diff --git a/routstr/upstream/messages_dispatch.py b/routstr/upstream/messages_dispatch.py
index 1951768d..6de5e57f 100644
--- a/routstr/upstream/messages_dispatch.py
+++ b/routstr/upstream/messages_dispatch.py
@@ -84,6 +84,34 @@ ALLOWED_MESSAGES_REQUEST_FIELDS: frozenset[str] = frozenset(
 )
 
 
+def prune_blank_system_blocks(body: dict) -> None:
+    """Drop whitespace-only ``system`` text.
+
+    Anthropic accepts a blank system prompt; OpenAI-compatible upstreams
+    reject it with ``text content blocks must contain non-whitespace text``.
+    """
+    system = body.get("system")
+    if isinstance(system, str):
+        if not system.strip():
+            body.pop("system", None)
+        return
+    if not isinstance(system, list):
+        return
+    kept = [
+        block
+        for block in system
+        if not (
+            isinstance(block, dict)
+            and block.get("type") == "text"
+            and not str(block.get("text") or "").strip()
+        )
+    ]
+    if kept:
+        body["system"] = kept
+    else:
+        body.pop("system", None)
+
+
 def coerce_litellm_payload(payload: object) -> dict:
     """Convert a litellm event into a plain dict.
 
@@ -511,6 +539,8 @@ async def dispatch_anthropic_messages(
         )
     body = {k: v for k, v in body.items() if k in ALLOWED_MESSAGES_REQUEST_FIELDS}
 
+    prune_blank_system_blocks(body)
+
     model_suffix = adapt_request(body) if adapt_request else ""
 
     # LiteLLM turns Anthropic's server-side web_search tool into the OpenAI
diff --git a/tests/unit/test_messages_litellm_dispatch.py b/tests/unit/test_messages_litellm_dispatch.py
index 7e2da476..47b3ccd9 100644
--- a/tests/unit/test_messages_litellm_dispatch.py
+++ b/tests/unit/test_messages_litellm_dispatch.py
@@ -23,6 +23,9 @@ from routstr.core.db import ApiKey  # noqa: E402
 from routstr.payment.cost_calculation import CostData  # noqa: E402
 from routstr.payment.models import Architecture, Model, Pricing  # noqa: E402
 from routstr.upstream.base import BaseUpstreamProvider  # noqa: E402
+from routstr.upstream.messages_dispatch import (  # noqa: E402
+    prune_blank_system_blocks,
+)
 from routstr.wallet import MintConnectionError, TokenConsumedError  # noqa: E402
 
 # ---------------------------------------------------------------------------
@@ -113,6 +116,39 @@ def _make_request(request_id: str | None = "req-test") -> Any:
 # ---------------------------------------------------------------------------
 
 
+def test_prune_blank_system_blocks_drops_blank_blocks() -> None:
+    body = {
+        "system": [
+            {"type": "text", "text": "  \n"},
+            {"type": "text", "text": "real prompt"},
+        ]
+    }
+    prune_blank_system_blocks(body)
+    assert body["system"] == [{"type": "text", "text": "real prompt"}]
+
+
+def test_prune_blank_system_blocks_drops_key_when_all_blank() -> None:
+    body = {"system": [{"type": "text", "text": "\n"}], "max_tokens": 8}
+    prune_blank_system_blocks(body)
+    assert body == {"max_tokens": 8}
+
+
+def test_prune_blank_system_blocks_handles_string_system() -> None:
+    blank = {"system": "   "}
+    prune_blank_system_blocks(blank)
+    assert blank == {}
+
+    kept = {"system": "be brief"}
+    prune_blank_system_blocks(kept)
+    assert kept == {"system": "be brief"}
+
+
+def test_prune_blank_system_blocks_keeps_non_text_blocks() -> None:
+    body = {"system": [{"type": "image", "source": {}}]}
+    prune_blank_system_blocks(body)
+    assert body["system"] == [{"type": "image", "source": {}}]
+
+
 def test_coerce_litellm_payload_handles_dict() -> None:
     out = BaseUpstreamProvider._coerce_litellm_payload({"a": 1})
     assert out == {"a": 1}