fix(acp): don't let a malformed tool argument abort the tool-call render

build_tool_start renders every ACP (Zed) tool call — on the live tool-progress
callback (acp_adapter/events.py) and during session history replay
(acp_adapter/server.py). It called build_tool_title and extract_locations
directly, so a model that emits a malformed argument crashed the render:

- terminal `command` as null/number -> TypeError (len() in build_tool_title)
- delegate_task `goal` as a number  -> TypeError (len())
- read_file `path` as a non-string  -> pydantic ValidationError building a
  ToolCallLocation

A live crash breaks the tool-call event; a persisted one breaks history replay
on every resume of that session. The sibling CLI label builder
get_cute_tool_message was already wrapped for exactly this reason
(agent/display.py: "display must never abort a turn").

Wrap build_tool_start the same way: on any builder failure, fall back to a
minimal, valid start event (tool name as title, resolved kind). The happy path
is unchanged.

Adds tests for the non-string command, path, and goal cases.

acp_adapter/tools.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import logging
 import uuid
 from typing import Any, Dict, List, Optional
 
@@ -14,6 +15,8 @@ from acp.schema import (
     ToolKind,
 )
 
+logger = logging.getLogger(__name__)
+
 # ---------------------------------------------------------------------------
 # Map hermes tool names -> ACP ToolKind
 # ---------------------------------------------------------------------------
@@ -1026,7 +1029,37 @@ def build_tool_start(
     *,
     edit_diff: Any = None,
 ) -> ToolCallStart:
-    """Create a ToolCallStart event for the given hermes tool invocation."""
+    """Create a ToolCallStart event for the given hermes tool invocation.
+
+    A malformed tool argument (e.g. a non-string ``command``/``path`` from a
+    model that ignores the schema) must never abort the ACP tool-call render —
+    ``build_tool_start`` runs on the live tool-progress callback and during
+    session history replay. On any failure in the title/content/location
+    builders, fall back to a minimal, valid start event. Mirrors
+    ``get_cute_tool_message`` in ``agent/display.py``, wrapped for the same
+    reason on the CLI side.
+    """
+    try:
+        return _build_tool_start(
+            tool_call_id, tool_name, arguments, edit_diff=edit_diff
+        )
+    except Exception as exc:  # noqa: BLE001 — a tool-call render must never abort the turn
+        logger.debug("ACP tool-start render failed for %r: %s", tool_name, exc)
+        safe_name = tool_name if isinstance(tool_name, str) and tool_name else "tool"
+        return acp.start_tool_call(
+            tool_call_id, safe_name, kind=get_tool_kind(safe_name),
+            content=None, locations=[], raw_input=None,
+        )
+
+
+def _build_tool_start(
+    tool_call_id: str,
+    tool_name: str,
+    arguments: Dict[str, Any],
+    *,
+    edit_diff: Any = None,
+) -> ToolCallStart:
+    """Build the ToolCallStart event (unguarded; see ``build_tool_start``)."""
     kind = get_tool_kind(tool_name)
     title = build_tool_title(tool_name, arguments)
     locations = extract_locations(arguments)

tests/acp/test_tools.py

@@ -227,6 +227,26 @@ class TestBuildToolStart:
         assert result.content is None
         assert result.raw_input is None
 
+    def test_build_tool_start_survives_non_string_command(self):
+        """A malformed (non-string) terminal command previously raised
+        TypeError in build_tool_title (len(None)) and aborted the render."""
+        result = build_tool_start("tc-bad-cmd", "terminal", {"command": None})
+        assert isinstance(result, ToolCallStart)
+        assert result.kind == "execute"  # tool identity preserved in the fallback
+
+    def test_build_tool_start_survives_non_string_path(self):
+        """A non-string read_file path previously raised a ToolCallLocation
+        pydantic ValidationError in extract_locations and aborted the render."""
+        result = build_tool_start("tc-bad-path", "read_file", {"path": {"p": "x"}})
+        assert isinstance(result, ToolCallStart)
+        assert result.kind == "read"
+
+    def test_build_tool_start_survives_non_string_goal(self):
+        """A non-string delegate_task goal previously raised TypeError
+        (len(123)) in build_tool_title and aborted the render."""
+        result = build_tool_start("tc-bad-goal", "delegate_task", {"goal": 123})
+        assert isinstance(result, ToolCallStart)
+
     def test_build_tool_start_for_web_extract_is_compact(self):
         """web_extract start should stay compact; title identifies URLs."""
         args = {"urls": ["https://example.com/docs"]}