Let the preset agent use the claude CLI's default model

mkdocs/docs/concepts/presets.md

@@ -91,13 +91,15 @@ To stop a creation and its runs, use `dstack preset stop`.
     export DSTACK_AGENT_ANTHROPIC_API_KEY=...
     ```
 
-    By default, the agent uses `claude-opus-4-8`. It doesn't set an effort level, so the `claude` CLI default applies. To override them, set:
+    By default, the agent sets neither a model nor an effort level, so the `claude` CLI's built-in defaults apply. To override them, set:
 
     ```shell
-    export DSTACK_AGENT_ANTHROPIC_MODEL=claude-opus-5
+    export DSTACK_AGENT_ANTHROPIC_MODEL=claude-fable-5-1
     export DSTACK_AGENT_CLAUDE_EFFORT=max
     ```
 
+    See the [Models overview](https://platform.claude.com/docs/en/models/overview) for the available models and their IDs.
+
     Supported effort levels are `low`, `medium`, `high`, `xhigh`, and `max`.
 
 ??? info "Presets directory"

mkdocs/docs/reference/cli/dstack/preset.md

@@ -50,7 +50,7 @@ Preset creation uses the existing `claude` login unless
 | --- | --- |
 | `DSTACK_AGENT_ANTHROPIC_API_KEY` | Anthropic API key used by the agent. |
 | `DSTACK_AGENT_CLAUDE_PATH` | `claude` executable name or path. Defaults to `claude` from `PATH`. |
-| `DSTACK_AGENT_ANTHROPIC_MODEL` | Claude model used by the agent. Defaults to `claude-opus-4-8`. |
+| `DSTACK_AGENT_ANTHROPIC_MODEL` | Claude model used by the agent. If unset, the `claude` CLI's built-in default is used. |
 | `DSTACK_AGENT_CLAUDE_EFFORT` | Claude effort level: `low`, `medium`, `high`, `xhigh`, or `max`. If unset, the `claude` CLI default is used. |
 
 Agent progress is written to `agent.log` under `~/.dstack/presets/<preset-id>/`,

mkdocs/docs/reference/env.md

@@ -219,5 +219,5 @@ $ find ~/.dstack/logs/cli/
 - `DSTACK_PROJECT`{ #DSTACK_PROJECT } – Has the same effect as `--project`. Defaults to `None`.
 - `DSTACK_AGENT_ANTHROPIC_API_KEY`{ #DSTACK_AGENT_ANTHROPIC_API_KEY } – The Anthropic API key used by the preset agent. If unset, the existing `claude` login is used.
 - `DSTACK_AGENT_CLAUDE_PATH`{ #DSTACK_AGENT_CLAUDE_PATH } – The `claude` executable name or path used by the preset agent. Defaults to `claude` from `PATH`.
-- `DSTACK_AGENT_ANTHROPIC_MODEL`{ #DSTACK_AGENT_ANTHROPIC_MODEL } – The Claude model used by the preset agent. Defaults to `claude-opus-4-8`.
+- `DSTACK_AGENT_ANTHROPIC_MODEL`{ #DSTACK_AGENT_ANTHROPIC_MODEL } – The Claude model used by the preset agent. If unset, the `claude` CLI's built-in default is used.
 - `DSTACK_AGENT_CLAUDE_EFFORT`{ #DSTACK_AGENT_CLAUDE_EFFORT } – The Claude effort level used by the preset agent. Can be `low`, `medium`, `high`, `xhigh`, or `max`. If unset, the `claude` CLI default is used.

src/dstack/_internal/cli/models/preset_agent.py

@@ -125,6 +125,18 @@ class PresetSessionState(CoreModel):
     run: Optional[PresetSessionRun]
 
 
+class PresetAgentInfo(CoreModel):
+    """`agent.json`: how the claude agent was launched. A debug record."""
+
+    executable: str
+    version: Optional[str]
+    auth_status: str
+    # None is the claude CLI's default.
+    effort: Optional[str]
+    # Reported by claude on its init line; None until then.
+    model: Optional[str]
+
+
 class ClaudeStreamEvent(CoreModel):
     """One line of the claude CLI's `--output-format stream-json`. Not our format:
     unknown fields are dropped and omitted fields default."""
@@ -135,6 +147,8 @@ class ClaudeStreamEvent(CoreModel):
     # Identifies the claude conversation, so an interrupted creation can be
     # resumed with `claude --resume`. Not every line carries it.
     session_id: Optional[str] = None
+    # The model claude runs with, e.g. "claude-opus-5[1m]". Set on the init line only.
+    model: Optional[str] = None
 
 
 class ClaudeResultEvent(ClaudeStreamEvent):

src/dstack/_internal/cli/services/presets/agent.py

@@ -18,6 +18,7 @@
     AnyClaudeStreamEvent,
     ClaudeResultEvent,
     PresetAgentFailure,
+    PresetAgentInfo,
     PresetAgentSuccess,
     PresetSessionProcess,
 )
@@ -90,9 +91,9 @@
 class ClaudeAuth:
     api_key: Optional[str]
     executable: str
-    # None uses the claude CLI's own default.
+    # None means the flag is not passed and claude uses its own default.
     effort: Optional[ClaudeEffort]
-    model: str
+    model: Optional[str]
 
 
 @dataclass
@@ -131,6 +132,16 @@ def _get_claude_auth_status(auth: "ClaudeAuth") -> str:
         return "unknown"
 
 
+def get_agent_info(auth: ClaudeAuth) -> PresetAgentInfo:
+    return PresetAgentInfo(
+        executable=auth.executable,
+        version=_get_claude_version(auth),
+        auth_status=_get_claude_auth_status(auth),
+        effort=auth.effort,
+        model=None,
+    )
+
+
 def get_claude_auth() -> ClaudeAuth:
     api_key = os.getenv("DSTACK_AGENT_ANTHROPIC_API_KEY") or None
     configured_path = os.getenv("DSTACK_AGENT_CLAUDE_PATH") or "claude"
@@ -146,7 +157,7 @@ def get_claude_auth() -> ClaudeAuth:
         api_key=api_key,
         executable=executable,
         effort=effort,
-        model=os.getenv("DSTACK_AGENT_ANTHROPIC_MODEL", "claude-opus-4-8"),
+        model=os.getenv("DSTACK_AGENT_ANTHROPIC_MODEL") or None,
     )
 
 
@@ -351,8 +362,6 @@ def _build_claude_command(*, auth: ClaudeAuth, resume_session_id: Optional[str])
         "Task,NotebookEdit",
         "--permission-mode",
         "bypassPermissions",
-        "--model",
-        auth.model,
         "--json-schema",
         json.dumps(_get_report_json_schema()),
     ]
@@ -362,6 +371,8 @@ def _build_claude_command(*, auth: ClaudeAuth, resume_session_id: Optional[str])
         command[2:2] = ["--bare"]
     if auth.effort is not None:
         command[2:2] = ["--effort", auth.effort]
+    if auth.model is not None:
+        command[2:2] = ["--model", auth.model]
     if resume_session_id is not None:
         command += ["--resume", resume_session_id]
     return command
@@ -559,6 +570,8 @@ async def _read_process_stream(
         if output.session_id is None and event.session_id:
             output.session_id = event.session_id
             session.record_claude_session_id(event.session_id)
+        if event.model:
+            session.record_agent_model(event.model)
         if event.type == "assistant":
             output.made_progress = True
         if not isinstance(event, ClaudeResultEvent):

src/dstack/_internal/cli/services/presets/create.py

@@ -28,6 +28,7 @@
     PresetAgentProcessOutput,
     attach_preset_agent,
     build_preset_agent_env,
+    get_agent_info,
     get_claude_auth,
     run_preset_agent,
     terminate_agent_process,
@@ -636,8 +637,8 @@ async def _create_preset(
         # while the listing and `--previous` read constraints from the session dir.
         session.write_constraints(constraints_text)
         session.write_prompt(prompt)
-        if setup.auth is not None:
-            session.write_agent_info(setup.auth)
+    if setup.auth is not None:
+        session.write_agent_info(get_agent_info(setup.auth))
     try:
         if mode == "attach":
             process_output = await attach_preset_agent(

src/dstack/_internal/cli/services/presets/session.py

@@ -10,14 +10,15 @@
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Iterator, Optional, Sequence
+from typing import Any, Iterator, Optional, Sequence
 
 import psutil
 import yaml
 from pydantic import ValidationError
 from rich.text import Text
 
 from dstack._internal.cli.models.preset_agent import (
+    PresetAgentInfo,
     PresetSessionFinalize,
     PresetSessionProcess,
     PresetSessionRun,
@@ -28,14 +29,10 @@
 from dstack._internal.cli.utils.common import console
 from dstack._internal.compat import IS_WINDOWS
 from dstack._internal.core.errors import CLIError
-from dstack._internal.core.models.common import validate_extra_ignore
+from dstack._internal.core.models.common import validate_extra_ignore, validate_json_extra_ignore
 from dstack._internal.core.models.configurations import PresetConfiguration
 from dstack._internal.utils.common import get_dstack_dir
 
-if TYPE_CHECKING:
-    from dstack._internal.cli.services.presets.agent import ClaudeAuth
-
-
 _PROGRESS_FILENAME = "progress.jsonl"
 _RUNS_FILENAME = "runs.jsonl"
 TRIALS_DIRNAME = "trials"
@@ -45,6 +42,7 @@
 _CONSTRAINTS_FILENAME = "constraints.json"
 _FINAL_REPORT_FILENAME = "final_report.json"
 _SESSION_FILENAME = "session.json"
+_AGENT_INFO_FILENAME = "agent.json"
 _USER_PROMPT_FILENAME = "user_prompt.md"
 
 
@@ -114,21 +112,25 @@ def write_constraints(self, constraints_text: str) -> None:
     def write_final_report(self, report_text: str) -> None:
         _write_private_text(self.path / _FINAL_REPORT_FILENAME, report_text)
 
-    def write_agent_info(self, auth: "ClaudeAuth") -> None:
-        from dstack._internal.cli.services.presets.agent import (
-            _get_claude_auth_status,
-            _get_claude_version,
+    def write_agent_info(self, info: PresetAgentInfo) -> None:
+        _write_private_text(
+            self.path / _AGENT_INFO_FILENAME,
+            info.model_dump_json(indent=2) + "\n",
         )
 
-        # `agent.json`: a debug document written once and read by nothing, so it
-        # is a plain dump, not a model.
-        info = {
-            "executable": auth.executable,
-            "version": _get_claude_version(auth),
-            "model": {"name": auth.model, "effort": auth.effort or "default"},
-            "auth_status": _get_claude_auth_status(auth),
-        }
-        _write_private_text(self.path / "agent.json", json.dumps(info, indent=2) + "\n")
+    def read_agent_info(self) -> Optional[PresetAgentInfo]:
+        try:
+            text = (self.path / _AGENT_INFO_FILENAME).read_text(encoding="utf-8")
+            return validate_json_extra_ignore(PresetAgentInfo, text)
+        except (OSError, ValidationError):
+            return None
+
+    def record_agent_model(self, model: str) -> None:
+        info = self.read_agent_info()
+        if info is None:
+            return
+        info.model = model
+        self.write_agent_info(info)
 
     def append_log(self, line: str) -> None:
         if not self._log_enabled: