diff --git a/src/features/agent-attachments/main/index.ts b/src/features/agent-attachments/main/index.ts
index f2f414c1..5a5dd085 100644
--- a/src/features/agent-attachments/main/index.ts
+++ b/src/features/agent-attachments/main/index.ts
@@ -1,3 +1,4 @@
 export * from './infrastructure/attachmentArtifactStore';
 export * from './providers/claudeAttachmentAdapter';
 export * from './providers/codexNativeAttachmentAdapter';
+export * from './providers/opencodeAttachmentAdapter';
diff --git a/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.test.ts b/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.test.ts
new file mode 100644
index 00000000..2749d3bc
--- /dev/null
+++ b/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.test.ts
@@ -0,0 +1,105 @@
+import {
+  buildOpenCodeAttachmentDeliveryParts,
+  redactOpenCodeFilePartsForDiagnostics,
+} from './opencodeAttachmentAdapter';
+
+import type { AttachmentPayload } from '@shared/types';
+
+function attachment(overrides: Partial<AttachmentPayload> = {}): AttachmentPayload {
+  return {
+    id: 'att_1',
+    filename: 'red.png',
+    mimeType: 'image/png',
+    size: 3,
+    data: Buffer.from([1, 2, 3]).toString('base64'),
+    ...overrides,
+  };
+}
+
+describe('OpenCode attachment adapter', () => {
+  it('keeps text-only messages on the legacy text path', () => {
+    expect(
+      buildOpenCodeAttachmentDeliveryParts({
+        text: 'hello',
+        model: 'openrouter/moonshotai/kimi-k2.6',
+      })
+    ).toEqual({
+      kind: 'legacy_text',
+      text: 'hello',
+      fileParts: [],
+      diagnostics: [],
+    });
+  });
+
+  it('serializes verified OpenCode vision models as file parts', () => {
+    const result = buildOpenCodeAttachmentDeliveryParts({
+      text: 'What color?',
+      model: 'openrouter/moonshotai/kimi-k2.6',
+      attachments: [attachment()],
+    });
+
+    expect(result.kind).toBe('text_with_file_parts');
+    expect(result.fileParts).toEqual([
+      {
+        type: 'file',
+        mime: 'image/png',
+        url: `data:image/png;base64,${attachment().data}`,
+        filename: 'red.png',
+      },
+    ]);
+    expect(result.diagnostics.join('\n')).not.toContain(attachment().data);
+  });
+
+  it('allows verified GLM 4.5V image delivery', () => {
+    expect(() =>
+      buildOpenCodeAttachmentDeliveryParts({
+        text: 'What color?',
+        model: 'openrouter/z-ai/glm-4.5v',
+        attachments: [attachment()],
+      })
+    ).not.toThrow();
+  });
+
+  it('blocks known non-vision OpenCode models before runtime send', () => {
+    expect(() =>
+      buildOpenCodeAttachmentDeliveryParts({
+        text: 'What color?',
+        model: 'openrouter/z-ai/glm-5.1',
+        attachments: [attachment()],
+      })
+    ).toThrow(/not verified for image attachments/);
+  });
+
+  it('blocks unknown OpenCode model image delivery by default', () => {
+    expect(() =>
+      buildOpenCodeAttachmentDeliveryParts({
+        text: 'What color?',
+        model: 'openrouter/example/new-model',
+        attachments: [attachment()],
+      })
+    ).toThrow(/unknown image support/);
+  });
+
+  it('rejects non-image attachments before provider send', () => {
+    expect(() =>
+      buildOpenCodeAttachmentDeliveryParts({
+        text: 'Read PDF',
+        model: 'openrouter/moonshotai/kimi-k2.6',
+        attachments: [attachment({ filename: 'a.pdf', mimeType: 'application/pdf' })],
+      })
+    ).toThrow(/OpenCode currently supports image attachments only/);
+  });
+
+  it('redacts data URLs from diagnostics', () => {
+    const redacted = redactOpenCodeFilePartsForDiagnostics([
+      {
+        type: 'file',
+        mime: 'image/png',
+        url: `data:image/png;base64,${attachment().data}`,
+        filename: 'red.png',
+      },
+    ]);
+
+    expect(redacted[0]!.url).toBe('[redacted data URL: image/png]');
+  });
+});
diff --git a/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.ts b/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.ts
new file mode 100644
index 00000000..d544f09a
--- /dev/null
+++ b/src/features/agent-attachments/main/providers/opencodeAttachmentAdapter.ts
@@ -0,0 +1,122 @@
+import {
+  AgentAttachmentError,
+  resolveAgentAttachmentCapability,
+} from '@features/agent-attachments/core/domain';
+
+import type { AttachmentPayload } from '@shared/types';
+
+export type OpenCodeFilePartMimeType = 'image/png' | 'image/jpeg' | 'image/webp';
+
+export interface OpenCodeFilePart {
+  type: 'file';
+  mime: OpenCodeFilePartMimeType;
+  url: string;
+  filename: string;
+}
+
+export interface OpenCodeAttachmentDeliveryParts {
+  kind: 'legacy_text' | 'text_with_file_parts';
+  text: string;
+  fileParts: OpenCodeFilePart[];
+  diagnostics: string[];
+}
+
+export interface BuildOpenCodeAttachmentDeliveryPartsInput {
+  text: string;
+  model: string;
+  attachments?: AttachmentPayload[];
+}
+
+function assertOpenCodeImageMimeType(
+  mimeType: string
+): asserts mimeType is OpenCodeFilePartMimeType {
+  if (mimeType === 'image/png' || mimeType === 'image/jpeg' || mimeType === 'image/webp') {
+    return;
+  }
+
+  throw new AgentAttachmentError(
+    'attachment_type_unsupported',
+    `OpenCode currently supports image attachments only; unsupported MIME: ${mimeType}`,
+    { providerId: 'opencode', retryable: false }
+  );
+}
+
+function assertOpenCodeVisionCapability(model: string): void {
+  const capability = resolveAgentAttachmentCapability({
+    providerId: 'opencode',
+    model,
+  });
+  if (capability.supportsImages) {
+    return;
+  }
+
+  const code =
+    capability.reason === 'known_non_vision_model' || capability.reason === 'unknown_model'
+      ? 'attachment_model_unsupported'
+      : 'attachment_type_unsupported';
+  throw new AgentAttachmentError(code, capability.displayText, {
+    providerId: 'opencode',
+    model,
+    retryable: false,
+    safeDetails: {
+      reason: capability.reason,
+    },
+  });
+}
+
+function formatBytes(bytes: number): string {
+  if (!Number.isFinite(bytes) || bytes <= 0) return '0 B';
+  if (bytes < 1024) return `${bytes} B`;
+  const kib = bytes / 1024;
+  if (kib < 1024) return `${kib.toFixed(1)} KB`;
+  return `${(kib / 1024).toFixed(1)} MB`;
+}
+
+export function buildOpenCodeAttachmentDeliveryParts(
+  input: BuildOpenCodeAttachmentDeliveryPartsInput
+): OpenCodeAttachmentDeliveryParts {
+  const attachments = input.attachments ?? [];
+  if (attachments.length === 0) {
+    return {
+      kind: 'legacy_text',
+      text: input.text,
+      fileParts: [],
+      diagnostics: [],
+    };
+  }
+
+  assertOpenCodeVisionCapability(input.model);
+
+  const fileParts: OpenCodeFilePart[] = [];
+  const diagnostics: string[] = [];
+  for (const attachment of attachments) {
+    assertOpenCodeImageMimeType(attachment.mimeType);
+    fileParts.push({
+      type: 'file',
+      mime: attachment.mimeType,
+      url: `data:${attachment.mimeType};base64,${attachment.data}`,
+      filename: attachment.filename,
+    });
+    diagnostics.push(
+      `prepared OpenCode image file part ${attachment.filename} (${attachment.mimeType}, ${formatBytes(
+        attachment.size
+      )}) for ${input.model}`
+    );
+  }
+
+  return {
+    kind: 'text_with_file_parts',
+    text: input.text,
+    fileParts,
+    diagnostics,
+  };
+}
+
+export function redactOpenCodeFilePartsForDiagnostics(
+  parts: OpenCodeFilePart[]
+): OpenCodeFilePart[] {
+  return parts.map((part) => ({
+    ...part,
+    url: `[redacted data URL: ${part.mime}]`,
+  }));
+}