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fix(coding-agent): chain system prompt in before_agent_start closes #3539

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Mario Zechner
2026-04-22 14:58:17 +02:00
parent 0e5b6e54f0
commit 4e919868f6
4 changed files with 66 additions and 4 deletions
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14
packages/coding-agent/docs/extensions.md
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@@ -466,7 +466,8 @@ Fired after user submits prompt, before agent loop. Can inject a message and/or
pi.on("before_agent_start", async (event, ctx) => {
// event.prompt - user's prompt text
// event.images - attached images (if any)
// event.systemPrompt - current system prompt
// event.systemPrompt - current chained system prompt for this handler
// (includes changes from earlier before_agent_start handlers)
// event.systemPromptOptions - structured options used to build the system prompt
// .customPrompt - any custom system prompt (from --system-prompt, SYSTEM.md, or custom templates)
// .selectedTools - tools currently active in the prompt
@@ -492,6 +493,8 @@ pi.on("before_agent_start", async (event, ctx) => {
The `systemPromptOptions` field gives extensions access to the same structured data Pi uses to build the system prompt. This lets you inspect what Pi has loaded — custom prompts, guidelines, tool snippets, context files, skills — without re-discovering resources or re-parsing flags. Use it when your extension needs to make deep, informed changes to the system prompt while respecting user-provided configuration.
Inside `before_agent_start`, `event.systemPrompt` and `ctx.getSystemPrompt()` both reflect the chained system prompt as of the current handler. Later `before_agent_start` handlers can still modify it again.
#### agent_start / agent_end
Fired once per user prompt.
@@ -580,6 +583,8 @@ pi.on("context", async (event, ctx) => {
Fired after the provider-specific payload is built, right before the request is sent. Handlers run in extension load order. Returning `undefined` keeps the payload unchanged. Returning any other value replaces the payload for later handlers and for the actual request.
This hook can rewrite provider-level system instructions or remove them entirely. Those payload-level changes are not reflected by `ctx.getSystemPrompt()`, which reports Pi's system prompt string rather than the final serialized provider payload.
```typescript
pi.on("before_provider_request", (event, ctx) => {
console.log(JSON.stringify(event.payload, null, 2));
@@ -918,7 +923,12 @@ ctx.compact({
### ctx.getSystemPrompt()
Returns the current effective system prompt. This includes any modifications made by `before_agent_start` handlers for the current turn.
Returns Pi's current system prompt string.
- During `before_agent_start`, this reflects chained system-prompt changes made so far for the current turn.
- It does not include later `context` message mutations.
- It does not include `before_provider_request` payload rewrites.
- If later-loaded extensions run after yours, they can still change what is ultimately sent.
```typescript
pi.on("before_agent_start", (event, ctx) => {