Read-only Python reader for the Orca trace format v0.

OrcaReplay's core is TypeScript, but the people who turn traces into evaluation datasets work in Python. This package makes a recorded run readable from pandas, PyTorch, or a notebook without shelling out to orca and parsing its output — and it is the evidence that v0 is a format rather than a TypeScript library with a document next to it.

• No third-party dependencies. Python 3.10+ standard library only, so it can be dropped into an environment with an already-pinned scientific stack.
• Read-only, deliberately. There is no writer here and there will not be one. The TypeScript implementation owns the write path, and a second writer means a second redaction implementation — which is how a secret leaks.
• Fully typed, ships py.typed.

pip install -e python/            # from an OrcaReplay checkout
# or, since there is nothing to build:
export PYTHONPATH="$PWD/python:$PYTHONPATH"

from orca_trace import TraceReader, derive_checkpoints, snap_to_checkpoint

reader = TraceReader.open("examples/traces/run_9f2c14a03b71")

reader.manifest.run_id          # 'run_9f2c14a03b71'
reader.manifest.adapter_id      # 'claude-code'
reader.verify_integrity()       # (True, '1a4bf78f…', '1a4bf78f…')   spec §6

events = reader.events()        # list[TraceEvent], or reader.stream() for a 500 MB trace
len(events)                     # 27
reader.problems()               # []  — everything skipped, and why. Empty means nothing was.

# Large payloads are spilled to content-addressed blobs (spec §2.2); .payload() reads them back.
request = next(e for e in events if e.type == "model.request" and e.blob)
reader.payload(request)         # {'messages': [...]}  — decoded JSON
diff = next(e for e in events if e.type == "fs.change")
reader.payload(diff)            # b'--- a/src/auth.ts\n…'  — bytes for non-JSON media types

# Checkpoints are derived from the log, never recorded (spec §3).
checkpoints = derive_checkpoints(events)
[c.seq for c in checkpoints]    # [1, 13, 14, 15, 16, 17, 18, 21, 24, 25, 26]

# A fork target that is not a checkpoint moves — and the reader tells you it moved.
snap_to_checkpoint(checkpoints, 20)
# (Checkpoint(seq=18, turn=3, fs_tree='9c1e4bd7…'), True)

Run the full example, which answers real questions from a trace:

$ python3 python/examples/analyze.py
run_9f2c14a03b71  adapter=claude-code  exit=1
integrity: OK
26 events, 6 turns, 10 checkpoints, 0 problems

tokens by model
  claude-opus-5      in   40350  out    934  calls   5

filesystem churn by turn
  turn 3   +18/-4  1 file(s)  src/auth.ts

fork targets
  seq 17  -> checkpoint 17 (exact), tree 9c1e4bd7f0a3821e5c94b6d2087fa31c4e5b9d63
  seq 20  -> checkpoint 18 (snapped back from 20), tree 9c1e4bd7f0a3821e5c94b6d2087fa31c4e5b9d63

Models: TraceEvent, Manifest, BlobRef, Checkpoint, Turn — all frozen dataclasses. Constants: EVENT_TYPES, ACTORS, SCHEMA_VERSION, RUN_ID_PATTERN, INLINE_PAYLOAD_LIMIT.

Spec §2 makes two tolerances mandatory, and this reader treats both as reportable, not silent:

tests/test_conformance.py reads examples/traces/run_9f2c14a03b71/ — written by the TypeScript implementation — and asserts this reader derives the same checkpoints, turns, and causal chains as packages/core/src/graph.ts. If that test ever fails, the disagreement is the bug: fix the spec first, then whichever implementation it says is wrong.

python3 -m pytest python/ -q

Places where v0 left real room, and what this reader chose (see the tests for each):

• format: date-time is an annotation in JSON Schema by default; the reference implementation asserts it via ajv-formats, so this reader asserts it too and rejects ts without an offset.
• causes[i] < seq is prose in §2.1 but is not encoded in the schema, so it is reported rather than enforced — dropping the event would make the two readers disagree about a trace's contents.
• §2.1 calls seq dense but no artifact checks it; this reader reports gaps and resynchronises.
• §3's "conversation prefix up to n" is inclusive: an unanswered model.request is itself a checkpoint, matching the TypeScript implementation — it is the point you want to retry.
• §6 defines integrity for events.jsonl only, so blob digests are checked on request (verify=True), not by default.

Apache-2.0.