fix(mobile): image-only messages ask the agent to respond, like desktop

[whoisaldo]

Replaces #14124, which was closed for missing verification evidence. The evidence is attached below, and the branch is merged with current main.

Closes #14049.

An image sent from the phone with no text reached the agent as only the image reference. Desktop sends the attachment-only prompt in that case ([User attached one or more files without additional text. …]), so the agent knows to respond to the image.

ATTACHMENT_ONLY_BOOTSTRAP_PROMPT moves from the web composer to @t3tools/shared/composerContextReferences, next to a new isImageReferencesOnly check. The mobile outbox's serializeComposerMessageForServer is used by both the existing-thread and new-task send paths. It now sends the prompt when the text is nothing but image references. The images stay in the message's attachments and context records. As the triage asks, text with a file reference or any prose is sent unchanged. Web behavior does not change; it only imports the constant from its new home.

Evidence

Setup:

• Device: an Android emulator on macOS (API 35, arm64). The development client is built from this branch.
• Server: an isolated serve --base-dir with one scratch Git project.
• Agent: Codex GPT-6-Astra, the default.
• Bundles: the "before" run loads the JS bundle from main at 5cc99e1. The "after" run loads this branch, merged with the same main. JS is the only difference, so one native client served both.

Steps, identical for both runs:

1. Tap New thread, then Demo app.
2. Tap +, then Photo Library, and pick one image.
3. Send without typing anything.

The composer looks the same in both runs:

Send result. Left: main. Right: this branch.

The user message text the server stored for each send, read from projection_thread_messages (both have the image attached):

main:         ![1000000018.jpg](t3-context://v1/image/d777fef4-…)
this branch:  [User attached one or more files without additional text. Respond using the conversation context and the attached files.]

On main the agent still acted, because the note itself contains an instruction. A plain photo would get no instruction to respond. With the change, the turn matches what desktop sends. The sent bubble now shows the image as an attachment with the bracketed prompt below it, the same way an image-only message from desktop appears.

Checks:

• Shared tests cover isImageReferencesOnly, and a mobile test covers the swap and the prose case. The mobile test fails without the change. Shared, mobile, and web tsc --noEmit pass, and lint adds no warnings.
• Not checked: iOS. This machine has Xcode 16.4, and the iOS client needs Xcode 26 for clerk-ios. iOS and Android share the changed React Native outbox code.
• Running the Android dev client locally required pinning com.facebook.fbjni:fbjni:0.7.0 in the generated android/build.gradle because of Android release builds crash on launch: react-native-shiki-engine resolves fbjni:+ to 0.8.1 #13710. That pin is not part of this PR.

[macroscopeapp]

Approvability

Verdict: Approved at ee84829

Macroscope's review found this PR approvable — This is a small, tested mobile bug fix that substitutes the established attachment-only prompt only for image-only messages, while preserving prose, file references, attachments, and context records. The other production edits are limited to relocating the unchanged prompt constant, with no schema, deployment, security, or static-analysis changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9bd1d80 and bd512a1.

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📒 Files selected for processing (8)

• apps/mobile/src/lib/composerContext.test.ts
• apps/mobile/src/lib/composerContext.ts
• apps/web/src/components/ChatView.tsx
• apps/web/src/components/chat/composerPromptHistory.test.ts
• apps/web/src/components/chat/composerPromptHistory.ts
• apps/web/src/components/chat/sendQueuedMessage.ts
• packages/shared/src/composerContextReferences.test.ts
• packages/shared/src/composerContextReferences.ts

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📝 Walkthrough

Walkthrough

The change adds shared detection for image-only composer text. Mobile serialization uses a shared bootstrap prompt for those messages. Web prompt-history code imports the same prompt from the shared package.

Changes

Composer message handling

Layer / File(s)	Summary
Shared image-only detection packages/shared/src/composerContextReferences.ts, packages/shared/src/composerContextReferences.test.ts	Adds a bootstrap prompt and a predicate that returns true when text contains one or more image references, no other reference types, and no remaining non-whitespace text. Tests cover matching and non-matching inputs.
Mobile message serialization apps/mobile/src/lib/composerContext.ts, apps/mobile/src/lib/composerContext.test.ts	Serialization uses the bootstrap prompt for image-only composer text. Tests verify that context remains attached and that prose-containing messages keep their original text.
Web shared prompt usage apps/web/src/components/ChatView.tsx, apps/web/src/components/chat/composerPromptHistory.ts, apps/web/src/components/chat/composerPromptHistory.test.ts, apps/web/src/components/chat/sendQueuedMessage.ts	Web code imports the bootstrap prompt from the shared package. composerPromptHistory.ts removes its local exported declaration and continues to use the shared prompt in recallable-prompt handling.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Suggested reviewers: chrisdeeming, juliusmarminge

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Merge Risk | ⚪ Minimal · up to bd512

Merge Risk: ⚪ Minimal · up to bd512

Mobile image-only messages receive the shared prompt while retaining their attachments, and web history filtering is unchanged. No material merge risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to bd512

The change reuses existing message-sending paths and does not introduce an identified privilege or credential exposure. Remaining uncertainty concerns end-to-end attachment ownership and downstream execution controls, rather than a demonstrated vulnerability.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated exposure is the submitting user's message in its selected thread and existing provider session. The changed classifier does not select another thread, resolve an asset, or alter model and interaction-mode fields in the inspected request path. Broader attachment authorization was not established by this review.

Security Findings and Attack Paths

• inferred — A user can trigger substitution with a fabricated image-kind reference, even without a corresponding attachment. This establishes a text-classification edge case, not an asset-access or privilege bypass: the inspected server path receives text as provider input and obtains attachments independently. No introduced security attack path was established from that edge case.

Trust Boundaries and Controls

• observed — The existing mobile upload mapping rebinds context attachment IDs to uploaded IDs, and the outbox passes prepared attachments separately. The provider request builder retains separate text and attachment fields. These are identity-preservation mechanisms, not evidence that server-side ownership authorization has been fully verified.

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🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the main change: mobile image-only messages prompt the agent to respond, matching desktop behavior.
Description check	✅ Passed	The description explains the problem and change, references the related issue and triage, and provides detailed reproduction steps, evidence, test results, and an iOS testing limitation.

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• Resolve merge conflict in branch fix/mobile-image-only-prompt

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