diff --git a/pipeline/haiku.py b/pipeline/haiku.py
index b0793d7..1f41c5c 100644
--- a/pipeline/haiku.py
+++ b/pipeline/haiku.py
@@ -97,9 +97,13 @@ def call_haiku(
         RuntimeError: If the subprocess times out or exits with a non-zero
             return code, or if the JSON response cannot be parsed.
     """
+    # Prompt goes on STDIN, not argv: a session extract can exceed Linux's
+    # MAX_ARG_STRLEN (128KB per single argument), which raises E2BIG ("Argument
+    # list too long") at exec time and silently kills saves of long sessions.
+    # `claude -p` with no positional prompt reads the prompt from stdin.
     cmd = [
         "claude",
-        "-p", prompt,
+        "-p",
         "--output-format", "json",
         "--no-session-persistence",
         "--exclude-dynamic-system-prompt-sections",
@@ -116,6 +120,7 @@ def call_haiku(
     try:
         result = subprocess.run(
             cmd,
+            input=prompt,
             capture_output=True,
             text=True,
             # claude emits UTF-8; without this, text=True decodes with the
diff --git a/tests/test_haiku.py b/tests/test_haiku.py
index 76134da..d41f856 100644
--- a/tests/test_haiku.py
+++ b/tests/test_haiku.py
@@ -140,6 +140,26 @@ def test_call_haiku_success(mock_run):
     assert "CLAUDECODE" not in env
 
 
+@patch("pipeline.haiku.subprocess.run")
+def test_call_haiku_sends_prompt_on_stdin_not_argv(mock_run):
+    """The prompt is delivered on STDIN, never as an argv string.
+
+    A session extract can exceed Linux's MAX_ARG_STRLEN (131072 bytes / 128KB
+    per single argument); the old ``claude -p <prompt>`` form fails at exec()
+    with OSError E2BIG ("Argument list too long"), silently losing the save.
+    Guard both halves so the regression can't silently return: the prompt must
+    arrive via ``input=`` and must NOT appear in the command argv (``-p`` is a
+    bare flag, immediately followed by the next option)."""
+    mock_run.return_value = MagicMock(
+        returncode=0, stdout=_mock_claude_response("ok"), stderr="")
+    call_haiku("the full prompt text")
+    args = mock_run.call_args
+    cmd = args[0][0]
+    assert args[1]["input"] == "the full prompt text"
+    assert "the full prompt text" not in cmd
+    assert cmd[cmd.index("-p") + 1] == "--output-format"
+
+
 @patch("pipeline.haiku.subprocess.run")
 def test_call_haiku_strips_parent_session_env(mock_run, monkeypatch):
     """The nested claude -p must not inherit the PARENT Claude Code session