diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 6f218f96..fced14d8 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -951,12 +951,9 @@ func applyClaudeToolPrefix(body []byte, prefix string) []byte {
 		return body
 	}
 
-	// Collect built-in tool names (those with a non-empty "type" field) so we can
-	// skip them consistently in both tools and message history.
-	builtinTools := map[string]bool{}
-	for _, name := range []string{"web_search", "code_execution", "text_editor", "computer"} {
-		builtinTools[name] = true
-	}
+	// Collect built-in tool names from the authoritative fallback seed list and
+	// augment it with any typed built-ins present in the current request body.
+	builtinTools := helps.AugmentClaudeBuiltinToolRegistry(body, nil)
 
 	if tools := gjson.GetBytes(body, "tools"); tools.Exists() && tools.IsArray() {
 		tools.ForEach(func(index, tool gjson.Result) bool {
diff --git a/internal/runtime/executor/claude_executor_test.go b/internal/runtime/executor/claude_executor_test.go
index c6220fe9..2cf969bb 100644
--- a/internal/runtime/executor/claude_executor_test.go
+++ b/internal/runtime/executor/claude_executor_test.go
@@ -739,6 +739,35 @@ func TestApplyClaudeToolPrefix_ToolChoiceBuiltin(t *testing.T) {
 	}
 }
 
+func TestApplyClaudeToolPrefix_KnownFallbackBuiltinsRemainUnprefixed(t *testing.T) {
+	for _, builtin := range []string{"web_search", "code_execution", "text_editor", "computer"} {
+		t.Run(builtin, func(t *testing.T) {
+			input := []byte(fmt.Sprintf(`{
+				"tools":[{"name":"Read"}],
+				"tool_choice":{"type":"tool","name":%q},
+				"messages":[{"role":"assistant","content":[{"type":"tool_use","name":%q,"id":"toolu_1","input":{}},{"type":"tool_reference","tool_name":%q},{"type":"tool_result","tool_use_id":"toolu_1","content":[{"type":"tool_reference","tool_name":%q}]}]}]
+			}`, builtin, builtin, builtin, builtin))
+			out := applyClaudeToolPrefix(input, "proxy_")
+
+			if got := gjson.GetBytes(out, "tool_choice.name").String(); got != builtin {
+				t.Fatalf("tool_choice.name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.0.name").String(); got != builtin {
+				t.Fatalf("messages.0.content.0.name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.1.tool_name").String(); got != builtin {
+				t.Fatalf("messages.0.content.1.tool_name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.2.content.0.tool_name").String(); got != builtin {
+				t.Fatalf("messages.0.content.2.content.0.tool_name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "tools.0.name").String(); got != "proxy_Read" {
+				t.Fatalf("tools.0.name = %q, want %q", got, "proxy_Read")
+			}
+		})
+	}
+}
+
 func TestStripClaudeToolPrefixFromResponse(t *testing.T) {
 	input := []byte(`{"content":[{"type":"tool_use","name":"proxy_alpha","id":"t1","input":{}},{"type":"tool_use","name":"bravo","id":"t2","input":{}}]}`)
 	out := stripClaudeToolPrefixFromResponse(input, "proxy_")
diff --git a/internal/runtime/executor/helps/claude_builtin_tools.go b/internal/runtime/executor/helps/claude_builtin_tools.go
new file mode 100644
index 00000000..5ee2b08d
--- /dev/null
+++ b/internal/runtime/executor/helps/claude_builtin_tools.go
@@ -0,0 +1,38 @@
+package helps
+
+import "github.com/tidwall/gjson"
+
+var defaultClaudeBuiltinToolNames = []string{
+	"web_search",
+	"code_execution",
+	"text_editor",
+	"computer",
+}
+
+func newClaudeBuiltinToolRegistry() map[string]bool {
+	registry := make(map[string]bool, len(defaultClaudeBuiltinToolNames))
+	for _, name := range defaultClaudeBuiltinToolNames {
+		registry[name] = true
+	}
+	return registry
+}
+
+func AugmentClaudeBuiltinToolRegistry(body []byte, registry map[string]bool) map[string]bool {
+	if registry == nil {
+		registry = newClaudeBuiltinToolRegistry()
+	}
+	tools := gjson.GetBytes(body, "tools")
+	if !tools.Exists() || !tools.IsArray() {
+		return registry
+	}
+	tools.ForEach(func(_, tool gjson.Result) bool {
+		if tool.Get("type").String() == "" {
+			return true
+		}
+		if name := tool.Get("name").String(); name != "" {
+			registry[name] = true
+		}
+		return true
+	})
+	return registry
+}
diff --git a/internal/runtime/executor/helps/claude_builtin_tools_test.go b/internal/runtime/executor/helps/claude_builtin_tools_test.go
new file mode 100644
index 00000000..d7badd19
--- /dev/null
+++ b/internal/runtime/executor/helps/claude_builtin_tools_test.go
@@ -0,0 +1,32 @@
+package helps
+
+import "testing"
+
+func TestClaudeBuiltinToolRegistry_DefaultSeedFallback(t *testing.T) {
+	registry := AugmentClaudeBuiltinToolRegistry(nil, nil)
+	for _, name := range defaultClaudeBuiltinToolNames {
+		if !registry[name] {
+			t.Fatalf("default builtin %q missing from fallback registry", name)
+		}
+	}
+}
+
+func TestClaudeBuiltinToolRegistry_AugmentsTypedBuiltinsFromBody(t *testing.T) {
+	registry := AugmentClaudeBuiltinToolRegistry([]byte(`{
+		"tools": [
+			{"type": "web_search_20250305", "name": "web_search"},
+			{"type": "custom_builtin_20250401", "name": "special_builtin"},
+			{"name": "Read"}
+		]
+	}`), nil)
+
+	if !registry["web_search"] {
+		t.Fatal("expected default typed builtin web_search in registry")
+	}
+	if !registry["special_builtin"] {
+		t.Fatal("expected typed builtin from body to be added to registry")
+	}
+	if registry["Read"] {
+		t.Fatal("expected untyped custom tool to stay out of builtin registry")
+	}
+}
diff --git a/test/claude_code_compatibility_sentinel_test.go b/test/claude_code_compatibility_sentinel_test.go
new file mode 100644
index 00000000..793b3c6a
--- /dev/null
+++ b/test/claude_code_compatibility_sentinel_test.go
@@ -0,0 +1,106 @@
+package test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+type jsonObject = map[string]any
+
+func loadClaudeCodeSentinelFixture(t *testing.T, name string) jsonObject {
+	t.Helper()
+	path := filepath.Join("testdata", "claude_code_sentinels", name)
+	data := mustReadFile(t, path)
+	var payload jsonObject
+	if err := json.Unmarshal(data, &payload); err != nil {
+		t.Fatalf("unmarshal %s: %v", name, err)
+	}
+	return payload
+}
+
+func mustReadFile(t *testing.T, path string) []byte {
+	t.Helper()
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read %s: %v", path, err)
+	}
+	return data
+}
+
+func requireStringField(t *testing.T, obj jsonObject, key string) string {
+	t.Helper()
+	value, ok := obj[key].(string)
+	if !ok || value == "" {
+		t.Fatalf("field %q missing or empty: %#v", key, obj[key])
+	}
+	return value
+}
+
+func TestClaudeCodeSentinel_ToolProgressShape(t *testing.T) {
+	payload := loadClaudeCodeSentinelFixture(t, "tool_progress.json")
+	if got := requireStringField(t, payload, "type"); got != "tool_progress" {
+		t.Fatalf("type = %q, want tool_progress", got)
+	}
+	requireStringField(t, payload, "tool_use_id")
+	requireStringField(t, payload, "tool_name")
+	requireStringField(t, payload, "session_id")
+	if _, ok := payload["elapsed_time_seconds"].(float64); !ok {
+		t.Fatalf("elapsed_time_seconds missing or non-number: %#v", payload["elapsed_time_seconds"])
+	}
+}
+
+func TestClaudeCodeSentinel_SessionStateShape(t *testing.T) {
+	payload := loadClaudeCodeSentinelFixture(t, "session_state_changed.json")
+	if got := requireStringField(t, payload, "type"); got != "system" {
+		t.Fatalf("type = %q, want system", got)
+	}
+	if got := requireStringField(t, payload, "subtype"); got != "session_state_changed" {
+		t.Fatalf("subtype = %q, want session_state_changed", got)
+	}
+	state := requireStringField(t, payload, "state")
+	switch state {
+	case "idle", "running", "requires_action":
+	default:
+		t.Fatalf("unexpected session state %q", state)
+	}
+	requireStringField(t, payload, "session_id")
+}
+
+func TestClaudeCodeSentinel_ToolUseSummaryShape(t *testing.T) {
+	payload := loadClaudeCodeSentinelFixture(t, "tool_use_summary.json")
+	if got := requireStringField(t, payload, "type"); got != "tool_use_summary" {
+		t.Fatalf("type = %q, want tool_use_summary", got)
+	}
+	requireStringField(t, payload, "summary")
+	rawIDs, ok := payload["preceding_tool_use_ids"].([]any)
+	if !ok || len(rawIDs) == 0 {
+		t.Fatalf("preceding_tool_use_ids missing or empty: %#v", payload["preceding_tool_use_ids"])
+	}
+	for i, raw := range rawIDs {
+		if id, ok := raw.(string); !ok || id == "" {
+			t.Fatalf("preceding_tool_use_ids[%d] invalid: %#v", i, raw)
+		}
+	}
+}
+
+func TestClaudeCodeSentinel_ControlRequestCanUseToolShape(t *testing.T) {
+	payload := loadClaudeCodeSentinelFixture(t, "control_request_can_use_tool.json")
+	if got := requireStringField(t, payload, "type"); got != "control_request" {
+		t.Fatalf("type = %q, want control_request", got)
+	}
+	requireStringField(t, payload, "request_id")
+	request, ok := payload["request"].(map[string]any)
+	if !ok {
+		t.Fatalf("request missing or invalid: %#v", payload["request"])
+	}
+	if got := requireStringField(t, request, "subtype"); got != "can_use_tool" {
+		t.Fatalf("request.subtype = %q, want can_use_tool", got)
+	}
+	requireStringField(t, request, "tool_name")
+	requireStringField(t, request, "tool_use_id")
+	if input, ok := request["input"].(map[string]any); !ok || len(input) == 0 {
+		t.Fatalf("request.input missing or empty: %#v", request["input"])
+	}
+}
diff --git a/test/testdata/claude_code_sentinels/control_request_can_use_tool.json b/test/testdata/claude_code_sentinels/control_request_can_use_tool.json
new file mode 100644
index 00000000..cafdb00a
--- /dev/null
+++ b/test/testdata/claude_code_sentinels/control_request_can_use_tool.json
@@ -0,0 +1,11 @@
+{
+  "type": "control_request",
+  "request_id": "req_123",
+  "request": {
+    "subtype": "can_use_tool",
+    "tool_name": "Bash",
+    "input": {"command": "npm test"},
+    "tool_use_id": "toolu_123",
+    "description": "Running npm test"
+  }
+}
diff --git a/test/testdata/claude_code_sentinels/session_state_changed.json b/test/testdata/claude_code_sentinels/session_state_changed.json
new file mode 100644
index 00000000..db411ace
--- /dev/null
+++ b/test/testdata/claude_code_sentinels/session_state_changed.json
@@ -0,0 +1,7 @@
+{
+  "type": "system",
+  "subtype": "session_state_changed",
+  "state": "requires_action",
+  "uuid": "22222222-2222-4222-8222-222222222222",
+  "session_id": "sess_123"
+}
diff --git a/test/testdata/claude_code_sentinels/tool_progress.json b/test/testdata/claude_code_sentinels/tool_progress.json
new file mode 100644
index 00000000..45a3a22e
--- /dev/null
+++ b/test/testdata/claude_code_sentinels/tool_progress.json
@@ -0,0 +1,10 @@
+{
+  "type": "tool_progress",
+  "tool_use_id": "toolu_123",
+  "tool_name": "Bash",
+  "parent_tool_use_id": null,
+  "elapsed_time_seconds": 2.5,
+  "task_id": "task_123",
+  "uuid": "11111111-1111-4111-8111-111111111111",
+  "session_id": "sess_123"
+}
diff --git a/test/testdata/claude_code_sentinels/tool_use_summary.json b/test/testdata/claude_code_sentinels/tool_use_summary.json
new file mode 100644
index 00000000..da3c4c3e
--- /dev/null
+++ b/test/testdata/claude_code_sentinels/tool_use_summary.json
@@ -0,0 +1,7 @@
+{
+  "type": "tool_use_summary",
+  "summary": "Searched in auth/",
+  "preceding_tool_use_ids": ["toolu_1", "toolu_2"],
+  "uuid": "33333333-3333-4333-8333-333333333333",
+  "session_id": "sess_123"
+}