diff --git a/headroom/learn/analyzer.py b/headroom/learn/analyzer.py
index 92bd4fb35..8fea17b4e 100644
--- a/headroom/learn/analyzer.py
+++ b/headroom/learn/analyzer.py
@@ -371,6 +371,26 @@ def _format_event(event: SessionEvent) -> str | None:
     return None
 
 
+_ERROR_PREVIEW_MAX = 200
+
+
+def _truncate_head_tail(text: str, max_chars: int = _ERROR_PREVIEW_MAX) -> str:
+    """Collapse newlines and truncate, keeping both the head and the tail.
+
+    A head-only slice drops the end of a traceback, which is exactly where the
+    root cause (``ExceptionType: message``) lives, so the digest would show only
+    the preamble and lose the diagnosis (see #2590). Keep both ends instead.
+    """
+    text = text.replace("\n", " ").strip()
+    if len(text) <= max_chars:
+        return text
+    sep = " … "
+    keep = max_chars - len(sep)
+    head = keep // 2
+    tail = keep - head
+    return f"{text[:head].rstrip()}{sep}{text[-tail:].lstrip()}"
+
+
 def _format_tool_call(tc: ToolCall) -> str:
     """Format a single tool call into a compact digest line."""
     status = "ERROR" if tc.is_error else "OK"
@@ -380,8 +400,9 @@ def _format_tool_call(tc: ToolCall) -> str:
     input_str = tc.input_summary[:120]
 
     if tc.is_error:
-        # Include truncated error output for failures
-        output_preview = tc.output[:200].replace("\n", " ").strip()
+        # Include truncated error output for failures, keeping the tail so a
+        # traceback's root cause survives (#2590).
+        output_preview = _truncate_head_tail(tc.output)
         return f"  [{tc.msg_index}] {tc.name}: {input_str} → {status}{error_cat}: {output_preview}"
     else:
         # Just indicate success with size
diff --git a/tests/test_learn/test_analyzer.py b/tests/test_learn/test_analyzer.py
index 4230755b4..2d711227f 100644
--- a/tests/test_learn/test_analyzer.py
+++ b/tests/test_learn/test_analyzer.py
@@ -155,6 +155,54 @@ class TestDigestBuilder:
         digest = _build_digest(_project(), [])
         assert "0 sessions" in digest or "test-project" in digest
 
+    def test_long_error_output_preserves_tail_root_cause(self):
+        # A traceback's diagnosis lives at the tail; a head-only slice would drop it (#2590).
+        traceback = (
+            "Traceback (most recent call last):\n"
+            + "\n".join(
+                f'  File "mod{i}.py", line {i}, in fn{i}\n    call_{i}()' for i in range(40)
+            )
+            + "\nKeyError: 'the-actual-root-cause'"
+        )
+        sessions = [
+            SessionData(
+                session_id="s1",
+                tool_calls=[
+                    _tc(
+                        name="Bash",
+                        output=traceback,
+                        is_error=True,
+                        error_category=ErrorCategory.UNKNOWN,
+                        msg_index=0,
+                    )
+                ],
+            )
+        ]
+        digest = _build_digest(_project(), sessions)
+        assert "Traceback" in digest
+        assert "KeyError: 'the-actual-root-cause'" in digest
+        assert "…" in digest
+
+    def test_short_error_output_not_truncated(self):
+        short = "ModuleNotFoundError: No module named 'foo'"
+        sessions = [
+            SessionData(
+                session_id="s1",
+                tool_calls=[
+                    _tc(
+                        name="Bash",
+                        output=short,
+                        is_error=True,
+                        error_category=ErrorCategory.MODULE_NOT_FOUND,
+                        msg_index=0,
+                    )
+                ],
+            )
+        ]
+        digest = _build_digest(_project(), sessions)
+        assert short in digest
+        assert "…" not in digest
+
 
 # =============================================================================
 # Prior Patterns Injection Tests