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Use this page as the source of truth for request auth and compatibility headers.

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Client-to-backend auth

All managed-agents requests to Synth backend require:
  • Authorization: Bearer <SYNTH_API_KEY>
Example: 
curl -sS https://api.usesynth.ai/api/managed-agents/anthropic/v1/health \
  -H "Authorization: Bearer $SYNTH_API_KEY" \
  -H "anthropic-version: 2023-06-01"

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Compatibility headers

Synth managed-agents Anthropic-compatible routes use:
  • anthropic-version (required)
  • anthropic-beta (required for beta-only managed-agents features)
Typical beta value used in current examples:
  • managed-agents-2026-04-01

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Backend-to-upstream auth policy

The backend proxy uses service-token mode upstream:
  • HORIZONS_PRIVATE_SERVICE_API_KEY is required on backend service
  • upstream auth is handled server-side by backend proxy
  • client bearer token is still validated at backend edge for org access

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Header forwarding policy

Managed-agents proxy behavior is intentionally strict:
  • forwards compatibility headers (anthropic-version, anthropic-beta)
  • forwards request body/query as-is for supported endpoints
  • drops non-allowlisted compatibility headers

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Common errors

SymptomLikely cause
401/403 at backend edgeMissing or invalid SYNTH_API_KEY
5xx config-style proxy errorMissing backend HORIZONS_PRIVATE_BASE_URL or HORIZONS_PRIVATE_SERVICE_API_KEY
Upstream rejects request version/betaInvalid or unsupported anthropic-version / anthropic-beta