diff --git a/src/lib/components/chat/Messages/OutputEditView.svelte b/src/lib/components/chat/Messages/OutputEditView.svelte
index 0fc07373ac..f3a2e4d3f2 100644
--- a/src/lib/components/chat/Messages/OutputEditView.svelte
+++ b/src/lib/components/chat/Messages/OutputEditView.svelte
@@ -138,7 +138,7 @@
 
 	function getMessageText(item: any): string {
 		return (item.content ?? [])
-			.filter((p: any) => p.type === 'output_text' || 'text' in p)
+			.filter((p: any) => p && (p.type === 'output_text' || 'text' in p))
 			.map((p: any) => p.text ?? '')
 			.join('\n');
 	}
@@ -146,7 +146,9 @@
 	function updateMessageText(idx: number, text: string) {
 		const next = [...output];
 		const item = { ...next[idx] };
-		const parts = (item.content ?? []).filter((p: any) => p.type === 'output_text' || 'text' in p);
+		const parts = (item.content ?? []).filter(
+			(p: any) => p && (p.type === 'output_text' || 'text' in p)
+		);
 		item.content = [{ ...(parts[0] ?? { type: 'output_text' }), text }];
 		next[idx] = item;
 		output = next;
@@ -155,7 +157,7 @@
 
 	function getReasoningText(item: any): string {
 		return (item.summary ?? item.content ?? [])
-			.filter((p: any) => 'text' in p)
+			.filter((p: any) => p && 'text' in p)
 			.map((p: any) => p.text ?? '')
 			.join('');
 	}
diff --git a/src/lib/components/chat/Messages/structuredOutput.ts b/src/lib/components/chat/Messages/structuredOutput.ts
index d4ebe0493e..612aca3dbe 100644
--- a/src/lib/components/chat/Messages/structuredOutput.ts
+++ b/src/lib/components/chat/Messages/structuredOutput.ts
@@ -374,7 +374,11 @@ export function buildOutputDisplayItems(output: OutputItem[] = []): OutputDispla
 	};
 
 	output.forEach((item, index) => {
-		if (item?.type === 'function_call_output') {
+		if (!item) {
+			return;
+		}
+
+		if (item.type === 'function_call_output') {
 			const inlineFile = getInlineFileFromToolOutput(toolCallByCallId[item.call_id ?? ''], item);
 			if (inlineFile) {
 				flushDetails();
@@ -388,14 +392,14 @@ export function buildOutputDisplayItems(output: OutputItem[] = []): OutputDispla
 		}
 
 		if (
-			item?.type === 'function_call' &&
+			item.type === 'function_call' &&
 			item.name === 'ask_user' &&
 			(item.status === 'pending' || item.status === 'in_progress')
 		) {
 			return;
 		}
 
-		if (item?.type && GROUPABLE_OUTPUT_TYPES.has(item.type)) {
+		if (item.type && GROUPABLE_OUTPUT_TYPES.has(item.type)) {
 			const token = buildDetailToken(item, index === output.length - 1, toolOutputByCallId);
 			if (token) {
 				currentDetailTokens.push(token);
@@ -403,7 +407,7 @@ export function buildOutputDisplayItems(output: OutputItem[] = []): OutputDispla
 			return;
 		}
 
-		if (item?.type === 'message') {
+		if (item.type === 'message') {
 			const text = getMessageText(item);
 			if (text.trim()) {
 				flushDetails();
@@ -481,6 +485,17 @@ function ensurePart(parts: OutputContentPart[], index: number, fallback?: Output
 	return parts[index];
 }
 
+function setPart(
+	parts: OutputContentPart[],
+	index: number,
+	part: OutputContentPart,
+	fallback?: OutputContentPart
+): void {
+	// Assigning past the end leaves a hole that later spreads turn into undefined parts.
+	ensurePart(parts, index, fallback);
+	parts[index] = part;
+}
+
 function findOutputItemIndex(output: OutputItem[], item: OutputItem): number {
 	return output.findIndex(
 		(existing) =>
@@ -529,7 +544,7 @@ export function applyResponseStreamEvent(
 		} else if (outputIndex < nextOutput.length) {
 			nextOutput.splice(outputIndex, 0, item);
 		} else {
-			nextOutput[outputIndex] = item;
+			nextOutput.push(item);
 		}
 		return nextOutput;
 	}
@@ -540,7 +555,13 @@ export function applyResponseStreamEvent(
 		}
 		const item = { ...event.item };
 		const existingIndex = findOutputItemIndex(nextOutput, item);
-		nextOutput[existingIndex >= 0 ? existingIndex : outputIndex] = item;
+		if (existingIndex >= 0) {
+			nextOutput[existingIndex] = item;
+		} else if (outputIndex < nextOutput.length) {
+			nextOutput[outputIndex] = item;
+		} else {
+			nextOutput.push(item);
+		}
 		return nextOutput;
 	}
 
@@ -565,7 +586,7 @@ export function applyResponseStreamEvent(
 			return nextOutput;
 		}
 		item.content = [...(item.content ?? [])];
-		item.content[event.content_index ?? item.content.length] = { ...event.part };
+		setPart(item.content, event.content_index ?? item.content.length, { ...event.part });
 		return nextOutput;
 	}
 
@@ -574,7 +595,8 @@ export function applyResponseStreamEvent(
 			return nextOutput;
 		}
 		item.summary = [...(item.summary ?? [])];
-		item.summary[event.summary_index ?? item.summary.length] = { ...event.part };
+		const summaryIndex = event.summary_index ?? item.summary.length;
+		setPart(item.summary, summaryIndex, { ...event.part }, { type: 'summary_text', text: '' });
 		return nextOutput;
 	}
 
@@ -604,7 +626,8 @@ export function applyResponseStreamEvent(
 		const typeName = eventType.split('.')[1];
 		if (typeName === 'content_part' && event.part) {
 			item.content = [...(item.content ?? [])];
-			item.content[event.content_index ?? Math.max(item.content.length - 1, 0)] = { ...event.part };
+			const contentIndex = event.content_index ?? Math.max(item.content.length - 1, 0);
+			setPart(item.content, contentIndex, { ...event.part });
 		} else if (typeName === 'function_call_arguments' && event.arguments !== undefined) {
 			item.arguments = event.arguments;
 		} else if (
@@ -636,7 +659,7 @@ export function replaceOutputMessageText(
 		}
 
 		const partIndex = item.content.findIndex(
-			(part) => typeof part.text === 'string' && part.text.includes(oldContent)
+			(part) => typeof part?.text === 'string' && part.text.includes(oldContent)
 		);
 		if (partIndex === -1) {
 			return item;