Build: ModelClient + the Append-Only Log

Maps to: Task 7. Kind: Build.

Objective

Create the panoptes-harness crate. Define the ModelClient trait (with #[async_trait]) and ModelResponse. Write the append-only JSONL writer and the test that guards its most important property: a second append must not overwrite the first.

Scaffold

Create (new crate — add it to workspace members):

• crates/panoptes-harness/Cargo.toml — [dependencies]: panoptes-core = { path = "../panoptes-core" }, plus serde, serde_json, reqwest, tokio, async-trait, chrono, sha2, clap, anyhow (all { workspace = true }); [dev-dependencies]: wiremock = "0.6", tempfile = "3". (sha2 is for the dispatch chapter's response-id hashing — the original task plan omits it there, so declare it now.)
• crates/panoptes-harness/src/lib.rs, src/client.rs (the trait), src/jsonl.rs (the writer).

Dependencies this chapter exercises: async-trait, serde_json (one record per line), tempfile (dev).

Expected result: cargo test -p panoptes-harness jsonl → 1 test passes (appends_without_truncating).

The spec (givens)

• ModelResponse fields: text: String, usage: Usage (from core). Derives: Debug, Clone.
• The trait: #[async_trait] pub trait ModelClient: Send + Sync with exactly two methods — fn model_name(&self) -> &str (the pinned model string) and async fn generate(&self, prompt: &str) -> anyhow::Result<ModelResponse> (single-turn; the prompt is the entire input).
• append_record(path: &Path, rec: &ResponseRecord) -> anyhow::Result<()> — opens with OpenOptions::new().create(true).append(true), writes one serde_json line.

Full code: Answer Key, Task 7.

Concepts exercised

• Defining an async trait with async_trait.
• std::fs::OpenOptions with .append(true) for append-only semantics.
• Testing filesystem behavior with tempfile.

The build loop (you drive)

1. Write the failing test appends_without_truncating: write two records, assert the file has two lines and each is valid JSON.
2. Predict: what would the test show if you opened the file with .create(true).write(true) instead of .append(true)? (This is the bug the test exists to catch.)
3. Run, check.
4. Implement the writer.
5. Run green, commit.

Done when

cargo test -p panoptes-harness jsonl passes and the append-only property is proven.