deepseek-harness/docs/rfc/implemented/testing/2026-06-11-property-based-testing.md

RFC: Property-based testing for protocol-shaped code

Status: implemented

Merges the original proposal and the decision record for one topic. It found a real BlockAssembler duplicate-block-end bug on first run.

Problem

Example-based tests pin the cases we thought of. The harness's core is protocol-shaped — chunk streams, event logs, schema conversion, inbox scheduling — where the input space is combinatorial and the interesting bugs live in interleavings nobody wrote an example for. The motivating evidence: a block-assembly ordering bug once survived 100% line coverage of the happy paths. Per-file 100% coverage proves every line ran, not that every interleaving is correct.

Decision

Adopt fast-check (a root devDependency) with one tests/properties.spec.ts per protocol-shaped package, generators tuned for realistic-but-adversarial inputs (not uniform noise) and numRuns kept so the suite stays well under ~10s locally. Failures print a reproducible seed. (The original proposal also sketched a nightly CI job running 100× the iterations; that was not shipped — the property suite runs only in the normal push/pull_request CI, and a scheduled high-iteration job remains possible future work.)

• dsh-llm / BlockAssembler: arbitrary chunk streams (valid + malformed: duplicate indices, stragglers, missing block-start). Invariants: blocks() count ≤ distinct indices seen; re-assembly idempotent (blocks() is stable across repeated calls and message().content mirrors it); blocks() never throws and yields only valid content-block tags; finish reflects the last finish chunk, defaulting to {kind:'stop'} when none arrives.
• dsh-session: arbitrary event logs. Invariants: deriveMessages deterministic; replay-from-seed identical; seq strictly monotonic; non-message events never affect derived history; derived content is decoupled from the log.
• dsh-tools: arbitrary SchemaSpec. Invariants: JSON Schema required equals the required:true keys at every level; conversion total; and the composition with runtime arg validation — generated args satisfying a spec pass validateArgs, and targeted corruptions (dropped required key, non-object top level) are rejected. This closes the validator/InferArgs drift risk.
• dsh-agent-loop: arbitrary send schedules against a never-exhausting adapter, driven through the agent/status settle signal (no wall-clock sleeps). Invariants: no message lost; turn numbers strictly increase; status transitions stay on the legal machine.

Consequences

• Generator quality is the value lever — the generators bias toward small index pools and short strings so collisions and interleavings are common.
• It already paid off: the BlockAssembler stream found a real bug — a duplicate block-end at the same index rewrote a completed block. Fixed (first close wins, matching the existing straggler rule) with a dedicated regression test.
• A property flake from a timeout is a finding, not something to retry away. The loop properties are deterministic by construction (settle on agent/status), so a hang is a real defect.
• Property tests supplement, not replace, the example tests that pin specific branches for the 100%-coverage gate.