diff --git a/src/claude_agent_sdk/_internal/session_mutations.py b/src/claude_agent_sdk/_internal/session_mutations.py
index 9350123d..7d68d3c4 100644
--- a/src/claude_agent_sdk/_internal/session_mutations.py
+++ b/src/claude_agent_sdk/_internal/session_mutations.py
@@ -22,6 +22,7 @@
 import json
 import os
 import re
+import shutil
 import unicodedata
 import uuid as uuid_mod
 from dataclasses import dataclass
@@ -170,11 +171,13 @@ def delete_session(
     session_id: str,
     directory: str | None = None,
 ) -> None:
-    """Delete a session by removing its JSONL file.
+    """Delete a session by removing its JSONL file and subagent transcripts.
 
-    This is a hard delete — the file is removed permanently. SDK users who
-    need soft-delete semantics can use ``tag_session(id, '__hidden')`` and
-    filter on listing instead.
+    This is a hard delete — the ``{session_id}.jsonl`` file is removed
+    permanently, along with the sibling ``{session_id}/`` subdirectory that
+    holds subagent transcripts (if it exists). SDK users who need soft-delete
+    semantics can use ``tag_session(id, '__hidden')`` and filter on listing
+    instead.
 
     Args:
         session_id: UUID of the session to delete.
@@ -206,6 +209,8 @@ def delete_session(
         if e.errno == errno.ENOENT:
             raise FileNotFoundError(f"Session {session_id} not found") from e
         raise
+    # Subagent transcripts live in a sibling {session_id}/ dir; often absent.
+    shutil.rmtree(path.parent / session_id, ignore_errors=True)
 
 
 @dataclass
diff --git a/tests/test_session_mutations.py b/tests/test_session_mutations.py
index 0575e5e3..18ad8aeb 100644
--- a/tests/test_session_mutations.py
+++ b/tests/test_session_mutations.py
@@ -490,6 +490,25 @@ def test_deletes_session_file(self, claude_config_dir: Path, tmp_path: Path):
         delete_session(sid, directory=project_path)
         assert not file_path.exists()
 
+    def test_removes_subagent_transcript_dir(
+        self, claude_config_dir: Path, tmp_path: Path
+    ):
+        """Cascades the sibling {sid}/ subagent dir alongside the .jsonl."""
+        project_path = str(tmp_path / "proj")
+        Path(project_path).mkdir(parents=True)
+        project_dir = _make_project_dir(
+            claude_config_dir, os.path.realpath(project_path)
+        )
+        sid, file_path = _make_session_file(project_dir)
+        subagent_dir = project_dir / sid
+        subagent_dir.mkdir()
+        (subagent_dir / f"{uuid.uuid4()}.jsonl").write_text("{}\n")
+
+        delete_session(sid, directory=project_path)
+
+        assert not file_path.exists()
+        assert not subagent_dir.exists()
+
     def test_deletes_without_directory(self, claude_config_dir: Path):
         """Searches all project directories when no directory is given."""
         project_dir = _make_project_dir(claude_config_dir, "/any/project")