session files

1 files changed, 64 insertions, 0 deletions

diff --git a/libpanto/src/openai_chat_json.zig b/libpanto/src/openai_chat_json.zig
index 9531131..37d347b 100644
--- a/libpanto/src/openai_chat_json.zig
+++ b/libpanto/src/openai_chat_json.zig
@@ -31,6 +31,31 @@ pub const StreamDelta = struct {
     /// treat the turn as failed.
     error_message: ?[]const u8 = null,
     error_type: ?[]const u8 = null,
+    /// Token usage from the final chunk's top-level `usage` block.
+    /// Only present on the final chunk when the request was sent with
+    /// `stream_options.include_usage: true`. Earlier chunks have null.
+    usage: ?StreamUsage = null,
+};
+
+/// Token usage payload from OpenAI's terminating SSE chunk. Field
+/// semantics:
+///
+///   - `prompt_tokens`: total input tokens, **including** cached tokens.
+///   - `completion_tokens`: output tokens (including reasoning tokens).
+///   - `cached_prompt_tokens`: subset of `prompt_tokens` served from
+///     the prompt cache (billed at a discount).
+///   - `reasoning_tokens`: subset of `completion_tokens` spent on
+///     internal reasoning (o-series models).
+///
+/// To map to `Usage`: `input = prompt_tokens - cached_prompt_tokens`,
+/// `cache_read = cached_prompt_tokens`, `output = completion_tokens`,
+/// `reasoning = reasoning_tokens`, `cache_write = 0` (OpenAI doesn't
+/// bill a cache-write premium).
+pub const StreamUsage = struct {
+    prompt_tokens: ?u64 = null,
+    completion_tokens: ?u64 = null,
+    cached_prompt_tokens: ?u64 = null,
+    reasoning_tokens: ?u64 = null,
 };
 
 /// A single entry from a streaming `tool_calls` array. Multiple parallel
@@ -66,6 +91,17 @@ pub fn serializeRequest(
     try s.objectField("stream");
     try s.write(true);
 
+    // Ask for the final-chunk usage block. Without this the server
+    // sends `usage: null` and we can't stamp token counts on the
+    // session log. Most OpenAI-compatible proxies accept this; ones
+    // that don't will either ignore it or 400 — in the latter case
+    // the user has bigger problems than missing token counts.
+    try s.objectField("stream_options");
+    try s.beginObject();
+    try s.objectField("include_usage");
+    try s.write(true);
+    try s.endObject();
+
     switch (cfg.reasoning) {
         .default => {},
         .off => {
@@ -285,6 +321,27 @@ pub fn parseStreamEvent(allocator: Allocator, payload: []const u8) !ParsedDelta
     // Top-level `error` field. Some providers (and OpenAI itself on rare
     // mid-stream failures) emit `data: {"error":{"message":...,"type":...}}`
     // with HTTP 200, so we look for this BEFORE the choices array.
+    // Top-level `usage` field on the terminating chunk. Independent of
+    // the (often empty) choices array.
+    if (root.object.get("usage")) |u| {
+        if (u == .object) {
+            var su: StreamUsage = .{};
+            su.prompt_tokens = readOptU64(u.object, "prompt_tokens");
+            su.completion_tokens = readOptU64(u.object, "completion_tokens");
+            if (u.object.get("prompt_tokens_details")) |ptd| {
+                if (ptd == .object) {
+                    su.cached_prompt_tokens = readOptU64(ptd.object, "cached_tokens");
+                }
+            }
+            if (u.object.get("completion_tokens_details")) |ctd| {
+                if (ctd == .object) {
+                    su.reasoning_tokens = readOptU64(ctd.object, "reasoning_tokens");
+                }
+            }
+            delta.usage = su;
+        }
+    }
+
     if (root.object.get("error")) |e| {
         switch (e) {
             .object => |obj| {
@@ -375,6 +432,13 @@ pub fn parseStreamEvent(allocator: Allocator, payload: []const u8) !ParsedDelta
     };
 }
 
+fn readOptU64(obj: std.json.ObjectMap, name: []const u8) ?u64 {
+    const v = obj.get(name) orelse return null;
+    if (v != .integer) return null;
+    if (v.integer < 0) return null;
+    return @intCast(v.integer);
+}
+
 // -----------------------------------------------------------------------------
 // Tests
 // -----------------------------------------------------------------------------