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

[cunninghamcard-bit]

Reading the diff: 43,300 of the 45,865 deleted lines are one commit, 37523b89, which deletes 34 hand-written mock-server test files. The type change itself is the other commits: 61 files, +2544 / −2565. Review by commit.

What

The prompt a provider receives is modelled by role, with one file part, as upstream's LanguageModelV4Message. Fourth of the protocol-type pull requests; it sits on the provider-options-type one and comes before the provider-factory PR (#214), because every vendor converter reads this type.

Before and after

Concept	Before	After
Message	LanguageModelPromptMessage { role, content: Vec<ContentPart>, provider_options } for every role	LanguageModelMessage, an enum by role: System { content: String }, User { content: Vec<UserPart> }, Assistant { content: Vec<AssistantPart> }, Tool { content: Vec<ToolPart> }
Parts	the user-facing ContentPart union, any part under any role	TextPart, FilePart, ReasoningPart, ToolCallPart, ToolResultPart, each defined once; each role lists the parts it allows
File input	five variants reach the provider: Image, File, FileBase64, FileUrl, FileReference	one FilePart { data: FileData, media_type, filename }
Conversion	strings wrapped, parts cloned	convert_to_language_model_prompt folds the five file variants into FilePart and returns InvalidPrompt for a part its role does not allow or a system message that is not plain text

The user-facing ModelMessage, ContentPart, Role and their JSON are unchanged.

Vendor code

Each converter matches on the message variant and the per-role part. Its five file arms become one arm that matches FilePart.data. Arms for role and part combinations that the types rule out are deleted. The request JSON for a given prompt is unchanged.

Tests

34 hand-written mock-server test files are deleted in the first commit. They build provider prompts from the removed struct and assert Rust's own earlier behaviour; the same files are deleted on the provider-factory branch. Recorded-cassette tests, upstream-sample tests and the remaining suites are converted and pass.

Not in this pull request

• ToolResultPart keeps result and is_error; upstream has a structured output union.
• ReasoningPart.signature and ToolCallPart.thought_signature stay as fields; upstream carries them in provider options.
• Upstream's reasoning-file, custom and tool-approval-response parts are not added. Nothing produces them in a single-step runtime.
• The user-facing message types are a later pull request, after feat(providers)!: AI SDK-shaped provider factories (RFC-0036 Part I, Rust) #214.

Wire format

Recordings store the provider prompt. Its JSON changes in two places: a system message's content is a string, and a file part is {"type":"file","data":…} for all five former variants. Cassettes and contract fixtures are unchanged. TypeScript: LanguageModelPromptMessage is replaced by LanguageModelMessage and the part types.

Verification

cargo fmt, cargo clippy --workspace --all-targets -D warnings, cargo doc, gen_ts_types.py --check; tests of core, provider-utils, providers, ffi, web, replay, cli: 2612 passed, 0 failed; Node and Python bindings cargo check. Without the deleted test files: 29 files, +1681 / −1859. With them and the changelog entry: 95 files, +2544 / −45865.

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https://claude.ai/code/session_01WGwanvo9WLU9HWD7sRR8TS