fix: raise proper Exception in ChainAgent instead of a string (#248) (#550)

raise/throw of a bare string is a TypeError in Python and produces an
unhelpful non-Error in TypeScript — neither can be caught by type,
neither carries a stack trace, and neither chains the original cause.

- Python: `raise f"..."` → `raise Exception(f"...") from error`
- TypeScript: `throw \`...\`` → `throw new Error(\`...\`)`
- Add test_chain_agent.py covering the fix (exception is a real
  Exception, __cause__ is the original error) plus basic routing and
  edge cases

Fixes #248

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cornelcroi

python/src/agent_squad/agents/chain_agent.py

@@ -62,7 +62,7 @@ async def process_request(
 
             except Exception as error:
                 Logger.error(f"Error processing request with agent {agent.name}:{str(error)}")
-                raise f"Error processing request with agent {agent.name}:{str(error)}" from error
+                raise Exception(f"Error processing request with agent {agent.name}:{str(error)}") from error
 
         return final_response
 

python/src/tests/agents/test_chain_agent.py

@@ -0,0 +1,103 @@
+"""Unit tests for ChainAgent."""
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+from agent_squad.agents.chain_agent import ChainAgent, ChainAgentOptions
+from agent_squad.types import ConversationMessage, ParticipantRole
+
+
+def _make_agent(name: str, response_text: str = "ok") -> MagicMock:
+    agent = MagicMock()
+    agent.name = name
+    agent.process_request = AsyncMock(return_value=ConversationMessage(
+        role=ParticipantRole.ASSISTANT.value,
+        content=[{"text": response_text}],
+    ))
+    return agent
+
+
+def _options(agents, **kwargs):
+    return ChainAgentOptions(
+        name="chain",
+        description="test chain",
+        agents=agents,
+        **kwargs,
+    )
+
+
+# ---------------------------------------------------------------------------
+# Basic routing
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_single_agent_returns_response():
+    agent = _make_agent("a", "hello")
+    chain = ChainAgent(_options([agent]))
+    result = await chain.process_request("hi", "u", "s", [])
+    assert result.content[0]["text"] == "hello"
+
+
+@pytest.mark.asyncio
+async def test_output_of_first_agent_fed_to_second():
+    a1 = _make_agent("a1", "step1")
+    a2 = _make_agent("a2", "step2")
+    chain = ChainAgent(_options([a1, a2]))
+    await chain.process_request("start", "u", "s", [])
+    a2.process_request.assert_awaited_once()
+    call_input = a2.process_request.call_args[0][0]
+    assert call_input == "step1"
+
+
+# ---------------------------------------------------------------------------
+# Exception handling — the core bug fix
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_agent_exception_raises_exception_not_string():
+    agent = MagicMock()
+    agent.name = "bad_agent"
+    agent.process_request = AsyncMock(side_effect=RuntimeError("boom"))
+    chain = ChainAgent(_options([agent]))
+
+    with pytest.raises(Exception) as exc_info:
+        await chain.process_request("hi", "u", "s", [])
+
+    # Must be a real Exception, not a string
+    assert isinstance(exc_info.value, Exception)
+    assert "bad_agent" in str(exc_info.value)
+    assert "boom" in str(exc_info.value)
+
+
+@pytest.mark.asyncio
+async def test_exception_chained_from_original():
+    original = RuntimeError("root cause")
+    agent = MagicMock()
+    agent.name = "bad_agent"
+    agent.process_request = AsyncMock(side_effect=original)
+    chain = ChainAgent(_options([agent]))
+
+    with pytest.raises(Exception) as exc_info:
+        await chain.process_request("hi", "u", "s", [])
+
+    assert exc_info.value.__cause__ is original
+
+
+# ---------------------------------------------------------------------------
+# Edge cases
+# ---------------------------------------------------------------------------
+
+def test_empty_agents_raises_value_error():
+    with pytest.raises(ValueError):
+        ChainAgent(_options([]))
+
+
+@pytest.mark.asyncio
+async def test_default_output_used_when_agent_returns_no_text():
+    agent = MagicMock()
+    agent.name = "empty"
+    agent.process_request = AsyncMock(return_value=ConversationMessage(
+        role=ParticipantRole.ASSISTANT.value,
+        content=[{"no_text": True}],
+    ))
+    chain = ChainAgent(_options([agent], default_output="fallback"))
+    result = await chain.process_request("hi", "u", "s", [])
+    assert result.content[0]["text"] == "fallback"

typescript/src/agents/chainAgent.ts

@@ -86,7 +86,7 @@ export class ChainAgent extends Agent {
         }
       } catch (error) {
         Logger.logger.error(`Error processing request with agent ${agent.name}:`, error);
-        throw `Error processing request with agent ${agent.name}:${String(error)}`;
+        throw new Error(`Error processing request with agent ${agent.name}:${String(error)}`);
       }
     }