Fix type errors in stream() by narrowing finalResponse after guard

The !gotDone check already throws, so finalResponse is guaranteed
defined after it. Added explicit !finalResponse to the guard and
removed optional chaining on all subsequent accesses.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tylergibbs1

src/core/run.ts

@@ -717,14 +717,14 @@ async function* streamInternal<TContext, TOutput = undefined>(
 				}
 			}
 
-			if (!gotDone) {
+			if (!gotDone || !finalResponse) {
 				throw new StratusError("Stream ended without a done event");
 			}
 
 			// Fire onLlmEnd hooks
 			const llmEndInfo = {
-				content: finalResponse?.content,
-				toolCallCount: finalResponse?.toolCalls.length,
+				content: finalResponse.content,
+				toolCallCount: finalResponse.toolCalls.length,
 			};
 			if (currentAgent.hooks.onLlmEnd) {
 				await currentAgent.hooks.onLlmEnd({
@@ -742,12 +742,12 @@ async function* streamInternal<TContext, TOutput = undefined>(
 			}
 
 			checkAborted(signal);
-			lastFinishReason = finalResponse?.finishReason;
-			if (finalResponse?.responseId) lastResponseId = finalResponse?.responseId;
-			ctx.addUsage(finalResponse?.usage);
+			lastFinishReason = finalResponse.finishReason;
+			if (finalResponse.responseId) lastResponseId = finalResponse.responseId;
+			ctx.addUsage(finalResponse.usage);
 			ctx.numTurns++;
 
-			applyTurnCost(ctx, finalResponse?.usage, costEstimator);
+			applyTurnCost(ctx, finalResponse.usage, costEstimator);
 
 			// Check budget after each model call
 			try {
@@ -765,16 +765,16 @@ async function* streamInternal<TContext, TOutput = undefined>(
 
 			const assistantMsg: AssistantMessage = {
 				role: "assistant",
-				content: finalResponse?.content,
-				...(finalResponse?.toolCalls.length > 0 ? { tool_calls: finalResponse?.toolCalls } : {}),
+				content: finalResponse.content ?? null,
+				...(finalResponse.toolCalls.length > 0 ? { tool_calls: finalResponse.toolCalls } : {}),
 			};
 			messages.push(assistantMsg);
 
-			if (finalResponse?.toolCalls.length === 0) {
+			if (finalResponse.toolCalls.length === 0) {
 				if (runHooks?.onAgentEnd) {
 					await runHooks.onAgentEnd({
 						agent: currentAgent,
-						output: finalResponse?.content ?? "",
+						output: finalResponse.content ?? "",
 						context: ctx.context,
 					});
 				}
@@ -795,7 +795,7 @@ async function* streamInternal<TContext, TOutput = undefined>(
 			const { toolMessages, handoffAgent } = await executeToolCallsWithHandoffs(
 				currentAgent,
 				ctx,
-				finalResponse?.toolCalls,
+				finalResponse.toolCalls,
 				trace,
 				signal,
 				toolErrorFmt,
@@ -806,7 +806,7 @@ async function* streamInternal<TContext, TOutput = undefined>(
 			messages.push(...toolMessages);
 
 			// Check toolUseBehavior
-			if (await shouldStopAfterToolCalls(currentAgent, finalResponse?.toolCalls, toolMessages)) {
+			if (await shouldStopAfterToolCalls(currentAgent, finalResponse.toolCalls, toolMessages)) {
 				const toolOutput = toolMessages.map((m) => m.content).join("\n");
 				resolveResult(
 					new RunResult<TOutput>({