diff --git a/packages/cli/src/acp-integration/session/Session.test.ts b/packages/cli/src/acp-integration/session/Session.test.ts
index 9480d4ebabd..74f5a3ac6b6 100644
--- a/packages/cli/src/acp-integration/session/Session.test.ts
+++ b/packages/cli/src/acp-integration/session/Session.test.ts
@@ -22183,6 +22183,73 @@ describe('Session', () => {
         expect(mockChatRecordingService.recordUserMessage).toHaveBeenCalled();
       });
 
+      it.each([
+        ['skill', CommandKind.SKILL],
+        ['custom command', CommandKind.FILE],
+      ])(
+        'keeps attachments when a %s expands into a model prompt',
+        async (_, kind) => {
+          vi.mocked(
+            nonInteractiveCliCommands.handleSlashCommand,
+          ).mockResolvedValueOnce({
+            type: 'submit_prompt',
+            content: [{ text: 'Expanded skill prompt' }],
+            resolvedCommand: {
+              name: 'price-sheet',
+              kind,
+            },
+          });
+
+          await session.prompt({
+            sessionId: 'test-session-id',
+            prompt: [
+              { type: 'text', text: '/price-sheet update these prices' },
+              { type: 'image', mimeType: 'image/png', data: 'QUJD' },
+              {
+                type: 'resource',
+                resource: {
+                  uri: 'attachment:///notes.txt',
+                  mimeType: 'text/plain',
+                  text: 'hello',
+                },
+              },
+            ],
+          });
+
+          expect(firstSentMessage()).toEqual([
+            { text: '@attachment:///notes.txt' },
+            { inlineData: { mimeType: 'image/png', data: 'QUJD' } },
+            { text: 'File: attachment:///notes.txt\nhello' },
+            { text: 'Expanded skill prompt' },
+          ]);
+        },
+      );
+
+      it('does not forward attachments from built-in commands', async () => {
+        vi.mocked(
+          nonInteractiveCliCommands.handleSlashCommand,
+        ).mockResolvedValueOnce({
+          type: 'submit_prompt',
+          content: [{ text: 'Expanded built-in prompt' }],
+          resolvedCommand: {
+            name: 'remember',
+            kind: CommandKind.BUILT_IN,
+          },
+        });
+
+        await session.prompt({
+          sessionId: 'test-session-id',
+          prompt: [
+            { type: 'text', text: '/remember this' },
+            { type: 'image', mimeType: 'image/png', data: 'QUJD' },
+          ],
+        });
+
+        expect(firstSentMessage()).toEqual([
+          { text: 'Expanded built-in prompt' },
+        ]);
+      });
+
       it('preserves an expanded slash prompt cancelled before model send', async () => {
         const finishedSpy = vi
           .spyOn(core, 'logConversationFinishedEvent')
diff --git a/packages/cli/src/acp-integration/session/Session.ts b/packages/cli/src/acp-integration/session/Session.ts
index de4d07bf691..56773c97fe6 100644
--- a/packages/cli/src/acp-integration/session/Session.ts
+++ b/packages/cli/src/acp-integration/session/Session.ts
@@ -13510,14 +13510,24 @@ export class Session implements SessionContext {
       : undefined;
 
     switch (result.type) {
-      case 'submit_prompt':
-        // Command wants to submit a prompt to the model
-        // Convert PartListUnion to Part[]
+      case 'submit_prompt': {
+        const expandedPrompt = normalizePartList(result.content);
+        const attachmentBlocks =
+          result.resolvedCommand?.kind === CommandKind.BUILT_IN
+            ? []
+            : originalPrompt.filter((block) => block.type !== 'text');
+        const attachmentParts =
+          attachmentBlocks.length === 0
+            ? []
+            : await this.#resolvePrompt(attachmentBlocks, abortSignal, {
+                deferBridgeConversions: true,
+              });
         return this.#applyBridgeConversionsIfNeeded(
-          normalizePartList(result.content),
+          [...attachmentParts, ...expandedPrompt],
           abortSignal,
           onFullTurnModel,
         );
+      }
 
       case 'message': {
         if (result.messageType === 'error') {
diff --git a/packages/web-shell/client/daemon/session/actions.test.ts b/packages/web-shell/client/daemon/session/actions.test.ts
index ac0e436a1b4..b53fbc34f2b 100644
--- a/packages/web-shell/client/daemon/session/actions.test.ts
+++ b/packages/web-shell/client/daemon/session/actions.test.ts
@@ -2290,6 +2290,7 @@ describe('createDaemonSessionActions', () => {
       connection: {
         status: 'connected',
         workspaceCwd: '/workspace',
+        commands: [commandInfo('help', 'builtin-command')],
         capabilities: {
           v: 1,
           mode: 'http-bridge',
@@ -2316,6 +2317,319 @@ describe('createDaemonSessionActions', () => {
     );
   });
 
+  it('does not upload attachments for built-in command aliases', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [commandInfo('compress', 'builtin-command', ['summarize'])],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/summarize', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).not.toHaveBeenCalled();
+    expect(session.submitPrompt).toHaveBeenCalledWith({
+      prompt: [{ type: 'text', text: '/summarize' }],
+    });
+  });
+
+  it.each(['/Compress', '/SUMMARIZE'])(
+    'keeps attachments for unresolved wrong-case command %s',
+    async (text) => {
+      const session = createMockSession('session-a');
+      const { actions } = createActionsHarness({
+        session,
+        connection: {
+          status: 'connected',
+          workspaceCwd: '/workspace',
+          commands: [commandInfo('compress', 'builtin-command', ['summarize'])],
+          capabilities: {
+            v: 1,
+            mode: 'http-bridge',
+            features: ['session_attachments'],
+            modelServices: [],
+          },
+        },
+      });
+
+      await actions.submitPrompt(text, {
+        images: [{ data: 'AQID', mimeType: 'image/png' }],
+      });
+
+      expect(session.uploadAttachment).toHaveBeenCalledOnce();
+    },
+  );
+
+  it('prefers a primary command name over another command alias', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [
+          commandInfo('compress', 'builtin-command', ['summarize']),
+          commandInfo('summarize', 'skill-dir-command'),
+        ],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/summarize', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).toHaveBeenCalledOnce();
+  });
+
+  it('recognizes built-in commands with whitespace after the slash', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [commandInfo('compress', 'builtin-command')],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/ compress', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).not.toHaveBeenCalled();
+  });
+
+  it('does not upload an unknown slash command before command metadata loads', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [commandInfo('price-sheet', 'skill-dir-command')],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/remember this API shape', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).not.toHaveBeenCalled();
+  });
+
+  it('does not upload a slash command before command metadata loads', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/remember this API shape', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).not.toHaveBeenCalled();
+  });
+
+  it.each([
+    '// stack trace from prod',
+    '/* crash note */',
+    '/var/log/app.log shows the crash',
+    '/var\\log\\app.log shows the crash',
+  ])('uploads attachments for non-command slash input %s', async (text) => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt(text, {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).toHaveBeenCalledOnce();
+    expect(session.submitPrompt).toHaveBeenCalledWith({
+      prompt: [
+        { type: 'text', text },
+        {
+          type: 'image',
+          attachmentId: 'image.png',
+          mimeType: 'image/png',
+          size: 3,
+        },
+      ],
+    });
+  });
+
+  it('uploads attachments for unknown commands after metadata loads', async () => {
+    const session = createMockSession('session-a');
+    const { actions } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [commandInfo('compress', 'builtin-command')],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/unlisted update', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).toHaveBeenCalledOnce();
+  });
+
+  it('uploads attachments used by skill slash commands', async () => {
+    const session = createMockSession('session-a');
+    const { actions, store } = createActionsHarness({
+      session,
+      connection: {
+        status: 'connected',
+        workspaceCwd: '/workspace',
+        commands: [commandInfo('price-sheet', 'skill-dir-command')],
+        capabilities: {
+          v: 1,
+          mode: 'http-bridge',
+          features: ['session_attachments'],
+          modelServices: [],
+        },
+      },
+    });
+
+    await actions.submitPrompt('/price-sheet update these prices', {
+      images: [{ data: 'AQID', mimeType: 'image/png' }],
+    });
+
+    expect(session.uploadAttachment).toHaveBeenCalledWith(
+      expect.any(Blob),
+      'image.png',
+      'image/png',
+      undefined,
+    );
+    expect(session.submitPrompt).toHaveBeenCalledWith({
+      prompt: [
+        { type: 'text', text: '/price-sheet update these prices' },
+        {
+          type: 'image',
+          attachmentId: 'image.png',
+          mimeType: 'image/png',
+          size: 3,
+        },
+      ],
+    });
+    expect(store.appendLocalUserMessage).toHaveBeenCalledWith(
+      '/price-sheet update these prices',
+      [{ data: 'AQID', mimeType: 'image/png' }],
+      undefined,
+      [],
+    );
+  });
+
+  it.each([
+    ['skill-dir-command', true],
+    ['builtin-command', false],
+  ] as const)(
+    'classifies %s command attachments on the sendPrompt path',
+    async (source, shouldUpload) => {
+      const session = createMockSession('session-a');
+      const { actions } = createActionsHarness({
+        session,
+        connection: {
+          status: 'connected',
+          workspaceCwd: '/workspace',
+          commands: [commandInfo('price-sheet', source)],
+          capabilities: {
+            v: 1,
+            mode: 'http-bridge',
+            features: ['session_attachments'],
+            modelServices: [],
+          },
+        },
+      });
+
+      const prompt = actions.sendPrompt('/price-sheet update', {
+        images: [{ data: 'AQID', mimeType: 'image/png' }],
+      });
+      await vi.waitFor(() => expect(session.submitPrompt).toHaveBeenCalled());
+
+      expect(session.uploadAttachment).toHaveBeenCalledTimes(
+        shouldUpload ? 1 : 0,
+      );
+      expect(session.submitPrompt).toHaveBeenCalledWith(
+        {
+          prompt: shouldUpload
+            ? [
+                { type: 'text', text: '/price-sheet update' },
+                {
+                  type: 'image',
+                  attachmentId: 'image.png',
+                  mimeType: 'image/png',
+                  size: 3,
+                },
+              ]
+            : [{ type: 'text', text: '/price-sheet update' }],
+        },
+        expect.any(AbortSignal),
+      );
+
+      await actions.cancel();
+      await expect(prompt).resolves.toEqual({ stopReason: 'cancelled' });
+    },
+  );
+
   it('uploads text attachments and submits attachment references', async () => {
     const session = createMockSession('session-a');
     const { actions, store } = createActionsHarness({
@@ -3912,11 +4226,13 @@ function createDeferred<T>() {
   return { promise, resolve, reject };
 }
 
-function commandInfo(name: string) {
+function commandInfo(name: string, source?: string, altNames?: string[]) {
   const raw = commandRaw(name);
   return {
     name,
     description: '',
+    ...(source ? { source } : {}),
+    ...(altNames ? { altNames } : {}),
     raw,
   };
 }
diff --git a/packages/web-shell/client/daemon/session/actions.ts b/packages/web-shell/client/daemon/session/actions.ts
index 57f0bb404b6..04178abd012 100644
--- a/packages/web-shell/client/daemon/session/actions.ts
+++ b/packages/web-shell/client/daemon/session/actions.ts
@@ -416,6 +416,28 @@ export function createDaemonSessionActions({
     );
   }
 
+  function discardsSlashCommandAttachments(text: string): boolean {
+    const trimmed = text.trim();
+    if (!trimmed.startsWith('/')) return false;
+    if (trimmed.startsWith('//') || trimmed.startsWith('/*')) return false;
+    const name = trimmed.slice(1).trim().split(/\s+/, 1)[0];
+    if (!name) return false;
+    if (name.includes('/') || name.includes('\\')) return false;
+    // Keep this lexical gate and two-pass lookup aligned with the daemon's
+    // isSlashCommand/parseSlashCommand behavior. A built-in entry marks a
+    // fully loaded command snapshot; until then unresolved commands fail closed.
+    const connection = getConnection();
+    const command =
+      connection.commands?.find((candidate) => candidate.name === name) ??
+      connection.commands?.find((candidate) =>
+        candidate.altNames?.includes(name),
+      );
+    if (command) return command.source === 'builtin-command';
+    return !connection.commands?.some(
+      (candidate) => candidate.source === 'builtin-command',
+    );
+  }
+
   async function promptContentWithUploadedAttachments(
     session: DaemonSessionClient,
     text: string,
@@ -432,7 +454,7 @@ export function createDaemonSessionActions({
     references: DaemonSessionAttachmentReference[];
     fileReferences: DaemonSessionAttachmentReference[];
   }> {
-    if (text.trimStart().startsWith('/')) {
+    if (discardsSlashCommandAttachments(text)) {
       return {
         content: toDaemonPromptContent(text),
         references: [],
@@ -857,9 +879,9 @@ export function createDaemonSessionActions({
             img.mimeType || img.mediaType || img.media_type || 'image/*',
         }));
         const normalizedFiles = normalizePromptFiles(options?.files);
-        const slashCommand = text.trimStart().startsWith('/');
-        const displayedImages = slashCommand ? [] : normalizedImages;
-        const displayedFiles = slashCommand ? [] : normalizedFiles;
+        const discardAttachments = discardsSlashCommandAttachments(text);
+        const displayedImages = discardAttachments ? [] : normalizedImages;
+        const displayedFiles = discardAttachments ? [] : normalizedFiles;
         const inputAnnotations =
           options?.inputAnnotations && options.inputAnnotations.length > 0
             ? options.inputAnnotations
@@ -1019,9 +1041,9 @@ export function createDaemonSessionActions({
         mimeType: img.mimeType || img.mediaType || img.media_type || 'image/*',
       }));
       const normalizedFiles = normalizePromptFiles(options?.files);
-      const slashCommand = text.trimStart().startsWith('/');
-      const displayedImages = slashCommand ? [] : normalizedImages;
-      const displayedFiles = slashCommand ? [] : normalizedFiles;
+      const discardAttachments = discardsSlashCommandAttachments(text);
+      const displayedImages = discardAttachments ? [] : normalizedImages;
+      const displayedFiles = discardAttachments ? [] : normalizedFiles;
       const inputAnnotations =
         options?.inputAnnotations && options.inputAnnotations.length > 0
           ? options.inputAnnotations
diff --git a/packages/web-shell/client/daemon/session/mappers.test.ts b/packages/web-shell/client/daemon/session/mappers.test.ts
index bd9b0058bf5..9ce0bb7c661 100644
--- a/packages/web-shell/client/daemon/session/mappers.test.ts
+++ b/packages/web-shell/client/daemon/session/mappers.test.ts
@@ -1176,6 +1176,31 @@ describe('updateConnectionFromDaemonEvent', () => {
     expect(next.skills).toEqual(['review']);
   });
 
+  it('maps command aliases from available_commands_update metadata', () => {
+    const next = applyEvent(
+      { status: 'connected', workspaceCwd: '/workspace' },
+      availableCommandsEvent(
+        [
+          {
+            name: 'compress',
+            description: 'Compress context',
+            input: null,
+            _meta: { source: 'builtin-command', altNames: ['summarize'] },
+          },
+        ],
+        [],
+      ),
+    );
+
+    expect(next.commands).toEqual([
+      expect.objectContaining({
+        name: 'compress',
+        source: 'builtin-command',
+        altNames: ['summarize'],
+      }),
+    ]);
+  });
+
   it('reads nested availableSkills from the daemon wire shape', () => {
     const next = applyEvent(
       { status: 'connected', workspaceCwd: '/workspace' },
diff --git a/packages/web-shell/client/daemon/session/mappers.ts b/packages/web-shell/client/daemon/session/mappers.ts
index 5618ed4abe1..5af18fc5e50 100644
--- a/packages/web-shell/client/daemon/session/mappers.ts
+++ b/packages/web-shell/client/daemon/session/mappers.ts
@@ -849,11 +849,17 @@ function mapAvailableCommandsUpdate(
 
 function mapCommandMeta(
   meta: Record<string, unknown> | null | undefined,
-): Pick<DaemonCommandInfo, 'source'> {
+): Pick<DaemonCommandInfo, 'source' | 'altNames'> {
   const record = meta ?? undefined;
   const source = getString(record, 'source');
+  const altNames = Array.isArray(record?.['altNames'])
+    ? record['altNames'].filter(
+        (name): name is string => typeof name === 'string',
+      )
+    : [];
   return {
     ...(source ? { source } : {}),
+    ...(altNames.length > 0 ? { altNames } : {}),
   };
 }
 
diff --git a/packages/web-shell/client/daemon/session/promptContent.ts b/packages/web-shell/client/daemon/session/promptContent.ts
index e69df182767..b8ca43a7b03 100644
--- a/packages/web-shell/client/daemon/session/promptContent.ts
+++ b/packages/web-shell/client/daemon/session/promptContent.ts
@@ -44,8 +44,8 @@ export function toDaemonPromptContent(
   files: readonly DaemonPromptFile[] = [],
 ): PromptContentBlock[] {
   // Token lines keep a visible trace in the daemon-recorded transcript
-  // (which stores text blocks only). Skipped for slash commands: the
-  // daemon's slash path drops attachment blocks, so a token would dangle.
+  // (which stores text blocks only). Slash commands resolve attachment blocks
+  // separately from their expanded text, so they do not need token lines.
   const withTokens = withAttachmentTokens(
     text,
     files.map((file) => attachmentUriForName(file.name)),
diff --git a/packages/web-shell/client/daemon/session/types.ts b/packages/web-shell/client/daemon/session/types.ts
index d2c1ca96b67..d7a1ec5e124 100644
--- a/packages/web-shell/client/daemon/session/types.ts
+++ b/packages/web-shell/client/daemon/session/types.ts
@@ -322,6 +322,7 @@ export interface DaemonCommandInfo {
   argumentHint?: string;
   autoSubmit?: boolean;
   source?: string;
+  altNames?: string[];
   raw: DaemonAvailableCommand;
 }