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Transport Adapter & Proxy reference

The @a5c-ai/transport-adapter package bridges API protocol differences between harnesses and upstream providers.

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Transport Adapter & Proxy

The @a5c-ai/transport-adapter package bridges API protocol differences between harnesses and upstream providers.

When Proxy is Needed

A harness needs the proxy when it speaks a different protocol than the upstream provider:

Harness Protocol	Upstream Protocol	Proxy?
Anthropic (Claude Code)	Anthropic	No
Anthropic (Claude Code)	OpenAI (Foundry)	Yes
Anthropic (Claude Code)	Google (Vertex)	Yes
OpenAI (Codex)	OpenAI (Foundry)	No
OpenAI (Pi via models.json)	OpenAI (Foundry)	Yes

Completion Engines

Engine	Upstream	Key Features
OpenAI	Azure Foundry	Tool normalization (`input_schema` → `parameters`), streaming `delta.tool_calls` accumulation
Google	Vertex AI	`functionCall`/`functionResponse` translation, `thoughtSignature` server-side store

Message Translation

The proxy translates between Anthropic and upstream formats bidirectionally:

**Anthropic → OpenAI**: tool_use → role:"assistant" + tool_calls, tool_result → role:"tool"
**Anthropic → Google**: tool_use → functionCall, tool_result → functionResponse (with toolIdToName mapping)
**OpenAI → Anthropic**: delta.tool_calls → content_block_start(tool_use) + input_json_delta
**Google → Anthropic**: functionCall → tool_use, thoughtSignature preserved via server-side store

Streaming Tool Support

The proxy handles streaming tool calls end-to-end:

Code

CompletionStreamEvent =
  | { type: 'text-delta', text: string }
  | { type: 'tool-call', id, name, arguments, metadata? }
  | { type: 'done', finishReason?, usage? }

See babysitter-plugin-flows for full sequence diagrams.