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A filler is a generated phrase such as “One moment, let me check.” It can cover a tool’s waiting period before the substantive answer arrives. It uses the session’s speech output, so it can be heard on phone and browser transports.

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Enable one filler

Keep the agent and slow lookup from Using tools. Add this line after the Agent(...) definition, preserving the existing greeting and language:
Restart the server and start a new session. Ask “Check order 101.” When the lookup remains pending, the assistant can say a short waiting phrase, then give the delivery result. Turn the browser thinking sound off for this test so you can hear the filler behavior on its own. The default delay is one second; generation has a five-second timeout. A fast tool gets no filler. If generation is too slow or the reply starts first, the phrase is skipped. The default is at most one filler per turn.

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Control the wording and timing

Replace that line with:
Generation starts when the tool call is detected and overlaps the delay. The phrase is spoken only while the turn still needs a filler and no substantive reply has started. Omit model to use the session’s LLM, or set a full provider/model ID to choose a separate generator.

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Longer waits

For tools that legitimately take longer, opt into repeats:
Repeats are bounded and stop being useful once the answer arrives or the turn is interrupted. Test with representative tool latency; repeated acknowledgements can distract from the actual answer. Fillers consume LLM and TTS usage and appear in transcripts as fillers. First-audio latency can measure a filler instead of the useful answer; see Events and usage. Set agent.voice_config["filler"] = {"enabled": False} to explicitly disable an inherited filler. Empty objects mean unset, rather than disabling a server default. Continue to Background audio for a separate, optional room-tone layer, or Turn-taking and interruptions.