deepseek-harness/packages/core/system-prompt

dsh-system-prompt

System prompt assembly registry. Plugins contribute ordered text sections, tool-schema providers, and named prompt variables; the agent loop calls assemble(context) once per step, and renderPrompt(assembly) is the full system prompt the model sees.

Service: SystemPrompt (ctx key: systemPrompt)

Public API

• ctx.systemPrompt.section(section: PromptSection): () => void Contribute a section. Duplicate names throw. Disposed with the calling fiber.
• ctx.systemPrompt.tools(provider: () => ToolSchema[]): () => void Contribute tool schemas (evaluated at each assembly). Disposed with the calling fiber.
• ctx.systemPrompt.variable(name: string, provider: (context) => string | undefined): () => void Contribute a prompt variable, referenced from section text as {{name}}. Duplicate or unreferenceable names throw; undefined means "no value for this assembly". Disposed with the calling fiber.
• ctx.systemPrompt.assemble(context?: AssembleContext): Promise<PromptAssembly> Assemble the prompt for one caller. Runs through the system-prompt/assemble waterfall.

Events

Event	Mode	Purpose
system-prompt/assemble	waterfall	Mutate/extend the assembly (with the caller's context) before it reaches the model
system-prompt/change	emit	A section, tool provider, or variable was registered or unregistered

Key types

• AssembleContext — what one assemble() call is FOR. Declared empty here and merge-extensible; dsh-agent declares agent?: Agent, so providers project per-agent facts. Providers must tolerate absent fields (a bare assemble() carries an empty context).
• PromptSection — { name, order, text: string | ((context) => string) }. Sections are concatenated in ascending order. Order bands: -100 is the harness identity, 0 the per-agent persona (both registered by the agent loop), tool guidance uses 100–199; other negative orders also render before the persona.
• PromptAssembly — { sections: AssembledSection[], tools: ToolSchema[], variables: Record<string, string | undefined> }. Section texts arrive resolved but not yet interpolated; variables holds every registered variable resolved against the context. Tool schemas are part of the assembly by design: "what the model is told it can do" is one coherent thing, even though adapters transmit schemas as a separate wire field.
• renderPrompt(assembly) — interpolates {{variable}} references in each section, drops empty sections, joins with blank lines. STRICT: an unknown reference (Object.hasOwn lookup — prototype names like {{constructor}} are unknown), a registered-but-valueless reference, a malformed complete {{…}} group, or a {{ that opens no complete group while a }} still follows ({{{model}}}) throws — fail loud beats shipping a malformed prompt. A lone {{ with no }} anywhere after it passes through verbatim; substituted values are never re-scanned.

Merge-extensible: plugins can declare extra fields on PromptAssembly and AssembleContext via declaration merging.

Extension points

• Section providers: tool packages own their cross-call guidance (tool:bash, tool:read, …); the agent loop owns harness:identity and agent:persona.
• Variable providers: the agent loop registers model and cwd; any plugin can register the facts it owns (a future date, git state, …).
• Tool schema providers: ToolRegistry registers itself as a tool provider automatically.
• The system-prompt/assemble waterfall: mutate or replace the assembly per caller (dynamic tool filtering, extra variables).

What is NOT here

• Any hardcoded prompt text — every section comes from plugins, every deployment-authored word from config.
• Prompt compaction (belongs on the agent/pre-step seam in dsh-agent).

Design rationale: the prompt-variables RFC.