diff --git a/packages/coding-agent/src/core/agent-session.ts b/packages/coding-agent/src/core/agent-session.ts
index c4c2d366..5083e60f 100644
--- a/packages/coding-agent/src/core/agent-session.ts
+++ b/packages/coding-agent/src/core/agent-session.ts
@@ -399,7 +399,7 @@ export class AgentSession {
 		throw new Error(formatNoApiKeyFoundMessage(model.provider));
 	}
 
-	private async _getCompactionRequestAuth(model: Model<any>): Promise<{
+	private async _getSummarizationRequestAuth(model: Model<any>): Promise<{
 		apiKey?: string;
 		headers?: Record<string, string>;
 		env?: Record<string, string>;
@@ -1744,7 +1744,7 @@ export class AgentSession {
 				throw new Error(formatNoModelSelectedMessage());
 			}
 
-			const { apiKey, headers, env } = await this._getCompactionRequestAuth(this.model);
+			const { apiKey, headers, env } = await this._getSummarizationRequestAuth(this.model);
 
 			const pathEntries = this.sessionManager.getBranch();
 			const settings = this.settingsManager.getCompactionSettings();
@@ -2012,7 +2012,7 @@ export class AgentSession {
 				headers = authResult.headers;
 				env = authResult.env;
 			} else {
-				({ apiKey, headers, env } = await this._getCompactionRequestAuth(this.model));
+				({ apiKey, headers, env } = await this._getSummarizationRequestAuth(this.model));
 			}
 
 			const pathEntries = this.sessionManager.getBranch();
@@ -2881,7 +2881,7 @@ export class AgentSession {
 			let summaryDetails: unknown;
 			if (options.summarize && entriesToSummarize.length > 0 && !extensionSummary) {
 				const model = this.model!;
-				const { apiKey, headers, env } = await this._getRequiredRequestAuth(model);
+				const { apiKey, headers, env } = await this._getSummarizationRequestAuth(model);
 				const branchSummarySettings = this.settingsManager.getBranchSummarySettings();
 				const result = await generateBranchSummary(entriesToSummarize, {
 					model,
diff --git a/packages/coding-agent/src/core/compaction/branch-summarization.ts b/packages/coding-agent/src/core/compaction/branch-summarization.ts
index 3f557c01..a96bb202 100644
--- a/packages/coding-agent/src/core/compaction/branch-summarization.ts
+++ b/packages/coding-agent/src/core/compaction/branch-summarization.ts
@@ -66,7 +66,7 @@ export interface GenerateBranchSummaryOptions {
 	/** Model to use for summarization */
 	model: Model<any>;
 	/** API key for the model */
-	apiKey: string;
+	apiKey?: string;
 	/** Request headers for the model */
 	headers?: Record<string, string>;
 	/** Provider-scoped environment values for the model */
diff --git a/packages/coding-agent/test/suite/regressions/6324-branch-summary-ambient-auth.test.ts b/packages/coding-agent/test/suite/regressions/6324-branch-summary-ambient-auth.test.ts
new file mode 100644
index 00000000..121cc006
--- /dev/null
+++ b/packages/coding-agent/test/suite/regressions/6324-branch-summary-ambient-auth.test.ts
@@ -0,0 +1,61 @@
+import { createAssistantMessageEventStream } from "@earendil-works/pi-ai";
+import { afterEach, describe, expect, it } from "vitest";
+import { assistantMsg, userMsg } from "../../utilities.ts";
+import { createHarness, type Harness } from "../harness.ts";
+
+describe("issue #6324 branch summary ambient auth", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	it("summarizes tree branches when request auth has no API key", async () => {
+		const harness = await createHarness({ withConfiguredAuth: false });
+		harnesses.push(harness);
+
+		let streamCallCount = 0;
+		harness.session.agent.streamFn = (model, _context, options) => {
+			streamCallCount++;
+			expect(options?.apiKey).toBeUndefined();
+
+			const stream = createAssistantMessageEventStream();
+			stream.push({
+				type: "done",
+				reason: "stop",
+				message: {
+					role: "assistant",
+					content: [{ type: "text", text: "branch summary text" }],
+					api: model.api,
+					provider: model.provider,
+					model: model.id,
+					usage: {
+						input: 1,
+						output: 1,
+						cacheRead: 0,
+						cacheWrite: 0,
+						totalTokens: 2,
+						cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+					},
+					stopReason: "stop",
+					timestamp: Date.now(),
+				},
+			});
+			return stream;
+		};
+
+		const targetId = harness.sessionManager.appendMessage(userMsg("first branch"));
+		harness.sessionManager.appendMessage(assistantMsg("first reply"));
+		harness.sessionManager.appendMessage(userMsg("abandoned branch work"));
+		harness.sessionManager.appendMessage(assistantMsg("abandoned reply"));
+
+		const result = await harness.session.navigateTree(targetId, { summarize: true });
+
+		expect(result.cancelled).toBe(false);
+		expect(streamCallCount).toBe(1);
+		expect(result.summaryEntry?.type).toBe("branch_summary");
+		expect(result.summaryEntry?.summary).toContain("branch summary text");
+	});
+});