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Breaking Change: CLI Now Subcommand-First

April 7, 2026 Breaking change in arize-phoenix 14.0.0 The Phoenix server CLI is now subcommand-first. Flags that previously preceded the subcommand must now follow it. Before: 
phoenix --dev serve
phoenix --host 0.0.0.0 --port 6006 serve
python -m phoenix.server.main --dev --dev-vite-port 5173 serve
After: 
phoenix serve --dev
phoenix serve --host 0.0.0.0 --port 6006
python -m phoenix.server.main serve --dev --dev-vite-port 5173
The db migrate subcommand is unchanged. Pass --database-url directly to the subcommand that needs it, or rely on PHOENIX_SQL_DATABASE_URL: 
phoenix serve --database-url "postgresql://..."
phoenix db migrate --database-url "postgresql://..."
The deprecated --enable-websockets flag has been removed. Use phoenix serve --help or phoenix db migrate --help for full option listings.

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Breaking Change: Legacy px.Client() Removed

April 7, 2026 Breaking change in arize-phoenix 14.0.0 phoenix.session.client.Client (accessed as px.Client()) has been removed. All client interactions now go through arize-phoenix-client. 
pip install arize-phoenix-client
Before: 
import phoenix as px

client = px.Client(endpoint="http://localhost:6006")
spans_df = client.get_spans_dataframe()
After: 
from phoenix.client import Client

client = Client(base_url="http://localhost:6006")
spans_df = client.spans.get_spans_dataframe()
The endpoint parameter is now base_url. When omitted, the client falls back to environment variables or http://localhost:6006. The new client organizes methods under resource namespaces (.spans, .traces, .datasets, .experiments) instead of flat methods on the client object. See the v14 migration guide for a full method-by-method mapping.

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Breaking Change: /v1/evaluations Endpoint Removed

April 7, 2026 Breaking change in arize-phoenix 14.0.0 The POST /v1/evaluations and GET /v1/evaluations REST endpoints have been removed. Use the annotations API instead:
PreviousReplacement
POST /v1/evaluations (span evals)POST /v1/span_annotations
POST /v1/evaluations (trace evals)POST /v1/trace_annotations
POST /v1/evaluations (document evals)POST /v1/document_annotations
GET /v1/evaluationsclient.spans.get_span_annotations(...)
Before: 
from phoenix.trace import SpanEvaluations
import phoenix as px

px.Client().log_evaluations(
    SpanEvaluations(eval_name="Hallucination", dataframe=results_df)
)
After: 
from phoenix.client import Client

Client().spans.log_span_annotations_dataframe(
    dataframe=results_df,
    annotation_name="Hallucination",
    annotator_kind="LLM",
)
protobuf is no longer a direct dependency of the Phoenix server (it remains a transitive dependency via OpenTelemetry gRPC packages).

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Breaking Change: Evals 1.0 Removed

April 7, 2026 Breaking change in arize-phoenix-evals 3.0.0, arize-phoenix-client 2.3.1+ arize-phoenix-evals 3.0.0 removes the legacy evals 1.0 module and the legacy experiments module from the Phoenix server package. Removed from arize-phoenix-evals:
  • The entire legacy/ subpackage and its models/ wrappers
  • MultimodalPrompt, PromptPartContentType, and PromptPart types — all adapter methods now use PromptLike
Removed from arize-phoenix:
  • phoenix.experiments — the legacy experiment execution module (functions.py, tracing.py, evaluators/)
  • phoenix.experiments.types — use phoenix.client.__generated__.v1.DatasetExample instead
Experiments migration: 
# Before
from phoenix.experiments.types import Example
from phoenix.experiments.evaluators import create_evaluator
from phoenix.experiments import run_experiment, evaluate_experiment

experiment = run_experiment(dataset, task, evaluators=[...])

# After
from phoenix.client.__generated__.v1 import DatasetExample as Example
from phoenix.client.experiments import create_evaluator, run_experiment, evaluate_experiment

experiment = run_experiment(dataset=dataset, task=task, evaluators=[...])
The phoenix-client experiments module (phoenix.client.experiments) is the replacement and has no dependency on arize-phoenix-evals.