fix: harden functionCall key detection in tool sieve

internal/adapter/openai/tool_sieve_functioncall.go

@@ -7,16 +7,16 @@ func findQuotedFunctionCallKeyStart(s string) int {
 	quotedIdx := findFunctionCallKeyStart(lower, `"functioncall"`)
 	bareIdx := findFunctionCallKeyStart(lower, "functioncall")
 
-	// Prefer quoted JSON keys when present. Bare-key detection is a fallback
-	// for loose payloads like {functionCall:{...}}.
-	//
-	// This avoids anchoring on earlier prose such as:
-	//   "... {note} functionCall: ... {\"functionCall\":{...}}"
-	// where choosing the earliest bare match can hide the real tool payload.
-	if quotedIdx >= 0 {
+	if quotedIdx < 0 {
+		return bareIdx
+	}
+	if bareIdx < 0 {
 		return quotedIdx
 	}
-	return bareIdx
+	if bareIdx < quotedIdx {
+		return bareIdx
+	}
+	return quotedIdx
 }
 
 func findFunctionCallKeyStart(lower, key string) int {
@@ -26,6 +26,10 @@ func findFunctionCallKeyStart(lower, key string) int {
 			return -1
 		}
 		idx := from + rel
+		if isInsideJSONString(lower, idx) {
+			from = idx + 1
+			continue
+		}
 		if !hasJSONObjectContextPrefix(lower[:idx]) {
 			from = idx + 1
 			continue
@@ -39,6 +43,14 @@ func findFunctionCallKeyStart(lower, key string) int {
 			j++
 		}
 		if j < len(lower) && lower[j] == ':' {
+			k := j + 1
+			for k < len(lower) && (lower[k] == ' ' || lower[k] == '\t' || lower[k] == '\r' || lower[k] == '\n') {
+				k++
+			}
+			if k < len(lower) && lower[k] != '{' {
+				from = idx + 1
+				continue
+			}
 			return idx
 		}
 		from = idx + 1
@@ -46,6 +58,26 @@ func findFunctionCallKeyStart(lower, key string) int {
 	return -1
 }
 
+func isInsideJSONString(s string, idx int) bool {
+	inString := false
+	escaped := false
+	for i := 0; i < idx; i++ {
+		c := s[i]
+		if escaped {
+			escaped = false
+			continue
+		}
+		if c == '\\' && inString {
+			escaped = true
+			continue
+		}
+		if c == '"' {
+			inString = !inString
+		}
+	}
+	return inString
+}
+
 func hasJSONObjectContextPrefix(prefix string) bool {
 	return strings.LastIndex(prefix, "{") >= 0
 }

internal/adapter/openai/tool_sieve_functioncall_test.go

@@ -0,0 +1,23 @@
+package openai
+
+import "testing"
+
+func TestFindQuotedFunctionCallKeyStart_PrefersEarlierBareKey(t *testing.T) {
+	input := `{functionCall:{"name":"a","arguments":"{}"},"message":"literal text: \"functionCall\": not a key"}`
+
+	got := findQuotedFunctionCallKeyStart(input)
+	want := 1
+	if got != want {
+		t.Fatalf("findQuotedFunctionCallKeyStart() = %d, want %d", got, want)
+	}
+}
+
+func TestFindQuotedFunctionCallKeyStart_PrefersEarlierQuotedKey(t *testing.T) {
+	input := `{"functionCall":{"name":"a","arguments":"{}"},"note":"functionCall appears in prose"}`
+
+	got := findQuotedFunctionCallKeyStart(input)
+	want := 1
+	if got != want {
+		t.Fatalf("findQuotedFunctionCallKeyStart() = %d, want %d", got, want)
+	}
+}