docs(agents): document the echo guard setting

[Him188]

Summary

Adds an Echo guard section to Agents > Build > Configuration, after Interruptions.

• Why it exists: callers' devices and the phone network normally cancel the agent's echo, but in a few calls (under 3%) some gets through in the agent's first seconds and cuts it off.
• What it does: for 3 seconds after the agent first starts talking, the caller can't interrupt it and what the caller says over the agent is dropped. Keypad entries and the web SDK's interrupt() still work.
• One key per call channel (web_voice, phone_inbound, phone_outbound), each true, false or null, in a table with the Builder labels and defaults (on, on, off).
• When to turn it off: when callers should be able to interrupt the agent within those 3 seconds.
• The config example in the same page shows a new agent's value (null).

Merge timing

The setting is live in production. The branch is rebased on main, whose api-reference/openapi.json already carries echo_guard from the scheduled schema update (#266), so no schema change is needed here.

Checks

• prettier --check agents/build/configuration.mdx passes.
• The anchors used (#system-variables, #keypad-input) exist.

Linear: FA-498.

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ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 7e0a3c89-dc15-4c67-95e3-5ed15ff4bad2

📥 Commits

Reviewing files that changed from the base of the PR and between 7d5908f and bfd413d.

📒 Files selected for processing: 1

• agents/build/configuration.mdx

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📝 Walkthrough

The configuration guide now describes conversation.echo_guard, its channel defaults and reset behavior, and which inputs remain effective during its three-second window. The draft response example includes "echo_guard": null.

Changes: Echo guard configuration

Layer / File(s)	Summary
Setting and example agents/build/configuration.mdx	The guide documents echo guard behavior, per-channel defaults, reset behavior, and inputs that remain effective during the guard window. The draft response example includes echo_guard: null.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to bfd41

The documentation matches the checked-in API schema for the described settings. No actionable merge risk is established by the available evidence.

Pre-merge checks: 5

Check name	Status	Explanation
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: documenting the conversation.echo_guard setting in the agents configuration documentation.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

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