fix(responses): report usage on response.completed so clients can auto-compact

Map upstream Chat Completions usage to the Responses API shape and attach it to response.completed. Capture chunk.usage before the empty-choices guard so the usage-only trailer chunk survives, and defer completion to flushEvents() when usage is not yet known — only on the direct openai:openai-responses route, since a pivoted stream never reaches flushEvents. Fixes #3432.

open-sse/translator/response/openai-responses.js

@@ -14,13 +14,47 @@ import { ROLE, OPENAI_BLOCK, RESPONSES_ITEM, OPENAI_FINISH, MODEL_FALLBACK } fro
  * Translate OpenAI chunk to Responses API events
  * @returns {Array} Array of events with { event, data } structure
  */
+// Upstream Chat Completions usage -> Responses API usage shape.
+// Without this, /v1/responses never reports usage: Responses clients (Codex CLI)
+// keep their "context used" gauge pinned at 0 and never auto-compact, so a long
+// session grows until the upstream context limit rejects it (9router issue #3432).
+//
+// Note this is stored under state.responsesUsage, NOT state.usage: state.usage is
+// owned by the stream layer, which fills it with normalizeUsage()-shaped counts
+// (prompt_tokens/prompt_tokens_details) and hands it to finalizeStream() for
+// logging and cost accounting. Overwriting it with this shape silently drops
+// cached/reasoning tokens from those stats.
+function toResponsesUsage(usage) {
+  if (!usage || typeof usage !== "object") return null;
+
+  const inputTokens = [usage.input_tokens, usage.prompt_tokens].find(Number.isFinite) ?? 0;
+  const outputTokens = [usage.output_tokens, usage.completion_tokens].find(Number.isFinite) ?? 0;
+  const responseUsage = {
+    input_tokens: inputTokens,
+    output_tokens: outputTokens,
+    total_tokens: Number.isFinite(usage.total_tokens) ? usage.total_tokens : inputTokens + outputTokens
+  };
+  const cachedTokens = [usage.input_tokens_details?.cached_tokens, usage.prompt_tokens_details?.cached_tokens].find(Number.isFinite);
+  const reasoningTokens = [usage.output_tokens_details?.reasoning_tokens, usage.completion_tokens_details?.reasoning_tokens].find(Number.isFinite);
+  if (Number.isFinite(cachedTokens)) responseUsage.input_tokens_details = { cached_tokens: cachedTokens };
+  if (Number.isFinite(reasoningTokens)) responseUsage.output_tokens_details = { reasoning_tokens: reasoningTokens };
+
+  return responseUsage;
+}
+
 export function openaiToOpenAIResponsesResponse(chunk, state) {
   if (!chunk) {
     return flushEvents(state);
   }
-  
+
+  // Capture upstream usage BEFORE the choices guard below: the last OpenAI chunk
+  // may carry usage together with an empty choices array, and it must not be dropped.
+  if (chunk.usage) {
+    state.responsesUsage = toResponsesUsage(chunk.usage);
+  }
+
   if (!chunk.choices?.length) return [];
-  
+
   const events = [];
   const nextSeq = () => ++state.seq;
   
@@ -112,7 +146,19 @@ export function openaiToOpenAIResponsesResponse(chunk, state) {
     for (const i in state.msgItemAdded) closeMessage(state, emit, i);
     closeReasoning(state, emit);
     for (const i in state.funcCallIds) closeToolCall(state, emit, i);
-    sendCompleted(state, emit);
+    // Upstreams report usage either on the finish chunk itself or on a trailing chunk
+    // whose `choices` array is empty (OpenAI does the latter). Emitting
+    // response.completed here would freeze the payload before that trailing chunk is
+    // parsed, so when usage is not known yet we leave completion to flushEvents(),
+    // which runs once the upstream stream ends and by then has seen every chunk.
+    //
+    // That only holds on the direct openai:openai-responses route. When this converter
+    // runs as the second hop of a pivot (Claude/Gemini/Kiro upstream), translateResponse()
+    // drops the terminal null chunk before reaching us — the first hop returns null for
+    // it, leaving nothing to iterate — so flushEvents() is never called and deferring
+    // would swallow the terminal event entirely. Keep the old behaviour there.
+    const flushReachesUs = state.targetFormat === FORMATS.OPENAI;
+    if (state.responsesUsage || !flushReachesUs) sendCompleted(state, emit);
   }
 
   return events;
@@ -376,7 +422,8 @@ function sendCompleted(state, emit) {
         created_at: state.created,
         status: "completed",
         background: false,
-        error: null
+        error: null,
+        ...(state.responsesUsage ? { usage: state.responsesUsage } : {})
       }
     });
   }

open-sse/utils/stream.js

@@ -60,7 +60,13 @@ export function createSSEStream(options = {}) {
   const decoder = new TextDecoder("utf-8", { fatal: false });
 
   const state = mode === STREAM_MODE.TRANSLATE
-    ? { ...initState(sourceFormat), provider, toolNameMap, customToolNames: new Set(customToolNames || []), model, sessionId: credentials?._clientSessionId || null }
+    ? { ...initState(sourceFormat), provider, toolNameMap, customToolNames: new Set(customToolNames || []), model, sessionId: credentials?._clientSessionId || null,
+        // Which upstream format this stream came from. A response translator can be
+        // reached either directly (target === its registered source) or as the second
+        // hop of a pivot, and on the terminal null chunk the pivot drops it — so a
+        // translator that defers closing events until flush needs to know which case
+        // it is in. Absent/undefined means "unknown", i.e. do not defer.
+        targetFormat }
     : null;
 
   let totalContentLength = 0;