diff --git a/src/lib/intake/interview.ts b/src/lib/intake/interview.ts
new file mode 100644
index 0000000..6a91946
--- /dev/null
+++ b/src/lib/intake/interview.ts
@@ -0,0 +1,138 @@
+import "server-only";
+
+import { getAdminPb } from "@/lib/pb/admin";
+import { getProvider, getSystemPrompt, AiError } from "@/lib/ai";
+import { aiConfigured } from "@/lib/ai/settings";
+import { canAskMore, sanitiseTurns, type Turn } from "@/lib/intake/turn";
+import { buildBriefPrompt, buildQuestionPrompt, type IntakeContext } from "@/lib/intake/prompt";
+import { parseBrief, type Brief } from "@/lib/intake/brief";
+import { deriveSizeBand, type SizeBand } from "@/lib/intake/size";
+import type { CommitRecord, ProjectRecord } from "@/lib/pb/types";
+
+export interface InterviewStep {
+  kind: "question" | "brief" | "unavailable";
+  question?: string;
+  brief?: Brief & { sizeBand: SizeBand };
+  message?: string;
+}
+
+/**
+ * The project's real commit scopes, used for the size band and to steer `area`
+ * toward something that exists rather than a name the model invents.
+ */
+async function intakeContext(projectId: string): Promise<{
+  context: IntakeContext;
+  knownScopes: string[];
+}> {
+  const pb = await getAdminPb();
+
+  const project = await pb.collection("projects").getOne<ProjectRecord>(projectId);
+
+  const commits = await pb.collection("commits").getFullList<CommitRecord>({
+    filter: `project = "${projectId}" && scope != "" && is_merge = false`,
+    fields: "scope",
+  });
+
+  const counts = new Map<string, number>();
+  for (const commit of commits) {
+    counts.set(commit.scope, (counts.get(commit.scope) ?? 0) + 1);
+  }
+
+  // Most-used first, capped: a list of sixty scopes is noise in a prompt.
+  const knownScopes = [...counts.entries()]
+    .sort((a, b) => b[1] - a[1])
+    .slice(0, 15)
+    .map(([scope]) => scope);
+
+  return {
+    context: { projectName: project.name, knownAreas: knownScopes },
+    knownScopes,
+  };
+}
+
+/**
+ * Advance the interview by one step.
+ *
+ * Stateless: the transcript arrives from the browser and is sanitised on every
+ * call. The question cap is enforced here, in code — a model that wants an
+ * eighth question does not get one.
+ *
+ * Never throws. A provider failure returns `unavailable`, and the caller offers
+ * the plain request box instead.
+ */
+export async function nextStep(input: {
+  projectId: string;
+  turns: Turn[];
+  finishNow: boolean;
+}): Promise<InterviewStep> {
+  if (!(await aiConfigured())) {
+    return { kind: "unavailable", message: "No AI provider is configured." };
+  }
+
+  const turns = sanitiseTurns(input.turns);
+
+  let provider;
+  let system: string;
+  let ctx: Awaited<ReturnType<typeof intakeContext>>;
+
+  try {
+    provider = await getProvider();
+    system = await getSystemPrompt();
+    ctx = await intakeContext(input.projectId);
+  } catch (error) {
+    return {
+      kind: "unavailable",
+      message: error instanceof AiError ? error.message : "The assistant is unavailable.",
+    };
+  }
+
+  // The cap, and the client's own "that's enough".
+  const finish = input.finishNow || !canAskMore(turns);
+
+  if (!finish) {
+    try {
+      const result = await provider.complete({
+        system,
+        user: buildQuestionPrompt(ctx.context, turns),
+        maxTokens: 1024,
+      });
+
+      const question = result.text.trim();
+      if (question) return { kind: "question", question };
+      // An empty question is not worth another round trip; write it up instead.
+    } catch (error) {
+      return {
+        kind: "unavailable",
+        message: error instanceof AiError ? error.message : "The assistant is unavailable.",
+      };
+    }
+  }
+
+  let raw = "";
+  try {
+    const result = await provider.complete({
+      system,
+      user: buildBriefPrompt(ctx.context, turns),
+      maxTokens: 4096,
+    });
+    raw = result.text;
+  } catch {
+    // Fall through: parseBrief degrades to the client's own answers rather
+    // than losing an interview they have already sat through.
+  }
+
+  const brief = parseBrief(raw, turns);
+
+  return {
+    kind: "brief",
+    brief: {
+      ...brief,
+      // Derived in code, never taken from the model's output.
+      sizeBand: deriveSizeBand({
+        subtaskCount: brief.subtasks.length,
+        knownScopes: ctx.knownScopes,
+        suggestedArea: brief.area,
+      }),
+    },
+  };
+}