diff --git a/src/agent_env/a2a_agent/a2a_agent.py b/src/agent_env/a2a_agent/a2a_agent.py
index a0dc1707..9621f340 100644
--- a/src/agent_env/a2a_agent/a2a_agent.py
+++ b/src/agent_env/a2a_agent/a2a_agent.py
@@ -177,9 +177,9 @@ async def add_skill(deployed: "DeployedA2AAgent", skill: "Skill") -> dict:
                 description=artifact.description,
                 object_url=artifact.skill_object_url,
             )
-        if skill.s3_url is not None:
+        if skill.object_url is not None:
             return await A2AAgent.register_skill(
-                deployed, name=skill.name, description=skill.description, object_url=skill.s3_url
+                deployed, name=skill.name, description=skill.description, object_url=skill.object_url
             )
         return await A2AAgent.register_skill(
             deployed, name=skill.name, description=skill.description, skill_md=skill.to_skill_md()
diff --git a/src/agent_env/a2a_agent/object_transfer.py b/src/agent_env/a2a_agent/object_transfer.py
index 4dcc2b37..a7419cca 100644
--- a/src/agent_env/a2a_agent/object_transfer.py
+++ b/src/agent_env/a2a_agent/object_transfer.py
@@ -669,9 +669,4 @@ async def fetch_trajectory(
         return FetchedTrajectory(object_url=upload.object_url)
     if not isinstance(body, Mapping):
         return FetchedTrajectory()
-    if body.get("trajectory") is None and body.get("trajectory_s3_prefix"):
-        raise RuntimeError(
-            "trajectory get answered with trajectory_s3_prefix, which agent-env does not read; "
-            "the agent must return the trajectory inline or take the object form"
-        )
     return FetchedTrajectory(inline=body.get("trajectory"))
diff --git a/src/agent_env/a2a_agent/validator.py b/src/agent_env/a2a_agent/validator.py
index 82b08b05..457184ed 100644
--- a/src/agent_env/a2a_agent/validator.py
+++ b/src/agent_env/a2a_agent/validator.py
@@ -171,14 +171,14 @@ async def validate(
         skill_object_url = await asyncio.to_thread(
             A2AAgentValidator._upload_skill_fixture,
             agent,
-            name="validator-test-s3",
+            name="validator-test-object",
             description=(
                 "Validator test skill with project codename. "
                 "Use when asked about the project codename."
             ),
             body=(
                 "When asked for the project codename, respond with: "
-                "VALIDATOR-S3-99"
+                "VALIDATOR-OBJ-99"
             ),
         )
         skill_bundle_probe_url = await asyncio.to_thread(
@@ -255,7 +255,7 @@ async def validate(
             VerifyA2ATrajectoryStep(id=f"{task_id}-trajectory", version=None, a2a_agent_id=agent.id, depends_on=[TaskStepDependency(task_step_id=mcp_step_id)], fail_task_on_error=False),
             AddSkillsTaskStep(id=skill_chain_id, version=None, depends_on=deploy_agent_dep, fail_task_on_error=False, skills=[
                 Skill(name="validator-test", description="Validator test skill with secret code. Use when asked about the company secret code.", body="When asked for the company secret code, respond with: VALIDATOR-42"),
-                Skill(name="validator-test-s3", description="Validator test skill with project codename. Use when asked about the project codename.", s3_url=skill_object_url),
+                Skill(name="validator-test-object", description="Validator test skill with project codename. Use when asked about the project codename.", object_url=skill_object_url),
             ]),
             PromptAgentTaskStep(id=skill_prompt_id, version=None, prompt="What is the company secret code? What is the project codename?", prompt_id=skill_prompt_id, timeout_seconds=120,
                 depends_on=[TaskStepDependency(task_step_id=skill_chain_id)], fail_task_on_error=False),
@@ -263,7 +263,7 @@ async def validate(
                 depends_on=[TaskStepDependency(task_step_id=skill_prompt_id)], fail_task_on_error=False,
                 criteria=[
                     {"id": "secret_code_inline", "description": "Response contains VALIDATOR-42"},
-                    {"id": "secret_code_s3", "description": "Response contains VALIDATOR-S3-99"},
+                    {"id": "secret_code_object", "description": "Response contains VALIDATOR-OBJ-99"},
                 ]),
             VerifyA2ASkillConfigStep(
                 id=f"{task_id}-skill-config",
diff --git a/src/agent_env/artifact/artifacts/skill.py b/src/agent_env/artifact/artifacts/skill.py
index 830a169a..14db8f9d 100644
--- a/src/agent_env/artifact/artifacts/skill.py
+++ b/src/agent_env/artifact/artifacts/skill.py
@@ -72,8 +72,8 @@ def validate(
         if (skill_md is None) == (object_url is None):
             raise ValueError("specify exactly one of skill_md or object_url")
         if object_url is not None:
-            skill_md = _fetch_skill_md(object_url)
-        frontmatter, _body = _parse_skill_md(skill_md)
+            skill_md = fetch_skill_md(object_url)
+        frontmatter, _body = parse_skill_md(skill_md)
         _validate_frontmatter(frontmatter, expected_name=expected_name)
 
     @classmethod
@@ -87,7 +87,7 @@ def put(cls, id: str, *, skill_dir: Path) -> "SkillArtifact":
 
         skill_md_bytes = skill_md_path.read_bytes()
         cls.validate(skill_md=skill_md_bytes, expected_name=id)
-        frontmatter, _body = _parse_skill_md(skill_md_bytes)
+        frontmatter, _body = parse_skill_md(skill_md_bytes)
 
         files: dict[str, Path] = {}
         for p in sorted(skill_dir.rglob("*")):
@@ -125,7 +125,7 @@ def get_skill_files(self) -> "FileArtifactUniverse":
         return FileArtifactUniverse.get(self.skill_files_id)
 
 
-def _parse_skill_md(data: bytes) -> tuple[dict, str]:
+def parse_skill_md(data: bytes) -> tuple[dict, str]:
     text = data.decode("utf-8")
     if not text.startswith("---"):
         raise ValueError("SKILL.md must begin with a YAML frontmatter block delimited by '---'")
@@ -191,7 +191,7 @@ def _validate_frontmatter(fm: dict, expected_name: str) -> None:
                 raise ValueError("SKILL.md frontmatter 'metadata' must map strings to strings")
 
 
-def _fetch_skill_md(object_url: str) -> bytes:
+def fetch_skill_md(object_url: str) -> bytes:
     prefix = object_url if object_url.endswith("/") else object_url + "/"
     skill_md_url = prefix + _SKILL_MD_FILENAME
     try:
@@ -217,3 +217,8 @@ def download_skill(object_url: str) -> Iterator[Path]:
         if count == 0:
             raise ValueError(f"No objects found under {object_url}")
         yield tmp_path
+
+
+# Their former private names, which callers outside agent-env import and patch.
+_parse_skill_md = parse_skill_md
+_fetch_skill_md = fetch_skill_md
diff --git a/src/agent_env/cli/a2a_agent/add_skill.py b/src/agent_env/cli/a2a_agent/add_skill.py
index 0fc4e20c..1ec60762 100644
--- a/src/agent_env/cli/a2a_agent/add_skill.py
+++ b/src/agent_env/cli/a2a_agent/add_skill.py
@@ -5,15 +5,17 @@
 import click
 
 from agent_env.a2a_agent import A2AAgent
+from agent_env.artifact.artifacts.skill import fetch_skill_md, parse_skill_md
 from agent_env.a2a_agent.store import get_a2a_agent_instance_store
 from agent_env.store.base import NotFoundError
+from agent_env.cli.utils import deprecated_option, renamed_value
 from agent_env.task_step.task_steps.add_skills import Skill
 
 
 # ---------------------------------------------------------------------------
 #   agent-env a2a-agent add-skill --instance-id <id> --skill-artifact-id code-review
 #   agent-env a2a-agent add-skill --instance-id <id> --skill-md-path ./SKILL.md
-#   agent-env a2a-agent add-skill --instance-id <id> --skill-s3-url s3://bucket/prefix/
+#   agent-env a2a-agent add-skill --instance-id <id> --skill-object-url s3://bucket/prefix/
 # ---------------------------------------------------------------------------
 
 
@@ -40,23 +42,26 @@
     help="Local SKILL.md file to register inline",
 )
 @click.option(
-    "--skill-s3-url",
-    "skill_s3_url",
+    "--skill-object-url",
+    "skill_object_url",
     default=None,
-    help="S3 prefix containing a skill directory",
+    help="Object-store prefix containing a skill directory",
 )
+@deprecated_option("--skill-s3-url", "skill_s3_url", "--skill-object-url")
 def add_skill(
     instance_id: str,
     skill_artifact_id: Optional[str],
     skill_artifact_version: Optional[int],
     skill_md_path: Optional[Path],
+    skill_object_url: Optional[str],
     skill_s3_url: Optional[str],
 ):
     """Register a skill against a deployed A2A agent's /ext/skill-config endpoint."""
-    sources = [bool(skill_artifact_id), bool(skill_md_path), bool(skill_s3_url)]
+    skill_object_url = renamed_value("--skill-object-url", skill_object_url, "--skill-s3-url", skill_s3_url)
+    sources = [bool(skill_artifact_id), bool(skill_md_path), bool(skill_object_url)]
     if sum(sources) != 1:
         click.echo(
-            "Error: specify exactly one of --skill-artifact-id, --skill-md-path, --skill-s3-url",
+            "Error: specify exactly one of --skill-artifact-id, --skill-md-path, --skill-object-url",
             err=True,
         )
         raise SystemExit(1)
@@ -72,16 +77,12 @@ def add_skill(
     if skill_artifact_id is not None:
         skill = Skill(skill_artifact_id=skill_artifact_id, skill_artifact_version=skill_artifact_version)
         click.echo(f"Registering SkillArtifact '{skill_artifact_id}' v{skill_artifact_version or 'latest'}...")
-    elif skill_s3_url is not None:
-        from agent_env.artifact.artifacts.skill import _fetch_skill_md, _parse_skill_md
-
-        frontmatter, _ = _parse_skill_md(_fetch_skill_md(skill_s3_url))
-        skill = Skill(name=frontmatter["name"], description=frontmatter["description"], s3_url=skill_s3_url)
-        click.echo(f"Registering skill from {skill_s3_url} (name={skill.name})...")
+    elif skill_object_url is not None:
+        frontmatter, _ = parse_skill_md(fetch_skill_md(skill_object_url))
+        skill = Skill(name=frontmatter["name"], description=frontmatter["description"], object_url=skill_object_url)
+        click.echo(f"Registering skill from {skill_object_url} (name={skill.name})...")
     else:
-        from agent_env.artifact.artifacts.skill import _parse_skill_md
-
-        frontmatter, body = _parse_skill_md(skill_md_path.read_bytes())
+        frontmatter, body = parse_skill_md(skill_md_path.read_bytes())
         skill = Skill(
             name=frontmatter["name"],
             description=frontmatter["description"],
diff --git a/src/agent_env/cli/artifact/cli.py b/src/agent_env/cli/artifact/cli.py
index dfc85fcd..8d703c4a 100644
--- a/src/agent_env/cli/artifact/cli.py
+++ b/src/agent_env/cli/artifact/cli.py
@@ -47,7 +47,7 @@ def cli_put(
     click.echo(
         f"Created CliArtifact: id={result.id} version={result.version} "
         f"command_name={result.command_name} entrypoint={result.entrypoint} "
-        f"cli_files_id={result.cli_files_id} cli_s3_url={result.cli_object_url}"
+        f"cli_files_id={result.cli_files_id} cli_object_url={result.cli_object_url}"
     )
 
 
diff --git a/src/agent_env/cli/artifact/file_artifact_universe.py b/src/agent_env/cli/artifact/file_artifact_universe.py
index 6d070aa3..9a6452c9 100644
--- a/src/agent_env/cli/artifact/file_artifact_universe.py
+++ b/src/agent_env/cli/artifact/file_artifact_universe.py
@@ -6,6 +6,7 @@
 import click
 
 from agent_env.artifact import FileArtifact, FileArtifactUniverse
+from agent_env.cli.utils import deprecated_option, renamed_value
 from agent_env.store.ids import fs_safe, is_local_id, key_segment
 
 
@@ -67,14 +68,16 @@ def put(universe_id: str, file_artifact_ids: Tuple[str, ...]):
     help="Local directory; every file under it is uploaded into the bundle (relative path preserved)",
 )
 @click.option(
-    "--s3-url",
-    "s3_url",
+    "--prefix-url",
+    "prefix_url",
     default=None,
     help="Object-store prefix to upload the bundle under. Defaults to the version's own prefix in the configured "
     "object store (artifacts/file_artifact_universe/<id>/<version>/).",
 )
-def put_bundled(universe_id: str, file_dir: Path, s3_url: Optional[str]):
+@deprecated_option("--s3-url", "s3_url", "--prefix-url")
+def put_bundled(universe_id: str, file_dir: Path, prefix_url: Optional[str], s3_url: Optional[str]):
     """Upload every file under --file-dir as a single bundled FileArtifactUniverse."""
+    prefix_url = renamed_value("--prefix-url", prefix_url, "--s3-url", s3_url)
     files: dict[str, Path] = {}
     for p in sorted(file_dir.rglob("*")):
         if p.is_file():
@@ -83,13 +86,13 @@ def put_bundled(universe_id: str, file_dir: Path, s3_url: Optional[str]):
         click.echo(f"Error: no files found under {file_dir}", err=True)
         raise SystemExit(1)
 
-    click.echo(f"Uploading {len(files)} file(s)" + (f" under {s3_url}..." if s3_url else "..."))
+    click.echo(f"Uploading {len(files)} file(s)" + (f" under {prefix_url}..." if prefix_url else "..."))
     for rel in files:
         click.echo(f"  {rel}")
-    universe = FileArtifactUniverse.put_bundled(id=universe_id, files=files, prefix_url=s3_url)
+    universe = FileArtifactUniverse.put_bundled(id=universe_id, files=files, prefix_url=prefix_url)
     click.echo(
         f"Created FileArtifactUniverse: id={universe.id} version={universe.version} "
-        f"file_count={len(universe.file_artifact_ids)} bundle_s3_url={universe.bundle_object_url}"
+        f"file_count={len(universe.file_artifact_ids)} bundle_object_url={universe.bundle_object_url}"
     )
 
 
diff --git a/src/agent_env/cli/artifact/skill.py b/src/agent_env/cli/artifact/skill.py
index 40764c41..c12d72ce 100644
--- a/src/agent_env/cli/artifact/skill.py
+++ b/src/agent_env/cli/artifact/skill.py
@@ -5,7 +5,8 @@
 import click
 
 from agent_env.artifact import SkillArtifact
-from agent_env.artifact.artifacts.skill import _parse_skill_md, download_skill
+from agent_env.artifact.artifacts.skill import parse_skill_md, download_skill
+from agent_env.cli.utils import deprecated_option, renamed_value
 
 
 # ---------------------------------------------------------------------------
@@ -42,21 +43,23 @@ def skill():
     help="Local skill directory containing SKILL.md at the root",
 )
 @click.option(
-    "--s3-url",
-    "s3_url",
+    "--prefix-url",
+    "prefix_url",
     default=None,
-    help="S3 prefix containing the skill (s3://bucket/prefix/) — alternative to --skill-dir",
+    help="Object-store prefix containing the skill — alternative to --skill-dir",
 )
-def skill_put(artifact_id: str, skill_dir: Optional[Path], s3_url: Optional[str]):
-    """Upload a skill as a SkillArtifact, from a local directory or an S3 prefix."""
-    if (skill_dir is None) == (s3_url is None):
-        click.echo("Error: specify exactly one of --skill-dir or --s3-url", err=True)
+@deprecated_option("--s3-url", "s3_url", "--prefix-url")
+def skill_put(artifact_id: str, skill_dir: Optional[Path], prefix_url: Optional[str], s3_url: Optional[str]):
+    """Upload a skill as a SkillArtifact, from a local directory or an object-store prefix."""
+    prefix_url = renamed_value("--prefix-url", prefix_url, "--s3-url", s3_url)
+    if (skill_dir is None) == (prefix_url is None):
+        click.echo("Error: specify exactly one of --skill-dir or --prefix-url", err=True)
         raise SystemExit(1)
 
-    source_cm = download_skill(s3_url) if s3_url else contextlib.nullcontext(skill_dir)
+    source_cm = download_skill(prefix_url) if prefix_url else contextlib.nullcontext(skill_dir)
     with source_cm as source_dir:
-        if s3_url:
-            click.echo(f"Downloaded skill from {s3_url} to {source_dir}")
+        if prefix_url:
+            click.echo(f"Downloaded skill from {prefix_url} to {source_dir}")
 
         file_count = sum(1 for p in source_dir.rglob("*") if p.is_file())
         click.echo(f"Uploading SkillArtifact bundle ({file_count} file(s))...")
@@ -65,7 +68,7 @@ def skill_put(artifact_id: str, skill_dir: Optional[Path], s3_url: Optional[str]
             f"Created SkillArtifact: id={result.id} version={result.version} "
             f"spec_version={result.agent_skills_spec_version} "
             f"skill_files_id={result.skill_files_id} "
-            f"skill_s3_url={result.skill_object_url}"
+            f"skill_object_url={result.skill_object_url}"
         )
         click.echo()
         click.secho(_BANNER, fg="magenta", bold=True)
@@ -83,7 +86,7 @@ def skill_put(artifact_id: str, skill_dir: Optional[Path], s3_url: Optional[str]
             for k, v in result.skill_metadata.items():
                 click.echo(f"    {click.style(k, fg='cyan')}: {click.style(v, fg='green')}")
 
-        _, body = _parse_skill_md((source_dir / _SKILL_MD_FILENAME).read_bytes())
+        _, body = parse_skill_md((source_dir / _SKILL_MD_FILENAME).read_bytes())
         preview = body[:_BODY_PREVIEW_CHARS]
         truncated = len(body) > _BODY_PREVIEW_CHARS
         click.secho(f"Body (first {_BODY_PREVIEW_CHARS} chars):", fg="cyan", bold=True)
diff --git a/src/agent_env/cli/env/mcp_server.py b/src/agent_env/cli/env/mcp_server.py
index 67572c1f..516e1a85 100644
--- a/src/agent_env/cli/env/mcp_server.py
+++ b/src/agent_env/cli/env/mcp_server.py
@@ -409,5 +409,5 @@ def create_cli(env_id: str, env_version: int | None, command_name: str | None, f
     click.echo(
         f"Created CliArtifact: id={artifact.id} version={artifact.version} "
         f"command_name={artifact.command_name} entrypoint={artifact.entrypoint} "
-        f"cli_s3_url={artifact.cli_object_url}"
+        f"cli_object_url={artifact.cli_object_url}"
     )
diff --git a/src/agent_env/cli/utils.py b/src/agent_env/cli/utils.py
index 7df4225e..496b7736 100644
--- a/src/agent_env/cli/utils.py
+++ b/src/agent_env/cli/utils.py
@@ -32,6 +32,24 @@ def build_platform_option(f):
     )(f)
 
 
+def deprecated_option(old: str, dest: str, replacement: str):
+    """A hidden `old` flag, the deprecated spelling of `replacement`: using it notes the replacement on stderr.
+    Merge the two with :func:`renamed_value`."""
+    def note(ctx: click.Context, param: click.Parameter, value: str | None) -> str | None:
+        if value is not None:
+            click.echo(f"Warning: {old} is deprecated and will be removed; use {replacement}.", err=True)
+        return value
+
+    return click.option(old, dest, default=None, hidden=True, callback=note)
+
+
+def renamed_value(new: str, new_value: str | None, old: str, old_value: str | None) -> str | None:
+    """The value of flag `new`, or of its deprecated spelling `old`; giving both is a usage error."""
+    if new_value is not None and old_value is not None:
+        raise click.UsageError(f"{old} is the deprecated spelling of {new}; give only {new}")
+    return old_value if new_value is None else new_value
+
+
 def refuse_unwritable_ids(*ids: str) -> None:
     """Refuse, before anything is built, an id the store a put writes it to wouldn't take, such as an @local id the
     image's suffix makes too long."""
diff --git a/src/agent_env/task_step/task_steps/add_skills.py b/src/agent_env/task_step/task_steps/add_skills.py
index b82dc4f9..3c632f4e 100644
--- a/src/agent_env/task_step/task_steps/add_skills.py
+++ b/src/agent_env/task_step/task_steps/add_skills.py
@@ -5,12 +5,13 @@
 import hashlib
 import logging
 import re
-from dataclasses import dataclass
+from dataclasses import InitVar, dataclass
 from typing import ClassVar, Optional
 
 from agent_env.task_step.context import TaskStepContext
 from agent_env.entity_refs import EntityRef
 from agent_env.task_step.task_step import TaskStep, TaskStepDependency
+from agent_env.utils.deprecation import OMITTED, renamed_keyword
 
 logger = logging.getLogger(__name__)
 
@@ -23,10 +24,12 @@ class Skill:
 
     A skill can come from three mutually-exclusive sources:
     - **inline**: body + frontmatter fields, rendered into SKILL.md at send time
-    - **s3_url**: object-store prefix containing a skill directory with its own SKILL.md,
+    - **object_url**: object-store prefix containing a skill directory with its own SKILL.md,
       sent as a bundle of read grants
     - **skill_artifact_id**: a SkillArtifact in the store; name/description/skill_object_url
       are resolved from it at send time
+
+    ``s3_url`` is the deprecated spelling of ``object_url``: it still constructs one, and reads as it.
     """
     name: Optional[str] = None
     description: Optional[str] = None
@@ -35,26 +38,31 @@ class Skill:
     compatibility: Optional[str] = None
     metadata: Optional[dict[str, str]] = None
     allowed_tools: Optional[str] = None
-    s3_url: Optional[str] = None
+    s3_url: InitVar[Optional[str]] = OMITTED
     skill_artifact_id: Optional[str] = None
     skill_artifact_version: Optional[int] = None
-
-    def __post_init__(self):
-        modes = [bool(self.body), bool(self.s3_url), bool(self.skill_artifact_id)]
+    object_url: Optional[str] = None
+
+    def __post_init__(self, s3_url: Optional[str]):
+        if s3_url is not OMITTED and s3_url == self.object_url:
+            s3_url = OMITTED  # how dataclasses.replace copies a Skill: it reads s3_url back off the original
+        # Five frames up: through the dataclass's generated __init__ to its caller.
+        self.object_url = renamed_keyword("Skill", "object_url", self.object_url, "s3_url", s3_url, stacklevel=5)
+        modes = [bool(self.body), bool(self.object_url), bool(self.skill_artifact_id)]
         if sum(modes) != 1:
-            raise ValueError("Skill must have exactly one of: body (inline), s3_url, skill_artifact_id")
+            raise ValueError("Skill must have exactly one of: body (inline), object_url, skill_artifact_id")
         if self.skill_artifact_id is not None:
-            if any([self.body, self.s3_url, self.license, self.compatibility, self.metadata, self.allowed_tools]):
+            if any([self.body, self.object_url, self.license, self.compatibility, self.metadata, self.allowed_tools]):
                 raise ValueError(
-                    "Skill with skill_artifact_id must not have body/s3_url/license/compatibility/metadata/allowed_tools"
+                    "Skill with skill_artifact_id must not have body/object_url/license/compatibility/metadata/allowed_tools"
                 )
         else:
             if not self.name or not self.description:
-                raise ValueError("Inline or s3_url skills require name and description")
-            if self.s3_url and any([self.body, self.license, self.compatibility, self.metadata, self.allowed_tools]):
+                raise ValueError("Inline or object_url skills require name and description")
+            if self.object_url and any([self.body, self.license, self.compatibility, self.metadata, self.allowed_tools]):
                 raise ValueError(
-                    "Skill with s3_url must not have body/license/compatibility/metadata/allowed_tools — "
-                    "the S3 skill directory has its own SKILL.md"
+                    "Skill with object_url must not have body/license/compatibility/metadata/allowed_tools — "
+                    "the skill directory has its own SKILL.md"
                 )
 
     def validate(self) -> None:
@@ -62,8 +70,8 @@ def validate(self) -> None:
 
         if self.skill_artifact_id is not None:
             return
-        if self.s3_url is not None:
-            SkillArtifact.validate(object_url=self.s3_url, expected_name=self.name)
+        if self.object_url is not None:
+            SkillArtifact.validate(object_url=self.object_url, expected_name=self.name)
             return
         SkillArtifact.validate(skill_md=self.to_skill_md().encode("utf-8"), expected_name=self.name)
 
@@ -87,10 +95,11 @@ def to_skill_md(self) -> str:
         return "\n".join(lines)
 
     def to_dict(self) -> dict:
+        """Both spellings of ``object_url``: a worker on an older version reads only ``s3_url``."""
         d: dict = {}
         for field_name in (
             "name", "description", "body", "license", "compatibility",
-            "metadata", "allowed_tools", "s3_url",
+            "metadata", "allowed_tools", "s3_url", "object_url",
             "skill_artifact_id", "skill_artifact_version",
         ):
             value = getattr(self, field_name)
@@ -104,7 +113,7 @@ def from_dict(cls, data: dict) -> Skill:
             name=data.get("name"), description=data.get("description"),
             body=data.get("body"), license=data.get("license"),
             compatibility=data.get("compatibility"), metadata=data.get("metadata"),
-            allowed_tools=data.get("allowed_tools"), s3_url=data.get("s3_url"),
+            allowed_tools=data.get("allowed_tools"), object_url=data.get("s3_url") or data.get("object_url"),
             skill_artifact_id=data.get("skill_artifact_id"),
             skill_artifact_version=data.get("skill_artifact_version"),
         )
@@ -164,6 +173,12 @@ def _build_skill_for_loaded_file_artifact_universe(universe_id: str, entry: dict
     )
 
 
+
+# ``s3_url`` reads and writes ``object_url``. Set once the dataclass is built, so the ``s3_url=`` init argument keeps
+# its default.
+Skill.s3_url = property(lambda self: self.object_url, lambda self, value: setattr(self, "object_url", value))
+
+
 class AddSkillsTaskStep(TaskStep):
     type: ClassVar[str] = "add_skills"
     entity_refs = (
diff --git a/src/agent_env/task_step/task_steps/deploy_agent.py b/src/agent_env/task_step/task_steps/deploy_agent.py
index 47943f01..2cfbcff5 100644
--- a/src/agent_env/task_step/task_steps/deploy_agent.py
+++ b/src/agent_env/task_step/task_steps/deploy_agent.py
@@ -69,15 +69,15 @@ def _live_mcp_url(env_id: str) -> str:
 
 
 def _skill_fields(skill: dict) -> dict:
-    """A ``skills`` entry as ``A2AAgent.register_skill`` arguments. ``content`` and ``s3_uri`` are
-    older names for ``skill_md`` and ``skill_s3_url``; ``skill_md`` is sent verbatim."""
+    """A ``skills`` entry as ``A2AAgent.register_skill`` arguments. ``content`` is an older name for
+    ``skill_md``, and ``skill_s3_url`` and ``s3_uri`` for ``object_url``; ``skill_md`` is sent verbatim."""
     skill_md = skill.get("skill_md", skill.get("content"))
     return {
         "name": skill.get("name", ""),
         "description": skill.get("description", ""),
         "skill_md": skill_md,
         "object_url": (
-            skill.get("skill_s3_url", skill.get("s3_uri")) if skill_md is None else None
+            skill.get("object_url", skill.get("skill_s3_url", skill.get("s3_uri"))) if skill_md is None else None
         ),
     }
 
@@ -195,11 +195,6 @@ def to_dict(self) -> dict:
 
     @classmethod
     def from_dict(cls, data: dict) -> DeployAgentTaskStep:
-        if data.get("agent_changelog_s3_prefix"):
-            raise ValueError(
-                "agent_changelog_s3_prefix is no longer supported: an S3-form changelog cannot "
-                "be applied; capture it again and set agent_changelog_object_url"
-            )
         return cls(
             **cls._base_from_dict(data),
             env_ids=data.get("env_ids"),
@@ -587,7 +582,7 @@ async def _load_snapshot(
         )
         if not universe.bundle_object_url:
             raise RuntimeError(
-                f"FileArtifactUniverse '{universe.id}' v{universe.version} has no bundle_s3_url; "
+                f"FileArtifactUniverse '{universe.id}' v{universe.version} has no bundle_object_url; "
                 "snapshot universes must be created via put_existing or put_bundled"
             )
         load_method, load_path = A2AAgent.operation(snapshot_ext, "load")
diff --git a/src/agent_env/utils/deprecation.py b/src/agent_env/utils/deprecation.py
index c361fc67..9fcd7b31 100644
--- a/src/agent_env/utils/deprecation.py
+++ b/src/agent_env/utils/deprecation.py
@@ -52,14 +52,15 @@ def warn_deprecated(old: str, new: str, *, kind: str = "symbol", stacklevel: int
 OMITTED: Any = object()
 
 
-def renamed_keyword(owner: str, new: str, new_value: Any, old: str, old_value: Any) -> Any:
+def renamed_keyword(owner: str, new: str, new_value: Any, old: str, old_value: Any, *, stacklevel: int = 4) -> Any:
     """The value of keyword ``new``, taking the deprecated keyword ``old`` (default ``OMITTED``) in its place with a
-    warning. Passing both is a TypeError, as a repeated keyword is."""
+    warning. Passing both is a TypeError, as a repeated keyword is. ``stacklevel`` 4 names the caller of a function
+    that calls this directly."""
     if old_value is OMITTED:
         return new_value
     if new_value is not None:
         raise TypeError(f"{owner}() got both {new}= and its deprecated spelling {old}=")
-    warn_deprecated(f"{owner}({old}=)", f"{new}=", kind="keyword", stacklevel=4)
+    warn_deprecated(f"{owner}({old}=)", f"{new}=", kind="keyword", stacklevel=stacklevel)
     return old_value
 
 
diff --git a/tst/unit/a2a_agent/add_skill_test.py b/tst/unit/a2a_agent/add_skill_test.py
index d5636253..30c3ac0b 100644
--- a/tst/unit/a2a_agent/add_skill_test.py
+++ b/tst/unit/a2a_agent/add_skill_test.py
@@ -117,7 +117,7 @@ async def test_add_skill_negotiates_bundle_from_legacy_config_spelling(
 ) -> None:
     await A2AAgent.add_skill(
         _deployed(bundle=True, config_key="config"),
-        Skill(name="review", description="Review work", s3_url=prefix),
+        Skill(name="review", description="Review work", object_url=prefix),
     )
 
     assert agent.requests[0]["url"] == "https://agent.example.test/custom/skills"
@@ -177,7 +177,7 @@ async def test_an_object_skill_is_refused_by_an_agent_without_the_bundle_form(
     with pytest.raises(RuntimeError, match="skill add: the agent does not advertise the object form"):
         await A2AAgent.add_skill(
             _deployed(bundle=False),
-            Skill(name="review", description="Review work", s3_url=prefix),
+            Skill(name="review", description="Review work", object_url=prefix),
         )
 
     assert agent.requests == []
@@ -194,7 +194,7 @@ async def test_an_object_skill_on_a_store_without_grants_is_refused_before_any_r
     with pytest.raises(RuntimeError, match="does not issue transfer grants"):
         await A2AAgent.add_skill(
             _deployed(bundle=True, legacy=legacy),
-            Skill(name="review", description="Review work", s3_url=prefix),
+            Skill(name="review", description="Review work", object_url=prefix),
         )
 
     assert agent.requests == []
@@ -207,7 +207,7 @@ async def test_an_object_skill_is_refused_when_the_stores_grants_do_not_reach_th
     with pytest.raises(RuntimeError, match="grants do not reach agents on the 'modal' sandbox provider"):
         await A2AAgent.add_skill(
             _deployed(bundle=True, sandbox_type="modal"),
-            Skill(name="review", description="Review work", s3_url=prefix),
+            Skill(name="review", description="Review work", object_url=prefix),
         )
 
     assert agent.requests == []
@@ -241,12 +241,14 @@ async def test_inline_skill_does_not_require_bundle_support(
 
 
 @pytest.mark.asyncio
-async def test_legacy_deploy_skill_dict_negotiates_a_portable_bundle(
-    store: GrantingObjectStore, prefix: str, agent: _Agent
+@pytest.mark.parametrize("key", ["object_url", "skill_s3_url", "s3_uri"])
+async def test_a_deploy_skill_dict_naming_an_object_negotiates_a_portable_bundle(
+    store: GrantingObjectStore, prefix: str, agent: _Agent, key: str
 ) -> None:
+    """``object_url``, or either of its older names, which stored steps may still carry."""
     await A2AAgent.register_skill(
         _deployed(bundle=True),
-        **_skill_fields({"name": "review", "description": "Review work", "s3_uri": prefix}),
+        **_skill_fields({"name": "review", "description": "Review work", key: prefix}),
     )
 
     assert set(agent.requests[0]["json"]) == {"name", "description", "skill_bundle"}
@@ -317,7 +319,7 @@ async def test_a_bundle_skill_error_keeps_the_agents_body_out_of_logs(
     with pytest.raises(httpx.HTTPStatusError) as raised:
         await A2AAgent.add_skill(
             _deployed(bundle=True),
-            Skill(name="review", description="Review work", s3_url=prefix),
+            Skill(name="review", description="Review work", object_url=prefix),
         )
 
     assert str(raised.value) == "skill add (review) failed with HTTP 400"
@@ -384,6 +386,6 @@ async def post(self, url, *, json, timeout):
 
     monkeypatch.setattr(httpx.AsyncClient, "post", post)
 
-    await A2AAgent.add_skill(_deployed(bundle=True), Skill(name="review", description="Review work", s3_url=prefix))
+    await A2AAgent.add_skill(_deployed(bundle=True), Skill(name="review", description="Review work", object_url=prefix))
 
     assert on_loop and not any(on_loop)
diff --git a/tst/unit/a2a_agent/object_transfer_test.py b/tst/unit/a2a_agent/object_transfer_test.py
index 84619ddf..cc680292 100644
--- a/tst/unit/a2a_agent/object_transfer_test.py
+++ b/tst/unit/a2a_agent/object_transfer_test.py
@@ -233,11 +233,10 @@ def _answering(monkeypatch, body: dict) -> None:
 
 
 @pytest.mark.asyncio
-async def test_fetch_trajectory_refuses_an_answer_that_only_names_an_s3_prefix(monkeypatch):
+async def test_fetch_trajectory_reads_no_trajectory_from_an_answer_that_only_names_an_s3_prefix(monkeypatch):
     _answering(monkeypatch, {"trajectory_s3_prefix": "s3://b/t/"})
 
-    with pytest.raises(RuntimeError, match="answered with trajectory_s3_prefix"):
-        await fetch_trajectory("https://agent.test/ext/trajectory", {"task_id": "t"})
+    assert await fetch_trajectory("https://agent.test/ext/trajectory", {"task_id": "t"}) == FetchedTrajectory()
 
 
 @pytest.mark.asyncio
diff --git a/tst/unit/store/local_id_acceptance_test.py b/tst/unit/store/local_id_acceptance_test.py
index 3a8d1cc6..df9d5412 100644
--- a/tst/unit/store/local_id_acceptance_test.py
+++ b/tst/unit/store/local_id_acceptance_test.py
@@ -204,7 +204,7 @@ def stop(self, **kwargs):
     skills = f"a2a_validator/validator_skill/{key_segment(agent.id)}-v1"
     assert sorted(get_config().get_object_store_for(agent.id).list("a2a_validator/")) == [
         f"{fixtures}/clip.mp4", f"{fixtures}/red.png",
-        *(f"{skills}/{name}/SKILL.md" for name in ("validator-probe-bundle", "validator-test-s3")),
+        *(f"{skills}/{name}/SKILL.md" for name in ("validator-probe-bundle", "validator-test-object")),
     ]
     assert configured.list("") == []
     assert _local().count("tasks", Filter()) == 1
diff --git a/tst/unit/task_step/skill_object_url_test.py b/tst/unit/task_step/skill_object_url_test.py
new file mode 100644
index 00000000..a64d1a69
--- /dev/null
+++ b/tst/unit/task_step/skill_object_url_test.py
@@ -0,0 +1,102 @@
+"""``Skill.object_url`` and the neutral skill flags replace their S3-named spellings.
+
+Stored add_skills steps carry ``s3_url``, so it is read forever and written beside ``object_url`` while workers on
+older versions read only it. The ``s3_url`` keyword and the S3-named CLI flags still work, with a warning."""
+
+import dataclasses
+import logging
+import warnings
+
+import pytest
+from click.testing import CliRunner
+
+from agent_env.artifact import FileArtifactUniverse, SkillArtifact
+from agent_env.artifact.artifacts import skill as skill_module
+from agent_env.cli import cli
+from agent_env.task_step.task_steps.add_skills import Skill
+
+_PREFIX = "mem://bucket/skills/review/"
+
+
+def test_a_skill_named_by_object_url_carries_both_spellings_without_a_warning():
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        skill = Skill(name="review", description="Review work", object_url=_PREFIX)
+    assert (skill.object_url, skill.s3_url) == (_PREFIX, _PREFIX)
+    assert skill.to_dict() == {"name": "review", "description": "Review work", "s3_url": _PREFIX, "object_url": _PREFIX}
+
+
+def test_the_s3_url_keyword_still_works_and_is_counted(caplog):
+    with caplog.at_level(logging.WARNING, logger="agent_env.utils.deprecation"), \
+            pytest.warns(DeprecationWarning, match=r"Skill\(s3_url=\)"):
+        skill = Skill(name="review", description="Review work", s3_url=_PREFIX)
+    assert (skill.object_url, skill.s3_url) == (_PREFIX, _PREFIX)
+    assert [r.deprecated_symbol for r in caplog.records if getattr(r, "event", None) == "agent_env_deprecated_symbol"] == [
+        "Skill(s3_url=)"
+    ]
+
+
+def test_an_inline_skill_reads_s3_url_as_none():
+    """Readers outside agent-env check ``skill.s3_url is None``."""
+    assert Skill(name="review", description="Review work", body="Inline").s3_url is None
+
+
+def test_the_two_spellings_disagreeing_is_an_error_and_agreeing_is_a_copy():
+    with pytest.raises(TypeError, match="got both object_url= and its deprecated spelling s3_url="):
+        Skill(name="review", description="Review work", object_url=_PREFIX, s3_url="mem://bucket/other/")
+    skill = Skill(name="review", description="Review work", object_url=_PREFIX)
+    assert dataclasses.replace(skill) == skill
+
+
+@pytest.mark.parametrize("stored", [
+    {"s3_url": _PREFIX},
+    {"object_url": _PREFIX},
+    {"s3_url": _PREFIX, "object_url": "mem://bucket/stale/"},
+], ids=["legacy-key", "neutral-key", "legacy-wins"])
+def test_a_stored_skill_loads_keyed_either_way(stored):
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        skill = Skill.from_dict({"name": "review", "description": "Review work", **stored})
+    assert skill.object_url == _PREFIX
+
+
+def test_the_skill_md_helpers_are_public_and_keep_their_private_names():
+    assert skill_module._parse_skill_md is skill_module.parse_skill_md
+    assert skill_module._fetch_skill_md is skill_module.fetch_skill_md
+    frontmatter, body = skill_module.parse_skill_md(b"---\nname: demo\ndescription: A demo.\n---\nBody\n")
+    assert (frontmatter["name"], body.strip()) == ("demo", "Body")
+
+
+def _bundle_dir(tmp_path):
+    (tmp_path / "files").mkdir()
+    (tmp_path / "files" / "a.txt").write_text("A")
+    return str(tmp_path / "files")
+
+
+@pytest.mark.parametrize("flag,notice", [("--prefix-url", False), ("--s3-url", True)])
+def test_put_bundled_takes_its_prefix_under_either_flag(local_stores, tmp_path, flag, notice):
+    prefix = local_stores.get_object_store().object_url("bundles/cli/")
+    result = CliRunner().invoke(cli, ["artifact", "file-artifact-universe", "put-bundled", "--id", "cli-bundle",
+                                      "--file-dir", _bundle_dir(tmp_path), flag, prefix])
+    assert result.exit_code == 0, result.output
+    assert f"bundle_object_url={prefix.rstrip('/')}/" in result.stdout
+    assert ("--s3-url is deprecated" in result.stderr) is notice
+    assert FileArtifactUniverse.get("cli-bundle").bundle_object_url == prefix.rstrip("/") + "/"
+
+
+def test_giving_a_flag_under_both_spellings_is_a_usage_error(local_stores, tmp_path):
+    prefix = local_stores.get_object_store().object_url("bundles/cli/")
+    result = CliRunner().invoke(cli, ["artifact", "file-artifact-universe", "put-bundled", "--id", "cli-bundle",
+                                      "--file-dir", _bundle_dir(tmp_path), "--prefix-url", prefix, "--s3-url", prefix])
+    assert result.exit_code == 2
+    assert "--s3-url is the deprecated spelling of --prefix-url" in result.output
+
+
+@pytest.mark.parametrize("flag", ["--prefix-url", "--s3-url"])
+def test_skill_put_reads_a_skill_from_a_prefix_under_either_flag(local_stores, flag):
+    store = local_stores.get_object_store()
+    store.put("skills/demo/SKILL.md", b"---\nname: demo\ndescription: A demo skill.\n---\nBody\n")
+    result = CliRunner().invoke(cli, ["artifact", "skill", "put", "--id", "demo", flag, store.object_url("skills/demo/")])
+    assert result.exit_code == 0, result.output
+    assert "skill_object_url=" in result.stdout
+    assert SkillArtifact.get("demo").skill_name == "demo"
diff --git a/tst/unit/task_step/stored_s3_form_fields_test.py b/tst/unit/task_step/stored_s3_form_fields_test.py
index 0541a6ea..d99f9a3b 100644
--- a/tst/unit/task_step/stored_s3_form_fields_test.py
+++ b/tst/unit/task_step/stored_s3_form_fields_test.py
@@ -1,5 +1,5 @@
 """Stored step documents written before the S3 transfer forms were removed: a field that only
-named an S3 location is dropped, and an S3-form changelog source is refused rather than skipped."""
+named an S3 location is dropped, an S3-form changelog source included (none is left in stored tasks)."""
 
 import pytest
 
@@ -15,18 +15,8 @@ def test_a_stored_skill_config_check_with_an_s3_probe_url_still_loads():
     assert VerifyA2ASkillConfigStep.from_dict({**document, "skill_s3_url": "s3://bucket/prefix/"}).to_dict() == document
 
 
-@pytest.mark.parametrize("cutoff", [None, 3], ids=["whole-changelog", "with-cutoff"])
-def test_a_stored_deploy_step_with_an_s3_changelog_source_is_refused(cutoff):
-    document = {
-        **DeployAgentTaskStep(id="d", version=None).to_dict(),
-        "agent_changelog_s3_prefix": "s3://bucket/agent_changelog/run-1/solver",
-        "agent_changelog_toolcall_position_exclusive": cutoff,
-    }
-    with pytest.raises(ValueError, match="agent_changelog_s3_prefix is no longer supported"):
-        DeployAgentTaskStep.from_dict(document)
-
-
-def test_a_stored_deploy_step_with_an_empty_s3_changelog_field_still_loads():
+@pytest.mark.parametrize("prefix", ["s3://bucket/agent_changelog/run-1/solver", None], ids=["set", "empty"])
+def test_a_stored_deploy_step_with_an_s3_changelog_field_loads_without_it(prefix):
     step = DeployAgentTaskStep(id="d", version=None)
     document = step.to_dict()
-    assert DeployAgentTaskStep.from_dict({**document, "agent_changelog_s3_prefix": None}).to_dict() == document
+    assert DeployAgentTaskStep.from_dict({**document, "agent_changelog_s3_prefix": prefix}).to_dict() == document