fix(gemini): filter noisy stderr output from gemini backend (#83)

* fix(gemini): filter noisy stderr output from gemini backend

- Add filteringWriter to filter [STARTUP], Warning, Session cleanup etc.
- Apply filter only for gemini backend stderr output
- Add unit tests for filtering logic

* fix: use defer for stderrFilter.Flush to cover all return paths

Address review feedback: ensure filter is flushed on failure paths

codeagent-wrapper/executor.go

@@ -683,8 +683,17 @@ func runCodexTaskWithContext(parentCtx context.Context, taskSpec TaskSpec, backe
 	if stderrLogger != nil {
 		stderrWriters = append(stderrWriters, stderrLogger)
 	}
+
+	// For gemini backend, filter noisy stderr output
+	var stderrFilter *filteringWriter
 	if !silent {
-		stderrWriters = append([]io.Writer{os.Stderr}, stderrWriters...)
+		stderrOut := io.Writer(os.Stderr)
+		if cfg.Backend == "gemini" {
+			stderrFilter = newFilteringWriter(os.Stderr, geminiNoisePatterns)
+			stderrOut = stderrFilter
+			defer stderrFilter.Flush()
+		}
+		stderrWriters = append([]io.Writer{stderrOut}, stderrWriters...)
 	}
 	if len(stderrWriters) == 1 {
 		cmd.SetStderr(stderrWriters[0])

codeagent-wrapper/filter.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"strings"
+)
+
+// geminiNoisePatterns contains stderr patterns to filter for gemini backend
+var geminiNoisePatterns = []string{
+	"[STARTUP]",
+	"Session cleanup disabled",
+	"Warning:",
+	"(node:",
+	"(Use `node --trace-warnings",
+	"Loaded cached credentials",
+	"Loading extension:",
+	"YOLO mode is enabled",
+}
+
+// filteringWriter wraps an io.Writer and filters out lines matching patterns
+type filteringWriter struct {
+	w        io.Writer
+	patterns []string
+	buf      bytes.Buffer
+}
+
+func newFilteringWriter(w io.Writer, patterns []string) *filteringWriter {
+	return &filteringWriter{w: w, patterns: patterns}
+}
+
+func (f *filteringWriter) Write(p []byte) (n int, err error) {
+	f.buf.Write(p)
+	for {
+		line, err := f.buf.ReadString('\n')
+		if err != nil {
+			// incomplete line, put it back
+			f.buf.WriteString(line)
+			break
+		}
+		if !f.shouldFilter(line) {
+			f.w.Write([]byte(line))
+		}
+	}
+	return len(p), nil
+}
+
+func (f *filteringWriter) shouldFilter(line string) bool {
+	for _, pattern := range f.patterns {
+		if strings.Contains(line, pattern) {
+			return true
+		}
+	}
+	return false
+}
+
+// Flush writes any remaining buffered content
+func (f *filteringWriter) Flush() {
+	if f.buf.Len() > 0 {
+		remaining := f.buf.String()
+		if !f.shouldFilter(remaining) {
+			f.w.Write([]byte(remaining))
+		}
+		f.buf.Reset()
+	}
+}

codeagent-wrapper/filter_test.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestFilteringWriter(t *testing.T) {
+	tests := []struct {
+		name     string
+		patterns []string
+		input    string
+		want     string
+	}{
+		{
+			name:     "filter STARTUP lines",
+			patterns: geminiNoisePatterns,
+			input:    "[STARTUP] Recording metric\nHello World\n[STARTUP] Another line\n",
+			want:     "Hello World\n",
+		},
+		{
+			name:     "filter Warning lines",
+			patterns: geminiNoisePatterns,
+			input:    "Warning: something bad\nActual output\n",
+			want:     "Actual output\n",
+		},
+		{
+			name:     "filter multiple patterns",
+			patterns: geminiNoisePatterns,
+			input:    "YOLO mode is enabled\nSession cleanup disabled\nReal content\nLoading extension: foo\n",
+			want:     "Real content\n",
+		},
+		{
+			name:     "no filtering needed",
+			patterns: geminiNoisePatterns,
+			input:    "Line 1\nLine 2\nLine 3\n",
+			want:     "Line 1\nLine 2\nLine 3\n",
+		},
+		{
+			name:     "empty input",
+			patterns: geminiNoisePatterns,
+			input:    "",
+			want:     "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			fw := newFilteringWriter(&buf, tt.patterns)
+			fw.Write([]byte(tt.input))
+			fw.Flush()
+
+			if got := buf.String(); got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestFilteringWriterPartialLines(t *testing.T) {
+	var buf bytes.Buffer
+	fw := newFilteringWriter(&buf, geminiNoisePatterns)
+
+	// Write partial line
+	fw.Write([]byte("Hello "))
+	fw.Write([]byte("World\n"))
+	fw.Flush()
+
+	if got := buf.String(); got != "Hello World\n" {
+		t.Errorf("got %q, want %q", got, "Hello World\n")
+	}
+}