1 files changed, 59 insertions, 10 deletions

diff --git a/libpanto/src/provider_anthropic_messages.zig b/libpanto/src/provider_anthropic_messages.zig
index 02277be..5c24b8f 100644
--- a/libpanto/src/provider_anthropic_messages.zig
+++ b/libpanto/src/provider_anthropic_messages.zig
@@ -181,6 +181,7 @@ const StreamState = struct {
     /// `onMessageComplete`, the latter stamps `null`.
     usage: provider_mod.Usage = .{},
     usage_seen: bool = false,
+    stop_reason: ?[]u8 = null,
 
     const ActiveBlock = struct {
         /// Index reported on the wire (Anthropic's content-array index).
@@ -210,6 +211,7 @@ const StreamState = struct {
         }
         for (self.blocks.items) |*b| b.deinit(self.allocator);
         self.blocks.deinit(self.allocator);
+        if (self.stop_reason) |s| self.allocator.free(s);
     }
 
     fn ensureStarted(self: *StreamState, receiver: *provider_mod.Receiver) !void {
@@ -322,6 +324,40 @@ const StreamState = struct {
         a.signature = try self.allocator.dupe(u8, sig);
     }
 
+    fn setStopReason(self: *StreamState, reason: ?[]const u8) !void {
+        if (self.stop_reason) |old| self.allocator.free(old);
+        self.stop_reason = if (reason) |r| try self.allocator.dupe(u8, r) else null;
+    }
+
+    fn isJSONObject(input: []const u8) bool {
+        if (input.len == 0) return true;
+        var parsed = std.json.parseFromSlice(std.json.Value, std.heap.page_allocator, input, .{}) catch return false;
+        defer parsed.deinit();
+        return parsed.value == .object;
+    }
+
+    fn appendAnthropicFailureResult(
+        allocator: Allocator,
+        blocks: *std.ArrayList(conversation.ContentBlock),
+        tool_use_id: []const u8,
+        input: []const u8,
+        stop_reason: ?[]const u8,
+    ) !void {
+        const id_copy = try allocator.dupe(u8, tool_use_id);
+        errdefer allocator.free(id_copy);
+        const msg = try std.fmt.allocPrint(
+            allocator,
+            "Tool call was not executed: Anthropic stopped before completing valid tool input JSON (stop_reason={s}). Partial input: {s}",
+            .{ stop_reason orelse "unknown", input },
+        );
+        errdefer allocator.free(msg);
+        var content: conversation.TextualBlock = .empty;
+        errdefer content.deinit(allocator);
+        try content.appendSlice(allocator, msg);
+        allocator.free(msg);
+        try blocks.append(allocator, .{ .ToolResult = .{ .tool_use_id = id_copy, .content = content } });
+    }
+
     /// Close the active block: append it to `blocks` and emit onBlockComplete.
     fn closeBlock(
         self: *StreamState,
@@ -355,11 +391,22 @@ const StreamState = struct {
                 .text = a.text_buf,
                 .signature = a.signature,
             } },
-            .tool_use => .{ .ToolUse = .{
-                .id = a.tool_id.?,
-                .name = a.tool_name.?,
-                .input = a.text_buf,
-            } },
+            .tool_use => blk: {
+                if (!isJSONObject(a.text_buf.items)) {
+                    try appendAnthropicFailureResult(
+                        self.allocator,
+                        &self.blocks,
+                        a.tool_id.?,
+                        a.text_buf.items,
+                        self.stop_reason,
+                    );
+                }
+                break :blk .{ .ToolUse = .{
+                    .id = a.tool_id.?,
+                    .name = a.tool_name.?,
+                    .input = a.text_buf,
+                } };
+            },
             .unsupported => unreachable,
         };
 
@@ -389,7 +436,8 @@ const StreamState = struct {
         self.finalized = true;
 
         if (self.active != null) {
-            // Wire dropped us mid-block. Close it cleanly anyway.
+            // Preserve an interrupted tool call so the agent can answer it
+            // with a synthetic error ToolResult instead of invoking it.
             try self.closeBlock(receiver);
         }
 
@@ -434,12 +482,13 @@ fn handleEvent(
             if (d.signature_delta) |sig| try state.setSignature(sig);
             if (d.input_json_delta) |j| try state.appendInputJsonDelta(receiver, j);
         },
-        .content_block_stop => {
-            try state.closeBlock(receiver);
+        .content_block_stop => |s| {
+            if (state.active) |a| {
+                if (a.wire_index == s.index) try state.closeBlock(receiver);
+            }
         },
         .message_delta => |d| {
-            // We don't act on stop_reason directly; message_stop is the
-            // authoritative end-of-stream signal.
+            try state.setStopReason(d.stop_reason);
             state.mergeUsage(d.usage);
         },
         .message_stop => {