tighten functionCall detection to quoted JSON keys

internal/adapter/openai/tool_sieve_core.go

@@ -183,7 +183,7 @@ func findToolSegmentStart(s string) int {
 		return -1
 	}
 	lower := strings.ToLower(s)
-	keywords := []string{"tool_calls", "\"function\"", "function.name:", "functioncall", "\"tool_use\""}
+	keywords := []string{"tool_calls", "\"function\"", "function.name:", "\"tool_use\""}
 	bestKeyIdx := -1
 	for _, kw := range keywords {
 		idx := strings.Index(lower, kw)
@@ -191,6 +191,9 @@ func findToolSegmentStart(s string) int {
 			bestKeyIdx = idx
 		}
 	}
+	if fnKeyIdx := findQuotedFunctionCallKeyStart(s); fnKeyIdx >= 0 && (bestKeyIdx < 0 || fnKeyIdx < bestKeyIdx) {
+		bestKeyIdx = fnKeyIdx
+	}
 	// Also detect XML tool call tags.
 	for _, tag := range xmlToolTagsToDetect {
 		idx := strings.Index(lower, tag)
@@ -240,13 +243,16 @@ func consumeToolCapture(state *toolStreamSieveState, toolNames []string) (prefix
 
 	lower := strings.ToLower(captured)
 	keyIdx := -1
-	keywords := []string{"tool_calls", "\"function\"", "function.name:", "functioncall", "\"tool_use\""}
+	keywords := []string{"tool_calls", "\"function\"", "function.name:", "\"tool_use\""}
 	for _, kw := range keywords {
 		idx := strings.Index(lower, kw)
 		if idx >= 0 && (keyIdx < 0 || idx < keyIdx) {
 			keyIdx = idx
 		}
 	}
+	if fnKeyIdx := findQuotedFunctionCallKeyStart(captured); fnKeyIdx >= 0 && (keyIdx < 0 || fnKeyIdx < keyIdx) {
+		keyIdx = fnKeyIdx
+	}
 
 	if keyIdx < 0 {
 		return "", nil, "", false
@@ -276,3 +282,32 @@ func consumeToolCapture(state *toolStreamSieveState, toolNames []string) (prefix
 	prefixPart, suffixPart = trimWrappingJSONFence(prefixPart, suffixPart)
 	return prefixPart, parsed.Calls, suffixPart, true
 }
+
+func findQuotedFunctionCallKeyStart(s string) int {
+	lower := strings.ToLower(s)
+	const key = "\"functioncall\""
+	for from := 0; from < len(lower); {
+		rel := strings.Index(lower[from:], key)
+		if rel < 0 {
+			return -1
+		}
+		idx := from + rel
+		if !hasJSONObjectContextPrefix(lower[:idx]) {
+			from = idx + 1
+			continue
+		}
+		j := idx + len(key)
+		for j < len(lower) && (lower[j] == ' ' || lower[j] == '\t' || lower[j] == '\r' || lower[j] == '\n') {
+			j++
+		}
+		if j < len(lower) && lower[j] == ':' {
+			return idx
+		}
+		from = idx + 1
+	}
+	return -1
+}
+
+func hasJSONObjectContextPrefix(prefix string) bool {
+	return strings.LastIndex(prefix, "{") >= 0
+}

internal/adapter/openai/tool_sieve_xml_test.go

@@ -120,6 +120,37 @@ func TestFindToolSegmentStartDetectsXMLToolCalls(t *testing.T) {
 	}
 }
 
+func TestFindToolSegmentStartIgnoresFunctionCallProse(t *testing.T) {
+	input := "Please explain the functionCall API field and how clients should parse it."
+	if got := findToolSegmentStart(input); got != -1 {
+		t.Fatalf("expected no tool segment start for prose, got %d", got)
+	}
+}
+
+func TestFindToolSegmentStartDetectsQuotedFunctionCallKey(t *testing.T) {
+	input := `prefix {"functionCall": {"name":"search_web","args":{"query":"x"}}}`
+	want := strings.Index(input, "{")
+	if got := findToolSegmentStart(input); got != want {
+		t.Fatalf("expected JSON object start %d, got %d", want, got)
+	}
+}
+
+func TestProcessToolSieveDoesNotBufferFunctionCallProse(t *testing.T) {
+	var state toolStreamSieveState
+	chunk := "Please explain the functionCall API field and keep streaming this sentence."
+	events := processToolSieveChunk(&state, chunk, []string{"search_web"})
+	var text string
+	for _, evt := range events {
+		text += evt.Content
+		if len(evt.ToolCalls) > 0 {
+			t.Fatalf("expected no tool calls for prose, got %#v", evt.ToolCalls)
+		}
+	}
+	if text != chunk {
+		t.Fatalf("expected prose to pass through immediately, got %q", text)
+	}
+}
+
 func TestProcessToolSieveDetectsGeminiFunctionCallPayload(t *testing.T) {
 	var state toolStreamSieveState
 	events := processToolSieveChunk(&state, `{"functionCall":{"name":"search_web","args":{"query":"latest"}}}`, []string{"search_web"})