Stream OpenAI tool_use as discrete blocks; drop BlockMeta

Close each tool_use on next-index delta (and at finalize) so receivers
see one start/delta/complete trio per tool, matching Anthropic. Log+drop
fragments for already-closed indices.

Remove BlockMeta from the Receiver interface: tool id/name can't be
reliably known at onBlockStart under OpenAI's streaming model. Receivers
get identity from the assembled ContentBlock at onBlockComplete. CLI
renders 'tool: {args} : (name)' accordingly.

1 files changed, 10 insertions, 9 deletions

diff --git a/libpanto/src/provider.zig b/libpanto/src/provider.zig
index 177fc2f..fd170f0 100644
--- a/libpanto/src/provider.zig
+++ b/libpanto/src/provider.zig
@@ -12,12 +12,6 @@ pub const ContentBlockType = enum {
     ToolResult,
 };
 
-pub const BlockMeta = struct {
-    /// Only populated for ToolUse blocks. Null for Text/Thinking.
-    tool_id: ?[]const u8 = null,
-    tool_name: ?[]const u8 = null,
-};
-
 /// Vtable for receiving streaming events from a Provider.
 ///
 /// The lifecycle callbacks (`onMessageStart` ... `onMessageComplete`) return
@@ -25,6 +19,13 @@ pub const BlockMeta = struct {
 /// stops streaming, calls `onError(err)`, and propagates `err` out of
 /// `streamStep`. No partial assistant message is appended to the conversation.
 ///
+/// Identity fields (tool id/name for ToolUse blocks) are intentionally *not*
+/// surfaced at `onBlockStart`. They cannot be reliably known at start time
+/// across all wire protocols — OpenAI streaming may deliver `id` and `name`
+/// in fragments across multiple deltas — so any "meta" we passed at start
+/// would be a phantom guarantee. Receivers that need identity get it from
+/// the fully-assembled `ContentBlock` delivered to `onBlockComplete`.
+///
 /// `onError` is the receiver's cleanup hook. It fires exactly once per failed
 /// turn, whether the error originated in the receiver itself (a write failure)
 /// or in the Provider (HTTP/parse/stream failure). It is the last callback the
@@ -32,7 +33,7 @@ pub const BlockMeta = struct {
 /// must swallow secondary failures during cleanup.
 pub const ReceiverVTable = struct {
     onMessageStart: *const fn (*anyopaque, conversation.MessageRole) anyerror!void,
-    onBlockStart: *const fn (*anyopaque, ContentBlockType, usize, ?BlockMeta) anyerror!void,
+    onBlockStart: *const fn (*anyopaque, ContentBlockType, usize) anyerror!void,
     onContentDelta: *const fn (*anyopaque, usize, []const u8) anyerror!void,
     onBlockComplete: *const fn (*anyopaque, usize, conversation.ContentBlock) anyerror!void,
     onMessageComplete: *const fn (*anyopaque, conversation.Message) anyerror!void,
@@ -47,8 +48,8 @@ pub const Receiver = struct {
         try self.vtable.onMessageStart(self.ptr, role);
     }
 
-    pub fn onBlockStart(self: Receiver, block_type: ContentBlockType, index: usize, meta: ?BlockMeta) !void {
-        try self.vtable.onBlockStart(self.ptr, block_type, index, meta);
+    pub fn onBlockStart(self: Receiver, block_type: ContentBlockType, index: usize) !void {
+        try self.vtable.onBlockStart(self.ptr, block_type, index);
     }
 
     pub fn onContentDelta(self: Receiver, block_index: usize, delta: []const u8) !void {