feat(coding-agent): add ctx.getSystemPrompt() to extension context

Adds a method to access the effective system prompt (after any per-turn
extension modifications) from the extension context.

Implementation:
- Add systemPrompt getter to AgentSession reading from agent.state.systemPrompt
- Wire getSystemPrompt through ExtensionContextActions to ExtensionRunner
- Add getSystemPrompt to interactive-mode's shortcut context
- Update docs with ctx.getSystemPrompt() section
- Add system-prompt-header.ts example
- Add example to docs reference table

Closes #1098

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added `ctx.getSystemPrompt()` to extension context for accessing the current effective system prompt ([#1098](https://github.com/badlogic/pi-mono/pull/1098) by [@kaofelix](https://github.com/kaofelix))
+
 ## [0.50.5] - 2026-01-30
 
 ## [0.50.4] - 2026-01-30

packages/coding-agent/docs/extensions.md

@@ -654,6 +654,17 @@ ctx.compact({
 });
 ```
 
+### ctx.getSystemPrompt()
+
+Returns the current effective system prompt. This includes any modifications made by `before_agent_start` handlers for the current turn.
+
+```typescript
+pi.on("before_agent_start", (event, ctx) => {
+  const prompt = ctx.getSystemPrompt();
+  console.log(`System prompt length: ${prompt.length}`);
+});
+```
+
 ## ExtensionCommandContext
 
 Command handlers receive `ExtensionCommandContext`, which extends `ExtensionContext` with session control methods. These are only available in commands because they can deadlock if called from event handlers.
@@ -1688,6 +1699,7 @@ All examples in [examples/extensions/](../examples/extensions/).
 | `dirty-repo-guard.ts` | Warn on dirty git repo | `on("session_before_*")`, `exec` |
 | `input-transform.ts` | Transform user input | `on("input")` |
 | `model-status.ts` | React to model changes | `on("model_select")`, `setStatus` |
+| `system-prompt-header.ts` | Display system prompt info | `on("agent_start")`, `getSystemPrompt` |
 | `claude-rules.ts` | Load rules from files | `on("session_start")`, `on("before_agent_start")` |
 | `file-trigger.ts` | File watcher triggers messages | `sendMessage` |
 | **Compaction & Sessions** |||

packages/coding-agent/examples/extensions/system-prompt-header.ts

@@ -0,0 +1,17 @@
+/**
+ * Displays a status widget showing the system prompt length.
+ *
+ * Demonstrates ctx.getSystemPrompt() for accessing the effective system prompt.
+ */
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("agent_start", (_event, ctx) => {
+		const prompt = ctx.getSystemPrompt();
+		ctx.ui.setStatus("system-prompt", `System: ${prompt.length} chars`);
+	});
+
+	pi.on("session_shutdown", (_event, ctx) => {
+		ctx.ui.setStatus("system-prompt", undefined);
+	});
+}

packages/coding-agent/src/core/agent-session.ts

@@ -514,6 +514,11 @@ export class AgentSession {
 		return this.agent.state.isStreaming;
 	}
 
+	/** Current effective system prompt (includes any per-turn extension modifications) */
+	get systemPrompt(): string {
+		return this.agent.state.systemPrompt;
+	}
+
 	/** Current retry attempt (0 if not retrying) */
 	get retryAttempt(): number {
 		return this._retryAttempt;
@@ -1756,6 +1761,7 @@ export class AgentSession {
 						}
 					})();
 				},
+				getSystemPrompt: () => this.systemPrompt,
 			},
 		);
 	}

packages/coding-agent/src/core/extensions/runner.ts

@@ -159,6 +159,7 @@ export class ExtensionRunner {
 	private hasPendingMessagesFn: () => boolean = () => false;
 	private getContextUsageFn: () => ContextUsage | undefined = () => undefined;
 	private compactFn: (options?: CompactOptions) => void = () => {};
+	private getSystemPromptFn: () => string = () => "";
 	private newSessionHandler: NewSessionHandler = async () => ({ cancelled: false });
 	private forkHandler: ForkHandler = async () => ({ cancelled: false });
 	private navigateTreeHandler: NavigateTreeHandler = async () => ({ cancelled: false });
@@ -203,6 +204,7 @@ export class ExtensionRunner {
 		this.shutdownHandler = contextActions.shutdown;
 		this.getContextUsageFn = contextActions.getContextUsage;
 		this.compactFn = contextActions.compact;
+		this.getSystemPromptFn = contextActions.getSystemPrompt;
 
 		// Process provider registrations queued during extension loading
 		for (const { name, config } of this.runtime.pendingProviderRegistrations) {
@@ -421,6 +423,7 @@ export class ExtensionRunner {
 			shutdown: () => this.shutdownHandler(),
 			getContextUsage: () => this.getContextUsageFn(),
 			compact: (options) => this.compactFn(options),
+			getSystemPrompt: () => this.getSystemPromptFn(),
 		};
 	}
 

packages/coding-agent/src/core/extensions/types.ts

@@ -260,6 +260,8 @@ export interface ExtensionContext {
 	getContextUsage(): ContextUsage | undefined;
 	/** Trigger compaction without awaiting completion. */
 	compact(options?: CompactOptions): void;
+	/** Get the current effective system prompt. */
+	getSystemPrompt(): string;
 }
 
 /**
@@ -1084,6 +1086,7 @@ export interface ExtensionContextActions {
 	shutdown: () => void;
 	getContextUsage: () => ContextUsage | undefined;
 	compact: (options?: CompactOptions) => void;
+	getSystemPrompt: () => string;
 }
 
 /**

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -1120,6 +1120,7 @@ export class InteractiveMode {
 					}
 				})();
 			},
+			getSystemPrompt: () => this.session.systemPrompt,
 		});
 
 		// Set up the extension shortcut handler on the default editor