diff --git a/src/features/agent-attachments/core/domain/budgets.test.ts b/src/features/agent-attachments/core/domain/budgets.test.ts
index 24a42a38..0031d454 100644
--- a/src/features/agent-attachments/core/domain/budgets.test.ts
+++ b/src/features/agent-attachments/core/domain/budgets.test.ts
@@ -1,4 +1,9 @@
-import { allocateImageBudgets, planResizeDimensions, sortAttachmentsForDelivery } from './budgets';
+import {
+  allocateImageBudgets,
+  estimateAgentAttachmentSerializedPayloadBytes,
+  planResizeDimensions,
+  sortAttachmentsForDelivery,
+} from './budgets';
 
 describe('agent attachment budgets', () => {
   it('does not upscale small images', () => {
@@ -36,4 +41,19 @@ describe('agent attachment budgets', () => {
     ]);
     expect(sorted.map((item) => item.id)).toEqual(['a', 'b']);
   });
+
+  it('estimates serialized provider payload bytes including base64 overhead', () => {
+    const bytes = estimateAgentAttachmentSerializedPayloadBytes({
+      text: 'hello',
+      attachments: [
+        {
+          filename: 'red.png',
+          mimeType: 'image/png',
+          data: 'a'.repeat(128),
+        },
+      ],
+    });
+
+    expect(bytes).toBeGreaterThan(128);
+  });
 });
diff --git a/src/features/agent-attachments/core/domain/budgets.ts b/src/features/agent-attachments/core/domain/budgets.ts
index 3280a7a0..3a901c1d 100644
--- a/src/features/agent-attachments/core/domain/budgets.ts
+++ b/src/features/agent-attachments/core/domain/budgets.ts
@@ -9,6 +9,47 @@ export const DEFAULT_AGENT_IMAGE_OPTIMIZATION_BUDGET: ImageOptimizationBudget =
   jpegQualityAttempts: [0.86, 0.82, 0.78, 0.74, 0.72],
 };
 
+export const MAX_AGENT_ATTACHMENT_SERIALIZED_PAYLOAD_BYTES = 7_500_000;
+
+const utf8Encoder = new TextEncoder();
+
+export function estimateAgentAttachmentSerializedPayloadBytes(input: {
+  text?: string;
+  attachments: Array<{
+    mimeType: string;
+    data: string;
+    filename?: string;
+  }>;
+}): number {
+  const contentBlocks: unknown[] = [{ type: 'text', text: input.text ?? '' }];
+  for (const attachment of input.attachments) {
+    const isImage = attachment.mimeType.startsWith('image/');
+    contentBlocks.push({
+      type: isImage ? 'image' : 'document',
+      ...(isImage
+        ? {}
+        : {
+            title: attachment.filename ?? 'attachment',
+          }),
+      source: {
+        type: 'base64',
+        media_type: attachment.mimeType,
+        data: attachment.data,
+      },
+    });
+  }
+
+  return utf8Encoder.encode(
+    JSON.stringify({
+      type: 'user',
+      message: {
+        role: 'user',
+        content: contentBlocks,
+      },
+    })
+  ).byteLength;
+}
+
 export function calculatePixelCount(dimensions: ImageDimensions): number {
   return dimensions.width * dimensions.height;
 }
diff --git a/src/main/ipc/teams.ts b/src/main/ipc/teams.ts
index e5f8ff60..647d386a 100644
--- a/src/main/ipc/teams.ts
+++ b/src/main/ipc/teams.ts
@@ -5,6 +5,10 @@ import { getAppIconPath } from '@main/utils/appIcon';
 import { getAppDataPath, getTeamsBasePath } from '@main/utils/pathDecoder';
 import { safeSendToRenderer } from '@main/utils/safeWebContentsSend';
 import { stripMarkdown } from '@main/utils/textFormatting';
+import {
+  estimateAgentAttachmentSerializedPayloadBytes,
+  MAX_AGENT_ATTACHMENT_SERIALIZED_PAYLOAD_BYTES,
+} from '@features/agent-attachments/core/domain';
 import {
   TEAM_ADD_MEMBER,
   TEAM_ADD_TASK_COMMENT,
@@ -2492,6 +2496,30 @@ function validateAttachments(
   return { valid: true, value: result };
 }
 
+function formatAttachmentBytes(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+function validateAttachmentSerializedPayload(input: {
+  text: string;
+  attachments: AttachmentPayload[];
+}): { valid: true } | { valid: false; error: string } {
+  const estimatedBytes = estimateAgentAttachmentSerializedPayloadBytes(input);
+  if (estimatedBytes <= MAX_AGENT_ATTACHMENT_SERIALIZED_PAYLOAD_BYTES) {
+    return { valid: true };
+  }
+  return {
+    valid: false,
+    error: `Attachment payload is too large after optimization: ${formatAttachmentBytes(
+      estimatedBytes
+    )} serialized. Limit is ${formatAttachmentBytes(
+      MAX_AGENT_ATTACHMENT_SERIALIZED_PAYLOAD_BYTES
+    )}. Remove an image or use a smaller screenshot.`,
+  };
+}
+
 function buildMessageDeliveryText(
   baseText: string,
   opts: {
@@ -2725,6 +2753,13 @@ async function handleSendMessage(
       return { success: false, error: attResult.error };
     }
     validatedAttachments = attResult.value;
+    const serializedResult = validateAttachmentSerializedPayload({
+      text: payload.text!,
+      attachments: validatedAttachments,
+    });
+    if (!serializedResult.valid) {
+      return { success: false, error: serializedResult.error };
+    }
   }
 
   const tn = validatedTeamName.value!;