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deepseek-harness/docs/adr/0015-structured-error-taxonomy.md
Tianyi Cui 825b57aff9 feat(llm): structured error taxonomy with a shared HarnessError base (RFC 005 pt 2)
Introduce HarnessError in dsh-llm (the leaf package): a stable machine-routable
code distinct from the message, cause chaining, name from the subclass, plus
isHarnessError. LlmError, ToolArgsError, and InvariantError now extend it.

Tool failures carry the structure end-to-end: ToolExecutionResult gains
error: { name, code } (populated from a thrown HarnessError), and the loop
forwards it onto the tool/result session event (which gained the same optional
field) for retry/sandbox plugins and replay. The loop's toError wraps non-Error
throws in a HarnessError(code: UNKNOWN, cause) instead of a bare Error.

Landed last and in isolation so it's a pure upgrade over the plain Error+code
the earlier PRs used — independently revertible. Graduates RFC 005 pt 2 ->
ADR 0015; RFC 005 now fully implemented.
2026-06-14 01:07:28 +08:00

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# ADR 0015: Structured error taxonomy
Status: accepted (2026-06-14)
## Context
Failures crossed seams as bare strings. A tool error flattened to a text block — name, code, and stack lost — so a future sandbox/retry plugin couldn't tell ENOENT from EACCES, and the model got less actionable feedback than it could. A non-Error throw degraded further: the loop wrapped it in `new Error(String(x))`, dropping any code. And `LlmError` was the only typed error in the system, with no shared base, so there was nothing for a consumer to `instanceof` against generically.
This is the last of the RFC 005 pieces and the one the user was most skeptical of, so it was deliberately built **last and in isolation**: the earlier PRs (arg validation, dev invariants) threw plain `Error`s with a `code` field, decoupled from any shared base, so this change is a pure upgrade and is independently revertible without unpicking them.
## Decision
A single `HarnessError extends Error` base in `dsh-llm` (the leaf package every other imports — no new dependency edge): a stable `code` distinct from `message`, `cause` chaining via `ErrorOptions`, and `name` defaulting to the subclass. `isHarnessError` narrows at seams.
- `LlmError`, `ToolArgsError` (dsh-tools), and `InvariantError` (dsh-invariants) now extend it, keeping their existing codes.
- `ToolExecutionResult` gains optional `error: { name, code }`, populated in the registry's catch when the thrown value is a `HarnessError`. The agent loop forwards it onto the `tool/result` session event (which gained the same optional field), so the structured failure survives into the log for retry/sandbox plugins and replay. The model-facing text block is unchanged.
- The loop's `toError` wraps a non-Error throw in a `HarnessError` (`code: 'UNKNOWN'`, original chained as `cause`) instead of a bare `Error`, so even a bad throw carries a routable code into the session `error` event (which already surfaced `code`).
## Consequences
- Errors are machine-routable end-to-end: a plugin can branch on `error.code` rather than substring-matching a message.
- One base class is imported widely, but it lives in the package everyone already depends on, so the cost is a single import, not a new edge.
- `deriveMessages` does not surface `error` into model history — the model still sees the text block; the structured field is for code and replay.
- Reverting this PR returns the earlier errors to plain `Error`+`code` form; nothing else in the stack depends on the shared base.