docs: enrich dynamic variables table and document function.parameters tool support

[dhruva-reddy]

Description

Two related additions surfaced by an FDE conversation about progressive authentication, where customers asked which Liquid variables are available and which tools support customer-defined function.parameters.

fern/assistants/dynamic-variables.mdx

• Add 5 new rows to the Default Variables table:
  • {{phoneNumber.number}} — the Vapi number that received/placed the call
  • {{phoneNumber.name}} — display name set on the number
  • {{call.id}} — Vapi-generated call UUID, useful for log correlation
  • {{call.type}} — inboundPhoneCall / outboundPhoneCall / webCall
  • {{transport.callSid}} — provider-side CallSid, useful for BYOT customers
• Tweak the {{now}} row's description to point at Advanced date and time usage below (since {{now}} is hardcoded UTC and the LiquidJS "now" | date: "...", "TZ" form is the only way to get other timezones).
• Add a callout linking to /tools/static-variables-and-aliases for the trust-tier framing and the within-assistant tool-chaining pattern.

fern/tools/custom-tools.mdx

• Add a new section "Other tool types that accept custom function parameters" with a matrix showing where customer-defined function.parameters works (function, apiRequest, code, handoff) vs. where the schema is Vapi-controlled or auto-derived (transferCall, endCall, dtmf, voicemail, sms, slack-send-message, GHL/Google integrations, mcp, make, and the Anthropic-native bash/computer/textEditor tools).
• Cross-link to /tools/static-variables-and-aliases for the static-parameters pattern that pairs with function.parameters.

Judgment calls — variables intentionally NOT exposed

Variable	Why skipped
{{phoneNumber.id}}, {{phoneNumber.provider}}	Internal IDs / infra fingerprints; no clear customer use case in prompts.
{{call.status}}, {{call.startedAt}}, {{call.assistantId}}	Internal state machine fields that are confusing if surfaced in customer prompts mid-call.
{{assistant.id}}, {{assistant.name}}	Encourages brittle customer code that hardcodes behavior on assistant identity (rename-fragility).
{{currentDateTime}}	Redundant with {{now}}; adds confusion ("which one?") for zero gain.
{{transport.provider}}	Internal-infra fingerprint (which transport vendor we use under the hood).

Testing Steps

• fern check -- 0 errors
• All internal links resolve (5 links across the two files)
• No JSON code blocks were modified
• Verify the new rows render correctly in the Default Variables table on the preview deployment
• Verify the cross-link callout renders as a Note block
• Verify the matrix on custom-tools.mdx renders correctly with column wrapping
• Style guide compliance (active voice, present tense, no marketing language)

Skipped

• test-writer / code-reviewer (docs-only PR, per write-pr skill skip rules)

Related

• Source: FDE conversation in this session about progressive auth and the variable bag
• Companion to merged feat: add "Passing data between assistants" page to squads #1033 (squads passing-data page) and merged docs(static-variables): add prompt-injection threat model and trust tiers #1035 (static-variables threat model and trust tiers)

[dhruva-reddy]

• docs: enrich dynamic variables table and document function.parameters tool support #1037 👈 (View in Graphite)
• main

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[github-actions]

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