fix(session): truncate context_messages when branching from fork point

Refs nesquena/hermes-webui#5096

Co-authored-by: b3nw <b3nw@users.noreply.github.com>

api/routes.py

@@ -12320,6 +12320,14 @@ def handle_post(handler, parsed) -> bool:
             branch_title = f"{source_title} (fork)"
 
         # Create new session inheriting workspace/model/profile
+        from api.session_ops import truncate_context_for_display_keep
+
+        fork_keep = keep_count if keep_count is not None else len(source_messages)
+        forked_context = truncate_context_for_display_keep(
+            getattr(source, "context_messages", None),
+            source_messages,
+            fork_keep,
+        )
         branch = Session(
             workspace=source.workspace,
             model=source.model,
@@ -12332,8 +12340,8 @@ def handle_post(handler, parsed) -> bool:
             enabled_toolsets=getattr(source, "enabled_toolsets", None),
             context_length=getattr(source, "context_length", None),
             threshold_tokens=getattr(source, "threshold_tokens", None),
-            # context_messages — deep copy so the branch has independent context
-            context_messages=copy.deepcopy(getattr(source, "context_messages", None) or []),
+            # context_messages — truncated to fork prefix (not full parent copy)
+            context_messages=copy.deepcopy(forked_context),
             # Gateway routing — inherit from source
             gateway_routing=copy.deepcopy(getattr(source, "gateway_routing", None)),
             # Context engine — inherit state so branch's context engine starts correctly

api/session_ops.py

@@ -67,6 +67,29 @@ def _truncation_watermark_for(messages):
         return 0.0
 
 
+def truncate_context_for_display_keep(
+    context_messages: list | None,
+    full_messages: list | None,
+    keep: int,
+) -> list:
+    """Align model context with display prefix ``full_messages[:keep]``."""
+    if keep <= 0:
+        return []
+    ctx = context_messages if isinstance(context_messages, list) else []
+    msgs = full_messages if isinstance(full_messages, list) else []
+    if not ctx:
+        return []
+    if len(ctx) == len(msgs):
+        return ctx[:keep]
+    if len(msgs) == 0:
+        return []
+    # Context tail aligns with full display transcript; preserve leading-only rows.
+    prefix_len = max(0, len(ctx) - len(msgs))
+    prefix = ctx[:prefix_len]
+    suffix = ctx[prefix_len:]
+    return prefix + suffix[:keep]
+
+
 def retry_last(session_id: str) -> dict[str, Any]:
     """Truncate the session to before the last user message, return its text.
 

tests/test_issue_branch_context_at_fork.py

@@ -0,0 +1,34 @@
+"""Branch must not inherit full parent context_messages (#5096 A)."""
+
+import copy
+
+from api.session_ops import truncate_context_for_display_keep
+
+
+def test_truncate_context_for_display_keep_drops_parent_tail():
+    msgs = [
+        {"role": "user", "content": "u1"},
+        {"role": "assistant", "content": "a1"},
+        {"role": "user", "content": "after-fork"},
+    ]
+    # Same-length context as display — fork at keep=2 matches messages[:2]
+    ctx = copy.deepcopy(msgs)
+    out = truncate_context_for_display_keep(ctx, msgs, 2)
+    assert len(out) == 2
+    assert out[-1]["content"] == "a1"
+    assert "after-fork" not in [m["content"] for m in out]
+
+
+def test_truncate_context_preserves_leading_compaction_row():
+    msgs = [
+        {"role": "user", "content": "u1"},
+        {"role": "assistant", "content": "a1"},
+    ]
+    ctx = [
+        {"role": "user", "content": "compaction-ref-only"},
+        {"role": "user", "content": "u1"},
+        {"role": "assistant", "content": "a1"},
+    ]
+    out = truncate_context_for_display_keep(ctx, msgs, 2)
+    assert len(out) == 3
+    assert out[0]["content"] == "compaction-ref-only"