AI Filters, Routing & Guardrails

Atmosphere’s AI layer provides three categories of infrastructure that sit between LLM responses and browser clients: filters that process the text stream, routing that directs requests to the right model, and guardrails that enforce safety policies before and after LLM calls.

AI stream filters

AI filters extend the same BroadcastFilter mechanism used by Atmosphere’s core, specialized for the AI streaming wire protocol. The base class AiStreamBroadcastFilter (in org.atmosphere.ai.filter) handles RawMessage unwrapping, JSON parsing via AiStreamMessage, and re-wrapping automatically. Subclasses only implement filterAiMessage().

AiStreamBroadcastFilter

This abstract class implements BroadcastFilterLifecycle. Its filter() method:

1. Checks if the message is a RawMessage wrapping a JSON string.
2. Parses it into an AiStreamMessage (which has fields: type, data, sessionId, seq, etc.).
3. Delegates to the abstract filterAiMessage() method.
4. Non-AI messages pass through unchanged.

public abstract class AiStreamBroadcastFilter implements BroadcastFilterLifecycle {

    protected abstract BroadcastAction filterAiMessage(
            String broadcasterId, AiStreamMessage msg,
            String originalJson, RawMessage rawMessage);
}

Subclasses return one of:

• new BroadcastAction(rawMessage) — pass through unchanged.
• new BroadcastAction(new RawMessage(modified.toJson())) — pass through modified.
• new BroadcastAction(ACTION.ABORT, rawMessage) — drop the message.
• new BroadcastAction(ACTION.SKIP, rawMessage) — stop filter chain, deliver as-is.

PiiRedactionFilter

Detects and redacts personally identifiable information from AI-generated text streams. Since streaming texts arrive one or a few words at a time, the filter buffers streaming texts per session until a sentence boundary (., !, ?, or newline) is detected. At that point it scans the buffered sentence, redacts matches, and emits the cleaned text.

Default patterns:

Pattern	What it matches
email	Standard email addresses
us-phone	US phone numbers (10+ digits, various formats)
ssn	US Social Security Numbers (NNN-NN-NNNN)
credit-card	Credit card numbers (13-19 digits with optional separators)

Usage:

broadcaster.getBroadcasterConfig().addFilter(new PiiRedactionFilter());

With a custom replacement string:

broadcaster.getBroadcasterConfig().addFilter(new PiiRedactionFilter("***"));

Adding custom patterns:

var filter = new PiiRedactionFilter();
filter.addPattern("uk-nino", Pattern.compile("[A-Z]{2}\\d{6}[A-Z]"));
broadcaster.getBroadcasterConfig().addFilter(filter);

Removing a default pattern:

filter.removePattern("credit-card");

On stream completion, any remaining buffered text is flushed with redaction applied. The filter uses a deferred broadcast on a virtual thread to emit the flushed streaming text before the terminal complete message.

ContentSafetyFilter

Scans AI-generated content for harmful patterns and blocks or replaces unsafe content mid-stream. Like PiiRedactionFilter, it buffers streaming texts into sentence-sized chunks for context-aware scanning. A pluggable SafetyChecker interface allows custom safety logic.

Safety outcomes are modeled as a sealed interface:

public sealed interface SafetyResult
        permits SafetyResult.Safe, SafetyResult.Unsafe, SafetyResult.Redacted {
    record Safe() implements SafetyResult {}
    record Unsafe(String reason) implements SafetyResult {}
    record Redacted(String cleanText) implements SafetyResult {}
}

• Safe: pass through unchanged.
• Unsafe: abort the entire stream and send an error message to the client.
• Redacted: replace the text with a cleaned version and continue streaming.

Built-in checker factories:

// Block the stream entirely when a keyword is found
var checker = ContentSafetyFilter.keywordChecker(Set.of("harmful-term"));
broadcaster.getBroadcasterConfig().addFilter(new ContentSafetyFilter(checker));

// Redact keywords instead of blocking
var checker = ContentSafetyFilter.redactingChecker(
    Set.of("sensitive-term"), "[FILTERED]");
broadcaster.getBroadcasterConfig().addFilter(new ContentSafetyFilter(checker));

For external moderation APIs, implement SafetyChecker directly:

SafetyChecker apiChecker = text -> {
    var result = moderationApi.check(text);
    if (result.isBlocked()) {
        return new SafetyResult.Unsafe(result.reason());
    }
    return new SafetyResult.Safe();
};

CostMeteringFilter

Tracks streaming text counts per session and per broadcaster, and optionally enforces streaming text budgets by aborting streams that exceed their allocation. This filter does not modify streaming text content.

var metering = new CostMeteringFilter();
metering.setBudget("user-123-broadcaster", 10000); // max 10K streaming texts
broadcaster.getBroadcasterConfig().addFilter(metering);

When a budget is exceeded, the filter marks the session as exceeded, drops subsequent streaming texts with ACTION.ABORT, and injects a single error message to notify the client.

Querying usage:

long sessionStreamingTexts = metering.getSessionStreamingTextCount("session-id");
long broadcasterStreamingTexts = metering.getBroadcasterStreamingTextCount("broadcaster-id");
metering.resetBroadcasterCount("broadcaster-id"); // rolling window reset

Wiring a StreamingTextBudgetManager for persistent budget tracking:

metering.setBudgetManager(budgetManager, sessionId -> lookupUserId(sessionId));

When setBudgetManager is configured, the filter calls budgetManager.recordUsage(ownerId, streamingTextCount) on every stream completion.

Streaming text budget management

The StreamingTextBudgetManager (in org.atmosphere.ai.budget) manages per-user or per-organization streaming text budgets with graceful degradation.

var budgetManager = new StreamingTextBudgetManager();
budgetManager.setBudget(new StreamingTextBudgetManager.Budget(
    "user-123", 100_000, "gemini-2.5-flash", 0.8));

The Budget record:

public record Budget(
    String ownerId,
    long maxStreamingTexts,
    String fallbackModel,
    double degradationThreshold
) {}

When usage approaches the degradationThreshold fraction (e.g., 80%), recommendedModel() returns the cheaper fallback model name. When the budget is fully exhausted, it throws BudgetExceededException:

try {
    Optional<String> fallback = budgetManager.recommendedModel("user-123");
    // fallback.isPresent() means "switch to the cheaper model"
} catch (BudgetExceededException e) {
    // Budget fully exhausted: e.ownerId(), e.budget(), e.used()
}

Other operations:

long remaining = budgetManager.remaining("user-123");
long used = budgetManager.currentUsage("user-123");
budgetManager.resetUsage("user-123"); // new billing period
budgetManager.removeBudget("user-123");

AiGuardrail

The AiGuardrail interface (in org.atmosphere.ai) provides pre-LLM and post-LLM inspection. Guardrails run in the interceptor chain:

Guardrails (pre) -> Rate Limit -> RAG -> [LLM call] -> Guardrails (post) -> Observability

public interface AiGuardrail {

    default GuardrailResult inspectRequest(AiRequest request) {
        return GuardrailResult.pass();
    }

    default GuardrailResult inspectResponse(String accumulatedResponse) {
        return GuardrailResult.pass();
    }
}

GuardrailResult is a sealed interface with three variants:

sealed interface GuardrailResult {
    record Pass() implements GuardrailResult {}
    record Modify(AiRequest modifiedRequest) implements GuardrailResult {}
    record Block(String reason) implements GuardrailResult {}

    static GuardrailResult pass() { return new Pass(); }
    static GuardrailResult modify(AiRequest req) { return new Modify(req); }
    static GuardrailResult block(String reason) { return new Block(reason); }
}

Example guardrail:

public class PiiGuardrail implements AiGuardrail {
    @Override
    public GuardrailResult inspectRequest(AiRequest request) {
        if (containsPii(request.message())) {
            return GuardrailResult.block("PII detected in request");
        }
        return GuardrailResult.pass();
    }
}

Register guardrails on an endpoint:

@AiEndpoint(path = "/chat", guardrails = {PiiGuardrail.class})

Model routing

The ModelRouter interface (in org.atmosphere.ai) mirrors Atmosphere’s transport failover pattern (WebSocket -> SSE -> long-polling) applied to the AI layer (GPT-4 -> Claude -> Gemini).

public interface ModelRouter {
    Optional<AiSupport> route(
        AiRequest request,
        List<AiSupport> availableBackends,
        Set<AiCapability> requiredCapabilities);

    void reportFailure(AiSupport backend, Throwable error);
    void reportSuccess(AiSupport backend);
}

FallbackStrategy

The ModelRouter.FallbackStrategy enum defines four strategies:

Strategy	Behavior
NONE	Use the primary model only
FAILOVER	On failure, try the next backend in priority order
ROUND_ROBIN	Distribute requests across backends
CONTENT_BASED	Route based on request characteristics (model hint, tool requirements)

DefaultModelRouter

The default implementation uses a circuit breaker pattern for health tracking:

• Consecutive failures increment a failure counter.
• After maxConsecutiveFailures (default 3), the backend is marked unhealthy.
• After a cooldown period (default 1 minute), the backend is eligible again.
• A success resets the failure counter.

var router = new DefaultModelRouter(FallbackStrategy.FAILOVER);
// or with custom thresholds:
var router = new DefaultModelRouter(FallbackStrategy.ROUND_ROBIN, 5, Duration.ofMinutes(2));

For CONTENT_BASED routing, the router checks request.model() for a model hint and request.tools() for tool requirements, preferring backends with AiCapability.TOOL_CALLING.

RoutingAiSupport

Wraps a ModelRouter and a list of backends into a single AiSupport instance. On failure, it attempts one retry with the next backend:

var routing = new RoutingAiSupport(router, List.of(
    springAiSupport,
    langChain4jSupport,
    adkSupport
));

The name() returns a descriptive string like routing(spring-ai,langchain4j,google-adk). Its capabilities() is the union of all backends’ capabilities.

RoutingLlmClient

For lower-level control, RoutingLlmClient (in org.atmosphere.ai.routing) routes at the LlmClient level with configurable rules:

var router = RoutingLlmClient.builder(defaultClient, "gemini-2.5-flash")
    .route(RoutingRule.contentBased(
        prompt -> prompt.contains("code"),
        openaiClient, "gpt-4o"))
    .route(RoutingRule.contentBased(
        prompt -> prompt.contains("translate"),
        claudeClient, "claude-3-haiku"))
    .build();

router.streamChatCompletion(request, session);

Rules are evaluated in order. The first matching rule determines the target client and model. If no rule matches, the default client is used.

Fan-out streaming

Fan-out sends the same prompt to multiple models simultaneously, with each model streaming texts through its own child session. The FanOutStreamingSession (in org.atmosphere.ai.fanout) orchestrates this.

FanOutStrategy

A sealed interface with three variants:

public sealed interface FanOutStrategy {
    record AllResponses() implements FanOutStrategy {}
    record FirstComplete() implements FanOutStrategy {}
    record FastestStreamingTexts(int streamingTextThreshold) implements FanOutStrategy {}
}

Strategy	Behavior
AllResponses	All models stream to completion. The client receives interleaved text streams distinguishable by session ID.
FirstComplete	First model to finish wins. All other in-flight calls are cancelled.
FastestStreamingTexts(n)	Observe streaming text production speed for n initial streaming texts, then keep the fastest model and cancel the rest.

ModelEndpoint

Describes one model to fan out to:

public record ModelEndpoint(String id, LlmClient client, String model) {}

FanOutResult

Available after fan-out completes:

public record FanOutResult(
    String modelId,
    String fullResponse,
    long timeToFirstStreamingTextMs,
    long totalTimeMs,
    int streamingTextCount
) {}

Usage

var endpoints = List.of(
    new ModelEndpoint("gemini", geminiClient, "gemini-2.5-flash"),
    new ModelEndpoint("gpt4", openaiClient, "gpt-4o")
);

try (var fanOut = new FanOutStreamingSession(session, endpoints,
        new FanOutStrategy.AllResponses(), resource)) {
    fanOut.fanOut(ChatCompletionRequest.of("ignored", userPrompt));

    // After completion, inspect results
    Map<String, FanOutResult> results = fanOut.getResults();
    var geminiResult = results.get("gemini");
    logger.info("Gemini TTFT: {}ms, total: {}ms, streaming texts: {}",
        geminiResult.timeToFirstStreamingTextMs(),
        geminiResult.totalTimeMs(),
        geminiResult.streamingTextCount());
}

Child sessions use IDs of the form parentSessionId + "-" + endpointId, so the client can distinguish which model produced each streaming text. The parent session receives metadata events: fanout.models (list of model IDs at start) and fanout.complete (boolean at end).

Putting it all together

A typical production setup combines filters, routing, and budget management:

// 1. Set up budget management
var budgetManager = new StreamingTextBudgetManager();
budgetManager.setBudget(new StreamingTextBudgetManager.Budget(
    "org-acme", 500_000, "gemini-2.5-flash", 0.8));

// 2. Set up routing with failover
var router = new DefaultModelRouter(FallbackStrategy.FAILOVER);
var routing = new RoutingAiSupport(router, List.of(
    springAiSupport, langChain4jSupport));

// 3. Add filters to the broadcaster
var metering = new CostMeteringFilter();
metering.setBudgetManager(budgetManager, sid -> lookupOrgId(sid));

broadcaster.getBroadcasterConfig().addFilter(new PiiRedactionFilter());
broadcaster.getBroadcasterConfig().addFilter(
    new ContentSafetyFilter(ContentSafetyFilter.keywordChecker(blockedTerms)));
broadcaster.getBroadcasterConfig().addFilter(metering);

The filter chain processes every streaming text in order: PII redaction first, then content safety, then cost metering. If PII redaction buffers a streaming text (waiting for a sentence boundary), it is not visible to downstream filters until the sentence is complete.

Samples

• samples/spring-boot-ai-tools/ — demonstrates the CostMeteringInterceptor that tracks streaming text usage and sends routing metadata to the client.
• samples/spring-boot-spring-ai-routing/ — demonstrates multi-model routing with RoutingAiSupport and DefaultModelRouter using Spring AI backends.