Retell → ElevenLabs

Two export shapes, and how to tell them apart

Dispatch on response_engine.type. It is always present.

• retell-llm — a prompt-driven agent. Tools live in retellLlmData.general_tools and in per-state tools.
• conversation-flow — a node graph. Tools live in conversationFlow.tools.

Do not dispatch on starting_state; it is frequently absent.

The two shapes need different work. A retell-llm source is usually one prompt plus tools, so an EL agent with a prompt and maybe procedures is the right target. A conversation-flow source has real states and usually wants an EL workflow.

Construct mapping

Retell constructs that are not what a mapping table might suggest

• press_digit and send_sms are tools, never node types. They appear only in tool definitions. If a mapping table tells you to convert an "SMS node" or a "DTMF node", it is describing something Retell does not have.
• conversation-flow sources have no begin_message. Derive the opening line from start_speaker plus the start node instead. Treating the absence as "this agent has no greeting" produces a silent agent.
• equation transitions are the deterministic ones. They lower to expression conditions. Natural-language prompt transitions lower to LLM conditions. Most edges in a typical flow are the latter, but the former are where the silent-inversion trap lives.
• Ambient sound presets are a closed set on both sides and they do not correspond. EL's presets are office1, office2, restaurant, city, typing, and elevator1–elevator4. Anything else in the source has to be hand-mapped to the nearest, or dropped and mentioned to the user. background_sound can also be overridden per node, which the source cannot do.
• Response variable paths need rewriting, not copying. Retell's properties. prefix is its own convention and is not part of the response — for a GraphQL endpoint it becomes data., and a bare field name needs that prefix added rather than removed. Bracketed indexes ([0]) must become dotted (.0.). See the assignment section of reference/expressions.md; a path that does not resolve leaves the variable unset, which then silently routes every condition reading it down the absent path.
• Voice emotion. EL v3 expression and mood tags are square-bracketed — [calm], [warmly], [excited], [sigh]. They belong in agent-level prompt text, not in workflow node prompts; describe the intended tone in prose inside a node.

Where the real gaps are

Treat any "Retell has X and ElevenLabs does not" claim as wrong until you have searched and failed to find the feature. The overwhelming majority of them are, including several that circulate widely. Before telling a customer to build a workaround, look for the construct — a workaround they do not need is worse than an unanswered question. These are the ones that hold up:

• Per-node fine-tuning examples. No equivalent construct. Put the guidance in the node's additional_prompt instead. This is a real loss of a mechanism, though usually not of behaviour.
• Free-navigation ("flex") mode. There is no mode switch. Approximate it with broad bidirectional edges — an EL edge carries a forward and a backward condition on the same object — and keep the graph small enough to reason about.
• cool_down on a global node. Retell throttles re-entry — 5, 10, 92 seconds. A procedure trigger has no throttle of any kind, so a global that Retell allowed back in once a minute becomes one that can fire on consecutive turns. This is a behaviour change with no config to soften it: the only lever is the trigger's own wording, so write it to describe a situation the caller is in rather than a topic they mentioned. Say so when a source uses long cool-downs; one export carried six of them.
• Entering a global node on a deterministic test. A trigger is a plain string of natural language — there is no expression object, so a global whose in-edges gate on {{Party Size}} >= 7 cannot be lowered onto a procedure and stay deterministic. Two options, both fine: keep that node in the graph and let the deterministic edges reach it normally, or fold the numeric test onto the edges leaving its predecessors (see traps.md — an edge condition can conjoin a model judgment with a variable test, which is the capability a trigger lacks). Do not lower it and hope the wording holds.
• The warm-transfer briefing message. Retell can speak a private message to the receiving human before bridging. No such field exists on either ElevenLabs transfer surface — a phone-transfer node takes custom_sip_headers, edge_order, position, post_dial_digits, require_acceptance, transfer_destination, transfer_type and uui, and the transfer-to-number system tool's entries add only condition and phone_number. Names like agent_message or whisper_message are accepted and silently dropped on both, so a config that looks right will simply not brief anyone. Closest workaround: conference transfer lets the agent address both legs, so put the briefing in the prompt. (Agent-to-agent transfer is different — that does carry a transfer message.)
• A dedicated transfer-negotiation agent. The transfer-type set is closed and has no entry for "a separate agent negotiates with the target, then bridges". But most of what customers use it for is covered: require_acceptance screens the receiver and auto-releases the caller if the target never accepts, and agent_message briefs the receiver before bridging. The residual gap is only the separate negotiating agent.

These were commonly claimed as gaps and are not: DTMF, SMS, reusable components, node-scoped knowledge, guardrails, ASR keyword boosting, A/B testing, version pinning, human detection before bridging, whisper messages, MCP, mid-call variable capture, and QA scoring. Each has a real EL construct — see the mapping table above, or ask the relevant sibling skill.

go_back_conditions is on that list, and the reason is worth knowing, because it looks like the scariest thing about lowering a global node and it is not a gap at all. Retell needs those conditions because its global is a node — entering it means leaving the node you were on, so something has to decide when to come back. ElevenLabs has no equivalent need, in either direction:

• A free-form procedure is injected into your context and moves you nowhere. You never left the node, so there is nothing to return to and no condition to write.
• A structured procedure does move you to its own entry node, and the node you came from is recorded at activation and restored when the procedure ends. The return is structural rather than condition-driven, so it cannot fail to fire the way a missing go_back_condition can.

So do not spend effort porting these, and do not tell a customer they lose the behaviour. What you should tell them is the reachability consequence in traps.md: the trigger is offered on every turn and the model chooses, so nothing guarantees your authored order. That is the real cost. Note also that the closest thing to scoping is per-conversation rather than per-node: the procedure set can be narrowed in conversation-initiation client data, where the workspace allows it.

Two more are narrower than usually stated rather than absent: hold music during a transfer exists but is not configurable, and automated QA scoring exists per conversation and per turn, with latency, interruption and hallucination scoring genuinely missing.

SMS

Retell's SMS is a tool. ElevenLabs has a native two-way SMS channel, so an SMS-first agent needs no external API — inbound SMS routing is on by default for an imported SMS-capable number. The one case that still needs a webhook tool is sending a single SMS from inside a voice call.

Sequencing a conversation-flow migration

1. Identify the shape and read the graph. Count nodes by type before deciding anything.
2. Handle credentials in every tool definition first (SKILL.md, step 2).
3. Decide what to collapse — see the folding trap in reference/traps.md before committing to it, and expect a faithful conversion to be roughly node-count-neutral. Semantic merging of duplicate nodes is a later, test-backed pass.
4. Create the agent shell directly, then port the base prompt and derive the first message.
5. Create the tools and record the id each returns, keyed by the Retell tool_id — a workflow node references a tool by id, and two Retell tools can share a name. See step 4 in SKILL.md.
6. Build nodes, then edges. Order only the conditional edges — unconditional ones are moved last for you.
7. Convert each equation transition with a type guard; convert each numeric comparison only after the variable is guaranteed numeric. Compose the guards from reference/expressions.md — most of a real flow's equations need one, and an unguarded neq fires on absent data.
8. Give every transfer node its failure edge.
9. Run the mocked smoke test (SKILL.md step 5) before reporting anything as done, then route the remaining config and rollout to the sibling skills.