diff --git a/scripts/probe_search_tool_choice.mjs b/scripts/probe_search_tool_choice.mjs
index 2e1084760..83e133551 100644
--- a/scripts/probe_search_tool_choice.mjs
+++ b/scripts/probe_search_tool_choice.mjs
@@ -8,16 +8,32 @@ const model = process.env.MODEL || 'model-f';
 const endpoint = process.env.ENDPOINT_URL || (() => { throw new Error("ENDPOINT_URL is required"); })();
 const prompts = ['Catch me up on the biggest AI developments this week. Explain why they matter and link your sources.', 'serch latest ai news pls'];
 const results = [];
+let system = 'You are Odysseus. Use web_search to find current information relevant to the user request.';
+if (process.env.CANONICAL_SYSTEM === '1') {
+  system = execFileSync((process.env.PYTHON || "python3"), ['-c', `
+import ast, sys
+from datetime import datetime, timezone
+from src.clean_agent_preview import native_input_files_clause
+tree = ast.parse(sys.stdin.read())
+fn = next(n for n in tree.body if isinstance(n, ast.AsyncFunctionDef) and n.name == 'stream_preview')
+assignment = next(n for n in fn.body if isinstance(n, ast.Assign) and any(isinstance(t, ast.Name) and t.id == 'system' for t in n.targets))
+runtime_scope_clause = 'This is a tool preview connected to the authenticated user’s real data. '
+native_workspace_enabled = False
+client_runtime_context = None
+shell_clause = 'Shell commands are disabled. '
+print(eval(compile(ast.Expression(assignment.value), '<canonical-system-expression>', 'eval')))
+`], {input:fs.readFileSync('src/clean_agent_preview.py','utf8'),encoding:'utf8'}).trim();
+}
 for (const prompt of prompts) {
   for (const choice of ['auto', 'required', {type:'function', function:{name:'web_search'}}]) {
     const started = performance.now();
     const response = await fetch(endpoint, {method:'POST', headers:{'Content-Type':'application/json'}, signal:AbortSignal.timeout(90000),
-      body:JSON.stringify({model, messages:[{role:'system',content:'You are Odysseus. Use web_search to find current information relevant to the user request.'},{role:'user',content:prompt}], tools, tool_choice:choice, temperature:0, max_tokens:256, stream:false, chat_template_kwargs:{enable_thinking:false}})});
+      body:JSON.stringify({model, messages:[{role:'system',content:system},{role:'user',content:prompt}], tools, tool_choice:choice, temperature:0, max_tokens:256, stream:false, chat_template_kwargs:{enable_thinking:false}})});
     const data = await response.json();
     const result = {prompt,choice,status:response.status,seconds:(performance.now()-started)/1000,message:data.choices?.[0]?.message,error:data.error};
     results.push(result); console.log(JSON.stringify(result));
   }
 }
 const target = `reports/search-tool-choice-probe-${Date.now()}.json`;
-fs.writeFileSync(target, JSON.stringify({model,tools,results},null,2)+'\n');
+fs.writeFileSync(target, JSON.stringify({model,system,tools,results},null,2)+'\n');
 console.log(target);
diff --git a/src/clean_agent_preview.py b/src/clean_agent_preview.py
index 840b674b0..ccc003345 100644
--- a/src/clean_agent_preview.py
+++ b/src/clean_agent_preview.py
@@ -162,6 +162,25 @@ SAFE_ACTIONS = {
 }
 
 
+def search_tool_choice_request(request):
+    """Enforce a search via one offered tool, not named-tool argument decoding.
+
+    The served model emits missing query fields under named search choice.
+    Required choice over the same single schema preserves the policy intent.
+    Other tools and auto/none requests retain their existing dispatch.
+    """
+    choice = request.get('tool_choice')
+    if not isinstance(choice, dict) or choice.get('type') != 'function':
+        return request
+    name = (choice.get('function') or {}).get('name')
+    if name != 'web_search':
+        return request
+    selected = [s for s in request.get('tools', []) if s.get('function', {}).get('name') == name]
+    if len(selected) != 1:
+        return request
+    return {**request, 'tools': selected, 'tool_choice': 'required'}
+
+
 def bounded_search_observation(output, budget=8000):
     """Share the observation budget across fetched sources, not prefix order.
 
@@ -4190,6 +4209,7 @@ async def stream_preview(*, endpoint_url, model, messages, headers, turn_contrac
                             }
                         force_private_browser_next_round = False
                 pending, content = {}, ''
+                request = search_tool_choice_request(request)
                 async with preview_model_response(client, endpoint_url, headers, request, context_recovery) as response:
                     response.raise_for_status()
                     async for line in response.aiter_lines():
diff --git a/tests/test_clean_agent_preview.py b/tests/test_clean_agent_preview.py
index 6639d5fe4..9cb2d912b 100644
--- a/tests/test_clean_agent_preview.py
+++ b/tests/test_clean_agent_preview.py
@@ -1283,9 +1283,8 @@ async def test_stream_retries_an_obviously_truncated_broad_web_answer(monkeypatc
     events = [json.loads(chunk[6:]) for chunk in raw if '[DONE]' not in chunk]
 
     assert len(requests) == (4 if embedded_article or empty_second_search else 5)
-    assert requests[2]['tool_choice'] == {
-        'type': 'function', 'function': {'name': 'web_search'},
-    }
+    assert requests[2]['tool_choice'] == 'required'
+    assert [s['function']['name'] for s in requests[2]['tools']] == ['web_search']
     if not embedded_article and not empty_second_search:
         assert requests[3]['tool_choice'] == {
             'type': 'function', 'function': {'name': 'web_fetch'},
@@ -4351,9 +4350,8 @@ async def test_blocked_search_engine_browser_forces_native_web_search(monkeypatc
     )]
 
     assert executions == ['private_browser', 'web_search']
-    assert requests[1]['tool_choice'] == {
-        'type': 'function', 'function': {'name': 'web_search'},
-    }
+    assert requests[1]['tool_choice'] == 'required'
+    assert [s['function']['name'] for s in requests[1]['tools']] == ['web_search']
     events = [json.loads(chunk[6:]) for chunk in raw if '[DONE]' not in chunk]
     assert any(
         event.get('type') == 'tool_loop_recovery'
diff --git a/tests/test_search_observation_budget.py b/tests/test_search_observation_budget.py
index ba7a222ea..cda66631d 100644
--- a/tests/test_search_observation_budget.py
+++ b/tests/test_search_observation_budget.py
@@ -4,6 +4,23 @@ import json
 import pytest
 
 
+def test_forced_search_dispatch_preserves_schema_without_mutating_request():
+    from src.clean_agent_preview import search_tool_choice_request
+    search = {'type': 'function', 'function': {'name': 'web_search', 'parameters': {'required': ['query']}}}
+    other = {'type': 'function', 'function': {'name': 'web_fetch'}}
+    request = {'tools': [search, other], 'tool_choice': {'type': 'function', 'function': {'name': 'web_search'}}, 'messages': []}
+    converted = search_tool_choice_request(request)
+    assert converted['tools'] == [search]
+    assert converted['tools'][0] is search
+    assert converted['tool_choice'] == 'required'
+    assert len(request['tools']) == 2
+    for choice in ['auto', 'none', 'required', {'type': 'function', 'function': {'name': 'web_fetch'}}]:
+        other_request = {**request, 'tool_choice': choice}
+        assert search_tool_choice_request(other_request) is other_request
+    missing = {**request, 'tools': [other]}
+    assert search_tool_choice_request(missing) is missing
+
+
 @pytest.mark.parametrize('prompt', [
     'Explain the settings and link the instructions, not just the homepage.',
     'Can you link to the original studies?',