Prompt

Prompt

This page describes the system prompt that the core plugin claudetto gives to each model. It is for a user who wants to know what Claude reads, and for a contributor who edits the prompt slice.

What the Slice Does

The prompt slice replaces the built-in sections of the system prompt with a shorter text. The text has the rules that each model shares and one variant for each model. The slice uses four hooks:

Hook	What it does
prompt.compose	Replaces the known built-in sections with the shared sections and the variant of the model.
prompt.section	Gives each built-in section that has a transform its shorter text.
agent.spawn	Keeps the model of a subagent for session.append.
session.append	Adds the model block to the end of the task of a subagent, one time.

The code is in src/plugins/claudetto/slices/prompt/. The texts are .md files in the folder prompts/, and just generate builds the module prompts.ts from them.

Variants

Each model family gets one variant:

Key	Model	Model notes
opus	Opus 5.5	One notes section.
sonnet	Sonnet 5.5	No notes section.
haiku	Haiku 5.5	One notes section with the step line.
fable	Fable 5.1	One notes section.

A model id that matches no family gets no variant. Its sections get only the transforms of prompt.section. The slice chooses the variant at the first prompt.compose of a session and keeps it. A model switch does not change the system prompt, because a changed system prompt breaks the prompt cache.

The intro section starts from the lean_body text that Claude Code 2.1.302 sends (domain/live-block.ts). A test fails when a line of that text has no transform.

Transforms

A transform finds each part of a built-in section with an anchor, and it replaces the part with a shorter text (transforms/parts.ts). The anchors match the text that Claude Code writes. This text is English in each language of the user, so an anchor does not depend on the language of the user. A new version of Claude Code can change the text. Then an anchor finds nothing, and that part stays as Claude Code wrote it. For each anchor that finds nothing, the slice writes one line to the debug log (missLines in domain/apply.ts). The line names the section and the anchor. Some parts are in the text only in some sessions, such as the line about fast mode, so a line in the log does not always mean a new version.

Subagents

A subagent gets its system prompt from its agent file, so it does not get the shared sections. The slice adds a short block for the model of the subagent to the end of its task. The block has the block line and the evidence line. Sonnet and Haiku also get the verification line, and Haiku also gets the step line.

Compaction Gate

The context_management section agrees with the compaction state of the session:

State	When	Text
handoff	The compaction option is handoff, the default.	It says that a handoff carries the work.
built-in	The compaction option is built-in.	The built-in text.
off	DISABLE_COMPACT is on.	It says nothing of compaction.

See Handoff for the compaction option.

Tool Descriptions

The surface slice gives shorter descriptions for these tools:

Tool	Change
Agent	A shorter description.
Bash	A shorter description, with the parts that Claude Code computes for each run.
Read	A shorter description.
ListAgents	Deferred behind ToolSearch.

A description replaces the built-in one only when it is shorter. The built-in Bash text has values that Claude Code computes for each run: the timeouts, the background line, and the # Machines section. The Bash text of claudetto copies each of these parts from the built-in text, so a value such as BASH_DEFAULT_TIMEOUT_MS stays correct.

Size

The profile makes the first request smaller for each model, by about 2,000 to 2,500 tokens. tests/claudetto/prompt/sizes.test.ts pins the size of each text, and a test fails when a text grows past its pin. To change a text, measure the first request again and update the pin.

Related Pages

• Trim: the built-in parts that claudetto turns off.
• Output Style: the Concise output style.
• Agents: the role agents and their model blocks.
• Handoff: the compaction option.
• Settings Snippet: the settings that the profile sets.