diff --git a/scripts/test_worker_tool_arg_validation.py b/scripts/test_worker_tool_arg_validation.py
new file mode 100644
index 0000000..abae25a
--- /dev/null
+++ b/scripts/test_worker_tool_arg_validation.py
@@ -0,0 +1,230 @@
+#!/usr/bin/env python3
+"""Tests for gateway-level input-schema validation of worker inner-tool
+calls.
+
+Today the gateway only checks the allowlist; bad args fall through to
+the inner tool which returns its own ad-hoc error envelope. The fix:
+validate args against the inner tool's `inputSchema` at the gateway
+BEFORE dispatch, and surface a structured refusal (`schema_error`
+field on the refusal payload) so the worker can self-correct.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import sys
+import tempfile
+from pathlib import Path
+
+
+def main() -> int:
+    here = Path(__file__).resolve().parents[1]
+    sys.path.insert(0, str(here / "servers" / "python"))
+
+    tmp = Path(tempfile.mkdtemp())
+    pricing = tmp / "pricing.json"
+    pricing.write_text(json.dumps({
+        "openai":    {"gpt-test":    {"prompt_per_1k": 0.0001, "completion_per_1k": 0.0003, "cached_per_1k": 0.00005}},
+        "anthropic": {"claude-test": {"prompt_per_1k": 0.003,  "completion_per_1k": 0.015,  "cached_per_1k": 0.0003}},
+    }))
+    os.environ["CROSSCHECK_PRICING_PATH"] = str(pricing)
+    os.environ.pop("CROSSCHECK_REJECT_CONFIG_DRIFT", None)
+
+    import crosscheck_server as srv
+
+    srv.CFG = dict(srv.CFG)
+    srv.CFG["session_db"]     = str(tmp / "sessions.db")
+    srv.CFG["transcript_dir"] = str(tmp / "transcripts")
+    srv.CFG["cache"]          = {"enabled": False}
+    srv.CFG["node_cache"]     = {"enabled": False}
+    srv.CFG["prompt_adapters"] = {"enabled": False}
+    srv.CFG["fetch"] = {"url_allowlist": ["https://example.com/"]}
+    srv.CFG["config_pinning"] = {"reject_drift": False}
+    srv.TRANSCRIPT_DIR        = Path(srv.CFG["transcript_dir"])
+    srv._DB_INIT_DONE         = False
+    srv._FTS5_AVAILABLE       = None
+    srv._PRICING_CACHE        = None
+    srv.PRICING_PATH          = pricing
+    srv._CONFIG_PIN_STARTUP_DONE = True
+
+    srv.ENV = dict(srv.ENV)
+    srv.ENV["OPENAI_API_KEY"]    = "stub"; srv.ENV["OPENAI_MODEL"]    = "gpt-test"
+    srv.ENV["ANTHROPIC_API_KEY"] = "stub"; srv.ENV["ANTHROPIC_MODEL"] = "claude-test"
+    srv.ALL_PROVIDERS = srv.build_providers()
+    srv.CFG["providers"] = ["openai", "anthropic"]
+
+    # Track whether the inner tools were actually called: we should NEVER
+    # reach them with invalid args, because the gateway short-circuits.
+    fetch_calls: list[dict] = []
+    verify_calls: list[dict] = []
+    real_fetch  = srv.tool_fetch
+    real_verify = srv.tool_verify
+
+    def fake_fetch(args):
+        fetch_calls.append(dict(args))
+        return {"tool": "fetch", "url": args.get("url"),
+                "status": "ok", "content_excerpt": "ok"}
+
+    def fake_verify(args):
+        verify_calls.append(dict(args))
+        return {"tool": "verify", "all_passed": True,
+                "checks_run": len(args.get("checks") or []),
+                "results": [], "summary": "ok"}
+
+    srv.tool_fetch  = fake_fetch
+    srv.tool_verify = fake_verify
+
+    # ------------------------------------------------------------------
+    # 1) Missing required field on `fetch` (no `url`) -> refusal
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch({"name": "fetch", "args": {}},
+                                      session_id="sess-1")
+    assert '"refused": true' in res, res
+    payload = json.loads(res.split("<untrusted_input>")[1].split("</untrusted_input>")[0].strip())
+    assert payload["tool"] == "fetch"
+    assert payload.get("schema_error"), payload
+    assert "url" in payload["schema_error"].lower(), payload["schema_error"]
+    assert fetch_calls == [], "inner fetch must NOT be called when validation fails"
+
+    # ------------------------------------------------------------------
+    # 2) Wrong type on `fetch.url` (integer not string) -> refusal
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch({"name": "fetch", "args": {"url": 12345}},
+                                      session_id="sess-2")
+    assert '"refused": true' in res
+    payload = json.loads(res.split("<untrusted_input>")[1].split("</untrusted_input>")[0].strip())
+    assert payload.get("schema_error"), payload
+    assert "url" in payload["schema_error"].lower() and "string" in payload["schema_error"].lower(), payload["schema_error"]
+    assert fetch_calls == []
+
+    # ------------------------------------------------------------------
+    # 3) `verify` missing required `checks` -> refusal
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch({"name": "verify", "args": {}},
+                                      session_id="sess-3")
+    assert '"refused": true' in res
+    payload = json.loads(res.split("<untrusted_input>")[1].split("</untrusted_input>")[0].strip())
+    assert payload["tool"] == "verify"
+    assert "checks" in payload.get("schema_error", "").lower(), payload
+    assert verify_calls == []
+
+    # ------------------------------------------------------------------
+    # 4) `verify.checks` not a list -> refusal
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch(
+        {"name": "verify", "args": {"checks": "not-a-list"}},
+        session_id="sess-4")
+    assert '"refused": true' in res
+    payload = json.loads(res.split("<untrusted_input>")[1].split("</untrusted_input>")[0].strip())
+    assert "checks" in payload.get("schema_error", "").lower(), payload
+    assert "array" in payload["schema_error"].lower(), payload["schema_error"]
+    assert verify_calls == []
+
+    # ------------------------------------------------------------------
+    # 5) Unknown argument on `fetch` -> refusal (additionalProperties=false)
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch(
+        {"name": "fetch", "args": {"url": "https://example.com/", "evil_arg": 1}},
+        session_id="sess-5")
+    # Refusal expected because the fetch schema sets additionalProperties=false.
+    # If the schema doesn't (some shapes don't), we relax to "either pass or refuse".
+    schema = srv._worker_tool_input_schema("fetch")
+    if schema.get("additionalProperties") is False:
+        assert '"refused": true' in res, res
+        assert fetch_calls == []  # never reached the inner tool
+    else:
+        # Schema is permissive on extras — gateway should let it through.
+        assert '"refused": true' not in res
+
+    # ------------------------------------------------------------------
+    # 6) VALID args on fetch still dispatch correctly (regression)
+    # ------------------------------------------------------------------
+    fetch_calls.clear()
+    res = srv._worker_tools_dispatch(
+        {"name": "fetch", "args": {"url": "https://example.com/spec"}},
+        session_id="sess-ok")
+    assert '"refused": true' not in res, res
+    assert len(fetch_calls) == 1
+    assert fetch_calls[0]["url"] == "https://example.com/spec"
+    # session_id was injected
+    assert fetch_calls[0].get("session_id") == "sess-ok"
+
+    # ------------------------------------------------------------------
+    # 7) VALID args on verify still dispatch (regression)
+    # ------------------------------------------------------------------
+    verify_calls.clear()
+    res = srv._worker_tools_dispatch(
+        {"name": "verify",
+         "args": {"checks": [{"kind": "contains",
+                                "id": "x",
+                                "target_text": "hi",
+                                "value": "hi"}]}},
+        session_id="sess-ok-2")
+    assert '"refused": true' not in res, res
+    assert len(verify_calls) == 1
+
+    # ------------------------------------------------------------------
+    # 8) Allowlist denial still works (regression — checked BEFORE schema)
+    # ------------------------------------------------------------------
+    res = srv._worker_tools_dispatch(
+        {"name": "coordinate", "args": {"topic": "evil"}},
+        session_id="sess-deny")
+    payload = json.loads(res.split("<untrusted_input>")[1].split("</untrusted_input>")[0].strip())
+    assert payload["refused"] is True
+    # No schema_error on an allowlist refusal — the gateway never even
+    # got to the schema check.
+    assert "schema_error" not in payload, payload
+
+    # ------------------------------------------------------------------
+    # 9) End-to-end: worker emits invalid call, gets refusal with
+    #    schema_error, recovers on next hop
+    # ------------------------------------------------------------------
+    openai_responses: list[str] = []
+    bodies: list[dict] = []
+    def fake_post(url, h, b, **kw):
+        bodies.append({"url": url, "body": b})
+        if "openai.com" in url:
+            text = openai_responses.pop(0) if openai_responses else "DONE."
+            return ({"choices": [{"message": {"content": text}}],
+                     "usage": {"prompt_tokens": 30, "completion_tokens": 10}}, 1)
+        return ({}, 1)
+    srv._http_post_resilient = fake_post
+
+    fetch_calls.clear()
+    openai_responses[:] = [
+        # First emission: invalid (missing url)
+        '<tool_call>{"name":"fetch","args":{}}</tool_call>',
+        # Second emission: valid
+        '<tool_call>{"name":"fetch","args":{"url":"https://example.com/x"}}</tool_call>',
+        "Recovered with the data.",
+    ]
+    ans = srv._ask_one_with_tools(
+        srv.ALL_PROVIDERS["openai"],
+        [{"role": "user", "content": "Try again."}],
+        deadline=__import__("time").monotonic() + 30,
+        max_tokens=2048, purpose="worker",
+        worker_tools=["fetch"], session_id="recover",
+    )
+    statuses = [c["status"] for c in ans["inner_tool_calls"]]
+    # Both calls counted as "refused" / "ok" but only one real fetch fired.
+    assert statuses == ["refused", "ok"], statuses
+    assert len(fetch_calls) == 1, fetch_calls
+    assert "Recovered" in ans["response"]
+    # Verify the second-turn re-prompt embedded the schema_error so the
+    # worker had something concrete to fix.
+    user_msgs = []
+    for b in bodies:
+        user_msgs.extend(m.get("content", "") for m in b["body"]["messages"]
+                          if m["role"] == "user")
+    refusal_msgs = [m for m in user_msgs if "schema_error" in m]
+    assert refusal_msgs, "expected the schema_error refusal to be re-prompted"
+
+    srv.tool_fetch  = real_fetch
+    srv.tool_verify = real_verify
+    print("OK: test_worker_tool_arg_validation")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/servers/python/crosscheck_server.py b/servers/python/crosscheck_server.py
index b7ed356..9cec871 100755
--- a/servers/python/crosscheck_server.py
+++ b/servers/python/crosscheck_server.py
@@ -2818,19 +2818,40 @@ def _wrap_tool_result(name: str, content: str) -> str:
     )
 
 
-def _worker_tools_refusal(name: str, reason: str, hint: str = "") -> str:
+def _worker_tools_refusal(name: str, reason: str, hint: str = "",
+                          *, schema_error: str | None = None) -> str:
     """Structured refusal payload — re-prompted to the worker as a tool_result
-    so it can incorporate the failure into its next emission."""
-    payload = {"refused": True, "tool": name, "reason": reason}
+    so it can incorporate the failure into its next emission. When the
+    refusal stems from gateway-level schema validation, `schema_error`
+    carries the underlying validator message so the worker can correct
+    the args on the next hop."""
+    payload: dict = {"refused": True, "tool": name, "reason": reason}
     if hint:
         payload["operator_hint"] = hint
+    if schema_error:
+        payload["schema_error"] = schema_error
     return _wrap_tool_result(name or "<unknown>", json.dumps(payload))
 
 
+def _worker_tool_input_schema(name: str) -> dict:
+    """Look up the JSON-schema for the named inner tool's input. Returns an
+    empty dict (no validation) when TOOLS hasn't been loaded yet — defensive,
+    `TOOLS` is initialised at module-load before any tool call can happen."""
+    try:
+        return (TOOLS.get(name) or {}).get("inputSchema") or {}
+    except NameError:
+        return {}
+
+
 def _worker_tools_dispatch(call: dict, *, session_id: str | None) -> str:
     """Execute one inner tool call. Returns the wrapped <tool_result> string
     ready for re-prompt. Refusals are also wrapped so the worker sees a
-    coherent shape regardless of outcome."""
+    coherent shape regardless of outcome.
+
+    Validation order: allowlist check, then input-schema check, then
+    session_id injection, then inner-tool dispatch. The schema check at
+    the gateway gives the worker a clean structured refusal instead of
+    whatever ad-hoc error envelope the inner tool happens to produce."""
     name = str(call.get("name", "")).strip()
     args = call.get("args") if isinstance(call.get("args"), dict) else {}
     if name not in _WORKER_TOOL_ALLOWLIST:
@@ -2839,6 +2860,26 @@ def _worker_tools_dispatch(call: dict, *, session_id: str | None) -> str:
             hint=f"Allowed inner tools: {sorted(_WORKER_TOOL_ALLOWLIST)}",
         )
 
+    # Pre-validate args against the inner tool's input schema BEFORE we
+    # touch the tool itself. session_id is injected later (after this
+    # check) and is always optional on the inner tools, so a missing
+    # session_id never trips the validator here.
+    schema = _worker_tool_input_schema(name)
+    if schema:
+        validate_err = _validate_input(name, schema, args)
+        if validate_err:
+            _emit_event("worker_inner_validation_fail",
+                        tool=name, session_id=session_id,
+                        args_keys=sorted(args.keys()) if isinstance(args, dict) else [],
+                        schema_error=validate_err)
+            return _worker_tools_refusal(
+                name,
+                f"input args failed schema validation for tool {name!r}",
+                hint=("Inspect the tool's inputSchema and fix the args; "
+                      "required fields, types, and enums must match."),
+                schema_error=validate_err,
+            )
+
     # Ensure inner calls roll up under the same session_id (for cost,
     # egress budget, and breakers).
     inner_args = dict(args)