Streaming inference

Spectre exposes streaming as an Instance-owned Run capability. The provider transport may live in another package provided it delivers provider data into the owning stream session's mailbox. Selection, budgets, cancellation, steering, terminal processing and recovery remain in the core runtime.

Streaming is intentionally opt-in. The first supported slice is text-only response generation without Action planning or structured output. Unsupported purposes and missing capabilities return typed errors; Spectre does not buffer a synchronous completion and pretend it was a stream.

An Agent with a configured Action planner is admitted only when Action planning is disabled for this call. A planner that also transforms visible reply text must implement incremental_cleaner?/0 and return true; otherwise streaming fails closed because a complete-response cleaner cannot truthfully certify individual deltas.

Start and consume a stream

Configure a provider package that implements Spectre.Inference.StreamAdapter, then pass its module and options through the ordinary model configuration or the call:

{:ok, stream} =
  Spectre.stream(instance, "Explain the result",
    plan_actions?: false,
    stream_adapter: MyApp.StreamAdapter,
    stream_adapter_opts: [profile: :fast]
  )

Enum.each(stream, fn
  %Spectre.Inference.StreamEvent{kind: :delta, payload: text} ->
    render_provisional(text)

  %Spectre.Inference.StreamEvent{kind: :usage, usage: usage} ->
    update_meter(usage)

  %Spectre.Inference.StreamEvent{kind: :inference_completed} ->
    mark_provider_complete()

  %Spectre.Inference.StreamEvent{kind: :result, payload: result} ->
    deliver_committed(result)

  %Spectre.Inference.StreamEvent{kind: kind}
  when kind in [:failed, :cancelled, :ambiguous, :interrupted] ->
    mark_terminal(kind)
end)

The handle is itself an Enumerable. It can be composed with Stream.transform/3, Enum.reduce/3 or a for comprehension. Enumeration is one-shot: a second consumer receives :already_consumed. Enum.take/2, an exception in the consumer, or consumer-process death runs cleanup and requests cancellation.

The handle contains a bearer token. Its custom Inspect output hides that token, but serialization does not turn it into safe durable state. Do not put a stream handle in a database, log, pub/sub payload or client response. It is valid only for the live runtime generation and bounded terminal-retention window.

Provisional data and the canonical result

A stream has two terminal milestones:

Never send a delta directly to an external user as though it were an approved Agent response. Action planning, policy, complete-reply sanitization and host guards operate on the complete response. Delta content_class is :provisional when incremental sanitization is active and :unsanitized when the host explicitly disables it.

The provisional lane is not promised to be a byte-for-byte prefix of the terminal Result. Its safety relation is deliberately one-way: the incremental sanitizer may suppress more text than the full-response sanitizer, but it must never emit text that the full-response sanitizer would later remove. This is checked across arbitrary UTF-8 chunk boundaries with a property test. For example, an unterminated control marker suppresses the remaining provisional tail even when the terminal sanitizer preserves it. Excess leading indentation is likewise suppressed once it fills the bounded lookahead; it does not fail the provider attempt. The committed Result is authoritative; consumers must not reconstruct it by concatenating deltas.

Optional sanitizer extensions

Spectre always owns the structural sanitizer that removes its control markup and bounds incremental lookahead. A host package can add model-specific cleanup without replacing that boundary:

reply_sanitizer: {Pulse.ModelSanitizer, model_family: :reasoning}

The module implements the Spectre.Reply.Sanitizer callbacks. sanitize/2 receives the core-cleaned terminal text. Streaming additionally requires init_stream/1, sanitize_chunk/2, and finish_stream/1; Spectre validates those callbacks before opening the provider and feeds them only valid, core-screened UTF-8. The extension owns any bounded lookahead needed for its model-specific syntax.

The same extension specification is stored in the portable inference descriptor, so recovery does not silently select a different policy. A stack or package such as Pulse may choose the module and its portable options per invocation or model family; provider adapters remain responsible only for transport decoding. Omitting :reply_sanitizer preserves the built-in behavior exactly. sanitize_reply: false explicitly bypasses both layers.

An extension must preserve the same one-way stream invariant as the built-in sanitizer and may suppress, but never synthesize or expand, visible text. Spectre rejects invalid UTF-8 and oversized extension deltas. A terminal callback failure falls back to the already-safe core result. An incremental callback failure terminates the attempt because Spectre cannot safely continue a partially transformed stream.

If only the canonical terminal matters, do not enumerate:

{:ok, stream} = Spectre.stream(instance, input, stream_options)
{:ok, %Spectre.Result{} = result} = Spectre.await_result(stream, 60_000)

The first await_result/2 claims an unattached handle as a result-only consumer and drives provider demand internally. If enumeration is already active, it waits for that consumer's terminal Result. Waiters are bounded and a timed-out waiter is removed without making the session retain an abandoned caller.

Backpressure

The preferred adapter mode is pull transport:

MapSet.new([:stream, :pull_transport, :incremental_usage])

The StreamSession issues at most one transport credit at a time. Provider chunks and logical events are different units: one chunk may contain several SSE events, or one event may span chunks. The adapter assembles transport framing and returns a bounded list of normalized ProviderEvent values.

A push adapter must declare:

MapSet.new([:stream, :push_transport, :bounded_push_transport])

:bounded_push_transport certifies a real bound before messages enter the session mailbox. A bounded session queue alone cannot backpressure an Erlang mailbox. Spectre rejects push adapters that omit the capability. Once inside the session, event count, bytes per delta, buffered bytes, buffered event count and events per transport item all have explicit limits. Overflow terminates the attempt; text is never discarded silently.

Delivery model

Provider delivery always uses the stream session's mailbox. open/2 and resume/3 run in the session process, so an asynchronous transport can capture self() as its destination. The :pull_transport and :push_transport capabilities describe demand and upstream buffering; they do not select a different delivery channel.

A callback-style HTTP client can be bridged through a monitored helper:

def open(descriptor, opts) do
  session = self()
  ref = make_ref()

  with {:ok, helper} <- start_helper(session, ref, descriptor, opts) do
    state = %{
      session: session,
      ref: ref,
      helper: helper,
      helper_monitor: Process.monitor(helper)
    }

    {:ok, state, %{}}
  end
end

The helper sends messages such as {:my_adapter, ref, chunk} to session. For a pull transport, request_transport_item/1 grants one credit to the helper; the helper must wait for that credit before asking the client for the next chunk. Prefer monitoring the helper rather than linking it. Stream sessions trap exits for orderly cancellation, so a linked helper's death is delivered as {:EXIT, pid, reason} to handle_transport/2 instead of killing the session.

A client that cannot pause callback delivery cannot truthfully declare :pull_transport. It must use :push_transport with :bounded_push_transport and enforce a real upstream bound before data enters the session mailbox.

Adapter contract

The core behaviour is Spectre.Inference.StreamAdapter:

@callback capabilities(profile, opts) :: MapSet.t(atom())
@callback open(descriptor, opts) :: {:ok, state, metadata} | {:error, term()}
@callback request_transport_item(state) :: {:ok, state} | {:error, term()}
@callback handle_transport(message, state) ::
            {:ok, [Spectre.Inference.ProviderEvent.t()], state}
            | {:ignore, state}
            | {:error, term(), state}
@callback cancel(state, reason) :: :ok | {:error, term()}

Optional resume/3 and reconcile/3 callbacks require matching declared capabilities. :cost_usage means cumulative cost is authoritative for the configured immutable pricing ref. Hard cost budgets are rejected when that capability is unavailable. :incremental_usage means cumulative usage can be enforced during generation.

Profile capability negotiation is fail-closed for selectors backed by a profile catalog: the selected metadata must include :stream in profile_supports. Spectre.Inference.Selector.Default is the narrow exception because it has no catalog to query and therefore cannot produce profile capability evidence. In that case the profile gate is not applicable and StreamAdapter.capabilities/2 is the authoritative gate; the adapter must still declare :stream and one valid transport mode or preparation fails. This is not a fallback from a known profile rejection.

Callbacks run in the session process and must initiate asynchronous transport work and return promptly. In particular, open/2 should start the request; the first owned transport item proves provider progress. For pull mode, {:ignore, state} means a mailbox message did not consume the outstanding credit. A consumed chunk that produces no complete logical event returns {:ok, [], state}.

Stream sessions trap exits so orderly supervisor shutdown can run cancel/2. A linked provider helper therefore delivers {:EXIT, pid, reason} to handle_transport/2; adapters should prefer monitoring helpers whose death is a provider failure, or explicitly ignore expected exit messages. Cancellation must return promptly: each session has a one-second shutdown grace period, after which the supervisor kills it and remote cancellation remains best-effort.

Provider sequencing is all-or-nothing for one attempt. An adapter may leave provider_sequence unset on every normalized event. Once it emits the first numbered event, every later delta, usage, started, completed or failed event must carry exactly the next non-negative integer. In particular, a numbered delta followed by unnumbered usage is a protocol violation. This rule spans transport batches and also applies after resume from the durable sequence floor.

Use Spectre.LLM.provider_opts/2 when translating the supplied keyword list to an HTTP/SDK request. Core state, control, budget and stream options must not be sent to a provider endpoint.

Run Spectre.Inference.StreamAdapter.Conformance.run/4 with a portable descriptor and deterministic transport messages in every adapter repository. It verifies capability negotiation, pull-credit calls, event batches, global ordering, terminal cardinality, optional cancel/reconcile reply shapes, and both adapter-owned byte bounds. To prove the raw bounds, implement the test-only conformance_fixture/4 callback: the runner supplies a binary one byte over the configured limit, the callback wraps it in the adapter's real transport-message shape, and the runner requires handle_transport/2 to return {:error, :provider_stream_overflow, state}. A real local TCP/SSE integration test is still required to prove socket flow control, the actual client/parser wiring and remote cancellation.

Cancellation and steering

Cancellation is idempotent:

:ok = Spectre.Inference.Stream.cancel(stream, :user_requested)

The Instance commits the cancellation command before the session asks the adapter to cancel remotely. A remote cancellation failure is not rewritten as success: the terminal receipt records the remote status as ambiguous.

Steering replaces the attempt:

{:ok, replacement} =
  Spectre.Inference.Stream.steer(stream, "Focus only on the last section")

The original Enumerable terminates with :superseded. It never follows the replacement epoch. Start enumerating replacement explicitly. This keeps sequence and fence validation local to one attempt and prevents a UI from silently joining text generated under different instructions.

Restart and resume

Instance death interrupts the live session and best-effort cancels the provider. Recovery inspects the canonical provider status:

• not started: dispatch may safely begin;
• cursor plus :resume: a successor Invocation is committed with a fresh epoch;
• provider request id plus :reconcile: the adapter can classify uncertain work;
• neither capability: the Run ends explicitly as interrupted or ambiguous.

The old handle is never updated in place. When recovery created a successor, the owner can request it using the old handle as a bearer/fence proof:

{:ok, replacement} = Spectre.resume_stream(instance, old_stream)

The call validates the previous consumer-token digest and recovery lineage. It cannot attach a stale or foreign handle.

Limits and budgets

The session enforces every value it can observe directly:

Two raw values are invisible to the core after parsing, so the adapter owns their enforcement:

These are defaults, not fixed ceilings. Override them on the inference when a provider's real event envelope requires more space:

stream_max_transport_chunk_bytes: 1_000_000,
stream_max_parser_residual_bytes: 1_000_000

Both values must remain finite positive integers. Raising the parser residual also raises the worst-case memory retained by every live stream, so configure it from the provider protocol's maximum event size rather than the model's total response size.

Capacity is enforced separately from byte and time limits:

Node-wide capacity is owned by an internal supervised capacity process. A slot is reserved before dispatch, transferred to the stream session, and released on every terminal path, including never-attached consumers and process death.

inference_budget accepts input_tokens, output_tokens, total_tokens, cost, attempts and duration_ms. The Instance owns aggregate reservation and settlement; each session receives an immutable BudgetSnapshot. A hard cost limit also requires inference_pricing_ref and authoritative cost usage. Heartbeat activity never extends the absolute deadline.

Usage-bearing stream events expose usage_quality as :provider, :estimated or :unavailable. Provider counters keep :provider only while Spectre can retain them unchanged. If conservative accounting raises a token counter from the reserved-input or output-byte floor, the attempt is labelled :estimated; that weaker label remains sticky because a later cumulative update cannot prove the retained maximum was token-exact.

The session translates those runtime option names and passes their resolved values to open/2 and resume/3 under an explicit namespace:

spectre_bounds: [
  max_transport_chunk_bytes: 256_000,
  max_parser_residual_bytes: 256_000
]

Adapters must fail with :provider_stream_overflow instead of retaining or truncating excess bytes. The conformance runner mechanically exercises both limits. Normalized delta binaries may split a UTF-8 codepoint; Spectre buffers the bounded trailing bytes and yields only complete, valid UTF-8 events.

Observer lane

Enable text-free committed progress separately:

opts = [
  inference_observer_lane: true,
  inference_progress_commit_interval: 5_000,
  inference_progress_limit: 256
]

{:ok, _subscription} = Spectre.Inference.Events.subscribe(instance)

Sessions send heartbeat snapshots to their Instance. The Instance updates its ephemeral liveness clock immediately, commits a throttled latest-value progress snapshot, and only then publishes an event. Raw deltas never use pub/sub. The same configured LRU limit is enforced when canonical checkpoints are restored or reconciled, so a forged progress map cannot bypass the live bound.

Receipt modes

Streaming works with :disabled, :observational and :required receipt modes. Required mode needs a durable Instance Checkpoint Store and a payload-capable Spectre.Receipt.Sink. Selection and dispatch intent cross the checkpoint/outbox barrier before provider start. See Boundary receipts.