How to handle empty-state and fallback greetings

Back to how-to guides Use this pattern when startup data can be missing, no ON_START branch can match, or a channel does not deliver a proactive welcome.

Concept

A fallback greeting is the safe default that keeps the first turn from feeling broken. You need one in three cases:

No branch condition matches.
A value that a branch or template reads is empty.
The channel does not send the ON_START welcome.

ABL gives you four ways to write a fallback. Choose by what the greeting must do next.

Goal	Use
Send a generic welcome, then run the first flow step in the same turn	A top-level `RESPOND` next to `BRANCHES`, with no `ELSE`
Send a generic welcome as the whole startup turn	An `ELSE` branch
Greet by name when the name exists, and generically when it does not	One `RESPOND` with a `{{#if}}` section
Send no proactive greeting	No `ON_START`. In a flow agent, the first flow step speaks first.

The difference between the first two rows matters when the agent has a FLOW. A branch-selected welcome, including ELSE, is the whole startup turn. The first flow step waits for the first customer message. A top-level RESPOND that no branch replaced is a prelude. The first flow step runs right after it, in the same turn. Branch conditions must be expressions. A plain-language condition such as "the customer is returning" never matches. It also produces missing-variable warnings at compile time.

Minimal working example

AGENT: Fallback_Greeting_Agent
GOAL: "Recover when startup conditions do not match"

ON_START:
  RESPOND: "Welcome. I can help with service, billing, or returns."
  BRANCHES:
    - IF: session.interaction.current.language == "fr"
      RESPOND: "Bonjour. Je peux vous aider avec le service, la facturation ou les retours."

FLOW:
  entry_point: ready
  steps:
    - ready

ready:
  REASONING: false
  RESPOND: "How can I help?"
  THEN: COMPLETE

session.interaction.current.language holds the language code the runtime resolved for the session. It comes from the session request, the contact profile, the channel, or the agent default language. It is empty when none of them supplies a language. An empty value never equals "fr", so the fallback runs. The ready step stands in for your own flow.

How it works

The runtime runs ON_START in this order:

Top-level SET and CALL run first.
A top-level SET_INTERACTION_LANGUAGE or CLEAR_INTERACTION_LANGUAGE runs next, before branch selection.
Branches are checked in order. The first matching branch wins.
If no branch matches, the runtime uses the top-level RESPOND and records a dsl_on_start_branch trace with matched: false.
If no branch matches and there is no top-level RESPOND, the runtime sends no welcome.

In the example, a French session gets only the French welcome. The ready step waits for the first customer message. Any other session gets the generic welcome followed by “How can I help?” in the same turn. A condition that fails at run time is skipped. The runtime records a dsl_on_start_branch_error trace and checks the next branch. A condition with invalid syntax in authored ABL fails compilation with INVALID_CEL_SYNTAX, so it does not reach a customer. In customer-visible text, a {{name}} that has no value renders as an empty string. “Welcome back, .” becomes “Welcome back, .”. Use a {{#if}} section when the value can be missing.

Common variations

Start without ON_START

Use the first flow step as the greeting when the channel or product experience should not send proactive content.

AGENT: No_Startup_Handler_Agent
GOAL: "Start safely even without a proactive startup response"

FLOW:
  entry_point: first_prompt
  steps:
    - first_prompt

first_prompt:
  REASONING: false
  RESPOND: "Hello. What can I help you with?"
  THEN: COMPLETE

Use an ELSE branch

Use ELSE when the generic welcome must replace the other branches and own the startup turn.

AGENT: Else_Greeting_Agent
GOAL: "Greet by language and keep a generic branch"

ON_START:
  BRANCHES:
    - IF: session.interaction.current.language == "fr"
      RESPOND: "Bonjour. Je peux vous aider avec le service, la facturation ou les retours."
    - ELSE:
      RESPOND: "Welcome. I can help with service, billing, or returns."

FLOW:
  entry_point: ready
  steps:
    - ready

ready:
  REASONING: false
  RESPOND: "How can I help?"
  THEN: COMPLETE

Both welcomes own the startup turn here. The ready step runs when the customer sends the first message. Put ELSE last. A top-level HANDOFF in ON_START never runs when an ELSE branch exists, and the compiler warns with ON_START_HANDOFF_UNREACHABLE.

Greet by name only when the name exists

Use one RESPOND with a {{#if}} section when the personal part is optional. The lookup_customer tool is a project-local assumption. It needs an execution binding in your project and returns a first_name field when the profile has one.

AGENT: Optional_Name_Greeting_Agent
GOAL: "Greet by name only when the profile lookup returns a name"

TOOLS:
  lookup_customer() -> object
    description: "Return the customer profile for this session"
    confirm: never

ON_START:
  CALL: lookup_customer
    AS: profile
  RESPOND: "{{#if profile.first_name}}Welcome back, {{profile.first_name}}. {{/if}}How can I help?"

FLOW:
  entry_point: ready
  steps:
    - ready

ready:
  REASONING: false
  RESPOND: "Tell me what you need."
  THEN: COMPLETE

If the lookup fails or returns no name, the customer sees “How can I help?”. If ON_START has no RESPOND and the tool returns a text content field, the runtime sends that text as the welcome. A RESPOND always takes priority over content.

Set the language before the greeting

Use SET_INTERACTION_LANGUAGE when a tool or profile holds the customer language. The runtime sets the language before it selects a branch. The branch then matches on session.interaction.current.locale.

AGENT: Profile_Language_Greeting_Agent
GOAL: "Greet in the saved language and fall back to a generic welcome"

TOOLS:
  load_profile() -> object
    description: "Return the customer profile, including preferred_language"
    confirm: never

ON_START:
  CALL: load_profile
    AS: profile
  SET_INTERACTION_LANGUAGE: "{{profile.preferred_language}}"
  RESPOND: "Welcome. I can help with service, billing, or returns."
  BRANCHES:
    - IF: session.interaction.current.locale == "fr-CA"
      RESPOND: "Bonjour. Je peux vous aider avec le service, la facturation ou les retours."

FLOW:
  entry_point: ready
  steps:
    - ready

ready:
  REASONING: false
  RESPOND: "How can I help?"
  THEN: COMPLETE

The load_profile tool is a project-local assumption. It needs an execution binding and returns a language tag such as fr-CA. The runtime rejects a tag that is invalid or not allowed for the session. Voice channels accept only the languages configured on the channel. Other sessions need language detection turned on in the project. A rejected or empty value leaves the language unchanged, so the generic fallback runs. A failed CALL leaves the language unchanged in the same way.

Know the channel fallbacks

Some channels apply their own fallback when ON_START provides no usable text.

Channel	Fallback order
Pipeline voice (KoreVG)	On inbound calls, a bound Greeting Audio clip plays first. The text chain is the channel Welcome Message, then the `ON_START` response, then the streamed startup text, then “Hello! How can I help you today?”. The text chain also runs when the clip cannot play.
Realtime voice (KoreVG)	`VOICE` `PLAIN_TEXT`, `RESPOND`, `MESSAGE_KEY`, a flow step named `greet_*`, a `welcome` or `greeting` template or message, then the channel Welcome Message. If none exists, the voice provider opens with its own prompt-driven greeting.
Agent Assist	The top-level `ON_START` `RESPOND`, then the `greeting` message, then “How can I help you?”. Branch responses are not used unless the welcome is rendered through a session.

Realtime voice never speaks a greeting that still contains an unresolved {{placeholder}}. Agent Assist skips it the same way.

Verification

Start a session that matches a branch. Confirm the branch response and a dsl_on_start_branch trace with matched: true.
Start a session that matches no branch, for example with no language set. Confirm the fallback response and a dsl_on_start_branch trace with matched: false.
Start a session where the optional value is missing. Confirm the greeting has no empty gap such as “Welcome back, .”.
Start a session without ON_START. Confirm the first flow step produces a clear first response.
On a channel that does not send the welcome, confirm a dsl_on_start_skipped trace. Confirm the first customer message still gets a response.
If you set the language, find interaction_language_override_changed. Confirm applied is true for a supported tag and false for an unsupported one.

Production readiness checklist

Define a generic fallback with a top-level RESPOND or an ELSE branch.
Keep fallback copy broad enough for unknown users.
Do not put sensitive personalization in fallback text.
Check that every value a condition reads has a source: a SET, a CALL, memory, or caller context.
Test the branch match, the branch miss, and the missing-value cases.
Test every deployed channel, including voice channels with a Welcome Message or Greeting Audio clip set.

Common mistakes

Mistake	Why it happens	How to avoid it
Omitting the generic fallback	Branch examples focus on the happy path.	Add a top-level `RESPOND`, an `ELSE` branch, or a safe first flow step.
Expecting the first flow step after a branch	A branch-selected welcome owns the startup turn.	Use a top-level `RESPOND` if the flow must continue in the same turn.
Writing a plain-language `IF:` condition	The text reads like routing guidance.	Write an expression. Plain text never matches.
Personalizing fallback text with unverified data	Startup data can be missing or client-asserted.	Keep fallback copy generic until identity and context are known.
Treating a branch miss as an error	Branches are conditional by design.	Test the miss path and confirm the fallback response.
Using `{{name}}` without a guard	A missing value renders as an empty string.	Wrap the personal part in a `{{#if}}` section.

Troubleshooting

Symptom	Likely cause	What to check
No greeting appears	The channel does not send the `ON_START` welcome, or there is no top-level `RESPOND`.	Look for `dsl_on_start_skipped` and `dsl_on_start_branch` in the trace.
The fallback shows when a branch should match	The condition reads a value that is empty at session start.	Check that a `SET`, `CALL`, or caller field produces the value before branches run.
A branch is skipped with an error trace	The condition failed to evaluate at run time.	Read the `dsl_on_start_branch_error` event for the condition and error.
The first flow step did not run after the greeting	A branch-selected welcome owns the startup turn.	Send a customer message, or move the greeting to a top-level `RESPOND`.
Greeting has a gap such as “Welcome back, .”	A template value was empty.	Use a `{{#if}}` section.
The language greeting never shows	The language tag was rejected, or the tool returned no tag.	Find `interaction_language_override_changed` in the trace. Check `applied` and `reason`.
Voice callers hear a different greeting	A channel Welcome Message or a Greeting Audio clip overrides it on pipeline voice.	Clear the Welcome Message. Turn off or unbind the Connect greeting.

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