From 9c64a7d4462a11674e2dea481b037b5f5d9c62fc Mon Sep 17 00:00:00 2001
From: t <t@tjp.lol>
Date: Tue, 2 Jun 2026 09:02:49 -0600
Subject: system prompt building and logging

---
 libpanto/src/anthropic_messages_json.zig | 81 ++++++++++++++++++++++++++++----
 1 file changed, 71 insertions(+), 10 deletions(-)

(limited to 'libpanto/src/anthropic_messages_json.zig')

diff --git a/libpanto/src/anthropic_messages_json.zig b/libpanto/src/anthropic_messages_json.zig
index 5b65c2b..f8bfc2e 100644
--- a/libpanto/src/anthropic_messages_json.zig
+++ b/libpanto/src/anthropic_messages_json.zig
@@ -111,20 +111,26 @@ fn writeRawJson(s: *std.json.Stringify, raw: []const u8) !void {
     try s.write(parsed.value);
 }
 
+/// Build the top-level Anthropic `system` string. Applies the shared
+/// append/replace derivation (see `conversation.effectiveSystemBlocks`),
+/// strips trailing newlines from each surviving block, and joins them with
+/// a horizontal rule (`\n\n---\n\n`). Anthropic's wire format requires a
+/// single string, so this rule is its concession to that constraint.
 fn collectSystemPrompt(
     conv: *const conversation.Conversation,
     out: *std.ArrayList(u8),
     allocator: Allocator,
 ) !void {
+    var blocks = try conversation.effectiveSystemBlocks(allocator, conv.messages.items);
+    defer blocks.deinit(allocator);
+
+    const sep = "\n\n---\n\n";
     var first = true;
-    for (conv.messages.items) |msg| {
-        if (msg.role != .system) continue;
-        for (msg.content.items) |block| {
-            if (block != .Text) continue;
-            if (!first) try out.append(allocator, '\n');
-            try out.appendSlice(allocator, block.Text.items);
-            first = false;
-        }
+    for (blocks.items) |text| {
+        const trimmed = std.mem.trimEnd(u8, text, "\n");
+        if (!first) try out.appendSlice(allocator, sep);
+        try out.appendSlice(allocator, trimmed);
+        first = false;
     }
 }
 
@@ -208,6 +214,11 @@ fn writeBlock(s: *std.json.Stringify, block: conversation.ContentBlock) !void {
             try s.write(tr.content.items);
             try s.endObject();
         },
+        // System blocks never reach here: `serializeRequest` filters out
+        // `.system` messages before emitting the `messages` array, and the
+        // system text is hoisted into the top-level `system` string by
+        // `collectSystemPrompt`. Present only to keep the switch exhaustive.
+        .System => {},
     }
 }
 
@@ -526,7 +537,7 @@ test "serializeRequest - system extracted into top-level field" {
     try testing.expectEqualStrings("Hello!", content[0].object.get("text").?.string);
 }
 
-test "serializeRequest - multiple system messages concatenated with newlines" {
+test "serializeRequest - multiple system messages joined with horizontal rule" {
     const allocator = testing.allocator;
 
     var conv = conversation.Conversation.init(allocator);
@@ -545,7 +556,57 @@ test "serializeRequest - multiple system messages concatenated with newlines" {
     var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
     defer parsed.deinit();
     try testing.expectEqualStrings(
-        "Be terse.\nBe accurate.",
+        "Be terse.\n\n---\n\nBe accurate.",
+        parsed.value.object.get("system").?.string,
+    );
+}
+
+test "serializeRequest - replace-mode system block wipes prior system text" {
+    const allocator = testing.allocator;
+
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("original seed");
+    try conv.addSystemMessage("original append");
+    try conv.replaceSystemMessage("fresh seed");
+    try conv.addSystemMessage("fresh append");
+    try conv.addUserMessage("Hi");
+
+    const cfg = testConfig("claude-x");
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    try testing.expectEqualStrings(
+        "fresh seed\n\n---\n\nfresh append",
+        parsed.value.object.get("system").?.string,
+    );
+}
+
+test "serializeRequest - trailing newlines stripped before the rule join" {
+    const allocator = testing.allocator;
+
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("line one\n\n");
+    try conv.addSystemMessage("line two\n");
+    try conv.addUserMessage("Hi");
+
+    const cfg = testConfig("claude-x");
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    try testing.expectEqualStrings(
+        "line one\n\n---\n\nline two",
         parsed.value.object.get("system").?.string,
     );
 }
-- 
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