[DOCS] Model configuration docs omit lean system prompt default scope and model exceptions

Documentation Type

Incorrect/outdated documentation

Documentation Location

https://code.claude.com/docs/en/env-vars

Section/Topic

CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT environment variable and model-specific default behavior

Current Documentation

The environment variable reference currently says:

CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT | Set to 1 to use a shorter system prompt and abbreviated tool descriptions on any model. Set to 0, false, no, or off to opt out even on models where the experiment or server configuration would otherwise enable it. The full tool set, hooks, MCP servers, and CLAUDE.md discovery remain enabled

The model configuration page documents model defaults, aliases, and model-specific behavior, but does not mention the lean system prompt default or its exceptions.

What's Wrong or Missing?

Claude Code v2.1.154 changed the lean system prompt rollout behavior: the lean system prompt is now the default for all models except Haiku, Sonnet, and Opus 4.7 and earlier.

The current docs only describe CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT as an opt-in shortcut and as an opt-out from experiments or server configuration. They do not state:

• that the lean system prompt is now the default for most models;
• which model families or versions are excluded from that default;
• whether users on excluded models should set CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT=1 to force the lean prompt;
• whether CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT=0 remains the supported opt-out for models where the lean prompt is default.

This leaves the documented behavior out of date for users trying to understand why their system prompt shape changed after upgrading to v2.1.154.

Suggested Improvement

Update the CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT row to describe the current default behavior and exceptions.

Suggested wording:

Uses Claude Code's lean system prompt with shorter built-in instructions and abbreviated tool descriptions. As of Claude Code v2.1.154, this prompt is the default for all models except Haiku, Sonnet, and Opus 4.7 and earlier. Set CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT=1 to force the lean prompt on models where it is not default. Set it to 0, false, no, or off to opt out on models where the lean prompt would otherwise be enabled. The full tool set, hooks, MCP servers, and CLAUDE.md discovery remain enabled.

Also add a cross-reference from the model configuration page's model-specific behavior section so users looking at model defaults can find this prompt-shape behavior.

Impact

Medium - Makes feature difficult to understand

Additional Context

Affected Pages:

| Page | Context |
|------|---------|
| https://code.claude.com/docs/en/env-vars | CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT documents opt-in and opt-out behavior but not the v2.1.154 default scope or model exceptions |
| https://code.claude.com/docs/en/model-config | Model-specific behavior is documented, but the lean system prompt default and exceptions are not cross-referenced |

Total scope: 2 pages affected