docs(changelog): add entry for fix(actor): Get responses from actor-run (#10861)

CHANGELOG.md

@@ -64,6 +64,19 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   validate-then-write approach: all model validation occurs in Phase 1, and
   state mutations only happen in Phase 2 after all validations succeed.
 
+- **`agents actor run` empty response for built-in LLM actors** (#10861): Fixed
+  `resolve_config_files` in `cli/commands/_resolve_actor.py` silently returning
+  empty output when invoked with a built-in actor name (e.g.
+  `anthropic/claude-sonnet-4-20250514`). Built-in actors generated from the
+  provider registry have a `config_blob` with `provider` and `model` fields but
+  no `type` field. Serialising this blob as-is produced YAML that
+  `ReactiveConfigParser` could not interpret (no agents, no routes → empty
+  `ReactiveConfig` → empty response). Fix: `_synthesize_llm_yaml()` now
+  synthesises a minimal v3 `type: llm` YAML when the actor has no `yaml_text`
+  and the `config_blob` has `provider` and `model` but no `type` field, allowing
+  the reactive config parser to create a working agent and graph route. BDD
+  regression coverage in `features/tdd_actor_run_response.feature`.
+
 - **ReactiveConfigParser route synthesis for v3 actors** (#10807): Fixed
   `agents actor run` silently returning empty output for v3 `type:llm` actors.
   `_build_from_v3()` and `_build()` now synthesise a default single-node