diff --git a/run_agent.py b/run_agent.py
index f2df8d85fb..f995a2886d 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -1406,6 +1406,12 @@ class AIAgent:
             else:
                 print(f"📊 Context limit: {self.context_compressor.context_length:,} tokens (auto-compression disabled)")
 
+        # Check immediately so CLI users see the warning at startup.
+        # Gateway status_callback is not yet wired, so any warning is stored
+        # in _compression_warning and replayed in the first run_conversation().
+        self._compression_warning = None
+        self._check_compression_model_feasibility()
+
         # Snapshot primary runtime for per-turn restoration.  When fallback
         # activates during a turn, the next turn restores these values so the
         # preferred model gets a fresh attempt each time.  Uses a single dict
@@ -1697,6 +1703,104 @@ class AIAgent:
             except Exception:
                 logger.debug("status_callback error in _emit_status", exc_info=True)
 
+    def _check_compression_model_feasibility(self) -> None:
+        """Warn at session start if the auxiliary compression model's context
+        window is smaller than the main model's compression threshold.
+
+        When the auxiliary model cannot fit the content that needs summarising,
+        compression will either fail outright (the LLM call errors) or produce
+        a severely truncated summary.
+
+        Called during ``__init__`` so CLI users see the warning immediately
+        (via ``_vprint``).  The gateway sets ``status_callback`` *after*
+        construction, so ``_replay_compression_warning()`` re-sends the
+        stored warning through the callback on the first
+        ``run_conversation()`` call.
+        """
+        if not self.compression_enabled:
+            return
+        try:
+            from agent.auxiliary_client import get_text_auxiliary_client
+            from agent.model_metadata import get_model_context_length
+
+            client, aux_model = get_text_auxiliary_client("compression")
+            if client is None or not aux_model:
+                msg = (
+                    "⚠ No auxiliary LLM provider configured — context "
+                    "compression will drop middle turns without a summary. "
+                    "Run `hermes setup` or set OPENROUTER_API_KEY."
+                )
+                self._compression_warning = msg
+                self._emit_status(msg)
+                logger.warning(
+                    "No auxiliary LLM provider for compression — "
+                    "summaries will be unavailable."
+                )
+                return
+
+            aux_base_url = str(getattr(client, "base_url", ""))
+            aux_api_key = str(getattr(client, "api_key", ""))
+            aux_context = get_model_context_length(
+                aux_model,
+                base_url=aux_base_url,
+                api_key=aux_api_key,
+            )
+
+            threshold = self.context_compressor.threshold_tokens
+            if aux_context < threshold:
+                # Suggest a threshold that would fit the aux model,
+                # rounded down to a clean percentage.
+                safe_pct = int((aux_context / self.context_compressor.context_length) * 100)
+                msg = (
+                    f"⚠ Compression model ({aux_model}) context "
+                    f"is {aux_context:,} tokens, but the main model's "
+                    f"compression threshold is {threshold:,} tokens. "
+                    f"Context compression will not be possible — the "
+                    f"content to summarise will exceed the auxiliary "
+                    f"model's context window.\n"
+                    f"  Fix options (config.yaml):\n"
+                    f"  1. Use a larger compression model:\n"
+                    f"       auxiliary:\n"
+                    f"         compression:\n"
+                    f"           model: <model-with-{threshold:,}+-context>\n"
+                    f"  2. Lower the compression threshold to fit "
+                    f"the current model:\n"
+                    f"       compression:\n"
+                    f"         threshold: 0.{safe_pct:02d}"
+                )
+                self._compression_warning = msg
+                self._emit_status(msg)
+                logger.warning(
+                    "Auxiliary compression model %s has %d token context, "
+                    "below the main model's compression threshold of %d "
+                    "tokens — compression summaries will fail or be "
+                    "severely truncated.",
+                    aux_model,
+                    aux_context,
+                    threshold,
+                )
+        except Exception as exc:
+            logger.debug(
+                "Compression feasibility check failed (non-fatal): %s", exc
+            )
+
+    def _replay_compression_warning(self) -> None:
+        """Re-send the compression warning through ``status_callback``.
+
+        During ``__init__`` the gateway's ``status_callback`` is not yet
+        wired, so ``_emit_status`` only reaches ``_vprint`` (CLI).  This
+        method is called once at the start of the first
+        ``run_conversation()`` — by then the gateway has set the callback,
+        so every platform (Telegram, Discord, Slack, etc.) receives the
+        warning.
+        """
+        msg = getattr(self, "_compression_warning", None)
+        if msg and self.status_callback:
+            try:
+                self.status_callback("lifecycle", msg)
+            except Exception:
+                pass
+
     def _is_direct_openai_url(self, base_url: str = None) -> bool:
         """Return True when a base URL targets OpenAI's native API."""
         url = (base_url or self._base_url_lower).lower()
@@ -7468,6 +7572,12 @@ class AIAgent:
                     )
             except Exception:
                 pass
+        # Replay compression warning through status_callback for gateway
+        # platforms (the callback was not wired during __init__).
+        if self._compression_warning:
+            self._replay_compression_warning()
+            self._compression_warning = None  # send once
+
         # NOTE: _turns_since_memory and _iters_since_skill are NOT reset here.
         # They are initialized in __init__ and must persist across run_conversation
         # calls so that nudge logic accumulates correctly in CLI mode.
diff --git a/tests/run_agent/test_compression_feasibility.py b/tests/run_agent/test_compression_feasibility.py
new file mode 100644
index 0000000000..1b4423414e
--- /dev/null
+++ b/tests/run_agent/test_compression_feasibility.py
@@ -0,0 +1,279 @@
+"""Tests for _check_compression_model_feasibility() — warns when the
+auxiliary compression model's context is smaller than the main model's
+compression threshold.
+
+Two-phase design:
+  1. __init__  → runs the check, prints via _vprint (CLI), stores warning
+  2. run_conversation (first call) → replays stored warning through
+     status_callback (gateway platforms)
+"""
+
+from unittest.mock import MagicMock, patch
+
+from run_agent import AIAgent
+from agent.context_compressor import ContextCompressor
+
+
+def _make_agent(
+    *,
+    compression_enabled: bool = True,
+    threshold_percent: float = 0.50,
+    main_context: int = 200_000,
+) -> AIAgent:
+    """Build a minimal AIAgent with a compressor, skipping __init__."""
+    agent = AIAgent.__new__(AIAgent)
+    agent.model = "test-main-model"
+    agent.provider = "openrouter"
+    agent.base_url = "https://openrouter.ai/api/v1"
+    agent.api_key = "sk-test"
+    agent.quiet_mode = True
+    agent.log_prefix = ""
+    agent.compression_enabled = compression_enabled
+    agent._print_fn = None
+    agent.suppress_status_output = False
+    agent._stream_consumers = []
+    agent._executing_tools = False
+    agent._mute_post_response = False
+    agent.status_callback = None
+    agent.tool_progress_callback = None
+    agent._compression_warning = None
+
+    compressor = MagicMock(spec=ContextCompressor)
+    compressor.context_length = main_context
+    compressor.threshold_tokens = int(main_context * threshold_percent)
+    agent.context_compressor = compressor
+
+    return agent
+
+
+# ── Core warning logic ──────────────────────────────────────────────
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=32_768)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_warns_when_aux_context_below_threshold(mock_get_client, mock_ctx_len):
+    """Warning emitted when aux model context < main model threshold."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    # threshold = 100,000 — aux has only 32,768
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "google/gemini-3-flash-preview")
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 1
+    assert "Compression model" in messages[0]
+    assert "32,768" in messages[0]
+    assert "100,000" in messages[0]
+    assert "will not be possible" in messages[0]
+    # Actionable fix guidance included
+    assert "Fix options" in messages[0]
+    assert "auxiliary:" in messages[0]
+    assert "compression:" in messages[0]
+    assert "threshold:" in messages[0]
+    # Warning stored for gateway replay
+    assert agent._compression_warning is not None
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=200_000)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_no_warning_when_aux_context_sufficient(mock_get_client, mock_ctx_len):
+    """No warning when aux model context >= main model threshold."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    # threshold = 100,000 — aux has 200,000 (sufficient)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "google/gemini-2.5-flash")
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 0
+    assert agent._compression_warning is None
+
+
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_warns_when_no_auxiliary_provider(mock_get_client):
+    """Warning emitted when no auxiliary provider is configured."""
+    agent = _make_agent()
+    mock_get_client.return_value = (None, None)
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 1
+    assert "No auxiliary LLM provider" in messages[0]
+    assert agent._compression_warning is not None
+
+
+def test_skips_check_when_compression_disabled():
+    """No check performed when compression is disabled."""
+    agent = _make_agent(compression_enabled=False)
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 0
+    assert agent._compression_warning is None
+
+
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_exception_does_not_crash(mock_get_client):
+    """Exceptions in the check are caught — never blocks startup."""
+    agent = _make_agent()
+    mock_get_client.side_effect = RuntimeError("boom")
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    # Should not raise
+    agent._check_compression_model_feasibility()
+
+    # No user-facing message (error is debug-logged)
+    assert len(messages) == 0
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=100_000)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_exact_threshold_boundary_no_warning(mock_get_client, mock_ctx_len):
+    """No warning when aux context exactly equals the threshold."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "test-model")
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 0
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=99_999)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_just_below_threshold_warns(mock_get_client, mock_ctx_len):
+    """Warning fires when aux context is one token below the threshold."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "small-model")
+
+    messages = []
+    agent._emit_status = lambda msg: messages.append(msg)
+
+    agent._check_compression_model_feasibility()
+
+    assert len(messages) == 1
+    assert "small-model" in messages[0]
+
+
+# ── Two-phase: __init__ + run_conversation replay ───────────────────
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=32_768)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_warning_stored_for_gateway_replay(mock_get_client, mock_ctx_len):
+    """__init__ stores the warning; _replay sends it through status_callback."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "google/gemini-3-flash-preview")
+
+    # Phase 1: __init__ — _emit_status prints (CLI) but callback is None
+    vprint_messages = []
+    agent._emit_status = lambda msg: vprint_messages.append(msg)
+    agent._check_compression_model_feasibility()
+
+    assert len(vprint_messages) == 1  # CLI got it
+    assert agent._compression_warning is not None  # stored for replay
+
+    # Phase 2: gateway wires callback post-init, then run_conversation replays
+    callback_events = []
+    agent.status_callback = lambda ev, msg: callback_events.append((ev, msg))
+    agent._replay_compression_warning()
+
+    assert any(
+        ev == "lifecycle" and "will not be possible" in msg
+        for ev, msg in callback_events
+    )
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=200_000)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_no_replay_when_no_warning(mock_get_client, mock_ctx_len):
+    """_replay_compression_warning is a no-op when there's no stored warning."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "big-model")
+
+    agent._emit_status = lambda msg: None
+    agent._check_compression_model_feasibility()
+
+    assert agent._compression_warning is None
+
+    callback_events = []
+    agent.status_callback = lambda ev, msg: callback_events.append((ev, msg))
+    agent._replay_compression_warning()
+
+    assert len(callback_events) == 0
+
+
+def test_replay_without_callback_is_noop():
+    """_replay_compression_warning doesn't crash when status_callback is None."""
+    agent = _make_agent()
+    agent._compression_warning = "some warning"
+    agent.status_callback = None
+
+    # Should not raise
+    agent._replay_compression_warning()
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=32_768)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_run_conversation_clears_warning_after_replay(mock_get_client, mock_ctx_len):
+    """After replay in run_conversation, _compression_warning is cleared
+    so the warning is not sent again on subsequent turns."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    mock_client = MagicMock()
+    mock_client.base_url = "https://openrouter.ai/api/v1"
+    mock_client.api_key = "sk-aux"
+    mock_get_client.return_value = (mock_client, "small-model")
+
+    agent._emit_status = lambda msg: None
+    agent._check_compression_model_feasibility()
+
+    assert agent._compression_warning is not None
+
+    # Simulate what run_conversation does
+    callback_events = []
+    agent.status_callback = lambda ev, msg: callback_events.append((ev, msg))
+    if agent._compression_warning:
+        agent._replay_compression_warning()
+        agent._compression_warning = None  # as in run_conversation
+
+    assert len(callback_events) == 1
+
+    # Second turn — nothing replayed
+    callback_events.clear()
+    if agent._compression_warning:
+        agent._replay_compression_warning()
+        agent._compression_warning = None
+
+    assert len(callback_events) == 0