResult {
-                    stdout: result.stdout,
-                    stderr: result.stderr,
-                    exit_code: result.exit_code,
-                    error: None,
-                }),
-                Err(e) => Ok(ExecResult {
-                    stdout: String::new(),
-                    stderr: String::new(),
-                    exit_code: 1,
-                    error: Some(e.to_string()),
-                }),
-            }
+        py.detach(|| {
+            rt.block_on(async move {
+                let mut bash = inner.lock().await;
+                match bash.exec(&commands).await {
+                    Ok(result) => Ok(ExecResult {
+                        stdout: result.stdout,
+                        stderr: result.stderr,
+                        exit_code: result.exit_code,
+                        error: None,
+                    }),
+                    Err(e) => Ok(ExecResult {
+                        stdout: String::new(),
+                        stderr: String::new(),
+                        exit_code: 1,
+                        error: Some(e.to_string()),
+                    }),
+                }
+            })
         })
     }
 
     /// Reset interpreter to fresh state.
-    fn reset(&self) -> PyResult<()> {
+    /// Releases GIL before blocking on tokio to prevent deadlock.
+    fn reset(&self, py: Python<'_>) -> PyResult<()> {
         let inner = self.inner.clone();
         let rt = tokio::runtime::Runtime::new()
             .map_err(|e| PyRuntimeError::new_err(format!("Failed to create runtime: {}", e)))?;
 
-        rt.block_on(async move {
-            let mut bash = inner.lock().await;
-            let builder = Bash::builder();
-            *bash = builder.build();
-            Ok(())
+        py.detach(|| {
+            rt.block_on(async move {
+                let mut bash = inner.lock().await;
+                let builder = Bash::builder();
+                *bash = builder.build();
+                Ok(())
+            })
         })
     }
 
@@ -378,40 +384,46 @@ impl BashTool {
         })
     }
 
-    fn execute_sync(&self, commands: String) -> PyResult<ExecResult> {
+    /// Releases GIL before blocking on tokio to prevent deadlock with callbacks.
+    fn execute_sync(&self, py: Python<'_>, commands: String) -> PyResult<ExecResult> {
         let inner = self.inner.clone();
         let rt = tokio::runtime::Runtime::new()
             .map_err(|e| PyRuntimeError::new_err(format!("Failed to create runtime: {}", e)))?;
 
-        rt.block_on(async move {
-            let mut bash = inner.lock().await;
-            match bash.exec(&commands).await {
-                Ok(result) => Ok(ExecResult {
-                    stdout: result.stdout,
-                    stderr: result.stderr,
-                    exit_code: result.exit_code,
-                    error: None,
-                }),
-                Err(e) => Ok(ExecResult {
-                    stdout: String::new(),
-                    stderr: String::new(),
-                    exit_code: 1,
-                    error: Some(e.to_string()),
-                }),
-            }
+        py.detach(|| {
+            rt.block_on(async move {
+                let mut bash = inner.lock().await;
+                match bash.exec(&commands).await {
+                    Ok(result) => Ok(ExecResult {
+                        stdout: result.stdout,
+                        stderr: result.stderr,
+                        exit_code: result.exit_code,
+                        error: None,
+                    }),
+                    Err(e) => Ok(ExecResult {
+                        stdout: String::new(),
+                        stderr: String::new(),
+                        exit_code: 1,
+                        error: Some(e.to_string()),
+                    }),
+                }
+            })
         })
     }
 
-    fn reset(&self) -> PyResult<()> {
+    /// Releases GIL before blocking on tokio to prevent deadlock.
+    fn reset(&self, py: Python<'_>) -> PyResult<()> {
         let inner = self.inner.clone();
         let rt = tokio::runtime::Runtime::new()
             .map_err(|e| PyRuntimeError::new_err(format!("Failed to create runtime: {}", e)))?;
 
-        rt.block_on(async move {
-            let mut bash = inner.lock().await;
-            let builder = Bash::builder();
-            *bash = builder.build();
-            Ok(())
+        py.detach(|| {
+            rt.block_on(async move {
+                let mut bash = inner.lock().await;
+                let builder = Bash::builder();
+                *bash = builder.build();
+                Ok(())
+            })
         })
     }
 
@@ -652,17 +664,20 @@ impl ScriptedTool {
     }
 
     /// Execute a bash script synchronously (blocking).
-    fn execute_sync(&self, commands: String) -> PyResult<ExecResult> {
+    /// Releases GIL before blocking on tokio to prevent deadlock with callbacks.
+    fn execute_sync(&self, py: Python<'_>, commands: String) -> PyResult<ExecResult> {
         let mut tool = self.build_rust_tool();
         let rt = tokio::runtime::Runtime::new()
             .map_err(|e| PyRuntimeError::new_err(format!("Failed to create runtime: {}", e)))?;
 
-        let resp = rt.block_on(async move {
-            tool.execute(ToolRequest {
-                commands,
-                timeout_ms: None,
+        let resp = py.detach(|| {
+            rt.block_on(async move {
+                tool.execute(ToolRequest {
+                    commands,
+                    timeout_ms: None,
+                })
+                .await
             })
-            .await
         });
         Ok(ExecResult {
             stdout: resp.stdout,
diff --git a/crates/bashkit-python/tests/test_bashkit.py b/crates/bashkit-python/tests/test_bashkit.py
index 76b369fb..8a15eea2 100644
--- a/crates/bashkit-python/tests/test_bashkit.py
+++ b/crates/bashkit-python/tests/test_bashkit.py
@@ -728,3 +728,123 @@ async def test_async_multiple_tools():
     assert r.exit_code == 0
     assert "A" in r.stdout
     assert "B" in r.stdout
+
+
+# ===========================================================================
+# GIL deadlock prevention tests
+# ===========================================================================
+
+
+def test_execute_sync_releases_gil_for_callback():
+    """execute_sync must release GIL before blocking on tokio runtime.
+
+    Without py.allow_threads(), a tool callback calling Python::attach()
+    inside rt.block_on() can deadlock when another thread holds the GIL.
+    This test validates the fix by running execute_sync with a callback
+    from a background thread while the main thread also holds the GIL.
+    """
+    import threading
+    import time
+
+    tool = ScriptedTool("api")
+
+    def slow_callback(params, stdin=None):
+        """Callback that briefly sleeps, requiring GIL reacquisition."""
+        time.sleep(0.01)
+        return f"ok-{params.get('id', 0)}\n"
+
+    tool.add_tool(
+        "slow",
+        "Slow command",
+        callback=slow_callback,
+        schema={"type": "object", "properties": {"id": {"type": "integer"}}},
+    )
+
+    results = [None, None]
+    errors = [None, None]
+
+    def run_in_thread(idx):
+        try:
+            r = tool.execute_sync(f"slow --id {idx}")
+            results[idx] = r
+        except Exception as e:
+            errors[idx] = e
+
+    t0 = threading.Thread(target=run_in_thread, args=(0,))
+    t1 = threading.Thread(target=run_in_thread, args=(1,))
+
+    t0.start()
+    t1.start()
+
+    # Timeout guards against deadlock — if GIL isn't released, threads block forever
+    t0.join(timeout=10)
+    t1.join(timeout=10)
+
+    assert not t0.is_alive(), "Thread 0 deadlocked (GIL not released in execute_sync)"
+    assert not t1.is_alive(), "Thread 1 deadlocked (GIL not released in execute_sync)"
+    assert errors[0] is None, f"Thread 0 error: {errors[0]}"
+    assert errors[1] is None, f"Thread 1 error: {errors[1]}"
+    assert results[0].exit_code == 0
+    assert results[1].exit_code == 0
+    assert "ok-0" in results[0].stdout
+    assert "ok-1" in results[1].stdout
+
+
+def test_bash_execute_sync_releases_gil():
+    """Bash.execute_sync must release GIL before blocking on tokio runtime."""
+    import threading
+
+    bash = Bash()
+    results = [None, None]
+    errors = [None, None]
+
+    def run_in_thread(idx):
+        try:
+            r = bash.execute_sync(f"echo thread-{idx}")
+            results[idx] = r
+        except Exception as e:
+            errors[idx] = e
+
+    t0 = threading.Thread(target=run_in_thread, args=(0,))
+    t1 = threading.Thread(target=run_in_thread, args=(1,))
+
+    t0.start()
+    t1.start()
+
+    t0.join(timeout=10)
+    t1.join(timeout=10)
+
+    assert not t0.is_alive(), "Thread 0 deadlocked (GIL not released)"
+    assert not t1.is_alive(), "Thread 1 deadlocked (GIL not released)"
+    assert errors[0] is None, f"Thread 0 error: {errors[0]}"
+    assert errors[1] is None, f"Thread 1 error: {errors[1]}"
+
+
+def test_bashtool_execute_sync_releases_gil():
+    """BashTool.execute_sync must release GIL before blocking on tokio runtime."""
+    import threading
+
+    tool = BashTool()
+    results = [None, None]
+    errors = [None, None]
+
+    def run_in_thread(idx):
+        try:
+            r = tool.execute_sync(f"echo thread-{idx}")
+            results[idx] = r
+        except Exception as e:
+            errors[idx] = e
+
+    t0 = threading.Thread(target=run_in_thread, args=(0,))
+    t1 = threading.Thread(target=run_in_thread, args=(1,))
+
+    t0.start()
+    t1.start()
+
+    t0.join(timeout=10)
+    t1.join(timeout=10)
+
+    assert not t0.is_alive(), "Thread 0 deadlocked (GIL not released)"
+    assert not t1.is_alive(), "Thread 1 deadlocked (GIL not released)"
+    assert errors[0] is None, f"Thread 0 error: {errors[0]}"
+    assert errors[1] is None, f"Thread 1 error: {errors[1]}"