refactor(core)!: model the provider prompt by role, with one file part

CHANGELOG.md

@@ -9,6 +9,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Breaking
 
+- The prompt a provider receives is modelled by role (`aimux-core`), as the AI
+  SDK's `LanguageModelV4Message`. `LanguageModelPrompt` is
+  `Vec<LanguageModelMessage>`: `System { content: String }`,
+  `User { content: Vec<UserPart> }`, `Assistant { content: Vec<AssistantPart> }`,
+  `Tool { content: Vec<ToolPart> }`. The parts are `TextPart`, `FilePart`,
+  `ReasoningPart`, `ToolCallPart`, `ToolResultPart`. A provider sees one file
+  part, `FilePart { data: FileData, media_type, filename }`; the five
+  user-facing file variants are folded into it by
+  `convert_to_language_model_prompt`, which now returns `Result` and rejects a
+  part its role does not allow. `LanguageModelPromptMessage` is removed. The
+  user-facing `ModelMessage` and `ContentPart` are unchanged. Recordings store
+  the new prompt JSON: a system message's `content` is a string and every file
+  input is a `file` part.
+
 - Provider options and provider metadata have one typed shape, the AI SDK's
   `Record<string, JSONObject>`: `SharedProviderOptions` and
   `SharedProviderMetadata` (`ProviderMetadata` is an alias) are

aimux-core/src/composite.rs

@@ -19,7 +19,7 @@
 use std::sync::Arc;
 
 use crate::language_model::LanguageModel;
-use crate::language_model_message::{LanguageModelPrompt, LanguageModelPromptMessage};
+use crate::language_model_message::{LanguageModelMessage, LanguageModelPrompt};
 use crate::result::GenerateContent;
 use crate::types::{InputTokenUsage, OutputTokenUsage, Usage};
 
@@ -117,10 +117,6 @@ pub fn build_aggregator_prompt(
         body.push('\n');
     }
 
-    out.push(LanguageModelPromptMessage {
-        role: crate::message::Role::User,
-        content: vec![crate::content::ContentPart::text(body)],
-        provider_options: None,
-    });
+    out.push(LanguageModelMessage::user_text(body));
     out
 }

aimux-core/src/generate.rs

@@ -527,12 +527,22 @@ pub async fn generate_text(
     prompt: impl Into<ModelPrompt>,
     options: GenerateTextOptions,
 ) -> Result<GenerateTextResult, AiMuxError> {
-    // 1. Convert user prompt to provider-facing prompt.
+    let (messages, instructions) = split_prompt(prompt.into(), options.instructions.as_deref());
+    let lm_prompt = convert_to_language_model_prompt(&messages, instructions)?;
+    generate_text_from_language_model_prompt(model, lm_prompt, messages, options).await
+}
+
+// Replay already has a provider prompt. Keep it intact instead of converting
+// through user content, which cannot represent every provider part.
+pub(crate) async fn generate_text_from_language_model_prompt(
+    model: &dyn LanguageModel,
+    lm_prompt: crate::language_model_message::LanguageModelPrompt,
+    messages: Vec<ModelMessage>,
+    options: GenerateTextOptions,
+) -> Result<GenerateTextResult, AiMuxError> {
     let repair_tool_call = options.repair_tool_call.clone();
     let tools = options.tools.clone();
     let operation_instructions = options.instructions.clone();
-    let (messages, instructions) = split_prompt(prompt.into(), operation_instructions.as_deref());
-    let lm_prompt = convert_to_language_model_prompt(&messages, instructions);
 
     // 2. Build CallOptions.
     let mut call_options = options.into_call_options(lm_prompt);
@@ -874,7 +884,7 @@ pub async fn stream_text(
     let tools = options.tools.clone();
     let operation_instructions = options.instructions.clone();
     let (messages, instructions) = split_prompt(prompt.into(), operation_instructions.as_deref());
-    let lm_prompt = convert_to_language_model_prompt(&messages, instructions);
+    let lm_prompt = convert_to_language_model_prompt(&messages, instructions)?;
 
     // 2. Build CallOptions.
     let mut call_options = options.into_call_options(lm_prompt);