dialt-sdk

The headless Python SDK for the Dialt realtime voice API. Dialt manages speech recognition, turn-taking, interruption handling, speech generation, reasoning and tool orchestration through a single API; this SDK connects telephony bridges, services, evaluations and custom devices to it.

Install the dialt-sdk distribution and import its public API from dialt.

uv add dialt-sdk

Register a Python tool

Define an async function once. ToolRegistry derives its name, description and JSON Schema; pass the generated definitions to the ordinary session configuration. The decorator leaves the function callable from your application.

from dialt import DialtMode, ToolRegistry

tools = ToolRegistry()

@tools.register(read_only=True, expected_duration="instant")
async def lookup_order(order_id: str) -> dict:
    """Look up the current order status.

    Args:
        order_id: The customer's order identifier.
    """
    return await order_store.lookup(order_id)

mode = DialtMode(instructions="Help callers with their orders.", tools=tools.definitions)
# In your application's handler for a broker-dispatched tool_call:
result = await tools.call(event.data["name"], event.data["args"])
# Set verified=True only after your application has established successful completion.
await session.send_tool_result(event.data["id"], result, outcome="succeeded", verified=True)

order_store, session and event above belong to your application. For a complete text-session example with tracked tasks, cancellation and quiet progress, see tool_updates.py. Run it with DIALT_API_KEY in the environment:

uv run --project sdk/python python -u sdk/python/examples/tool_updates.py

Keep the event consumer running while tools execute so audio, cancellation and other calls can still be handled. The registry does not consume events, schedule tasks, retry operations, infer success or grant permission. Your host owns these decisions and sends exactly one terminal result per dispatched call. Only execute a broker-dispatched tool_call; declaring a permissioned tool does not authorize running it directly.

Registration contract

• tools.register(function, **options) and @tools.register(**options) register async functions; @tools.register uses defaults. Bound async methods work too.
• Parameter annotations support str, int, float, bool, None, Literal values (strings, integers, booleans, null), unions, list[T] and dict[str, T], recursively. Future annotations are resolved with Python's get_type_hints; referenced types must be resolvable there.
• The cleaned docstring is the tool description, including any parameter explanations. Override name or description if needed. Required parameters have no Python default: str | None still requires an argument unless a default is provided. Return annotations do not define the result envelope.
• Registration rejects duplicate names, missing/unsupported parameter types, non-async functions, positional-only parameters, variadic parameters and invalid defaults. call rejects missing, extra or incorrectly typed arguments before execution, without coercion; booleans are not numbers and non-finite floats are rejected. Each invocation receives its own argument copies.
• Existing tool policy options such as requires_permission or deferred pass through unchanged. Safety and duration are never inferred from the function name or annotation.
• tools.definitions returns an independent JSON-serializable manifest, usable at session start, with set_tools, or in eval cases. Custom JSON Schemas still use ToolDefinition directly; a registry's parameters cannot be overridden. No broker frame or dependency is added.

Choose what the caller hears

Routine updates should normally stay quiet. Use speech for information that helps the caller at that moment, and send the final result once the operation has finished.

Progress and partials never complete the call. Do not send a spoken partial and then repeat the same information as a terminal result. reply must be a boolean, so a string such as "false" cannot accidentally request speech. A request to speak is not a playback receipt.

For a deferred tool, notify_on_complete=False keeps its final result in context without a proactive completion wake. It does not suppress an ordinary call's answer. A partial with interaction is a separate, durable request for a caller decision; use it only when the work needs an answer. See the background tool guide for deferral, decision interactions and their acknowledgements.

A string greeting is the conversation's first assistant turn, not presentation-only audio. It enters model context and is bargeable in voice mode; when interrupted, playback stops and only the heard prefix remains in context. Use greeting=False to disable it.

deferred: True permits a tool call to become a background job after the host sends send_tool_deferred: the call can outlive the voice turn, the conversation carries on while the host works, and with notify_on_complete the agent delivers the result when the floor is free, even if the caller has moved on to another topic. For work that should release the voice turn immediately, call send_tool_deferred(id, handle, status_label=...) before continuing it in the background. The handle identifies that call, not your worker: mint a fresh one per call (e.g. f"job-{call_id}") and check the returned acknowledgement — re-using a live handle is rejected as handle_in_use, and a rejected defer leaves the call on the ordinary tool timeout, so it expires while you think it is backgrounded. Keep a handle-to-worker map if several calls should feed one long-running worker. For proactive host announcements, call await session.inject_context(text, role="context", reply=True, message_id="job-42"). It returns the broker's authoritative acknowledgement (accepted plus optional retryable/detail). For a typed user turn, the same message_id and input_source="text" appear on the final ASR event; spoken input uses input_source="voice". The role defaults to "context", reply defaults to False, and omitted message IDs are generated by the SDK. Text is limited to 1–2000 characters. An accepted message_id is an idempotency key for the logical session. After an acknowledgement loss, retry the identical payload and ID (including after resume) to replay delivery proof without injecting a duplicate turn. Reusing an accepted ID with different content is rejected; the broker retains up to 512 accepted receipts per logical session.

Save session.resume_token from the connected session; after a transport loss, pass it as resume_token=... to the replacement DialtSession.connect(...) call so pending jobs are re-announced as tool_deferred_resume events within the broker's bounded resume window.

Automatic session recovery

DialtSession.connect(..., auto_reconnect=True) (the default) handles that transport loss for you: on any abnormal close (a WS close code other than 1000, e.g. 1006 loss, 1011 upstream lost, 1012/1013 broker drain/handoff), the SDK redials the same URL with the retained start-frame configuration (voice, instructions, tools, mode, user, timezone, capabilities) plus the latest resume_token, so the broker (possibly a different instance) restores the session from its Postgres stash. Redials back off exponentially (0.5s base, capped at 5s, up to 12 attempts by default; tune with reconnect_base_s, reconnect_max_s, max_reconnect_attempts), and resume_token keeps rotating on every accepted ready, including a resumed one.

session.state reports "connecting", "live", "reconnecting", or "closed", and session.events() surfaces the recovery itself:

• reconnecting ({"attempt": 1, "code": <close code>}): the connection dropped and a redial is under way. events() keeps running, it does not end here.
• reconnected ({"attempt": <n>}): the redial succeeded and the session is live again.
• resume_failed ({"code": "resume_failed", "detail": ..., "retryable": False}): the broker rejected the resume (the window closed, or the token was already consumed). Terminal: resume_token is cleared and events() ends right after, with no session_end.
• error ({"code": "reconnect_failed", "detail": ...}): every redial attempt failed and the chain gave up. Terminal, events() ends right after.
• A clean server close (WS code 1000, e.g. an idle sign-off or end_call) still ends the session with session_end and never reconnects, same as before.

While state == "reconnecting", send_audio/stream_audio silently drop the frame instead of raising or buffering it (buffered stale audio must never flush into the resumed session once it comes back), and every other control method (send_text, inject_context, set_voice, set_tools, handoff_agent, and the rest) raises DialtError(code="reconnecting", retryable=True) so a caller can retry once reconnected arrives instead of getting the ambiguous connection_closed. set_voice, set_instructions, and set_tools fold their change into the retained mode so a later reconnect replays it; set_tool_choice folds only a non-one_shot choice, and only when the retained mode already has a non-empty tools list (matching set_tools resetting tool_choice to "auto" on the wire). Pass auto_reconnect=False to get the pre-recovery behavior instead: any drop just ends events().

The SDK deliberately does not own capture, playback, pacing or echo cancellation. Live playback integrations must implement the playback contract.

For a text session, keep the same conversation configuration and replace the media loop with committed turns:

async with await DialtSession.connect(
    "wss://api.dialt.com/v1/realtime",
    api_key=os.environ["DIALT_API_KEY"],
    mode=DialtMode(modality="text", instructions="Help customers with their orders."),
) as session:
    await session.send_text("Where is order A123?")
    async for event in session.events():
        if event.type == "utterance":
            print(event.data["text"])

Text mode is WebSocket-only and emits no audio. Model behavior, tools, greeting, history and conversation lifecycle events remain the same.

DialtMode(end_call=True) lets the agent end the session itself through the managed end_call(farewell) tool: the farewell is sent, then session_end_requested arrives with it. That advisory does not confirm that the peer heard or acted on the farewell. Stop automatic new-turn injection but keep any local output already queued; a client that needs post-close audio drain owns that policy. The server waits a short, cancellable grace for a real user turn before it closes. Off by default; the host then ends the session with wrap_up or by closing. DialtMode(end_call_when="the user asks to end the call") states your own ending condition instead of Dialt's default (the caller asks you to end the call); the condition is declared to the agent as part of the tool.

Mid-session agent handoff, instructions, voice switch, tool swap and wrap-up

set_instructions(text) replaces the session instructions from the next reply on; a reply already in flight, tool answer included, finishes under the old ones, and the platform persona above them is untouched. set_voice(key) switches the roster voice from the next reply on, confirmed by a voice event. set_tools(tools) replaces the client tool manifest for an agent whose capabilities change by call phase.

For an atomic same-session handoff, use await session.handoff_agent(instructions=..., tools=..., voice=..., context=..., operation_id=...). The SDK generates operation_id when omitted; it only correlates this live request and is not an idempotency or retry token. The call waits through an optional queued acknowledgement for the final applied or rejected acknowledgement. On timeout or disconnect its outcome is uncertain and it is never retried. Await it before a mode setter. After an applied handoff, the caller recipe may use its separately acknowledged inject_context(..., reply=True) call to make the new agent respond first.

The individual setters remain available when you need a narrower change. Send them between replies (after done); with new_speaker the server refuses set_instructions (instructions_busy, retryable) from the moment a turn is accepted until its reply's done. Passing the caller to a human is a different operation: a permission-gated client tool whose host moves the call leg, then request_wrap_up.

async for event in session.events():
    if event.type == "tool_call" and event.data["name"] == "handoff_to_agent":
        await session.send_tool_result(event.data["id"], {"handoff_complete": True},
                                       outcome="succeeded", verified=True)
    elif event.type == "done" and handoff_pending:
        await session.handoff_agent(
            instructions=SPECIALIST_INSTRUCTIONS, tools=SPECIALIST_TOOLS,
            voice="southern_us_female")
        await session.inject_context(
            f"The caller {name} has just been passed to you by intake; reason: {reason}.",
            role="context", reply=True)

request_wrap_up(reason) asks the model for a graceful sign-off in persona, for a host-enforced time limit or a transfer the host is about to make. It never cuts a reply in flight; the session closes with session_end once the sign-off has played.

See the complete Python reference and the Twilio bridge quickstart.

Policy guidance

Define each rule once in mode.policy with id, when, and do:

{"include_instructions": true, "background_guidance": true,
 "rules": [{"id": "supervisor", "when": "The caller requests a supervisor.",
            "do": "Call request_supervisor and explain the tool result to the caller."}]}

Both switches default true and work independently. Declare and handle the tool normally; existing permission checks still apply. Background checks quietly inject guidance about the reviewed caller or assistant turn without requesting another reply or executing tools. Source revisions, flag updates/retractions, errors and completion events are reported through the ordinary event interface. Delivery is not proof the agent complied.

Previous action configurations remain accepted temporarily during migration. See the policy contract.

Hosted evals

dialt.evals creates cases and starts runs on the Dialt evals dashboard with your account API key (dk_...; existing ck_... keys remain valid). Cases are JSON files in your repository; upsert_case keeps the hosted copy in step by name, so a re-push updates rather than duplicates.

from dialt import EvalsClient, load_cases

evals = EvalsClient(api_key=os.environ["DIALT_API_KEY"])
cases = evals.upsert_cases(load_cases("evals/"))          # one case per *.json file
run = evals.start_run([c["id"] for c in cases], modality="text")
print(evals.dashboard_url(run["id"]))
result = evals.wait(run["id"])                            # polls until terminal
print(result["status"], [a["status"] for a in result["attempts"]])

A case has name, starter (or target.greeting, when the agent opens the call), target and simulator (each a session mode document, the shape DialtMode.to_wire() produces, so every session option is a case option), fixtures, checks and limits; the field reference is in the evals guide. converse-recipes ships a converse-evals push evals/ command built on this client.

Relaying two sessions (simulations)

dialt.relay cross-pipes two sessions so one can play the user for the other: the building blocks behind converse-recipes' converse-sim and the hosted evals in the Dialt webapp.

from dialt.relay import TextTurnRelay, VoiceTurnRelay

# text: forward each committed target utterance to the simulator once tool work has settled
relay = TextTurnRelay(simulator.send_text)
relay.utterance(event.data["text"])      # on the target's `utterance`
relay.working(active)                    # on `working`
relay.done()                             # on `done`: forwards after TURN_RELAY_SETTLE_S

# voice: the simulator's virtual microphone, a paced stream that never stops
relay = VoiceTurnRelay(simulator)
relay.start()                            # line noise flows from now on, like a live line
await relay.audio(event.audio)           # on each target `audio` event: queued, played at real time
await target.send_client_event(          # on the target's `interrupted`: what the simulator
    "playback_stopped", **relay.interrupted(event.data))   # never heard, as a real client reports it
relay.canceled()                         # on the target's `canceled`: drop the rescinded audio
await relay.close()                      # at the end; a line that hangs up first just stops the mic

The broker measures turn-end silence in received audio, so a simulated caller's mic must keep streaming between turns and after an interrupted clip, not only while someone speaks; the receiving side's own endpointer then closes each turn on the trailing silence. Both relays take on_error to surface background failures. An eval harness that expects a simulated caller to make a decision must leave that caller an opportunity to respond before ending its relay. That is an eval-harness decision, not a general done or session_end_requested API guarantee.

WebRTC transport (experimental)

Experimental: the API is stable, but this transport is newly shipped and still being hardened on real networks; ws remains the default and recommended fallback.

Pass transport="webrtc" to DialtSession.connect(...) to carry the session over WebRTC (UDP) instead of the default WebSocket. Requires uv add "dialt-sdk[webrtc]". ws remains the default; most headless callers are fine on it. See the Python guide's WebRTC section.

Licensed under the Apache License 2.0. This license applies to the SDK, not to the hosted Dialt service, its models, or its server-side implementation. Runtime dependencies remain under their own licenses.

Application tool approval

The permission source vocabulary is caller (the default) or application; the old conversation and external values are not accepted.

Set requires_permission: True and permission_source: "application" on a tool to require approval from your application. Omit the source for caller consent verified by AI. Forward permission_pending events whose source is "application" to your approval application, displaying the exact tool, arguments, status_label and expires_at. Keep the session event consumer running while the reviewer decides. In your authenticated approval callback:

await session.resolve_tool_permission(request_id, "approve")  # or "decline"

The authenticated session host is trusted to submit decisions; authenticate the reviewer in your application. Observe permission_resolution in session.events() and check event.data["accepted"] and any rejection reason. Sending is not acceptance or completion. An accepted approval releases the exact stored tool call; handle tool_call and return its result as usual. Caller consent does not satisfy application approval, and the assistant does not ask the caller for it.

Voice selection uses the server roster from GET /v1/voices. Irish Male keeps the stable key classic. Circuit (circuit) is the default when no voice or saved preference is supplied. Empty keys are rejected locally; unknown, retired or unavailable keys return the server's nonretryable invalid_voice error. A rejected mid-session switch leaves the confirmed voice unchanged, including on reconnect. Retired keys must be replaced explicitly.