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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.

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Breaking Change: GraphQL Forward Pagination Requires first

April 7, 2026 Breaking change in arize-phoenix 14.0.0 Three GraphQL connection fields now require an explicit first argument and no longer accept backward pagination (last/before):
TypeFieldMax first
Projectspans1000
Tracespans1000
ProjectSessiontraces1000
Before: 
query {
  project(id: "...") {
    spans {
      edges { node { name } }
    }
  }
}
After: 
query {
  project(id: "...") {
    spans(first: 100) {
      edges { node { name } }
    }
  }
}
Queries that omit first will fail with "first" is required. Queries that pass first greater than 1000 will fail with "first" must be less than or equal to 1000. Backward pagination with last/before is no longer supported on these fields. This change protects the server from unbounded queries that could cause excessive memory usage and slow response times.

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New: Resume Interrupted SDK Experiments

April 7, 2026 New in arize-phoenix 14.0.0 SDK experiments still run in the caller’s Python process. Phoenix v14 adds dedicated resume APIs so you can continue interrupted experiments without starting over from scratch. Additional capabilities:
  • Resume API — use resume_experiment(experiment_id=..., task=...) to re-run only missing or failed task runs.
  • Async resume API — use async_resume_experiment(...) for async tasks.
  • Evaluator support — pass evaluators to resume_experiment to run or re-run evaluations on completed task runs after resuming.
The run_experiment call signature is unchanged. See Resume Interrupted Experiments for full documentation.