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To stream AI responses in Flutter or React Native, use a two-hop design: the mobile app sends the prompt to your authenticated application server, the server calls the OpenAI Responses API with stream: true, and the server forwards a controlled stream to the app. The provider API key stays on the server. Your client decodes incoming bytes, handles typed events such as response.output_text.delta, appends each text delta to local state, and renders the growing answer.
HTTP server-sent events (SSE) are the documented fit for one-way text generation. WebSockets make more sense for a persistent, bidirectional product such as interactive voice. The difficult parts are not printing the first token: they are framing SSE records correctly, handling terminal and failure events, cancelling work, surviving dropped connections, and deciding whether partial output may be shown before moderation.
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What architecture should a Flutter or React Native AI chat use?
The safest general architecture is a mobile client plus an application-server proxy. The client never calls the model provider with a secret key. It authenticates to your server, which validates the request, starts the provider stream, and relays only the data your app needs.
The two-hop request path
- Mobile app to your server: Flutter or React Native sends the conversation input, an application-session token, and any product options your server permits.
- Server to the Responses API: Your server authenticates the user, checks quotas and prompt limits, selects an allowed model, and calls the Responses API with
stream: true. - Server back to mobile: The server forwards SSE records or converts them to a normalized newline-delimited JSON contract that is stable for your app.
- Incremental rendering: The app decodes bytes, parses complete records, appends text deltas to one accumulated answer, and updates the chat view in controlled batches.
This separation lets you revoke sessions, enforce per-user limits, change models without shipping a new app, redact logs, and abort an upstream request when a user taps Stop. It also prevents anyone who extracts the mobile bundle from obtaining the provider credential.
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Why a direct mobile-to-provider call is unsafe
A key embedded in an Android or iOS bundle is recoverable through reverse engineering, runtime inspection, or a compromised device. Restricting the key by package name, certificate, or bundle identifier does not make it a secret; those values can be copied or spoofed. Keep the provider key in server-side configuration and authenticate the mobile request to your own endpoint instead.
How does stream: true work in the Responses API?
With streaming enabled, the API returns an HTTP event stream rather than waiting for one complete response. The stream contains typed semantic events. Text arrives in response.output_text.delta events, whose delta value is the next piece of visible text; a successful generation ends with response.completed.
Event state machine
Treat event names as state transitions, not as text to display. A practical state machine looks like this:
| Event | Client state | Action |
|---|---|---|
response.created or response.queued |
Started | Show a running indicator and create an application request ID. |
response.output_text.delta |
Receiving | Append the event’s delta string to the answer buffer. Ignore empty deltas. |
response.completed |
Success | Stop the spinner, mark the message complete, and enable follow-up actions. |
response.incomplete |
Recoverable partial | Keep the text already received, label it incomplete according to your product policy, and offer retry. |
response.failed or an error event |
Failed | Stop loading, retain or discard partial text according to policy, and show a retry action. |
Responses events include monotonically increasing sequence_number values. Preserve them, or at least preserve your own request ID, if your server will deduplicate records or implement resume behavior. Do not wait for a supposed final token: a terminal event is the authoritative end of the operation.
How SSE records are framed
SSE is a text protocol carried over an HTTP response. A record can contain one or more data: lines and ends at a blank line. Network packets do not respect those boundaries, so one read may contain half a record or several records. Decode UTF-8 incrementally, append decoded text to a byte/string buffer, split only when the blank-line separator is present, then parse the data: payload as JSON.
Never assume that each callback from the networking library equals one event. Likewise, do not append the entire JSON event to the chat bubble: append only the text in a delta event. Keep protocol errors and terminal states separate from visible answer text.
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How do I build the authenticated streaming proxy?
Your server endpoint should expose a small, app-specific contract rather than leaking every provider event. It can pass through the provider’s SSE format, or emit one newline-delimited JSON object per logical event. The latter is useful when a mobile networking stack handles ordinary chunked HTTP more reliably than native SSE.
Server responsibilities
- Authenticate the mobile session before contacting the model provider.
- Validate prompt size, conversation shape, user identity, quota, and the requested model against a server-side allow-list.
- Set the response content type appropriate to your chosen contract and flush data as soon as a complete event is available.
- Abort the upstream request when the client disconnects or sends a cancellation request.
- Attach an application request ID and, when useful, preserve provider sequence numbers for diagnostics and deduplication.
- Log timing, status, and request IDs without writing prompts or generated text that may contain sensitive information.
Illustrative server flow
The exact SDK method and event iterator depend on your server language and package version, but the control flow is consistent:
authenticate mobile request
validate input, quota, and allowed model
upstream = Responses API(input, model, stream=true)
for each provider event in upstream:
if event is output_text.delta:
send normalized delta to the mobile client
else if event is a terminal or error event:
send that state and close the response
abort upstream when the mobile connection closesIf you pass through SSE, preserve record boundaries and send the provider event type and JSON data. If you normalize to newline-delimited JSON, define a versioned schema such as {"type":"delta","text":"..."}, {"type":"completed"}, and {"type":"error","code":"..."}. Keep provider-specific fields on the server unless the client genuinely needs them.
Connection and timeout behavior
Configure infrastructure so it does not buffer the entire response or terminate an otherwise active stream. Reverse proxies and serverless platforms may impose idle or maximum-duration limits; send only protocol-valid keep-alive data if your chosen deployment requires it, and document the behavior for the mobile client. A timeout must become an explicit incomplete or error state, not an endless spinner.
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Flutter needs an HTTP client that exposes the response byte stream. The implementation is a parser and state-management problem: decode incrementally, buffer until a complete SSE record, process the event, and update the widget tree without rebuilding on every tiny token.
- Call your endpoint, not the provider: Include the app’s authentication token and conversation payload in the request. Do not put a provider key in Dart constants, platform resources, or build-time variables shipped to users.
- Read bytes incrementally: Use a Dart HTTP client whose response exposes a stream of bytes. Feed chunks through an incremental UTF-8 decoder so a multibyte character split across packets is reconstructed correctly.
- Frame records: Append decoded text to a buffer, locate the SSE blank-line separator, remove one complete record at a time, and collect its
data:lines. - Parse and dispatch: Decode the data as JSON, inspect the typed event, append only
response.output_text.deltavalues, and transition state for completed, incomplete, failed, or error events. - Batch UI notifications: Keep a
StringBufferor immutable message value in application state, but notify widgets at a controlled cadence. A very fast stream can otherwise cause excessive rebuilds and battery use. - Close deliberately: Cancel the subscription and tell the server to abort upstream work when the user leaves the screen or taps Stop. Mark the local message as cancelled rather than completed.
Using typed Dart events
The openai_dart documentation describes typed response events and helpers named accumulate() and collectText(). Those helpers can remove parser code when the package version and endpoint match your server contract. A custom parser remains preferable when your server normalizes events, needs explicit cancellation, records telemetry, redacts fields, or supports a resume protocol.
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Flutter lifecycle details
Keep stream ownership outside a short-lived widget when a route can rebuild or be replaced. A controller, notifier, or equivalent state object should own the subscription, expose started/receiving/completed/incomplete/failed states, and dispose the subscription during cancellation. If the app backgrounds, decide whether to cancel immediately, allow a bounded background operation, or reconnect later; do not silently create duplicate requests when the screen resumes.
How do I show ChatGPT-style tokens in React Native?
Use a React Native networking stack that truly exposes incremental response bytes or SSE events on every platform you ship. Support differs between React Native versions, JavaScript engines, and networking libraries, so verify Android and iOS behavior instead of assuming browser fetch semantics.
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- Start one request: Store an application request ID in the component or state store, and disable duplicate send actions until the request is explicitly finished or cancelled.
- Decode incrementally: Feed each available byte chunk into a UTF-8 decoder and retain incomplete record fragments between callbacks.
- Append deltas: For each parsed
response.output_text.delta, update one accumulated string. Do not replace the string with the newest delta, or the UI will appear to lose earlier words. - Batch state updates: Queue several deltas and publish them together on a short cadence appropriate to your interface. This keeps React rendering responsive on long answers.
- Handle terminal states: Mark completion only on
response.completed; expose incomplete and failed states separately and offer retry without automatically duplicating the original prompt.
The official JavaScript reference documents access to a raw fetch response for custom streaming logic, while a Node streaming guide demonstrates converting a server stream to newline-separated JSON for browser clients. In React Native, the deciding factor is whether the selected library exposes reliable incremental data on your actual platform targets. If it does not, keep provider streaming on the server and send coalesced chunks through a transport the client supports.
Rendering markdown safely while text is incomplete
Token boundaries can split a markdown code fence, link, or emphasis span. A renderer that expects a complete document may flicker or throw while those constructs are unfinished. During streaming, render plain text or a tolerant subset, then run the full markdown pass after the completed event; this also reduces expensive re-parsing.
Should I use SSE or WebSockets for AI text?
For one-way text generation, SSE over HTTP is usually the simplest choice and matches the documented Responses API pattern. WebSockets are justified when the product needs persistent bidirectional messaging, interactive voice, or frequent client-to-server events.
| Decision axis | SSE | WebSockets |
|---|---|---|
| Direction | Server to client stream, with normal HTTP requests in the opposite direction | Full bidirectional channel |
| Connection lifetime | One HTTP response per generation; reconnect per request or by application policy | Long-lived connection that can carry many messages |
| Reconnect and resume | Requires request IDs, sequence handling, and an explicit replay policy | Requires connection recovery, session state, and message replay policy |
| Intermediaries | Uses familiar HTTP infrastructure but may encounter buffering or idle timeouts | Needs WebSocket upgrade support and compatible proxies, load balancers, and mobile libraries |
| Implementation complexity | Lower for text deltas and standard HTTP authentication | Higher lifecycle, heartbeat, reconnect, and backpressure complexity |
| Other realtime modalities | Best for text or other server-to-client events | Better when audio, control messages, or interactive input share one session |
Do not choose WebSockets merely because the answer arrives quickly. Choose them when the product requirement is genuinely bidirectional or long-lived. For a normal prompt-and-answer screen, an authenticated HTTP endpoint that streams SSE is easier to observe, secure, and recover.
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How do I keep the provider API key safe?
Keep the key exclusively in server-side secrets management and make the mobile app authenticate to your server. The official guidance is explicit: “Keep your API key on the server and proxy the response stream to the browser.” The same rule applies to Flutter and React Native bundles.
- Use short-lived app sessions or access tokens and verify them on every streaming request.
- Apply per-user and per-device quotas before opening an upstream stream.
- Allow-list model names and generation options; never trust a client-supplied model or spending limit.
- Validate maximum prompt size and conversation length to prevent accidental resource exhaustion.
- Redact prompts, deltas, authorization headers, and provider responses from routine logs.
- Separate development and production credentials and rotate the provider key without releasing a mobile update.
- Rate-limit retries so a poor connection does not multiply provider requests.
What reliability features are essential for streamed responses?
Streaming changes failure handling because the user can see a useful partial answer before the request ends. Design every request as a finite state machine and make cancellation, timeout, and reconnect behavior explicit.
Cancellation
When the user taps Stop or navigates away, cancel the client subscription and notify the server. The server should abort the provider request and close the downstream response. If cancellation races with a final event, treat the first terminal state recorded for that application request as authoritative and ignore duplicate terminal events.
Reconnect and deduplication
A dropped connection does not tell you whether the provider finished. Preserve an application request ID and, where your backend supports it, provider sequence numbers. On reconnect, either request replay from a known sequence or start a new generation with a clear “retry” action; never blindly append a second answer to the first partial one.
Bounded buffers and backpressure
Set a maximum visible answer size and a maximum time or event count for a single request. Keep the full provider payload off the client if the product does not need it. If the UI cannot keep up, coalesce deltas on the server or client rather than allowing an unbounded queue to grow.
Tests that expose real bugs
- Split an SSE record across several byte chunks, including in the middle of a multibyte UTF-8 character.
- Deliver multiple records in one network callback.
- Send an empty delta, malformed JSON, an unknown event type, and duplicate terminal events.
- Drop the connection before completion and verify that partial text is labeled incomplete.
- Cancel during generation and confirm the upstream request is aborted.
- Delay events long enough to trigger proxy and client timeout paths.
- Run Android and iOS tests on slow networks, not only on a local simulator.
How should streaming affect moderation and privacy?
Streaming makes moderation harder because partial completions are evaluated before the complete output is available. OpenAI warns that “Streaming makes it more difficult to moderate the content of the completions.” Requested moderation scores arrive after the full generated output rather than alongside every partial delta.
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If your product must approve content before any of it appears, do not display deltas immediately. Buffer the completion on the server, run the required review workflow, and then release approved text. If low latency is more important, document that moderation is post-generation, limit the contexts in which streaming is allowed, and provide a user-visible report or stop mechanism.
What common streaming failures mean and how do I fix them?
| Symptom | Likely cause | Fix |
|---|---|---|
| The answer appears only at the end | A proxy, server framework, or client buffers the response | Disable response buffering where supported, flush each complete record, and verify that the client receives incremental bytes. |
| Text is duplicated | Reconnect logic replays already applied events | Track request and sequence IDs, or reset the local buffer before a deliberate retry. |
| Words are corrupted | Each chunk was decoded as an independent UTF-8 string | Use one incremental decoder for the lifetime of the response. |
| The spinner never stops | Terminal events are ignored or a transport error is swallowed | Handle completed, incomplete, failed, and error events explicitly, and add a client timeout. |
| Only some platforms stream | The React Native or Dart networking stack exposes buffering on one target | Test the exact library and platform combination; switch transports or send coalesced server chunks. |
| The app shows provider metadata as prose | The parser appends every event instead of filtering by type | Append only the delta field from text-delta events and route state events separately. |
| Costs or latency spike after retries | Reconnect creates a second provider generation | Use an idempotent application request policy, rate-limit retries, and require a deliberate retry after an uncertain disconnect. |
What should I verify before deploying?
- The provider key is absent from every mobile artifact and is available only to the server process.
- Authentication, prompt limits, quotas, and model allow-lists are enforced before upstream work starts.
- The server emits valid SSE records or a documented newline-delimited JSON schema without buffering.
- Flutter and React Native clients incrementally decode UTF-8 and correctly frame records across arbitrary packet boundaries.
- State transitions cover started, receiving, completed, incomplete, failed, cancelled, and transport-error paths.
- Stop, screen disposal, app backgrounding, timeout, reconnect, and retry behavior have explicit tests.
- Logs contain request IDs and timings but not sensitive prompts, generated text, or credentials.
- Your moderation policy states whether partial output may be shown and what happens to incomplete text.
- Infrastructure limits for idle connections, maximum response duration, proxy buffering, and concurrent streams are known and monitored.
Frequently Asked Questions
Can I use the same stream parser for Flutter and React Native?
You can share the event contract and state machine, but not necessarily the networking code. Dart and React Native libraries expose different byte-stream and cancellation APIs, so keep framing rules identical while implementing platform-specific transport adapters.
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What should I store if a mobile stream disconnects?
Store an application request ID, the last applied sequence number when available, and the accumulated text. That lets the server decide whether replay is safe and lets the UI distinguish a recoverable partial answer from a brand-new generation.
How can I test streaming without making a model request every time?
Record representative SSE or newline-delimited JSON fixtures, including split records, empty deltas, malformed payloads, and every terminal state. Feed those fixtures into the same decoder and reducer used in production, then reserve a small number of end-to-end tests for authentication and proxy behavior.
Is it safe to render streamed markdown immediately?
Only if the renderer tolerates incomplete syntax. Code fences, links, and emphasis can be split across deltas; a plain-text or limited renderer during generation is safer, followed by full markdown rendering after completion.
Should a backgrounded phone keep generating an answer?
Choose a policy rather than relying on accidental OS behavior. Cancel on background, continue only within platform-approved limits, or let the server finish and provide a request-status path for the app to reconcile when it returns.
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