feat: add support for data-* prefixed types in messages and attachments

[adi-bradwell]

Summary

Adds full support for data-* prefixed types in user messages and attachments, addressing all issues from #3358.

Changes (17 files, +66 / -29)

New types (packages/core/src/types/):

• Added DataUIPart type: { type: \data-${string}`; data: unknown; id?: string }`
• Updated ThreadUserMessagePart union to include DataUIPart
• Split BaseAttachment into a discriminated union: BaseFileAttachment (with required contentType: string) and BaseDataAttachment (with optional contentType?: string)

Graceful degradation in switch statements:

• thread-message-like.ts: user handler returns data-* parts as-is (using isDataUIPart type guard); assistant handler returns null (skips)
• MessageParts.tsx / MessagePartsGrouped.tsx: returns null for data-* parts
• ComposerAttachments.tsx / MessageAttachments.tsx: falls back to components?.Attachment
• auiV0.ts: skips data-* parts via return null + .filter(Boolean)
• All downstream packages (react-a2a, react-langgraph, react-ai-sdk, ui, docs) updated

Adapter guards:

• CompositeAttachmentAdapter.remove(): early return for data-* attachments
• CloudFileAttachmentAdapter.send(): graceful handling when contentType is absent
• vercelAttachmentAdapter.send(): runtime check for contentType

Exports:

• DataUIPart exported from both @assistant-ui/core and @assistant-ui/react

Design principles

• Add-only: no existing behavior is changed; guards are added in default cases before existing throws
• Uses type.startsWith("data-") consistently (not "data") to avoid changing behavior for the existing bare "data" type
• No as assertions — uses proper type guard (isDataUIPart) instead

Verified

• All 44 build tasks pass
• All 35 test tasks pass

Test plan

• Full monorepo build passes (44/44 tasks)
• Full test suite passes (35/35 tasks)
• Verify data-* parts in user messages don't crash rendering
• Verify data-* attachments can be created and removed without errors
• Verify existing "data", "image", "file", etc. types continue working unchanged

Made with Cursor

Summary by CodeRabbit

• New Features

  • Added support for custom data-* attachments and data-* message parts across messaging and UI, allowing extensible data-prefixed types to be accepted and rendered or preserved.

• Bug Fixes

  • Improved validation for attachments with missing content type and reduced crashes by skipping or safely handling unknown data-* parts.
  • Converter now preserves specific data-* type labels for data parts.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds explicit support for data-* prefixed attachment and message-part types across types, runtimes, adapters, and UI: new discriminated types and guards let data- parts be recognized, skipped, or handled instead of triggering exhaustive-type errors.

Changes

Cohort / File(s)	Summary
Type system & exports packages/core/src/types/attachment.ts, packages/core/src/types/message.ts, packages/core/src/types/index.ts, packages/react/src/types/index.ts	Adds DataAttachmentType/FileAttachmentType, BaseDataAttachment/BaseFileAttachment, DataUIPart type; updates unions and re-exports to include data- UI parts.
Runtime message utilities packages/core/src/runtime/utils/thread-message-like.ts	Adds isDataUIPart guard; includes DataUIPart in ThreadMessageLike content union; adjusts assistant/user part handling to skip or pass through data- parts.
Adapters & attachment logic packages/core/src/adapters/attachment.ts, packages/react-ai-sdk/src/ui/utils/vercelAttachmentAdapter.ts, packages/react/src/legacy-runtime/runtime-cores/adapters/attachment/CloudFileAttachmentAdapter.ts	Early returns for data-* attachments in remove/send flows; require/validate contentType where relevant; avoid creating content for attachments missing contentType.
UI attachment rendering apps/docs/components/legacy/attachment-old.tsx, packages/ui/src/components/assistant-ui/attachment.tsx, packages/react/src/primitives/composer/ComposerAttachments.tsx, packages/react/src/primitives/message/MessageAttachments.tsx	Switch/default branches updated to allow data-* attachment types to render (or be used as labels) instead of throwing unknown-type errors.
Message part rendering packages/react/src/primitives/message/MessageParts.tsx, packages/react/src/primitives/message/MessagePartsGrouped.tsx	Default cases now return null for data-* message parts, filtering/skipping them rather than throwing unhandled-type errors.
Message conversion & runtime guards packages/react-ai-sdk/src/ui/utils/convertMessage.ts, packages/react-a2a/src/useA2ARuntime.ts, packages/react-langgraph/src/useLangGraphRuntime.ts, packages/react/src/legacy-runtime/cloud/auiV0.ts, apps/docs/components/legacy/attachment-old.tsx	Converters and getMessageContent/encoding paths now check for data-* prefixes: convert uses the specific data- type as the discriminator; runtimes either throw early for unsupported data- types or return/filter them instead of relying on exhaustive-check fallthroughs.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 A tiny hop for code and type,
I sniffed out data- in the night,
Now parts that once would cause a fright
Are handled gentle, quiet, light—
A rabbit cheers: the build runs right! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title clearly and concisely summarizes the main change: adding support for data-* prefixed types across messages and attachments, which aligns with all 17 modified files.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/data-star-types-support

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

packages/react-ai-sdk/src/ui/utils/convertMessage.ts (1)

143-149: ⚠️ Potential issue | 🔴 Critical

Type mismatch: code cannot satisfy DataMessagePart with template literal types.

The code at line 145 assigns type: part.type (where part.type is a value like "data-attachment" matching data-${string}), but uses satisfies DataMessagePart which requires type: "data" as a literal string. These are incompatible:

• DataMessagePart.type is defined as the literal "data"
• DataUIPart.type is defined as data-${string}

The object being returned matches DataUIPart, not DataMessagePart. Either change the assertion to satisfies DataUIPart (though this removes the name field) or revert to hardcoding type: "data" to match the actual DataMessagePart contract. The current code has a type incompatibility.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/react-ai-sdk/src/ui/utils/convertMessage.ts` around lines 143 - 149,
The returned object in convertMessage.ts inside the branch if
(part.type.startsWith("data-")) currently uses part.type (e.g.,
"data-attachment") but asserts satisfies DataMessagePart which requires type:
"data"; fix this by making the returned object's type property the literal
"data" (keep name: part.type.substring(5) and data: (part as any).data) so the
shape truly matches DataMessagePart, and keep the satisfies DataMessagePart
assertion; update the object returned in that branch accordingly.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@packages/react-ai-sdk/src/ui/utils/convertMessage.ts`:
- Around line 143-149: The returned object in convertMessage.ts inside the
branch if (part.type.startsWith("data-")) currently uses part.type (e.g.,
"data-attachment") but asserts satisfies DataMessagePart which requires type:
"data"; fix this by making the returned object's type property the literal
"data" (keep name: part.type.substring(5) and data: (part as any).data) so the
shape truly matches DataMessagePart, and keep the satisfies DataMessagePart
assertion; update the object returned in that branch accordingly.