fix: treat a silent agent turn as silence, not a run_input error

[theomonnom]

Problem

SessionHost turns a run that produced no chat items into an error:

if not items_list and not error:
    error = "agent produced no response items"

The caller sees it as a raised RuntimeError, so the turn — and, in simulations, the whole job — is scored as an agent failure.

An empty turn is a legitimate outcome. An LLM can return a completion with no text and no tool calls, and every item-add site in agent_activity is guarded on non-empty text (if forwarded_text:), so nothing lands in the chat context or in RunResult.events.

Agents that induce this on purpose hit it reliably. A close-the-call tool whose output tells the model it has already said goodbye and should now say nothing gets exactly that:

OUT : "You've already said goodbye - do NOT give another farewell, sign-off,
       or filler. If the caller just asked a real question, answer it in one
       short sentence; otherwise say nothing."
USER: "Okay, thank you."        <- agent emits nothing, turn fails

Agent-side traces for that turn confirm the model, not the framework, produced the silence:

[agent_turn]  dur=0.28s  lk.interrupted=False
  [llm_node]    lk.response.text=''
    [llm_request] gen_ai.usage.output_tokens=1

One output token, empty response text, no tool call, not interrupted — and the transcript up to that point is complete and correct.

Fix

Report the empty item list and let the caller decide what the silence means. Simulation drivers already model a silent turn explicitly, so they can end the call cleanly instead of failing it.

Test

test_run_silent_turn_is_not_an_error drives a real AgentSession whose LLM returns an empty completion through SessionHost → RemoteSession. Without the fix it fails with the exact production error:

RuntimeError: session request run_input failed: agent produced no response items

[devin-ai-integration]

✅ Devin Review: No Issues Found

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