1 files changed, 46 insertions, 18 deletions

diff --git a/libpanto/src/compaction.zig b/libpanto/src/compaction.zig
index 254b876..3a1c012 100644
--- a/libpanto/src/compaction.zig
+++ b/libpanto/src/compaction.zig
@@ -146,19 +146,33 @@ fn priorAssistantUsage(
 fn countWords(msg: Message) u64 {
     var total: u64 = 0;
     for (msg.content.items) |block| {
-        const text: []const u8 = switch (block) {
-            .Text => |b| b.items,
-            .Thinking => |b| b.text.items,
-            .ToolUse => |b| b.input.items,
-            .ToolResult => |b| b.content.items,
-            .System => |b| b.text.items,
-            .CompactionSummary => |b| b.text.items,
-        };
-        total += countWordsIn(text);
+        switch (block) {
+            .Text => |b| total += countWordsIn(b.items),
+            .Thinking => |b| total += countWordsIn(b.text.items),
+            .ToolUse => |b| total += countWordsIn(b.input.items),
+            .ToolResult => |b| {
+                // Text parts count by words. Media parts (base64 image /
+                // PDF blobs) would wildly distort the retention window if
+                // counted by length, so each is charged a fixed estimate
+                // (matching the pi reference impl's ~4800-char image cost,
+                // divided back out to words via `word_to_token_factor`).
+                for (b.parts.items) |p| switch (p) {
+                    .text => |t| total += countWordsIn(t.items),
+                    .media => total += media_part_word_estimate,
+                };
+            },
+            .System => |b| total += countWordsIn(b.text.items),
+            .CompactionSummary => |b| total += countWordsIn(b.text.items),
+        }
     }
     return total;
 }
 
+/// Fixed per-media-part word estimate. The pi reference impl sizes each
+/// inline image at ~4800 characters for compaction token math; expressed
+/// as words (≈4800/6 chars-per-word) this lands near 800.
+const media_part_word_estimate: u64 = 800;
+
 fn countWordsIn(text: []const u8) u64 {
     var count: u64 = 0;
     var in_word = false;
@@ -351,8 +365,18 @@ pub fn serializeTranscript(
                     try w.appendSlice(allocator, line);
                 },
                 .ToolResult => |b| {
+                    var body: conversation.TextualBlock = .empty;
+                    defer body.deinit(allocator);
+                    for (b.parts.items) |p| switch (p) {
+                        .text => |t| try body.appendSlice(allocator, t.items),
+                        .media => |m| {
+                            const note = try std.fmt.allocPrint(allocator, "[{s} attachment]", .{m.media_type});
+                            defer allocator.free(note);
+                            try body.appendSlice(allocator, note);
+                        },
+                    };
                     const line = try std.fmt.allocPrint(allocator, "[Tool result ({s})]: {s}", .{
-                        b.tool_use_id, b.content.items,
+                        b.tool_use_id, body.items,
                     });
                     defer allocator.free(line);
                     try w.appendSlice(allocator, line);
@@ -436,6 +460,16 @@ fn freeMsgs(alloc: Allocator, msgs: []Message) void {
     for (msgs) |*m| m.deinit(alloc);
 }
 
+/// Build a `.ToolResult` content block with a single text part.
+fn textToolResult(alloc: Allocator, id: []const u8, text: []const u8) !conversation.ContentBlock {
+    var parts: std.ArrayList(conversation.ResultPartStored) = .empty;
+    try parts.append(alloc, .{ .text = try conversation.textualBlockFromSlice(alloc, text) });
+    return .{ .ToolResult = .{
+        .tool_use_id = try alloc.dupe(u8, id),
+        .parts = parts,
+    } };
+}
+
 test "countWordsIn - basic word counting" {
     try testing.expectEqual(@as(u64, 0), countWordsIn(""));
     try testing.expectEqual(@as(u64, 0), countWordsIn("   \n\t "));
@@ -530,10 +564,7 @@ test "computeSplit - tool-result user messages attach to the current turn" {
 
     // Turn 1: user q1 -> assistant(tool_use) -> user(tool_result) -> assistant a1
     var tr_content: std.ArrayList(conversation.ContentBlock) = .empty;
-    try tr_content.append(a, .{ .ToolResult = .{
-        .tool_use_id = try a.dupe(u8, "t1"),
-        .content = try conversation.textualBlockFromSlice(a, "result"),
-    } });
+    try tr_content.append(a, try textToolResult(a, "t1", "result"));
 
     var msgs = [_]Message{
         try userMsg(a, "q1"),
@@ -689,10 +720,7 @@ test "serializeTranscript - tool call and result framing" {
         .input = try conversation.textualBlockFromSlice(a, "{\"cmd\":\"ls\"}"),
     } });
     var tr_content: std.ArrayList(conversation.ContentBlock) = .empty;
-    try tr_content.append(a, .{ .ToolResult = .{
-        .tool_use_id = try a.dupe(u8, "tc1"),
-        .content = try conversation.textualBlockFromSlice(a, "file.txt"),
-    } });
+    try tr_content.append(a, try textToolResult(a, "tc1", "file.txt"));
     var msgs = [_]Message{
         .{ .role = .assistant, .content = tu_content },
         .{ .role = .user, .content = tr_content },