diff --git a/agent/agent_runtime_helpers.py b/agent/agent_runtime_helpers.py
index 09c4aec5cb06..d1803048babf 100644
--- a/agent/agent_runtime_helpers.py
+++ b/agent/agent_runtime_helpers.py
@@ -672,6 +672,39 @@ def strip_think_blocks(agent, content: str) -> str:
     """
     if not content:
         return ""
+    # Coerce non-string content to text before any regex runs.  Providers
+    # that return assistant ``content`` as a list of blocks (Anthropic via
+    # OpenRouter emits ``[{"type":"text",...}, {"type":"thinking",...}]``) or
+    # as a dict flow into this shared helper from several callers — most
+    # notably ``_interim_assistant_visible_text`` reading a *stored* history
+    # message whose content was persisted as a list.  A raw list/dict reaching
+    # ``re.sub`` below raises ``TypeError: expected string or bytes-like
+    # object, got 'list'``, which the outer conversation loop swallows and
+    # retries forever (observed as an infinite "preparing terminal…" loop on
+    # Anthropic models via OpenRouter).  Flatten here so every caller is safe.
+    if not isinstance(content, str):
+        if isinstance(content, list):
+            _parts: list[str] = []
+            for _part in content:
+                if isinstance(_part, str):
+                    _parts.append(_part)
+                elif isinstance(_part, dict):
+                    _ptype = str(_part.get("type") or "").strip().lower()
+                    # Drop reasoning/thinking blocks outright — this function's
+                    # whole job is to strip them, and their text lives under
+                    # different keys ("thinking", "reasoning") per provider.
+                    if _ptype in {"thinking", "reasoning", "redacted_thinking"}:
+                        continue
+                    _text = _part.get("text")
+                    if isinstance(_text, str) and _text:
+                        _parts.append(_text)
+            content = "".join(_parts)
+        elif isinstance(content, dict):
+            content = str(content.get("text") or content.get("content") or "")
+        else:
+            content = str(content)
+        if not content:
+            return ""
     # 1. Closed tag pairs — case-insensitive for all variants so
     #    mixed-case tags (<THINK>, <Thinking>) don't slip through to
     #    the unterminated-tag pass and take trailing content with them.
diff --git a/tests/run_agent/test_run_agent.py b/tests/run_agent/test_run_agent.py
index 2b0db24f6b73..24c1030dc089 100644
--- a/tests/run_agent/test_run_agent.py
+++ b/tests/run_agent/test_run_agent.py
@@ -445,6 +445,40 @@ class TestStripThinkBlocks:
     def test_none_returns_empty(self, agent):
         assert agent._strip_think_blocks(None) == ""
 
+    def test_list_content_flattened_no_crash(self, agent):
+        """Anthropic-via-OpenRouter returns content as a block list.
+
+        A raw list reaching ``re.sub`` raised ``TypeError: expected string
+        or bytes-like object, got 'list'``, which the outer conversation
+        loop swallowed and retried forever (infinite "preparing terminal…"
+        loop). ``strip_think_blocks`` must flatten list content to visible
+        text and drop reasoning blocks.
+        """
+        result = agent._strip_think_blocks(
+            [
+                {"type": "text", "text": "visible answer"},
+                {"type": "thinking", "thinking": "internal reasoning"},
+            ]
+        )
+        assert isinstance(result, str)
+        assert "visible answer" in result
+        assert "internal reasoning" not in result
+
+    def test_dict_content_flattened_no_crash(self, agent):
+        """Some servers return content as a single dict block."""
+        result = agent._strip_think_blocks({"type": "text", "text": "hello world"})
+        assert isinstance(result, str)
+        assert "hello world" in result
+
+    def test_list_of_only_thinking_returns_empty(self, agent):
+        """A list carrying only reasoning blocks yields no visible text."""
+        assert (
+            agent._strip_think_blocks([{"type": "thinking", "thinking": "x"}]) == ""
+        )
+
+    def test_empty_list_returns_empty(self, agent):
+        assert agent._strip_think_blocks([]) == ""
+
     def test_no_blocks_unchanged(self, agent):
         assert agent._strip_think_blocks("hello world") == "hello world"