fix(stream): skip [DONE] terminator for Claude SSE clients (#2190)

Integrated into release/v3.8.0 after syncing the contributor branch and validating tests/unit/stream-utils.test.ts locally.

open-sse/utils/stream.ts

@@ -550,6 +550,23 @@ export function createSSEStream(options: StreamOptions = {}) {
       ? clientResponseFormat === FORMATS.OPENAI_RESPONSES
       : sourceFormat === FORMATS.OPENAI_RESPONSES) === true;
 
+  // Clients whose SSE protocol terminates naturally on the last
+  // provider-shape event (not on a `data: [DONE]` line). Emitting
+  // `[DONE]` to these clients produces a parser error in the SDK and
+  // breaks follow-up turns (Capy/Anthropic SDK: text gets stuck in the
+  // "Thought" area; subsequent /v1/messages calls retry into a corrupt
+  // state). Skip the `[DONE]` for these formats.
+  const clientExpectsClaudeStream =
+    (mode === STREAM_MODE.PASSTHROUGH
+      ? clientResponseFormat === FORMATS.CLAUDE
+      : sourceFormat === FORMATS.CLAUDE) === true;
+
+  // Single source of truth for the [DONE] decision, used at both emission
+  // sites below. Only OpenAI Chat Completions clients expect [DONE];
+  // Responses API and Anthropic SSE terminate on their own protocol events
+  // (response.completed / message_stop respectively).
+  const shouldEmitDoneTerminator = !clientExpectsResponsesStream && !clientExpectsClaudeStream;
+
   let buffer = "";
   let usage: UsageTokenRecord | null = null;
   /** Passthrough (OpenAI CC shape): saw tool_calls in stream before finish_reason */
@@ -1604,7 +1621,7 @@ export function createSSEStream(options: StreamOptions = {}) {
             if (!doneSent) {
               await emitFinalSseMetadata(controller, usage);
               doneSent = true;
-              if (!clientExpectsResponsesStream) {
+              if (shouldEmitDoneTerminator) {
                 clientPayloadCollector.push({ done: true });
                 const doneOutput = "data: [DONE]\n\n";
                 reqLogger?.appendConvertedChunk?.(doneOutput);
@@ -1800,7 +1817,7 @@ export function createSSEStream(options: StreamOptions = {}) {
           if (!doneSent) {
             await emitFinalSseMetadata(controller, state?.usage as Record<string, unknown> | null);
             doneSent = true;
-            if (!clientExpectsResponsesStream) {
+            if (shouldEmitDoneTerminator) {
               clientPayloadCollector.push({ done: true });
               const doneOutput = "data: [DONE]\n\n";
               reqLogger?.appendConvertedChunk?.(doneOutput);

tests/unit/stream-utils.test.ts

@@ -561,6 +561,62 @@ test("createSSEStream passthrough injects a synthetic Claude text block for empt
   );
 });
 
+test("createSSEStream passthrough does not emit [DONE] for Claude SSE clients", async () => {
+  const text = await readTransformed(
+    [
+      `event: message_start\ndata: ${JSON.stringify({
+        type: "message_start",
+        message: {
+          id: "msg_claude_done_gate",
+          type: "message",
+          role: "assistant",
+          model: "claude-sonnet-4",
+          content: [],
+          stop_reason: null,
+          stop_sequence: null,
+          usage: { input_tokens: 3, output_tokens: 0 },
+        },
+      })}\n\n`,
+      `event: content_block_start\ndata: ${JSON.stringify({
+        type: "content_block_start",
+        index: 0,
+        content_block: { type: "text", text: "" },
+      })}\n\n`,
+      `event: content_block_delta\ndata: ${JSON.stringify({
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "text_delta", text: "Claude client stream" },
+      })}\n\n`,
+      `event: content_block_stop\ndata: ${JSON.stringify({
+        type: "content_block_stop",
+        index: 0,
+      })}\n\n`,
+      `event: message_delta\ndata: ${JSON.stringify({
+        type: "message_delta",
+        delta: { stop_reason: "end_turn", stop_sequence: null },
+        usage: { output_tokens: 3 },
+      })}\n\n`,
+      `event: message_stop\ndata: ${JSON.stringify({
+        type: "message_stop",
+      })}\n\n`,
+    ],
+    {
+      mode: "passthrough",
+      sourceFormat: FORMATS.CLAUDE,
+      clientResponseFormat: FORMATS.CLAUDE,
+      provider: "claude",
+      model: "claude-sonnet-4",
+      body: {
+        messages: [{ role: "user", content: "hello" }],
+      },
+    }
+  );
+
+  assert.match(text, /event: message_stop/);
+  assert.match(text, /Claude client stream/);
+  assert.doesNotMatch(text, /\[DONE\]/);
+});
+
 test("createSSEStream translate mode injects a synthetic Claude text block when OpenAI finishes empty", async () => {
   let onCompletePayload = null;
   const text = await readTransformed(