RFC: Prompt variables and tool-guidance ownership

Status: implemented

Problem

The assembled system prompt had four defects, all of one family: facts the harness already knows were restated by hand somewhere else, and drifted.

The model could not know its own name. AgentOptions.model drives every request, but no prompt text carried it — and nothing COULD carry it: sections in dsh-system-prompt were context-global while the model name is per-agent, and assemble() took no per-agent input at all.

Tool guidance was hand-written prose in leaf YAML. The bash/subagent/todo_write usage guidance lived in the systemPrompt strings of examples/coding-agent/cordis.yml and examples/acp-agent/cordis.yml — two drifting copies (the ACP one was already abridged) — while dsh-tool-fs and dsh-tool-web owned their guidance as ctx.systemPrompt.section() contributions. Loading or dropping a tool plugin meant editing every deployment's persona by hand; both YAMLs carried a FIXME(config-comments) apologizing for a symptom of the split, and the stdio welcome banner hand-enumerated the tool set too.

The persona rendered after tool guidance. The loop string-joined agent.options.systemPrompt AFTER the assembled sections, so the model read "Use the read tool…" before "You are coding-agent" — backwards relative to the identity-first convention (Claude Code, Codex) and a second composition path besides the section pipeline.

The fork tool's description was false. dsh-tool-subagent hardcoded one description written for spawn semantics — "a separate agent that works in its own context … it does not see this conversation" — and the subagent_fork instance (whose child inherits the parent's completed turns) got the same words; the YAML prose corrected the lie out-of-band. Minor kin: PromptSection.name was documented "(diagnostics / dedup)" but duplicates were silently accepted.

Decision

One principle: every fact in the prompt has exactly one owner. The model name and workspace are config/session facts → the harness exposes them as variables and the persona references them. Per-tool semantics and when-to-use → the tool's description. Cross-call habits a description cannot carry → the tool package's prompt section. Harness provenance → the static harness:identity section. Deployment role and behavior → the deployment's persona.

Assemble context

SystemPrompt.assemble(context) takes a merge-extensible AssembleContext. dsh-system-prompt declares the optional scope selector used for scoped routing, while dsh-agent declaration-merges the optional typed agent field onto it (a type-level edge agent → system-prompt, with no runtime dependency cycle). The loop calls assembleContextFor(agent) each step so both fields identify the same agent; section text providers may read that context, and the system-prompt/assemble waterfall receives it so a listener can filter or extend per agent.

Prompt variables

Plugins contribute named values via ctx.systemPrompt.variable(name, provider); prompt text references them as {{name}}. Providers are functions of the AssembleContext and may return undefined — "no value for THIS assembly". assemble() resolves every registered variable into PromptAssembly.variables (waterfall listeners can see, add, or override); renderPrompt interpolates. Rendering is STRICT — fail loud beats shipping a malformed prompt: a reference to an unregistered name throws (listing what exists; lookup is Object.hasOwn, so a prototype property like {{constructor}} is unknown, not a function spliced into the prompt), a registered-but-valueless reference throws, a complete {{…}} group that is not a well-formed name ([a-z][a-z0-9_]*, e.g. {{ model }}) throws, and a {{ that opens no complete group while a }} still follows ({{{model}}}, {{a{b}}) throws. A lone {{ with no }} anywhere after it is ordinary prose and passes through verbatim; substituted values are never re-scanned. Registration rejects duplicate and unreferenceable names, mirroring the tool registry — and section() now rejects duplicate section names, making the documented dedup real.

dsh-agent-loop registers the two built-ins, both pure projections of the context agent: model (= options.model) and cwd (= session.header.cwd). The example personas write powered by the {{model}} model — the model name is stated once, in the model: config key. {{cwd}} is demonstrated in the ACP example only: every ACP session carries the client's cwd, while config-pre-created stdio agents have none (a persona claiming {{cwd}} there fails the turn — by design). The variables stay on the loop plugin (unlike the sections below): they are runtime facts of the agents THIS loop drives, and a replacement loop supplies its own.

Persona as the order-0 section

dsh-system-prompt itself registers the two harness-owned sections (they must survive a swapped loop plugin, so they do NOT live on dsh-agent-loop): the static harness:identity at order -100 — every prompt opens by stating the agent is powered by the DeepSeek Harness SDK — and the global default deployment:persona at order 0, whose text is the plugin's own persona config. AgentOptions.systemPrompt and the loop's special-case join are gone: fullSystemPrompt ≡ renderPrompt(assembly), one ordered pipeline for everything the model sees, and agent/pre-step (compaction's token-pressure input) measures exactly the real prompt. An agent-scoped section with the same deployment:persona name shadows the default for that agent; programmatic setup may register one directly, and the subagent persona feature installs one before publishing an in-process child when the selected provider supports it. Order bands are convention: harness identity -100, persona 0, tool guidance 100–199; other negative orders also render before the persona.

Tool guidance ownership

Per-tool semantics and when-to-use live in tool DESCRIPTIONS, which already ship in every request — the YAML prose was ~fully redundant with them. Sections carry only the cross-call habits a single call's description cannot: dsh-tool-bash contributes tool:bash (order 105) — check the [exit code: N] marker on every result; dsh-tool-fs's read section gains the "not shell commands like cat" contrast. todo_write and the subagent tools need NO section — their descriptions already carry the whole contract. The leaf personas shrink to identity + behavior (verify your work; keep answers brief), and the welcome banner stops enumerating tools.

The subagent conversation-history descriptor

SubagentProvider gains readonly inheritsParentContext: boolean — a DESCRIPTIVE conversation-history fact beside capabilities, not in it (capabilities are start-time validation; nothing validates against this flag). Spawn and ACP declare false, fork declares true. The name refers only to conversation seeding, not Cordis scope, services, tools, or authority. dsh-tool-subagent derives both the tool description and the prompt parameter description from the flag (providerWording): the fork instance now tells the model the child is seeded with the conversation's completed turns (not the in-flight turn) and that its prompt should state only what is new. Deriving the description from a provider that arrives on its own fiber is what forced the provider-lifecycle events and the tool's reactive registration — that mechanism, its Loader-concurrency rationale, and its rejected alternatives are recorded in the provider-lifecycle-events RFC.

Alternatives considered

The loop composes an identity line itself — hardcodes model-facing prose in the one package that must stay thin ("plugins, not loop changes"), and outside the section pipeline it would be a second composition path. (The identity DOES ship as a code literal — but as an ordinary section registered by dsh-system-prompt, whose system-prompt/assemble waterfall remains the escape valve for a deployment that must drop it.)
Inject the model name via the agent/request waterfall — prompt text composed in two places, and agent/pre-step's fullSystemPrompt would omit it, so compaction would measure a prompt that is not what the model sees.
Hand-write the model name in each persona — duplicates the model: key one line above and silently lies after a config edit; the exact disease this RFC cures.
Lenient interpolation (leave unknown refs verbatim, or substitute empty) — a typo ships {{modle}} (or a hole) to the model and nobody notices until transcript review.
Per-instance subagent wording in config — returns model-facing prose to every deployment × instance, the P2 disease again. Keying wording off the provider NAME — providerName is itself config, so a renamed provider silently gets the wrong words.
Resolving the provider at apply time (a load-order requirement) and section-only subagent wording (lazily resolved at assemble) — the alternatives to the provider-lifecycle events; both rejected in the provider-lifecycle-events RFC.

Out of scope

Further variables (date, platform, git state) — the registry makes each a one-line contribution by whichever plugin owns the fact; none is claimed here.
A config cwd for pre-created stdio agents (would let the stdio persona use {{cwd}} and partition persistence by real path) — deferred until the session-cwd story is revisited.

Shipped invariants

renderPrompt(await assemble(assembleContextFor(agent))) for the coding-agent example renders the harness identity, then the persona (with the agent's model name interpolated), then the fs/bash/web guidance sections; the loop has no other prompt-composition path.
The subagent_fork schema description says the child inherits the conversation; the subagent one says it does not. The tool follows its provider: absent before the backend activates, present after, gone when the backend unloads, re-worded from the fresh provider on reload.
Unknown/valueless/malformed/unbalanced {{…}} references throw with the section name in the message; duplicate section, variable, and tool-name registrations all throw.
Snapshot goldens are prompt-independent by construction: llm-replay keys replay on (turn, step) chunk streams and never re-verifies the outgoing request.

Consequences

Every fact in the assembled prompt now has exactly one owner, and the hand-maintained tool prose in leaf YAML is gone: loading or dropping a tool plugin no longer means editing any deployment's persona.
{{model}} reflects AgentOptions.model at assembly time. A plugin that switches models in the agent/request waterfall makes the prompt's claim stale for that step, and one that SUPPLIES the model there (options.model unset — the loop's documented fallback) leaves the variable valueless at render, failing a {{model}} persona before the waterfall runs. Both have the same remedy, and it is the ownership rule itself: the plugin that owns the late-bound model fact states it early on the system-prompt/assemble waterfall (assembly.variables['model'] = …) — one owner, both statements; a loop test pins the supply path end-to-end. Accepted.
While a bound provider is absent (not yet activated, unloaded, mid-HMR-reload), the subagent tool does not exist and a model request in that window simply lacks it. That is the honest state — the alternative was a registered tool whose description or execution could not be trusted.
Strictness means a persona can fail a turn at render (e.g. {{cwd}} on a cwd-less session). The failure is contained — the turn ends error, the loop survives — and it is an authoring error we WANT loud.
No escape syntax for a literal {{name}} in prompt prose yet; add one if a real prompt ever needs it.

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