fix(compaction): preserve OpenCode session attribution

packages/coding-agent/src/core/changes.md

@@ -1,5 +1,23 @@
 # changes
 
+## 2026-09-11 - Preserve OpenCode session attribution for direct compaction summaries
+
+### What changed
+
+- `packages/coding-agent/src/core/provider-attribution.ts`: exports the existing OpenCode session-header predicate as `getOpenCodeSessionHeaders()` so non-AgentSession request paths can reuse the provider/host test without duplicating it. `mergeProviderAttributionHeaders()` continues to apply the same generated defaults and caller override order.
+
+### Why
+
+- Builtin compaction dispatches its summary directly through `ModelRuntime.stream()` and therefore cannot reuse the AgentSession wrapper that normally attaches `x-opencode-session`; exposing the narrow helper lets that alternate request boundary preserve the same routing identity.
+
+### Why an extension could not handle it
+
+- The session-attribution predicate is core provider request policy used before or beneath extension header transforms; duplicating it in an extension would allow the provider/host rules to drift from normal AgentSession traffic.
+
+### Expected merge conflict zones
+
+- LOW: `packages/coding-agent/src/core/provider-attribution.ts` around the OpenCode session-header helper and `mergeProviderAttributionHeaders()`.
+
 ## 2026-09-10 - Atomic account display-name metadata with shape-keyed sentinel guard (senpi#1495)
 
 ### What changed

packages/coding-agent/src/core/extensions/builtin/compaction/changes.md

@@ -1,5 +1,24 @@
 # changes.md — builtin compaction policy
 
+## 2026-09-11 - Preserve OpenCode session routing on builtin summary requests
+
+### What changed
+
+- `packages/coding-agent/src/core/extensions/builtin/compaction/speculative-summary.ts`: seeds the current `SessionManager` id through `getOpenCodeSessionHeaders()` before resolved auth/configured headers and the existing provider-request header transform.
+- `packages/coding-agent/test/suite/regressions/issue-1523-opencode-session-attribution.test.ts`: pins provider-id and host routing, configured-header precedence, extension-transform precedence, and non-OpenCode neutrality.
+
+### Why
+
+- Issue #1523: the compaction summarizer bypasses `sdk.ts`'s normal AgentSession stream wrapper, so OpenCode Go requests could omit `x-opencode-session` and fail with `400 MissingSessionID` while ordinary turns succeeded.
+
+### Why an extension could not handle it
+
+- The omission occurs at the builtin compaction request-construction boundary before `ModelRuntime.stream()`. The fix must seed the normal routing defaults before the already-supported provider header transform, which remains authoritative.
+
+### Expected merge conflict zones
+
+- LOW: `speculative-summary.ts` at request-header construction immediately before `summarizationStream()`.
+
 ## Deterministic resume slice for an over-window restored context (2026-09-10)
 
 ### What changed

packages/coding-agent/src/core/extensions/builtin/compaction/speculative-summary.ts

@@ -18,6 +18,7 @@ import {
 	summarizationMaxDurationMs,
 } from "../../../compaction/stream-watchdog.ts";
 import { convertToLlm } from "../../../messages.ts";
+import { getOpenCodeSessionHeaders } from "../../../provider-attribution.ts";
 import type { buildPrompt } from "./prompts.ts";
 import { repairOrphanedToolResults } from "./repair-tool-pairs.ts";
 import type { SpeculativeCompactionContext, SpeculativeCompactionSnapshot } from "./speculative.ts";
@@ -143,9 +144,18 @@ export async function generateSummaryMessage(options: {
 			),
 			...(options.snapshot.tools && options.snapshot.tools.length > 0 ? { tools: options.snapshot.tools } : {}),
 		};
+		// Builtin compaction bypasses AgentSession's sdk.ts stream wrapper. Seed only
+		// the OpenCode routing defaults here, then keep resolved auth/configured
+		// headers and the provider-request transform authoritative as on normal turns.
+		const requestHeaders: ProviderHeaders = {
+			...getOpenCodeSessionHeaders(options.snapshot.model, options.context.sessionManager.getSessionId()),
+			...(options.auth.headers ?? {}),
+		};
 		const headers = providerRequest
-			? await providerRequest.transformHeaders(options.auth.headers ?? {})
-			: options.auth.headers;
+			? await providerRequest.transformHeaders(requestHeaders)
+			: Object.keys(requestHeaders).length > 0
+				? requestHeaders
+				: undefined;
 		const responseStream = summarizationStream(options.context, options.snapshot.model, requestContext, {
 			apiKey: options.auth.apiKey,
 			headers,

packages/coding-agent/src/core/provider-attribution.ts

@@ -65,7 +65,10 @@ function getDefaultAttributionHeaders(
 	return undefined;
 }
 
-function getSessionHeaders(model: Model<Api>, sessionId: string | undefined): Record<string, string> | undefined {
+export function getOpenCodeSessionHeaders(
+	model: Model<Api>,
+	sessionId: string | undefined,
+): Record<string, string> | undefined {
 	if (!sessionId) return undefined;
 	if (
 		model.provider !== "opencode" &&
@@ -84,7 +87,7 @@ export function mergeProviderAttributionHeaders(
 	...headerSources: Array<ProviderHeaders | undefined>
 ): ProviderHeaders | undefined {
 	const merged: ProviderHeaders = {
-		...getSessionHeaders(model, sessionId),
+		...getOpenCodeSessionHeaders(model, sessionId),
 		...getDefaultAttributionHeaders(model, settingsManager),
 	};
 

packages/coding-agent/test/suite/regressions/issue-1523-opencode-session-attribution.test.ts

@@ -0,0 +1,148 @@
+import {
+	createAssistantMessageEventStream,
+	fauxAssistantMessage,
+	type Model,
+	type ProviderHeaders,
+} from "@earendil-works/pi-ai";
+import { describe, expect, it } from "vitest";
+import { generateSummaryMessage } from "../../../src/core/extensions/builtin/compaction/speculative-summary.ts";
+import type {
+	SpeculativeCompactionContext,
+	SpeculativeCompactionSnapshot,
+} from "../../../src/core/extensions/builtin/compaction/speculative.ts";
+import type { ModelRegistry } from "../../../src/core/model-registry.ts";
+import { SessionManager } from "../../../src/core/session-manager.ts";
+
+type TestModel = Model<"openai-completions">;
+
+function createModel(provider = "opencode-go", baseUrl = "https://opencode.ai/zen/go/v1/messages"): TestModel {
+	return {
+		id: "test-model",
+		name: "Test Model",
+		api: "openai-completions",
+		provider,
+		baseUrl,
+		reasoning: false,
+		input: ["text"],
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 128_000,
+		maxTokens: 4_096,
+	};
+}
+
+function completedStream(model: TestModel) {
+	const stream = createAssistantMessageEventStream();
+	stream.end({
+		...fauxAssistantMessage("summary"),
+		api: model.api,
+		provider: model.provider,
+		model: model.id,
+	});
+	return stream;
+}
+
+async function captureSummaryHeaders(options: {
+	model: TestModel;
+	authHeaders?: ProviderHeaders;
+	transformHeaders?: (headers: ProviderHeaders) => ProviderHeaders | Promise<ProviderHeaders>;
+}): Promise<ProviderHeaders | undefined> {
+	let capturedHeaders: ProviderHeaders | undefined;
+	const sessionManager = SessionManager.inMemory();
+	sessionManager.newSession({ id: "session-1523" });
+
+	const modelRuntime = {
+		stream: (model: TestModel, _context: unknown, streamOptions?: { headers?: ProviderHeaders }) => {
+			capturedHeaders = streamOptions?.headers;
+			return completedStream(model);
+		},
+	};
+	const context: SpeculativeCompactionContext = {
+		model: options.model,
+		modelRegistry: { modelRuntime } as unknown as ModelRegistry,
+		sessionManager,
+		getContextUsage: () => undefined,
+		getMessageRevision: () => 1,
+		prepareProviderRequest: async (messages) => ({
+			messages,
+			transformPayload: async (payload) => payload,
+			transformHeaders: async (headers) =>
+				options.transformHeaders ? await options.transformHeaders(headers) : headers,
+		}),
+		applyCompaction: async () => ({ applied: true, reason: "ok" }),
+	};
+	const snapshot = {
+		model: options.model,
+		contextWindow: options.model.contextWindow,
+		systemPrompt: "system",
+		tools: [],
+	} as unknown as SpeculativeCompactionSnapshot;
+
+	await generateSummaryMessage({
+		context,
+		messages: [{ role: "user", content: [{ type: "text", text: "history" }], timestamp: 1 }],
+		prompt: { system: "system", user: "summarize" } as never,
+		snapshot,
+		auth: { headers: options.authHeaders },
+	});
+	return capturedHeaders;
+}
+
+// Regression for #1523: builtin compaction bypasses sdk.ts's normal AgentSession
+// provider-attribution wrapper but must preserve the same OpenCode routing semantics.
+describe("issue #1523: compaction provider session attribution", () => {
+	it("sends the OpenCode session headers on builtin summarization", async () => {
+		const headers = await captureSummaryHeaders({ model: createModel() });
+
+		expect(headers?.["x-opencode-session"]).toBe("session-1523");
+		expect(headers?.["x-opencode-client"]).toBe("pi");
+	});
+
+	it("recognizes OpenCode routing by opencode.ai host", async () => {
+		const headers = await captureSummaryHeaders({
+			model: createModel("custom-openai", "https://opencode.ai/zen/go/v1/messages"),
+		});
+
+		expect(headers?.["x-opencode-session"]).toBe("session-1523");
+		expect(headers?.["x-opencode-client"]).toBe("pi");
+	});
+
+	it("keeps resolved auth or configured headers above generated session defaults", async () => {
+		const headers = await captureSummaryHeaders({
+			model: createModel(),
+			authHeaders: {
+				"x-opencode-session": "configured-session",
+				"x-opencode-client": "configured-client",
+			},
+		});
+
+		expect(headers?.["x-opencode-session"]).toBe("configured-session");
+		expect(headers?.["x-opencode-client"]).toBe("configured-client");
+	});
+
+	it("keeps before-provider header transforms authoritative", async () => {
+		const headers = await captureSummaryHeaders({
+			model: createModel(),
+			authHeaders: {
+				"x-opencode-session": "configured-session",
+				"x-opencode-client": "configured-client",
+			},
+			transformHeaders: (input) => ({
+				...input,
+				"x-opencode-session": "extension-session",
+				"x-opencode-client": "extension-client",
+			}),
+		});
+
+		expect(headers?.["x-opencode-session"]).toBe("extension-session");
+		expect(headers?.["x-opencode-client"]).toBe("extension-client");
+	});
+
+	it("does not add OpenCode session headers for unrelated providers", async () => {
+		const headers = await captureSummaryHeaders({
+			model: createModel("example", "https://example.test/v1"),
+		});
+
+		expect(headers?.["x-opencode-session"]).toBeUndefined();
+		expect(headers?.["x-opencode-client"]).toBeUndefined();
+	});
+});