diff --git a/agent/conversation_loop.py b/agent/conversation_loop.py
index d3cad0c35fd..fdf65c07558 100644
--- a/agent/conversation_loop.py
+++ b/agent/conversation_loop.py
@@ -989,6 +989,7 @@ def run_conversation(
         copilot_auth_retry_attempted=False
         thinking_sig_retry_attempted = False
         image_shrink_retry_attempted = False
+        multimodal_tool_content_retry_attempted = False
         oauth_1m_beta_retry_attempted = False
         llama_cpp_grammar_retry_attempted = False
         has_retried_429 = False
@@ -2060,6 +2061,31 @@ def run_conversation(
                             "or shrink didn't reduce size; surfacing original error."
                         )
 
+                # Multimodal-tool-content recovery: providers that follow
+                # the OpenAI spec strictly (tool message content must be a
+                # string) reject our list-type content with a 400.  Strip
+                # image parts from any list-type tool messages, mark the
+                # (provider, model) as no-list-tool-content for the rest
+                # of this session so future tool results preemptively
+                # downgrade, and retry once.  See issue #27344.
+                if (
+                    classified.reason == FailoverReason.multimodal_tool_content_unsupported
+                    and not multimodal_tool_content_retry_attempted
+                ):
+                    multimodal_tool_content_retry_attempted = True
+                    if agent._try_strip_image_parts_from_tool_messages(api_messages):
+                        agent._vprint(
+                            f"{agent.log_prefix}📐 Provider rejected list-type tool content — "
+                            f"downgraded screenshots to text and retrying...",
+                            force=True,
+                        )
+                        continue
+                    else:
+                        logger.info(
+                            "multimodal-tool-content recovery: no list-type tool "
+                            "messages with image parts found; surfacing original error."
+                        )
+
                 # Anthropic OAuth subscription rejected the 1M-context beta
                 # header ("long context beta is not yet available for this
                 # subscription"). Disable the beta for the rest of this
diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index 42eb42d6803..7fa38bbcf70 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -50,6 +50,7 @@ class FailoverReason(enum.Enum):
 
     # Request format
     format_error = "format_error"        # 400 bad request — abort or strip + retry
+    multimodal_tool_content_unsupported = "multimodal_tool_content_unsupported"  # Provider rejected list-type content in tool messages (e.g. Xiaomi MiMo) — downgrade to text and retry
 
     # Provider-specific
     thinking_signature = "thinking_signature"  # Anthropic thinking block sig invalid
@@ -165,6 +166,32 @@ _IMAGE_TOO_LARGE_PATTERNS = [
     # the likely culprit; we still try the shrink path before giving up.
 ]
 
+# Providers that follow the OpenAI spec strictly require tool message
+# ``content`` to be a string.  Some (Anthropic native, Codex Responses,
+# Gemini native, first-party OpenAI) extend this to accept a content-parts
+# list (text + image_url) so screenshots from computer_use survive.  Others
+# (Xiaomi MiMo, some Alibaba endpoints, a long tail of OpenAI-compatible
+# providers) reject the list with a 400 — the patterns below are the most
+# common error shapes we see.  Recovery: strip image parts from tool
+# messages in-place, record the (provider, model) for the rest of the
+# session so we don't waste another call learning the same lesson, retry.
+#
+# See: https://github.com/NousResearch/hermes-agent/issues/27344
+_MULTIMODAL_TOOL_CONTENT_PATTERNS = [
+    # Xiaomi MiMo: {"error":{"code":"400","message":"Param Incorrect","param":"text is not set"}}
+    "text is not set",
+    # Generic "tool message must be string" shapes
+    "tool message content must be a string",
+    "tool content must be a string",
+    "tool message must be a string",
+    # OpenAI-compat servers that reject list-type tool content with a
+    # schema-validation message
+    "expected string, got list",
+    "expected string, got array",
+    # Alibaba/DashScope variant
+    "tool_call.content must be string",
+]
+
 # Context overflow patterns
 _CONTEXT_OVERFLOW_PATTERNS = [
     "context length",
@@ -781,6 +808,19 @@ def _classify_400(
 ) -> ClassifiedError:
     """Classify 400 Bad Request — context overflow, format error, or generic."""
 
+    # Multimodal tool content rejected from 400.  Must be checked BEFORE
+    # image_too_large because the recovery is different (strip image parts
+    # from tool messages, mark the model as no-list-tool-content for the
+    # rest of the session) and BEFORE context_overflow because some of the
+    # patterns ("text is not set") are ambiguous in isolation but become
+    # specific when combined with a 400 on a request known to contain
+    # multimodal tool content.
+    if any(p in error_msg for p in _MULTIMODAL_TOOL_CONTENT_PATTERNS):
+        return result_fn(
+            FailoverReason.multimodal_tool_content_unsupported,
+            retryable=True,
+        )
+
     # Image-too-large from 400 (Anthropic's 5 MB per-image check fires this way).
     # Must be checked BEFORE context_overflow because messages can trip both
     # patterns ("exceeds" + "image") and image-shrink is a cheaper recovery.
@@ -922,6 +962,13 @@ def _classify_by_message(
             should_compress=True,
         )
 
+    # Multimodal tool content patterns (from message text when no status_code)
+    if any(p in error_msg for p in _MULTIMODAL_TOOL_CONTENT_PATTERNS):
+        return result_fn(
+            FailoverReason.multimodal_tool_content_unsupported,
+            retryable=True,
+        )
+
     # Image-too-large patterns (from message text when no status_code)
     if any(p in error_msg for p in _IMAGE_TOO_LARGE_PATTERNS):
         return result_fn(
diff --git a/run_agent.py b/run_agent.py
index 001d03784ad..5b89839b645 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -3357,6 +3357,25 @@ class AIAgent:
             return content
 
         if self._model_supports_vision():
+            # Vision-capable on paper — but if we've already learned in this
+            # session that the active (provider, model) rejects list-type
+            # tool content (e.g. Xiaomi MiMo's 400 "text is not set"),
+            # short-circuit to a text summary so we don't burn another
+            # round-trip relearning the same lesson.  Cache populated by
+            # the 400 recovery path in agent.conversation_loop.  Transient
+            # per-session; next session retries.
+            key = (
+                (getattr(self, "provider", "") or "").strip().lower(),
+                (getattr(self, "model", "") or "").strip(),
+            )
+            no_list = getattr(self, "_no_list_tool_content_models", None)
+            if no_list and key in no_list:
+                logger.debug(
+                    "Tool %s: model %s/%s known to reject list-type tool "
+                    "content this session — sending text summary",
+                    tool_name, key[0], key[1],
+                )
+                return _multimodal_text_summary(result)
             return content
 
         summary = _multimodal_text_summary(result)
@@ -3385,6 +3404,80 @@ class AIAgent:
         from agent.conversation_compression import try_shrink_image_parts_in_messages
         return try_shrink_image_parts_in_messages(api_messages)
 
+    def _try_strip_image_parts_from_tool_messages(self, api_messages: list) -> bool:
+        """Downgrade list-type tool messages to text summaries in-place.
+
+        Recovery path for providers that reject list-type tool message content
+        (e.g. Xiaomi MiMo's 400 "text is not set"; see issue #27344).  Walks
+        ``api_messages`` for any ``role: "tool"`` message whose ``content`` is
+        a list containing image parts, replaces the content with the existing
+        text part(s) (or a minimal placeholder if none survive), and records
+        the active (provider, model) in ``self._no_list_tool_content_models``
+        so subsequent ``_tool_result_content_for_active_model`` calls in this
+        session preemptively downgrade screenshots without a round-trip.
+
+        Returns True when at least one tool message was downgraded — the
+        caller (the 400 recovery branch in ``agent.conversation_loop``) uses
+        this to decide whether to retry the API call with the modified
+        history or surface the original error.
+        """
+        if not isinstance(api_messages, list):
+            return False
+
+        # Record (provider, model) so we don't relearn this lesson.
+        key = (
+            (getattr(self, "provider", "") or "").strip().lower(),
+            (getattr(self, "model", "") or "").strip(),
+        )
+        if not hasattr(self, "_no_list_tool_content_models"):
+            self._no_list_tool_content_models = set()
+        if key[1]:  # only record when we actually have a model id
+            self._no_list_tool_content_models.add(key)
+
+        changed = False
+        for msg in api_messages:
+            if not isinstance(msg, dict) or msg.get("role") != "tool":
+                continue
+            content = msg.get("content")
+            if not isinstance(content, list):
+                continue
+
+            # Salvage any text parts so the model still sees some signal.
+            text_parts: List[str] = []
+            had_image = False
+            for part in content:
+                if not isinstance(part, dict):
+                    if isinstance(part, str) and part.strip():
+                        text_parts.append(part.strip())
+                    continue
+                ptype = part.get("type")
+                if ptype == "image_url" or ptype == "input_image":
+                    had_image = True
+                    continue
+                if ptype in {"text", "input_text"}:
+                    text = str(part.get("text") or "").strip()
+                    if text:
+                        text_parts.append(text)
+
+            if not had_image:
+                # List-type content but no image parts — leave alone (some
+                # providers reject ANY list content, but stripping a
+                # text-only list doesn't reduce ambiguity; let the caller
+                # surface the original error if this turns out to be the
+                # case).
+                continue
+
+            if text_parts:
+                msg["content"] = "\n\n".join(text_parts)
+            else:
+                msg["content"] = (
+                    "[image content removed — provider does not accept "
+                    "list-type tool message content]"
+                )
+            changed = True
+
+        return changed
+
     def _anthropic_preserve_dots(self) -> bool:
         """True when using an anthropic-compatible endpoint that preserves dots in model names.
         Alibaba/DashScope keeps dots (e.g. qwen3.5-plus).
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index a6fb56a7075..eef3650347c 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -56,6 +56,7 @@ class TestFailoverReason:
             "overloaded", "server_error", "timeout",
             "context_overflow", "payload_too_large", "image_too_large",
             "model_not_found", "format_error",
+            "multimodal_tool_content_unsupported",
             "provider_policy_blocked",
             "thinking_signature", "long_context_tier",
             "oauth_long_context_beta_forbidden",
@@ -1256,3 +1257,66 @@ class TestRateLimitErrorWithoutStatusCode:
         e.status_code = None
         result = classify_api_error(e, provider="copilot", model="gpt-4o")
         assert result.reason != FailoverReason.rate_limit
+
+
+
+# ── Test: multimodal_tool_content_unsupported pattern ───────────────────
+
+class TestMultimodalToolContentUnsupported:
+    """Issue #27344 — providers that reject list-type tool message content
+    should be classified as ``multimodal_tool_content_unsupported`` so the
+    retry loop can downgrade screenshots to text and try again.
+    """
+
+    def test_xiaomi_mimo_text_is_not_set_pattern(self):
+        """The actual Xiaomi MiMo 400 wording from the bug report."""
+        e = MockAPIError(
+            "Error code: 400 - {'error': {'code': '400', 'message': 'Param Incorrect', 'param': 'text is not set', 'type': ''}}",
+            status_code=400,
+        )
+        result = classify_api_error(e, provider="xiaomi", model="mimo-v2.5")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+        assert result.retryable is True
+
+    def test_generic_tool_message_must_be_string(self):
+        e = MockAPIError(
+            "tool message content must be a string",
+            status_code=400,
+        )
+        result = classify_api_error(e, provider="custom", model="some-model")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+
+    def test_expected_string_got_list(self):
+        e = MockAPIError(
+            "Schema validation failed: expected string, got list",
+            status_code=400,
+        )
+        result = classify_api_error(e, provider="custom", model="some-model")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+
+    def test_multimodal_tool_content_takes_priority_over_context_overflow(self):
+        """Some providers return a 400 whose message contains BOTH
+        'text is not set' and a length-shaped phrase; the tool-content
+        recovery is cheaper than compression so it must win the priority.
+        """
+        e = MockAPIError(
+            "text is not set; context length exceeded",
+            status_code=400,
+        )
+        result = classify_api_error(e, provider="xiaomi", model="mimo-v2.5")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+
+    def test_no_status_code_path_also_classifies(self):
+        """When the error reaches us without a status code (transport
+        layer ate it) the message-only classifier branch must also
+        recognise the pattern.
+        """
+        e = MockTransportError("tool_call.content must be string")
+        result = classify_api_error(e, provider="alibaba", model="qwen3.5-plus")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+
+    def test_unrelated_400_is_not_misclassified(self):
+        """Make sure the patterns don't false-positive on normal 400s."""
+        e = MockAPIError("bad request: missing field 'model'", status_code=400)
+        result = classify_api_error(e, provider="openrouter", model="anthropic/claude-sonnet-4")
+        assert result.reason != FailoverReason.multimodal_tool_content_unsupported
diff --git a/tests/run_agent/test_multimodal_tool_content_recovery.py b/tests/run_agent/test_multimodal_tool_content_recovery.py
new file mode 100644
index 00000000000..63ee49f97c0
--- /dev/null
+++ b/tests/run_agent/test_multimodal_tool_content_recovery.py
@@ -0,0 +1,260 @@
+"""Tests for reactive multimodal-tool-content recovery.
+
+Covers the full chain for providers that reject list-type content in
+``role: "tool"`` messages (Xiaomi MiMo's 400 "text is not set", etc.):
+
+  1. agent/error_classifier.py: 400 with the right wording classifies as
+     ``FailoverReason.multimodal_tool_content_unsupported``.
+  2. run_agent._try_strip_image_parts_from_tool_messages downgrades tool
+     messages whose ``content`` is a list-with-image to a string text
+     summary, in-place, and records the active (provider, model) in
+     ``self._no_list_tool_content_models`` so future tool results in this
+     session preemptively downgrade.
+  3. run_agent._tool_result_content_for_active_model short-circuits to a
+     text summary when the (provider, model) is in the cache, even though
+     ``_model_supports_vision`` returns True — avoiding a wasted round
+     trip on every subsequent screenshot in the session.
+
+The end-to-end retry loop wiring (`conversation_loop.py`) is exercised by
+the classifier signal + helper-mutation tests; the integration only adds
+a trivial flag-and-continue around the existing pattern used for
+``image_too_large`` recovery.
+
+See: https://github.com/NousResearch/hermes-agent/issues/27344
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from agent.error_classifier import FailoverReason, classify_api_error
+
+
+class _FakeApiError(Exception):
+    """Stand-in for an openai.BadRequestError with status_code + body."""
+
+    def __init__(self, status_code: int, message: str, body: dict | None = None):
+        super().__init__(message)
+        self.status_code = status_code
+        self.body = body or {"error": {"message": message}}
+        self.response = None
+
+
+def _make_agent(provider: str = "xiaomi", model: str = "mimo-v2.5"):
+    """Build a bare AIAgent for method-level testing, no provider setup."""
+    from run_agent import AIAgent
+    agent = object.__new__(AIAgent)
+    agent.provider = provider
+    agent.model = model
+    return agent
+
+
+# ─── Strip helper ────────────────────────────────────────────────────────────
+
+
+class TestStripImagePartsHelper:
+    def test_no_messages_returns_false(self):
+        agent = _make_agent()
+        assert agent._try_strip_image_parts_from_tool_messages([]) is False
+        assert agent._try_strip_image_parts_from_tool_messages(None) is False
+
+    def test_no_tool_messages_returns_false(self):
+        agent = _make_agent()
+        msgs = [
+            {"role": "user", "content": "plain text"},
+            {"role": "assistant", "content": "ack"},
+        ]
+        assert agent._try_strip_image_parts_from_tool_messages(msgs) is False
+
+    def test_tool_message_with_string_content_unchanged(self):
+        agent = _make_agent()
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": "plain string result"},
+        ]
+        assert agent._try_strip_image_parts_from_tool_messages(msgs) is False
+        assert msgs[0]["content"] == "plain string result"
+
+    def test_tool_message_list_without_image_unchanged(self):
+        """List content with only text parts is left alone — caller surfaces
+        the original error if this turns out to also be rejected."""
+        agent = _make_agent()
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "text", "text": "hello"},
+            ]},
+        ]
+        assert agent._try_strip_image_parts_from_tool_messages(msgs) is False
+
+    def test_tool_message_list_with_image_downgrades(self):
+        agent = _make_agent()
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "text", "text": "AX summary: 5 buttons visible"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,iVBOR..."}},
+            ]},
+        ]
+        assert agent._try_strip_image_parts_from_tool_messages(msgs) is True
+        # Image stripped; text preserved as a string.
+        assert isinstance(msgs[0]["content"], str)
+        assert "AX summary" in msgs[0]["content"]
+        assert "image_url" not in msgs[0]["content"]
+        assert "iVBOR" not in msgs[0]["content"]
+
+    def test_tool_message_image_only_gets_placeholder(self):
+        """If the list had nothing but image parts, leave a placeholder so
+        the assistant message has something to reference."""
+        agent = _make_agent()
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,iVBOR..."}},
+            ]},
+        ]
+        assert agent._try_strip_image_parts_from_tool_messages(msgs) is True
+        assert isinstance(msgs[0]["content"], str)
+        assert "image content removed" in msgs[0]["content"]
+
+    def test_records_provider_model_in_session_cache(self):
+        agent = _make_agent(provider="xiaomi", model="mimo-v2.5")
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "text", "text": "summary"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,X"}},
+            ]},
+        ]
+        agent._try_strip_image_parts_from_tool_messages(msgs)
+        assert ("xiaomi", "mimo-v2.5") in agent._no_list_tool_content_models
+
+    def test_only_tool_messages_get_downgraded(self):
+        """User / assistant messages with list-type content are out of
+        scope — they're handled by the existing image-routing path."""
+        agent = _make_agent()
+        msgs = [
+            {"role": "user", "content": [
+                {"type": "text", "text": "describe"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,X"}},
+            ]},
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "text", "text": "summary"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,Y"}},
+            ]},
+        ]
+        agent._try_strip_image_parts_from_tool_messages(msgs)
+        # User message untouched.
+        assert isinstance(msgs[0]["content"], list)
+        assert any(p.get("type") == "image_url" for p in msgs[0]["content"])
+        # Tool message downgraded.
+        assert isinstance(msgs[1]["content"], str)
+        assert "summary" in msgs[1]["content"]
+
+    def test_skips_recording_when_no_model_id(self):
+        """Don't poison the cache with empty keys when provider/model is
+        unset (e.g. lazy-initialised mid-handshake)."""
+        agent = _make_agent(provider="", model="")
+        msgs = [
+            {"role": "tool", "tool_call_id": "x", "content": [
+                {"type": "text", "text": "summary"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,X"}},
+            ]},
+        ]
+        agent._try_strip_image_parts_from_tool_messages(msgs)
+        assert agent._no_list_tool_content_models == set()
+
+
+# ─── Short-circuit on cached models ──────────────────────────────────────────
+
+
+class TestToolResultContentShortCircuit:
+    """Once the session has learned that (provider, model) rejects list
+    content, ``_tool_result_content_for_active_model`` returns a text
+    summary even though ``_model_supports_vision`` reports True.
+    """
+
+    def _multimodal_result(self, png_b64: str = "iVBORw0KGgoAAAA"):
+        return {
+            "_multimodal": True,
+            "content": [
+                {"type": "text", "text": "capture mode=som 800x600 app=Safari"},
+                {"type": "image_url",
+                 "image_url": {"url": f"data:image/png;base64,{png_b64}"}},
+            ],
+            "text_summary": "capture mode=som 800x600 app=Safari",
+            "meta": {"mode": "som", "width": 800, "height": 600, "elements": 5,
+                     "png_bytes": 1024},
+        }
+
+    def test_returns_list_when_cache_empty_and_vision_supported(self, monkeypatch):
+        agent = _make_agent(provider="xiaomi", model="mimo-v2.5")
+        agent._no_list_tool_content_models = set()  # explicit empty
+        monkeypatch.setattr(agent, "_model_supports_vision", lambda: True)
+        out = agent._tool_result_content_for_active_model(
+            "computer_use", self._multimodal_result()
+        )
+        # Native multimodal path: returns the content parts list.
+        assert isinstance(out, list)
+        assert any(p.get("type") == "image_url" for p in out)
+
+    def test_returns_text_summary_when_model_in_cache(self, monkeypatch):
+        agent = _make_agent(provider="xiaomi", model="mimo-v2.5")
+        agent._no_list_tool_content_models = {("xiaomi", "mimo-v2.5")}
+        monkeypatch.setattr(agent, "_model_supports_vision", lambda: True)
+        out = agent._tool_result_content_for_active_model(
+            "computer_use", self._multimodal_result()
+        )
+        # Short-circuit: a plain string summary, no image_url present.
+        assert isinstance(out, str)
+        assert "data:image" not in out
+        assert "image_url" not in out
+
+    def test_cache_miss_on_different_model(self, monkeypatch):
+        """Cache is per (provider, model). A cached entry for mimo-v2.5
+        must NOT affect a session running on a different model.
+        """
+        agent = _make_agent(provider="xiaomi", model="mimo-v2.5-pro")
+        agent._no_list_tool_content_models = {("xiaomi", "mimo-v2.5")}
+        monkeypatch.setattr(agent, "_model_supports_vision", lambda: True)
+        out = agent._tool_result_content_for_active_model(
+            "computer_use", self._multimodal_result()
+        )
+        assert isinstance(out, list)
+
+    def test_missing_cache_attribute_falls_through(self, monkeypatch):
+        """Tests that build agents via ``object.__new__`` without calling
+        ``__init__`` must not crash — the cache attribute may be absent.
+        """
+        agent = _make_agent()
+        # Deliberately do not assign _no_list_tool_content_models.
+        monkeypatch.setattr(agent, "_model_supports_vision", lambda: True)
+        out = agent._tool_result_content_for_active_model(
+            "computer_use", self._multimodal_result()
+        )
+        assert isinstance(out, list)
+
+
+# ─── Classifier ──────────────────────────────────────────────────────────────
+
+
+class TestRecoveryEndToEndClassification:
+    """Lock in that the patterns used by the recovery path classify to
+    the right ``FailoverReason``. (The recovery hook in
+    ``agent.conversation_loop`` consumes this reason directly.)
+    """
+
+    def test_xiaomi_mimo_classifies(self):
+        err = _FakeApiError(
+            status_code=400,
+            message=(
+                "Error code: 400 - {'error': {'code': '400', 'message': "
+                "'Param Incorrect', 'param': 'text is not set', 'type': ''}}"
+            ),
+        )
+        result = classify_api_error(err, provider="xiaomi", model="mimo-v2.5")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported
+        assert result.retryable is True
+
+    def test_alibaba_variant_classifies(self):
+        err = _FakeApiError(
+            status_code=400,
+            message="tool_call.content must be string",
+        )
+        result = classify_api_error(err, provider="alibaba", model="qwen3.5-plus")
+        assert result.reason == FailoverReason.multimodal_tool_content_unsupported