fix(system-prompt): reject a toolOrder that names an unregistered tool

Review follow-up (#196): a listed name with no registered tool was silently
ignored; misconfiguration must block work instead. The check lives in the
assembly — the earliest moment the registered tool set exists (tool plugins
register after the service constructs) and the only universal one (cordis has
no "all plugins loaded" event; registrations change at any time). assemble()
is now async so the throw surfaces as a rejection rather than a synchronous
escape from a Promise-returning method.

Blast radius, pinned by a loop-level test: the rejection reaches the turn's
outer catch — the turn closes balanced with an `error` reason, agent/error
mirrors it, no step opens, no request/header is logged, no request reaches
the adapter, and the agent returns to idle; every turn fails identically
until the config is fixed. A boot-time validation pass was considered and
rejected (recorded in the RFC). The general principle — misconfiguration
fails loud, never a silent skip — is added to AGENTS.md.

packages/core/agent-loop/tests/tool-order.spec.ts

@@ -91,4 +91,27 @@ describe('loop-level canonical tool order', () => {
     expect(adapter.requests[0]?.tools?.map(tool => tool.name)).toEqual(['zulu', 'alpha', 'mike'])
     expect(Object.isFrozen(adapter.requests[0])).toBe(true)
   })
+
+  it('fails the turn — no model request — when toolOrder names an unregistered tool', async () => {
+    // The assemble rejection escapes to runTurn's outer catch: the open turn
+    // closes with an `error` reason (agent/error mirrors it), no step opens,
+    // no request/header is logged, the adapter never sees a request, and the
+    // agent returns to idle — a misconfigured deployment fails every turn
+    // deterministically instead of silently reordering nothing.
+    const adapter = new MockAdapter([textResponse('never sent')])
+    const ctx = await harness(adapter, ['ghost', TOOL_ORDER_REST])
+    registerNamed(ctx, 'alpha')
+    const errors: Error[] = []
+    ctx.on('agent/error', (_agent, _turn, _step, error) => void errors.push(error))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+    agent.send([{ type: 'text', text: 'go' }])
+    await waitForIdle(ctx, agent)
+    expect(adapter.requests).toHaveLength(0)
+    expect(errors.map(e => e.message)).toEqual(['toolOrder lists unregistered tool "ghost"; registered tools: alpha'])
+    expect(foldRequestHeader(agent.session.events)).toBeUndefined()
+    const end = agent.session.events.find(e => e.type === 'turn/end')
+    expect(end?.type === 'turn/end' && end.data.reason).toMatchObject({ kind: 'error', step: 1 })
+    // The turn is balanced (turn/start → turn/end) with no step events inside.
+    expect(agent.session.events.some(e => e.type === 'step/start')).toBe(false)
+  })
 })

packages/core/system-prompt/README.md

@@ -7,7 +7,7 @@ System prompt assembly registry. Plugins contribute ordered text sections, tool-
 | Key | Default | Meaning |
 |---|---|---|
 | `persona` | `''` | The deployment persona: the ONE deployment-authored prompt fragment, rendered as the order-0 `deployment:persona` section and shared by every agent in the context (subagents included). A template — complete `{{…}}` groups are interpreted strictly against the registered variables (the shipped loop registers `{{model}}`/`{{cwd}}`), with no escape syntax for literal braces yet. Empty ⇒ the section is dropped at render. |
-| `toolOrder` | — | Explicit model-facing tool order, as a list of `ToolSchema.name`s with one `'<unlisted-tools>'` rest entry (`TOOL_ORDER_REST`): listed tools take their listed position, names with no registered tool are ignored, unlisted tools land at the rest entry in lexicographic name order. Absent ⇒ plain lexicographic name order. Applied to the collected tools BEFORE the `system-prompt/assemble` waterfall — like the sections' `order` sort, it canonicalizes what the registry contributed (registration order is a plugin-load artifact), and a waterfall listener that mutates the list owns the determinism of what it emits. A list without exactly one rest entry, or with duplicates, throws at load. Why a central list and not per-plugin weights: [Explicit model-facing tool order](../../../docs/rfc/implemented/feature/2026-07-06-explicit-tool-order.md). |
+| `toolOrder` | — | Explicit model-facing tool order, as a list of `ToolSchema.name`s with one `'<unlisted-tools>'` rest entry (`TOOL_ORDER_REST`): listed tools take their listed position, unlisted tools land at the rest entry in lexicographic name order. Absent ⇒ plain lexicographic name order. Applied to the collected tools BEFORE the `system-prompt/assemble` waterfall — like the sections' `order` sort, it canonicalizes what the registry contributed (registration order is a plugin-load artifact), and a waterfall listener that mutates the list owns the determinism of what it emits. Misconfiguration fails loud: a list without exactly one rest entry, or with duplicates, throws at load; a listed name with no registered tool rejects every `assemble()` — under the shipped loop the turn fails before any model request. Why a central list and not per-plugin weights: [Explicit model-facing tool order](../../../docs/rfc/implemented/feature/2026-07-06-explicit-tool-order.md). |
 
 ## Service: `SystemPrompt` (ctx key: `systemPrompt`)
 
@@ -16,7 +16,7 @@ System prompt assembly registry. Plugins contribute ordered text sections, tool-
 - `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.
+- `ctx.systemPrompt.assemble(context?: AssembleContext): Promise<PromptAssembly>` Assemble the prompt for one caller. Runs through the `system-prompt/assemble` waterfall. Rejects when a configured `toolOrder` names a tool no provider contributed.
 
 ### Events
 

packages/core/system-prompt/src/index.ts

@@ -120,10 +120,13 @@ const GROUP_AT = /^\{\{([^{}]*)\}\}/
 export const TOOL_ORDER_REST = '<unlisted-tools>'
 
 /**
- * Validate a configured tool-order list at service construction: the
- * {@link TOOL_ORDER_REST} rest entry exactly once, no duplicate names. Returns the list
- * (or undefined when unconfigured); throws otherwise, failing the service at
- * load — a bad order config must never reach an assembly.
+ * Validate a configured tool-order list's shape at service construction:
+ * the {@link TOOL_ORDER_REST} rest entry exactly once, no duplicate names.
+ * Returns the list (or undefined when unconfigured); throws otherwise,
+ * failing the service at load — a bad order config must never reach an
+ * assembly. Whether every listed name matches a registered tool is checked
+ * at each assembly instead ({@link orderTools}): tool plugins register after
+ * this service constructs, so the tool set does not exist yet here.
  */
 function validateToolOrder(toolOrder: string[] | undefined): string[] | undefined {
   if (toolOrder === undefined) return undefined
@@ -141,12 +144,22 @@ function validateToolOrder(toolOrder: string[] | undefined): string[] | undefine
 /**
  * Order collected tool schemas by the validated policy: with no configured
  * list, plain lexicographic name order; with one, listed names take their
- * listed position and every unlisted tool lands at the {@link TOOL_ORDER_REST} rest entry in
- * lexicographic name order. Never drops a tool, and both sorts are stable, so
- * tools sharing a name keep their collection order.
+ * listed position and every unlisted tool lands at the
+ * {@link TOOL_ORDER_REST} rest entry in lexicographic name order. A listed
+ * name with no collected tool throws — misconfiguration fails loud, and this
+ * is the earliest moment the registered tool set exists to check against
+ * (tool plugins register after the service constructs, so load time is too
+ * early): the assembly rejects, failing the caller's turn before any model
+ * request. Never drops a tool, and both sorts are stable, so tools sharing a
+ * name keep their collection order.
  */
 function orderTools(tools: ToolSchema[], toolOrder: string[] | undefined): ToolSchema[] {
   if (toolOrder === undefined) return tools.sort(compareToolNames)
+  const registered = new Set(tools.map(tool => tool.name))
+  const unknown = toolOrder.filter(name => name !== TOOL_ORDER_REST && !registered.has(name))
+  if (unknown.length > 0) {
+    throw new Error(`toolOrder lists unregistered tool${unknown.length > 1 ? 's' : ''} ${unknown.map(name => `"${name}"`).join(', ')}; registered tools: ${[...registered].sort().join(', ') || '(none)'}`)
+  }
   const listed = new Set(toolOrder)
   const rest = tools.filter(tool => !listed.has(tool.name)).sort(compareToolNames)
   return toolOrder.flatMap(name =>
@@ -174,13 +187,17 @@ export interface Config {
   persona?: string
   /**
    * Explicit model-facing tool order, as a list of `ToolSchema.name`s: listed
-   * tools take their listed position, names with no registered tool are
-   * ignored, and tools absent from the list are inserted at the
-   * {@link TOOL_ORDER_REST} (`'<unlisted-tools>'`) entry in lexicographic name order. A
-   * configured list must contain the rest entry exactly once and no duplicate names —
-   * anything else throws at load; a bad order config must never reach a
-   * model request. When omitted, tools are ordered lexicographically by name.
-   * Applied to the tools {@link SystemPrompt.assemble} collects, BEFORE the
+   * tools take their listed position, and tools absent from the list are
+   * inserted at the {@link TOOL_ORDER_REST} (`'<unlisted-tools>'`) entry in
+   * lexicographic name order. A configured list must contain the rest entry
+   * exactly once, no duplicate names, and no name without a registered tool —
+   * a misconfigured order blocks work instead of silently reaching a model
+   * request: shape violations throw at load, and an unregistered name rejects
+   * every assembly (failing the turn before any model request — the earliest
+   * moment the registered tool set exists to check against, since tool
+   * plugins register after this service constructs). When omitted, tools are
+   * ordered lexicographically by name. Applied to the tools
+   * {@link SystemPrompt.assemble} collects, BEFORE the
    * `system-prompt/assemble` waterfall — like the sections' `order` sort, it
    * canonicalizes what the registry contributed (registration order is a
    * plugin-load artifact); a waterfall listener that mutates the tool list
@@ -394,7 +411,8 @@ export class SystemPrompt extends Service {
    * against `context` and sorted by order, tools collected from all providers
    * and put in the canonical model-facing order ({@link Config.toolOrder}, or
    * lexicographic name order when unconfigured — provider registration order
-   * is a plugin-load artifact and never reaches the assembly), and every
+   * is a plugin-load artifact and never reaches the assembly; a configured
+   * order naming a tool no provider contributed rejects the assembly), and every
    * registered variable resolved against `context` into `assembly.variables`.
    * Tool schemas are deep-cloned because adapters and request waterfalls may
    * mutate schema objects. Runs through the `system-prompt/assemble`
@@ -408,7 +426,10 @@ export class SystemPrompt extends Service {
    *   see {@link AssembleContext}).
    * @returns the assembly after the waterfall has run.
    */
-  assemble(context: AssembleContext = {}): Promise<PromptAssembly> {
+  // async so the misconfigured-toolOrder throw in orderTools surfaces as a
+  // rejection: a Promise-returning method must not throw synchronously
+  // (`assemble().catch(...)` would miss it).
+  async assemble(context: AssembleContext = {}): Promise<PromptAssembly> {
     const variables: Record<string, string | undefined> = {}
     for (const [name, provider] of this.variableProviders) {
       variables[name] = provider(context)

packages/core/system-prompt/tests/tool-order.spec.ts

@@ -18,6 +18,8 @@ function names(assembly: PromptAssembly): string[] {
 }
 
 describe('SystemPrompt tool order', () => {
+  // The ONE place the public constant's value is pinned; everything else
+  // (tests and deployment configs alike) references TOOL_ORDER_REST.
   it('exports the rest entry as "<unlisted-tools>"', () => {
     expect(TOOL_ORDER_REST).toBe('<unlisted-tools>')
   })
@@ -40,12 +42,25 @@ describe('SystemPrompt tool order', () => {
     expect(names(await backward.systemPrompt.assemble())).toEqual(['alpha', 'zulu'])
   })
 
-  it('applies a configured toolOrder: listed positions, rest at the rest entry lexicographically, absent names ignored', async () => {
-    const ctx = await mount({ toolOrder: ['todo_write', 'ghost', TOOL_ORDER_REST, 'bash'] })
+  it('applies a configured toolOrder: listed positions, rest at the rest entry lexicographically', async () => {
+    const ctx = await mount({ toolOrder: ['todo_write', TOOL_ORDER_REST, 'bash'] })
     ctx.systemPrompt.tools(() => [tool('bash'), tool('echo_b'), tool('todo_write'), tool('echo_a')])
     expect(names(await ctx.systemPrompt.assemble())).toEqual(['todo_write', 'echo_a', 'echo_b', 'bash'])
   })
 
+  it('rejects the assembly when toolOrder names a tool that is not registered (misconfiguration blocks work)', async () => {
+    const ctx = await mount({ toolOrder: ['todo_write', 'ghost', TOOL_ORDER_REST, 'wraith'] })
+    ctx.systemPrompt.tools(() => [tool('bash'), tool('todo_write')])
+    await expect(ctx.systemPrompt.assemble()).rejects.toThrow(
+      'toolOrder lists unregistered tools "ghost", "wraith"; registered tools: bash, todo_write')
+  })
+
+  it('names the single unregistered tool when no tools are registered at all', async () => {
+    const ctx = await mount({ toolOrder: ['ghost', TOOL_ORDER_REST] })
+    await expect(ctx.systemPrompt.assemble()).rejects.toThrow(
+      'toolOrder lists unregistered tool "ghost"; registered tools: (none)')
+  })
+
   it('keeps collection order between tools that share a name (stable sort)', async () => {
     const ctx = await mount()
     ctx.systemPrompt.tools(() => [tool('dup', 'first'), tool('anchor'), tool('dup', 'second')])