diff --git a/packages/core/src/core/compression/utils.test.ts b/packages/core/src/core/compression/utils.test.ts
index 4fd536cc58..980d1c7b6a 100644
--- a/packages/core/src/core/compression/utils.test.ts
+++ b/packages/core/src/core/compression/utils.test.ts
@@ -14,7 +14,7 @@
  */
 
 import { describe, it, expect } from 'vitest';
-import type { IContent } from '../../services/history/IContent.js';
+import type { IContent, MediaBlock } from '../../services/history/IContent.js';
 import {
   adjustForToolCallBoundary,
   findForwardValidSplitPoint,
@@ -65,6 +65,38 @@ function toolResponseMsg(
     ],
   };
 }
+function mediaBlock(
+  mimeType: string,
+  filename?: string,
+  data = 'base64data',
+  caption?: string,
+): MediaBlock {
+  return {
+    type: 'media',
+    mimeType,
+    filename,
+    data,
+    encoding: 'base64',
+    caption,
+  };
+}
+
+function humanMsgWithMedia(
+  text: string,
+  ...mediaBlocks: MediaBlock[]
+): IContent {
+  return {
+    speaker: 'human',
+    blocks: [{ type: 'text', text }, ...mediaBlocks],
+  };
+}
+
+function humanMsgOnlyMedia(...mediaBlocks: MediaBlock[]): IContent {
+  return {
+    speaker: 'human',
+    blocks: mediaBlocks,
+  };
+}
 
 // ---------------------------------------------------------------------------
 // adjustForToolCallBoundary
@@ -620,4 +652,185 @@ describe('sanitizeHistoryForCompression', () => {
     const result = sanitizeHistoryForCompression([msg]);
     expect(result[0].speaker).toBe('human');
   });
+
+  // Media block tests (Issue #1875)
+  it('converts media blocks to text placeholders with filename', () => {
+    const history = [
+      humanMsgOnlyMedia(mediaBlock('application/pdf', 'document.pdf')),
+    ];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result).toHaveLength(1);
+    expect(result[0].speaker).toBe('human');
+    expect(result[0].blocks).toHaveLength(1);
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached PDF: document.pdf]',
+    );
+  });
+
+  it('converts media blocks to text placeholders using mimeType when no filename', () => {
+    const history = [humanMsgOnlyMedia(mediaBlock('image/png'))];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result).toHaveLength(1);
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached image: image/png]',
+    );
+  });
+
+  it('converts media blocks with empty filename to text placeholders using mimeType', () => {
+    const history = [humanMsgOnlyMedia(mediaBlock('image/png', ''))];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result).toHaveLength(1);
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached image: image/png]',
+    );
+  });
+
+  it('handles different media categories (image, pdf, audio, video, unknown)', () => {
+    const imageBlock = mediaBlock('image/jpeg', 'photo.jpg');
+    const pdfBlock = mediaBlock('application/pdf', 'report.pdf');
+    const audioBlock = mediaBlock('audio/mp3', 'song.mp3');
+    const videoBlock = mediaBlock('video/mp4', 'movie.mp4');
+    const unknownBlock = mediaBlock('application/octet-stream', 'data.bin');
+
+    const history = [
+      humanMsgOnlyMedia(
+        imageBlock,
+        pdfBlock,
+        audioBlock,
+        videoBlock,
+        unknownBlock,
+      ),
+    ];
+    const result = sanitizeHistoryForCompression(history);
+    const texts = result[0].blocks.map((b) => (b as { text: string }).text);
+
+    expect(texts).toContain('[Attached image: photo.jpg]');
+    expect(texts).toContain('[Attached PDF: report.pdf]');
+    expect(texts).toContain('[Attached audio: song.mp3]');
+    expect(texts).toContain('[Attached video: movie.mp4]');
+    expect(texts).toContain('[Attached unknown: data.bin]');
+  });
+
+  it('handles mixed content with text + media blocks in same message', () => {
+    const history = [
+      humanMsgWithMedia(
+        'Please analyze this document',
+        mediaBlock('application/pdf', 'report.pdf'),
+      ),
+    ];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result[0].blocks).toHaveLength(2);
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      'Please analyze this document',
+    );
+    expect(result[0].blocks[1].type).toBe('text');
+    expect((result[0].blocks[1] as { text: string }).text).toBe(
+      '[Attached PDF: report.pdf]',
+    );
+  });
+
+  it('converts messages with only media blocks properly', () => {
+    const history = [
+      humanMsgOnlyMedia(mediaBlock('image/png', 'screenshot.png')),
+    ];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result).toHaveLength(1);
+    expect(result[0].speaker).toBe('human');
+    expect(result[0].blocks).toHaveLength(1);
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached image: screenshot.png]',
+    );
+  });
+
+  it('does not change speaker for media block messages (unlike tool messages)', () => {
+    // Media blocks keep original speaker, only tool messages get re-tagged
+    const msg: IContent = {
+      speaker: 'ai',
+      blocks: [mediaBlock('image/jpeg', 'photo.jpg')],
+    };
+    const result = sanitizeHistoryForCompression([msg]);
+    expect(result[0].speaker).toBe('ai'); // unchanged
+    expect(result[0].blocks[0].type).toBe('text');
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached image: photo.jpg]',
+    );
+  });
+
+  it('handles mixed tool and media blocks in same message', () => {
+    const msg: IContent = {
+      speaker: 'ai',
+      blocks: [
+        { type: 'text', text: 'Analyzing file and document' },
+        mediaBlock('application/pdf', 'document.pdf'),
+        {
+          type: 'tool_call',
+          id: 'c1',
+          name: 'read_file',
+          parameters: { path: '/tmp/test' },
+        },
+      ],
+    };
+    const result = sanitizeHistoryForCompression([msg]);
+    expect(result[0].blocks).toHaveLength(3);
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      'Analyzing file and document',
+    );
+    expect((result[0].blocks[1] as { text: string }).text).toBe(
+      '[Attached PDF: document.pdf]',
+    );
+    expect((result[0].blocks[2] as { text: string }).text).toContain(
+      '[Tool Call: read_file]',
+    );
+  });
+
+  it('prefers media caption over filename in compression placeholders', () => {
+    const history = [
+      humanMsgOnlyMedia(
+        mediaBlock(
+          'image/png',
+          'diagram.png',
+          'base64data',
+          'Architecture diagram',
+        ),
+      ),
+    ];
+    const result = sanitizeHistoryForCompression(history);
+    expect(result).toHaveLength(1);
+    expect(result[0].blocks).toHaveLength(1);
+    expect((result[0].blocks[0] as { text: string }).text).toBe(
+      '[Attached image: Architecture diagram]',
+    );
+  });
+
+  it('re-tags tool speaker to human and placeholderizes media blocks in tool messages', () => {
+    const msg: IContent = {
+      speaker: 'tool',
+      blocks: [
+        {
+          type: 'tool_response',
+          callId: 'c1',
+          toolName: 'read_file',
+          result: 'file contents here',
+        },
+        mediaBlock('image/png', 'screenshot.png'),
+      ],
+    };
+    const result = sanitizeHistoryForCompression([msg]);
+    expect(result[0].speaker).toBe('human');
+    expect(result[0].blocks).toHaveLength(2);
+    expect((result[0].blocks[0] as { text: string }).text).toContain(
+      '[Tool Result: read_file]',
+    );
+    expect((result[0].blocks[0] as { text: string }).text).toContain(
+      'file contents here',
+    );
+    expect((result[0].blocks[1] as { text: string }).text).toBe(
+      '[Attached image: screenshot.png]',
+    );
+  });
 });
diff --git a/packages/core/src/core/compression/utils.ts b/packages/core/src/core/compression/utils.ts
index 1a7ea98918..704785b2a4 100644
--- a/packages/core/src/core/compression/utils.ts
+++ b/packages/core/src/core/compression/utils.ts
@@ -18,9 +18,11 @@
 import type {
   ContentBlock,
   IContent,
+  MediaBlock,
   TextBlock,
 } from '../../services/history/IContent.js';
 import type { IProvider } from '../../providers/IProvider.js';
+import { classifyMediaBlock } from '../../providers/utils/mediaUtils.js';
 
 /**
  * Aggregate text from content blocks, handling spacing between text and
@@ -337,15 +339,40 @@ export async function runVerificationPass(
 }
 
 /**
- * Convert tool_call and tool_response blocks to plain text representations
- * so the compression request doesn't trip Anthropic's strict tool_use /
- * tool_result pairing validation.  Orphaned tool blocks (from interrupted
- * loops or the loop-detector halting mid-tool-call) would otherwise cause
- * 400 errors when sent to the LLM for summarisation.
+ * Convert a MediaBlock to a concise text placeholder for compression.
+ * This prevents provider-specific media types (like PDF "file" parts) from
+ * reaching the compression LLM call, which would cause 400 errors on providers
+ * that don't support certain media types.
+ *
+ * Format: [Attached <category>: <caption | filename | mimeType | unknown>]
+ * The identifier prefers caption for accessibility/context, then falls back to
+ * filename, mimeType, and finally "unknown".
+ */
+export function mediaBlockToCompressionPlaceholder(media: MediaBlock): string {
+  const category = classifyMediaBlock(media);
+  // Prefer caption first (for accessibility/context), then filename, then mimeType, then 'unknown'
+  const identifier =
+    media.caption?.trim() ||
+    media.filename?.trim() ||
+    media.mimeType ||
+    'unknown';
+  // Capitalize PDF label for display, keep other categories as-is
+  const label = category === 'pdf' ? 'PDF' : category;
+  return `[Attached ${label}: ${identifier}]`;
+}
+
+/**
+ * Convert tool_call, tool_response, and media blocks to plain text representations
+ * so the compression request doesn't trip provider-specific validation errors.
+ *
+ * - Tool blocks: Anthropic's strict tool_use / tool_result pairing validation
+ *   would reject orphaned tool blocks (from interrupted loops).
+ * - Media blocks: Providers like Kimi don't support certain media types (e.g.,
+ *   PDF "file" parts) and would return 400 errors.
  *
  * Messages whose speaker is 'tool' are re-tagged as 'human' since they
- * no longer carry structural tool_result blocks.  All other block types
- * (text, thinking, code, media) pass through unchanged.
+ * no longer carry structural tool_result blocks. Messages with media blocks
+ * keep their original speaker since media is not speaker-specific.
  */
 export function sanitizeHistoryForCompression(
   messages: readonly IContent[],
@@ -354,7 +381,8 @@ export function sanitizeHistoryForCompression(
     const hasToolBlocks = msg.blocks.some(
       (b) => b.type === 'tool_call' || b.type === 'tool_response',
     );
-    if (!hasToolBlocks && msg.speaker !== 'tool') {
+    const hasMediaBlocks = msg.blocks.some((b) => b.type === 'media');
+    if (!hasToolBlocks && !hasMediaBlocks && msg.speaker !== 'tool') {
       return msg;
     }
 
@@ -390,6 +418,10 @@ export function sanitizeHistoryForCompression(
           }
           return { type: 'text', text } as TextBlock;
         }
+        if (block.type === 'media') {
+          const text = mediaBlockToCompressionPlaceholder(block);
+          return { type: 'text', text } as TextBlock;
+        }
         return block;
       })
       .filter((b): b is ContentBlock => b !== null);