1 files changed, 49 insertions, 5 deletions

diff --git a/libpanto/src/anthropic_messages_json.zig b/libpanto/src/anthropic_messages_json.zig
index f7df80b..1b1cf6e 100644
--- a/libpanto/src/anthropic_messages_json.zig
+++ b/libpanto/src/anthropic_messages_json.zig
@@ -138,7 +138,7 @@ pub fn serializeRequest(
     for (window, 0..) |msg, mi| {
         if (msg.role == .system) continue;
         const mark: ?usize = if (cache_at) |c| (if (c.message == mi) c.block else null) else null;
-        try writeMessage(&s, msg, mark);
+        try writeMessage(&s, msg, mark, cfg);
     }
     try s.endArray();
 
@@ -251,6 +251,7 @@ fn writeMessage(
     s: *std.json.Stringify,
     msg: conversation.Message,
     cache_block: ?usize,
+    cfg: *const config_mod.AnthropicMessagesConfig,
 ) !void {
     try s.beginObject();
 
@@ -261,7 +262,7 @@ fn writeMessage(
     try s.beginArray();
     for (msg.content.items, 0..) |block, bi| {
         const mark = if (cache_block) |cb| cb == bi else false;
-        try writeBlock(s, block, mark);
+        try writeBlock(s, block, mark, cfg);
     }
     try s.endArray();
 
@@ -278,7 +279,12 @@ fn writeCacheControl(s: *std.json.Stringify) !void {
     try s.endObject();
 }
 
-fn writeBlock(s: *std.json.Stringify, block: conversation.ContentBlock, mark_cache: bool) !void {
+fn writeBlock(
+    s: *std.json.Stringify,
+    block: conversation.ContentBlock,
+    mark_cache: bool,
+    cfg: *const config_mod.AnthropicMessagesConfig,
+) !void {
     switch (block) {
         .Text => |tb| {
             try s.beginObject();
@@ -292,9 +298,14 @@ fn writeBlock(s: *std.json.Stringify, block: conversation.ContentBlock, mark_cac
         .Thinking => |tb| {
             // Anthropic requires the signature field to round-trip a thinking
             // block. If we don't have one (e.g. block was synthesized from a
-            // non-Anthropic provider), skip the block: Anthropic will reject
-            // unsigned thinking on incoming messages.
+            // non-Anthropic provider), or if the signature came from a
+            // different provider/model than the current request, skip the
+            // block: Anthropic will reject unsigned thinking on incoming
+            // messages, and opaque signatures are not portable across
+            // provider/model boundaries.
             const sig = tb.signature orelse return;
+            const origin = tb.signature_origin orelse return;
+            if (!origin.matches(.anthropic_messages, cfg.base_url, cfg.model)) return;
             try s.beginObject();
             try s.objectField("type");
             try s.write("thinking");
@@ -789,6 +800,7 @@ test "serializeRequest - cache breakpoint skips a trailing thinking block" {
             .signature = sig,
         } },
     }, null);
+    try conversation.setThinkingOrigins(allocator, conv.messages.items[0].content.items, .anthropic_messages, "u", "claude-x");
 
     var cfg = testConfig("claude-x");
     var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
@@ -935,6 +947,7 @@ test "serializeRequest - signed assistant Thinking blocks round-trip" {
         } },
         .{ .Text = try conversation.textualBlockFromSlice(allocator, "the answer is 42") },
     }, null);
+    try conversation.setThinkingOrigins(allocator, conv.messages.items[0].content.items, .anthropic_messages, "u", "claude-x");
 
     const cfg = testConfig("claude-x");
     var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
@@ -962,6 +975,37 @@ test "serializeRequest - signed assistant Thinking blocks round-trip" {
     try testing.expectEqualStrings("the answer is 42", content[1].object.get("text").?.string);
 }
 
+test "serializeRequest - mismatched thinking origin drops the block" {
+    const allocator = testing.allocator;
+
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+
+    const sig = try allocator.dupe(u8, "EqQBCgIYAhIM1gbcDa9GJwZA");
+    try conv.addAssistantMessage(&.{
+        .{ .Thinking = .{
+            .text = try conversation.textualBlockFromSlice(allocator, "other provider thinking"),
+            .signature = sig,
+        } },
+        .{ .Text = try conversation.textualBlockFromSlice(allocator, "answer") },
+    }, null);
+    try conversation.setThinkingOrigins(allocator, conv.messages.items[0].content.items, .openai_responses, "https://api.individual.githubcopilot.com", "gpt-5.4-mini");
+
+    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();
+
+    const content = parsed.value.object.get("messages").?.array.items[0]
+        .object.get("content").?.array.items;
+    try testing.expectEqual(@as(usize, 1), content.len);
+    try testing.expectEqualStrings("text", content[0].object.get("type").?.string);
+}
+
 test "serializeRequest - unsigned Thinking blocks are dropped" {
     // Anthropic rejects thinking blocks without a valid signature on inbound
     // messages. If a block lacks one (e.g. from a different provider or an