feat(coding-agent): add provider payload hook

packages/agent/src/agent.ts

@@ -8,6 +8,7 @@ import {
 	type ImageContent,
 	type Message,
 	type Model,
+	type SimpleStreamOptions,
 	streamSimple,
 	type TextContent,
 	type ThinkingBudgets,
@@ -74,6 +75,11 @@ export interface AgentOptions {
 	 */
 	getApiKey?: (provider: string) => Promise<string | undefined> | string | undefined;
 
+	/**
+	 * Inspect or replace provider payloads before they are sent.
+	 */
+	onPayload?: SimpleStreamOptions["onPayload"];
+
 	/**
 	 * Custom token budgets for thinking levels (token-based providers only).
 	 */
@@ -117,6 +123,7 @@ export class Agent {
 	public streamFn: StreamFn;
 	private _sessionId?: string;
 	public getApiKey?: (provider: string) => Promise<string | undefined> | string | undefined;
+	private _onPayload?: SimpleStreamOptions["onPayload"];
 	private runningPrompt?: Promise<void>;
 	private resolveRunningPrompt?: () => void;
 	private _thinkingBudgets?: ThinkingBudgets;
@@ -132,6 +139,7 @@ export class Agent {
 		this.streamFn = opts.streamFn || streamSimple;
 		this._sessionId = opts.sessionId;
 		this.getApiKey = opts.getApiKey;
+		this._onPayload = opts.onPayload;
 		this._thinkingBudgets = opts.thinkingBudgets;
 		this._transport = opts.transport ?? "sse";
 		this._maxRetryDelayMs = opts.maxRetryDelayMs;
@@ -429,6 +437,7 @@ export class Agent {
 			model,
 			reasoning,
 			sessionId: this._sessionId,
+			onPayload: this._onPayload,
 			transport: this._transport,
 			thinkingBudgets: this._thinkingBudgets,
 			maxRetryDelayMs: this._maxRetryDelayMs,

packages/ai/src/providers/amazon-bedrock.ts

@@ -146,7 +146,7 @@ export const streamBedrock: StreamFunction<"bedrock-converse-stream", BedrockOpt
 			const client = new BedrockRuntimeClient(config);
 
 			const cacheRetention = resolveCacheRetention(options.cacheRetention);
-			const commandInput = {
+			let commandInput = {
 				modelId: model.id,
 				messages: convertMessages(context, model, cacheRetention),
 				system: buildSystemPrompt(context.systemPrompt, model, cacheRetention),
@@ -154,7 +154,10 @@ export const streamBedrock: StreamFunction<"bedrock-converse-stream", BedrockOpt
 				toolConfig: convertToolConfig(context.tools, options.toolChoice),
 				additionalModelRequestFields: buildAdditionalModelRequestFields(model, options),
 			};
-			options?.onPayload?.(commandInput);
+			const nextCommandInput = await options?.onPayload?.(commandInput, model);
+			if (nextCommandInput !== undefined) {
+				commandInput = nextCommandInput as typeof commandInput;
+			}
 			const command = new ConverseStreamCommand(commandInput);
 
 			const response = await client.send(command, { abortSignal: options.signal });

packages/ai/src/providers/anthropic.ts

@@ -235,8 +235,11 @@ export const streamAnthropic: StreamFunction<"anthropic-messages", AnthropicOpti
 				options?.headers,
 				copilotDynamicHeaders,
 			);
-			const params = buildParams(model, context, isOAuthToken, options);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, isOAuthToken, options);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as MessageCreateParamsStreaming;
+			}
 			const anthropicStream = client.messages.stream({ ...params, stream: true }, { signal: options?.signal });
 			stream.push({ type: "start", partial: output });
 

packages/ai/src/providers/azure-openai-responses.ts

@@ -85,8 +85,11 @@ export const streamAzureOpenAIResponses: StreamFunction<"azure-openai-responses"
 			// Create Azure OpenAI client
 			const apiKey = options?.apiKey || getEnvApiKey(model.provider) || "";
 			const client = createClient(model, apiKey, options);
-			const params = buildParams(model, context, options, deploymentName);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, options, deploymentName);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as ResponseCreateParamsStreaming;
+			}
 			const openaiStream = await client.responses.create(
 				params,
 				options?.signal ? { signal: options.signal } : undefined,

packages/ai/src/providers/google-gemini-cli.ts

@@ -369,8 +369,11 @@ export const streamGoogleGeminiCli: StreamFunction<"google-gemini-cli", GoogleGe
 			const baseUrl = model.baseUrl?.trim();
 			const endpoints = baseUrl ? [baseUrl] : isAntigravity ? ANTIGRAVITY_ENDPOINT_FALLBACKS : [DEFAULT_ENDPOINT];
 
-			const requestBody = buildRequest(model, context, projectId, options, isAntigravity);
-			options?.onPayload?.(requestBody);
+			let requestBody = buildRequest(model, context, projectId, options, isAntigravity);
+			const nextRequestBody = await options?.onPayload?.(requestBody, model);
+			if (nextRequestBody !== undefined) {
+				requestBody = nextRequestBody as CloudCodeAssistRequest;
+			}
 			const headers = isAntigravity ? getAntigravityHeaders() : GEMINI_CLI_HEADERS;
 
 			const requestHeaders = {

packages/ai/src/providers/google-vertex.ts

@@ -87,8 +87,11 @@ export const streamGoogleVertex: StreamFunction<"google-vertex", GoogleVertexOpt
 			const project = resolveProject(options);
 			const location = resolveLocation(options);
 			const client = createClient(model, project, location, options?.headers);
-			const params = buildParams(model, context, options);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, options);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as GenerateContentParameters;
+			}
 			const googleStream = await client.models.generateContentStream(params);
 
 			stream.push({ type: "start", partial: output });

packages/ai/src/providers/google.ts

@@ -74,8 +74,11 @@ export const streamGoogle: StreamFunction<"google-generative-ai", GoogleOptions>
 		try {
 			const apiKey = options?.apiKey || getEnvApiKey(model.provider) || "";
 			const client = createClient(model, apiKey, options?.headers);
-			const params = buildParams(model, context, options);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, options);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as GenerateContentParameters;
+			}
 			const googleStream = await client.models.generateContentStream(params);
 
 			stream.push({ type: "start", partial: output });

packages/ai/src/providers/mistral.ts

@@ -69,8 +69,11 @@ export const streamMistral: StreamFunction<"mistral-conversations", MistralOptio
 			const normalizeMistralToolCallId = createMistralToolCallIdNormalizer();
 			const transformedMessages = transformMessages(context.messages, model, (id) => normalizeMistralToolCallId(id));
 
-			const payload = buildChatPayload(model, context, transformedMessages, options);
-			options?.onPayload?.(payload);
+			let payload = buildChatPayload(model, context, transformedMessages, options);
+			const nextPayload = await options?.onPayload?.(payload, model);
+			if (nextPayload !== undefined) {
+				payload = nextPayload as ChatCompletionStreamRequest;
+			}
 			const mistralStream = await mistral.chat.stream(payload, buildRequestOptions(model, options));
 			stream.push({ type: "start", partial: output });
 			await consumeChatStream(model, output, stream, mistralStream);

packages/ai/src/providers/openai-codex-responses.ts

@@ -140,8 +140,11 @@ export const streamOpenAICodexResponses: StreamFunction<"openai-codex-responses"
 			}
 
 			const accountId = extractAccountId(apiKey);
-			const body = buildRequestBody(model, context, options);
-			options?.onPayload?.(body);
+			let body = buildRequestBody(model, context, options);
+			const nextBody = await options?.onPayload?.(body, model);
+			if (nextBody !== undefined) {
+				body = nextBody as RequestBody;
+			}
 			const headers = buildHeaders(model.headers, options?.headers, accountId, apiKey, options?.sessionId);
 			const bodyJson = JSON.stringify(body);
 			const transport = options?.transport || "sse";

packages/ai/src/providers/openai-completions.ts

@@ -86,8 +86,11 @@ export const streamOpenAICompletions: StreamFunction<"openai-completions", OpenA
 		try {
 			const apiKey = options?.apiKey || getEnvApiKey(model.provider) || "";
 			const client = createClient(model, context, apiKey, options?.headers);
-			const params = buildParams(model, context, options);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, options);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as OpenAI.Chat.Completions.ChatCompletionCreateParamsStreaming;
+			}
 			const openaiStream = await client.chat.completions.create(params, { signal: options?.signal });
 			stream.push({ type: "start", partial: output });
 

packages/ai/src/providers/openai-responses.ts

@@ -89,8 +89,11 @@ export const streamOpenAIResponses: StreamFunction<"openai-responses", OpenAIRes
 			// Create OpenAI client
 			const apiKey = options?.apiKey || getEnvApiKey(model.provider) || "";
 			const client = createClient(model, context, apiKey, options?.headers);
-			const params = buildParams(model, context, options);
-			options?.onPayload?.(params);
+			let params = buildParams(model, context, options);
+			const nextParams = await options?.onPayload?.(params, model);
+			if (nextParams !== undefined) {
+				params = nextParams as ResponseCreateParamsStreaming;
+			}
 			const openaiStream = await client.responses.create(
 				params,
 				options?.signal ? { signal: options.signal } : undefined,

packages/ai/src/providers/register-builtins.ts

@@ -1,13 +1,7 @@
 import { clearApiProviders, registerApiProvider } from "../api-registry.js";
-import type {
-	AssistantMessage,
-	AssistantMessageEvent,
-	Context,
-	Model,
-	SimpleStreamOptions,
-	StreamOptions,
-} from "../types.js";
+import type { AssistantMessage, AssistantMessageEvent, Context, Model, SimpleStreamOptions } from "../types.js";
 import { AssistantMessageEventStream } from "../utils/event-stream.js";
+import type { BedrockOptions } from "./amazon-bedrock.js";
 import { streamAnthropic, streamSimpleAnthropic } from "./anthropic.js";
 import { streamAzureOpenAIResponses, streamSimpleAzureOpenAIResponses } from "./azure-openai-responses.js";
 import { streamGoogle, streamSimpleGoogle } from "./google.js";
@@ -22,7 +16,7 @@ interface BedrockProviderModule {
 	streamBedrock: (
 		model: Model<"bedrock-converse-stream">,
 		context: Context,
-		options?: StreamOptions,
+		options?: BedrockOptions,
 	) => AsyncIterable<AssistantMessageEvent>;
 	streamSimpleBedrock: (
 		model: Model<"bedrock-converse-stream">,
@@ -83,7 +77,7 @@ function createLazyLoadErrorMessage(model: Model<"bedrock-converse-stream">, err
 function streamBedrockLazy(
 	model: Model<"bedrock-converse-stream">,
 	context: Context,
-	options?: StreamOptions,
+	options?: BedrockOptions,
 ): AssistantMessageEventStream {
 	const outer = new AssistantMessageEventStream();
 

packages/ai/src/types.ts

@@ -79,9 +79,10 @@ export interface StreamOptions {
 	 */
 	sessionId?: string;
 	/**
-	 * Optional callback for inspecting provider payloads before sending.
+	 * Optional callback for inspecting or replacing provider payloads before sending.
+	 * Return undefined to keep the payload unchanged.
 	 */
-	onPayload?: (payload: unknown) => void;
+	onPayload?: (payload: unknown, model: Model<Api>) => unknown | undefined | Promise<unknown | undefined>;
 	/**
 	 * Optional custom HTTP headers to include in API requests.
 	 * Merged with provider defaults; can override default headers.

packages/coding-agent/docs/extensions.md

@@ -243,6 +243,7 @@ user sends prompt ────────────────────
   │   │                                            │       │
   │   ├─► turn_start                               │       │
   │   ├─► context (can modify messages)            │       │
+  │   ├─► before_provider_request (can inspect or replace payload)
   │   │                                            │       │
   │   │   LLM responds, may call tools:            │       │
   │   │     ├─► tool_call (can block)              │       │
@@ -489,6 +490,21 @@ pi.on("context", async (event, ctx) => {
 });
 ```
 
+#### before_provider_request
+
+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.
+
+```typescript
+pi.on("before_provider_request", (event, ctx) => {
+  console.log(JSON.stringify(event.payload, null, 2));
+
+  // Optional: replace payload
+  // return { ...event.payload, temperature: 0 };
+});
+```
+
+This is mainly useful for debugging provider serialization and cache behavior.
+
 ### Model Events
 
 #### model_select
@@ -1934,6 +1950,7 @@ 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` |
+| `provider-payload.ts` | Inspect or patch provider payloads | `on("before_provider_request")` |
 | `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` |

packages/coding-agent/examples/extensions/provider-payload.ts

@@ -0,0 +1,14 @@
+import { appendFileSync } from "node:fs";
+import { join } from "node:path";
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	const logFile = join(process.cwd(), ".pi", "provider-payload.log");
+
+	pi.on("before_provider_request", (event) => {
+		appendFileSync(logFile, `${JSON.stringify(event.payload, null, 2)}\n\n`, "utf8");
+
+		// Optional: replace the payload instead of only logging it.
+		// return { ...event.payload, temperature: 0 };
+	});
+}

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

@@ -32,6 +32,8 @@ export type {
 	BashToolResultEvent,
 	BeforeAgentStartEvent,
 	BeforeAgentStartEventResult,
+	BeforeProviderRequestEvent,
+	BeforeProviderRequestEventResult,
 	// Context
 	CompactOptions,
 	// Events - Agent

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

@@ -13,6 +13,7 @@ import type { SessionManager } from "../session-manager.js";
 import type {
 	BeforeAgentStartEvent,
 	BeforeAgentStartEventResult,
+	BeforeProviderRequestEvent,
 	CompactOptions,
 	ContextEvent,
 	ContextEventResult,
@@ -105,6 +106,7 @@ type RunnerEmitEvent = Exclude<
 	| ToolResultEvent
 	| UserBashEvent
 	| ContextEvent
+	| BeforeProviderRequestEvent
 	| BeforeAgentStartEvent
 	| ResourcesDiscoverEvent
 	| InputEvent
@@ -710,6 +712,40 @@ export class ExtensionRunner {
 		return currentMessages;
 	}
 
+	async emitBeforeProviderRequest(payload: unknown): Promise<unknown> {
+		const ctx = this.createContext();
+		let currentPayload = payload;
+
+		for (const ext of this.extensions) {
+			const handlers = ext.handlers.get("before_provider_request");
+			if (!handlers || handlers.length === 0) continue;
+
+			for (const handler of handlers) {
+				try {
+					const event: BeforeProviderRequestEvent = {
+						type: "before_provider_request",
+						payload: currentPayload,
+					};
+					const handlerResult = await handler(event, ctx);
+					if (handlerResult !== undefined) {
+						currentPayload = handlerResult;
+					}
+				} catch (err) {
+					const message = err instanceof Error ? err.message : String(err);
+					const stack = err instanceof Error ? err.stack : undefined;
+					this.emitError({
+						extensionPath: ext.path,
+						event: "before_provider_request",
+						error: message,
+						stack,
+					});
+				}
+			}
+		}
+
+		return currentPayload;
+	}
+
 	async emitBeforeAgentStart(
 		prompt: string,
 		images: ImageContent[] | undefined,

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

@@ -493,6 +493,12 @@ export interface ContextEvent {
 	messages: AgentMessage[];
 }
 
+/** Fired before a provider request is sent. Can replace the payload. */
+export interface BeforeProviderRequestEvent {
+	type: "before_provider_request";
+	payload: unknown;
+}
+
 /** Fired after user submits prompt but before agent loop. */
 export interface BeforeAgentStartEvent {
 	type: "before_agent_start";
@@ -807,6 +813,7 @@ export type ExtensionEvent =
 	| ResourcesDiscoverEvent
 	| SessionEvent
 	| ContextEvent
+	| BeforeProviderRequestEvent
 	| BeforeAgentStartEvent
 	| AgentStartEvent
 	| AgentEndEvent
@@ -832,6 +839,8 @@ export interface ContextEventResult {
 	messages?: AgentMessage[];
 }
 
+export type BeforeProviderRequestEventResult = unknown;
+
 export interface ToolCallEventResult {
 	block?: boolean;
 	reason?: string;
@@ -944,6 +953,10 @@ export interface ExtensionAPI {
 	on(event: "session_before_tree", handler: ExtensionHandler<SessionBeforeTreeEvent, SessionBeforeTreeResult>): void;
 	on(event: "session_tree", handler: ExtensionHandler<SessionTreeEvent>): void;
 	on(event: "context", handler: ExtensionHandler<ContextEvent, ContextEventResult>): void;
+	on(
+		event: "before_provider_request",
+		handler: ExtensionHandler<BeforeProviderRequestEvent, BeforeProviderRequestEventResult>,
+	): void;
 	on(event: "before_agent_start", handler: ExtensionHandler<BeforeAgentStartEvent, BeforeAgentStartEventResult>): void;
 	on(event: "agent_start", handler: ExtensionHandler<AgentStartEvent>): void;
 	on(event: "agent_end", handler: ExtensionHandler<AgentEndEvent>): void;

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

@@ -292,6 +292,13 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: [],
 		},
 		convertToLlm: convertToLlmWithBlockImages,
+		onPayload: async (payload, _model) => {
+			const runner = extensionRunnerRef.current;
+			if (!runner?.hasHandlers("before_provider_request")) {
+				return payload;
+			}
+			return runner.emitBeforeProviderRequest(payload);
+		},
 		sessionId: sessionManager.getSessionId(),
 		transformContext: async (messages) => {
 			const runner = extensionRunnerRef.current;

packages/coding-agent/src/index.ts

@@ -56,6 +56,8 @@ export type {
 	AppAction,
 	BashToolCallEvent,
 	BeforeAgentStartEvent,
+	BeforeProviderRequestEvent,
+	BeforeProviderRequestEventResult,
 	CompactOptions,
 	ContextEvent,
 	ContextUsage,