fix(env): head+tail preview for truncated tool output

environment/large_output.go

@@ -122,21 +122,16 @@ func ShouldSkipForOutputsDir(command string) bool {
 	return strings.Contains(command, OutputsDir+"/") || strings.Contains(command, OutputsDir+"\\")
 }
 
-// truncateContent creates a truncated preview with optional file reference.
+// truncateContent creates a head+tail preview with an optional file reference.
+// Showing the first AND last lines (around an elision marker) means a line-rich
+// oversized output — a directory listing, a log, a JSON array — surfaces both
+// ends rather than only the head, which previously forced the agent to read the
+// spilled file just to see the tail.
 func (p *LargeOutputProcessor) truncateContent(content string, filePath string) string {
 	lines := strings.Split(content, "\n")
 	totalLines := len(lines)
+	preview, headShown, omitted := p.buildPreview(lines)
 
-	// Calculate preview
-	previewLines := min(p.config.PreviewLines, totalLines)
-	preview := strings.Join(lines[:previewLines], "\n")
-
-	// Truncate preview if too long
-	if len(preview) > p.config.PreviewSize {
-		preview = preview[:p.config.PreviewSize] + "\n... [preview truncated]"
-	}
-
-	// Build truncation message
 	var sb strings.Builder
 	sb.WriteString("<output_truncated>\n")
 	fmt.Fprintf(&sb, "Output too large (%d chars, %d lines).", len(content), totalLines)
@@ -147,12 +142,67 @@ func (p *LargeOutputProcessor) truncateContent(content string, filePath string)
 		sb.WriteString(" Could not save full content.\n\n")
 	}
 
-	fmt.Fprintf(&sb, "Preview (first %d lines):\n```\n%s\n```\n\n", previewLines, preview)
-
-	if filePath != "" {
-		fmt.Fprintf(&sb, "To read more: read(\"%s\", offset=%d, limit=100)\n", filePath, previewLines)
+	if omitted > 0 {
+		fmt.Fprintf(&sb, "Preview (first %d + last lines; %d lines omitted in the middle):\n```\n%s\n```\n\n", headShown, omitted, preview)
+		if filePath != "" {
+			fmt.Fprintf(&sb, "To read the omitted middle: read(\"%s\", offset=%d, limit=%d)\n", filePath, headShown, omitted)
+		}
+	} else {
+		fmt.Fprintf(&sb, "Preview (first %d lines):\n```\n%s\n```\n\n", headShown, preview)
+		if filePath != "" {
+			fmt.Fprintf(&sb, "To read more: read(\"%s\", offset=%d, limit=100)\n", filePath, headShown)
+		}
 	}
 
 	sb.WriteString("</output_truncated>")
 	return sb.String()
 }
+
+// buildPreview returns a char-bounded preview of a truncated output. When the
+// output has more lines than the line budget it shows a head AND a tail around
+// an elision marker — each end gets half the character budget so a few very long
+// head lines can't starve the tail. Otherwise (few lines, or one giant line) it
+// falls back to a head-only, char-capped preview. Returns the preview text, the
+// number of leading lines shown (an accurate read-more offset), and the number
+// of lines omitted in the middle (0 when none).
+func (p *LargeOutputProcessor) buildPreview(lines []string) (text string, headShown, omitted int) {
+	total := len(lines)
+	maxLines := p.config.PreviewLines
+	maxChars := p.config.PreviewSize
+
+	if total <= maxLines {
+		head := strings.Join(lines, "\n")
+		if len(head) > maxChars {
+			// One or few very long lines: head-only, char-capped. headShown=0 so
+			// the read-more offset doesn't claim whole lines the cap chopped.
+			return head[:maxChars] + "\n... [preview truncated]", 0, 0
+		}
+		return head, total, 0
+	}
+
+	headN := maxLines / 2
+	if headN < 1 {
+		headN = 1
+	}
+	tailN := maxLines - headN
+	omitted = total - headN - tailN
+	head := capHead(strings.Join(lines[:headN], "\n"), maxChars/2)
+	tail := capTail(strings.Join(lines[total-tailN:], "\n"), maxChars/2)
+	return head + fmt.Sprintf("\n... [%d lines omitted] ...\n", omitted) + tail, headN, omitted
+}
+
+// capHead keeps at most maxChars characters from the start of s.
+func capHead(s string, maxChars int) string {
+	if len(s) <= maxChars {
+		return s
+	}
+	return s[:maxChars] + " …"
+}
+
+// capTail keeps at most maxChars characters from the end of s.
+func capTail(s string, maxChars int) string {
+	if len(s) <= maxChars {
+		return s
+	}
+	return "… " + s[len(s)-maxChars:]
+}

environment/large_output_test.go

@@ -0,0 +1,63 @@
+package environment
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func headTailTestProcessor() *LargeOutputProcessor {
+	// Small budgets so the head+tail paths are exercised deterministically.
+	// truncateContent ignores ws, so a nil-ws processor is fine here.
+	return &LargeOutputProcessor{config: LargeOutputConfig{
+		MaxOutputSize: 200,
+		PreviewSize:   120,
+		PreviewLines:  10,
+	}}
+}
+
+// A line-rich oversized output shows BOTH a head and a tail around an elision,
+// with an accurate omitted-line count and a read-more offset past the head.
+func TestTruncateContent_LineRichShowsHeadAndTail(t *testing.T) {
+	p := headTailTestProcessor()
+	var b strings.Builder
+	for i := 0; i < 100; i++ {
+		fmt.Fprintf(&b, "line-%03d\n", i)
+	}
+	out := p.truncateContent(b.String(), ".outputs/x.txt")
+
+	assert.Contains(t, out, "line-000", "head must be shown")
+	assert.Contains(t, out, "line-099", "tail must be shown")
+	assert.Contains(t, out, "lines omitted", "must mark the omitted middle")
+	// PreviewLines=10 → 5 head + 5 tail; split() yields 101 lines (trailing ""),
+	// so omitted = 101 - 5 - 5 = 91 and the read-more offset starts past the head.
+	assert.Contains(t, out, "offset=5")
+	assert.Contains(t, out, "limit=91")
+}
+
+// The head+tail preview stays char-bounded even with long lines: a few very
+// long head lines must not starve the tail (both end markers survive).
+func TestTruncateContent_HeadTailIsCharBounded(t *testing.T) {
+	p := headTailTestProcessor()
+	lines := make([]string, 30)
+	for i := range lines {
+		lines[i] = fmt.Sprintf("head%02d-%s-tail%02d", i, strings.Repeat("x", 500), i)
+	}
+	out := p.truncateContent(strings.Join(lines, "\n"), ".outputs/x.txt")
+
+	assert.Contains(t, out, "lines omitted")
+	assert.Contains(t, out, "head00", "first line's head prefix must survive")
+	assert.Contains(t, out, "tail29", "last line's tail suffix must survive (not starved by long head lines)")
+}
+
+// A single very long line falls back to a head-only, char-capped preview with
+// no false "omitted" claim.
+func TestTruncateContent_OneGiantLineHeadOnly(t *testing.T) {
+	p := headTailTestProcessor()
+	out := p.truncateContent(strings.Repeat("y", 1000), ".outputs/x.txt")
+
+	assert.NotContains(t, out, "lines omitted")
+	assert.Contains(t, out, "preview truncated")
+}