feat(helexa-acp): anthropic-messages provider

Stage 6b. Third provider impl, completing the wire-format trio
(openai-chat, openai-responses, anthropic-messages). Lets a
helexa-acp endpoint configured with `wire_api = "anthropic-messages"`
drive Claude models — either against Anthropic directly or via
cortex's /v1/messages translation surface.

## Encoder (CompletionRequest → Anthropic body)

- System messages flatten to the top-level `system` field
  (concatenated with blank lines when there are multiple).
- User text → `{role:"user", content:"..."}`.
- User MultiPart (text + images) → `content` array with Anthropic's
  distinct image shape: `{type:"image", source:{type:"base64",
  media_type, data}}` — structurally different from OpenAI's
  `image_url` data URI.
- Assistant text → `{role:"assistant", content:"..."}`.
- Assistant tool_calls → `content` array with optional `{type:"text"}`
  block plus one `{type:"tool_use", id, name, input:<parsed json>}`
  per call. The internal arguments JSON string is parsed back to a
  Value before encoding (Anthropic requires the parsed form);
  malformed JSON falls back to a String input so the request body
  still serialises.
- Tool result → `{role:"user", content:[{type:"tool_result",
  tool_use_id, content}]}` per Anthropic's convention (no separate
  `tool` role).
- `max_tokens` is required by Anthropic; defaults to 8192 when the
  request doesn't specify.

## Decoder (Anthropic SSE → CompletionEvent)

Named SSE events:

- `message_start` → captures input_tokens from `usage` for the
  eventual UsageStats.
- `content_block_start` (type=text) → TextDelta (initial text, if any).
- `content_block_start` (type=tool_use) → ToolCallStart; if a
  pre-buffered `input` is present, also emits a single
  ToolCallArgsDelta.
- `content_block_start` (type=thinking, for extended-thinking
  models) → ReasoningDelta.
- `content_block_delta` (text_delta) → TextDelta.
- `content_block_delta` (input_json_delta) → ToolCallArgsDelta,
  correlated by block index.
- `content_block_delta` (thinking_delta) → ReasoningDelta.
- `message_delta` → Usage (final output_tokens) + Finish with
  stop_reason mapped: end_turn/stop_sequence → "stop", max_tokens
  → "length", tool_use → "tool_calls".
- `message_stop` → stream terminates.
- `ping` ignored (Anthropic's keep-alive).
- `error` → yields Err and ends the stream.

## Wiring

- Authentication: `x-api-key` + `anthropic-version: 2023-06-01`
  headers (not Bearer). Both ship when api_key is configured;
  servers that don't care (cortex) ignore them.
- `WireApi::AnthropicMessages` in build_provider now constructs
  the provider instead of erroring "reserved for future".
- `provider::mod.rs` registers the new module.

18 new unit tests: encoder (system collapse, multi-system concat,
default max_tokens, multipart with image, tool_use blocks, tool
results, malformed JSON arg fallback), decoder (text streaming,
tool_use lifecycle, max_tokens→length mapping, empty deltas, ping
events, error events, cancellation, malformed payload skip,
thinking blocks).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/helexa-acp/src/main.rs

@@ -30,7 +30,8 @@ mod tools;
 use agent::Agent;
 use config::{Config, EndpointConfig, WireApi};
 use provider::{
-    Provider, openai_chat::OpenAIChatProvider, openai_responses::OpenAIResponsesProvider,
+    Provider, anthropic_messages::AnthropicMessagesProvider, openai_chat::OpenAIChatProvider,
+    openai_responses::OpenAIResponsesProvider,
 };
 
 /// Set up tracing. Logs go to stderr by default — stdout is
@@ -94,10 +95,7 @@ fn build_provider(endpoint: EndpointConfig) -> anyhow::Result<Arc<dyn Provider>>
     match endpoint.wire_api {
         WireApi::OpenAiChat => Ok(Arc::new(OpenAIChatProvider::new(endpoint)?)),
         WireApi::OpenAiResponses => Ok(Arc::new(OpenAIResponsesProvider::new(endpoint)?)),
-        WireApi::AnthropicMessages => Err(anyhow::anyhow!(
+        WireApi::AnthropicMessages => Ok(Arc::new(AnthropicMessagesProvider::new(endpoint)?)),
-            "endpoint '{}' wire_api 'anthropic-messages' is reserved for a future provider",
-            endpoint.name
-        )),
     }
 }
 

crates/helexa-acp/src/provider/mod.rs

@@ -17,6 +17,7 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 use tokio_util::sync::CancellationToken;
 
+pub mod anthropic_messages;
 pub mod openai_chat;
 pub mod openai_responses;