feat: add fake phase harness

packages/session/src/fake.test.ts

@@ -97,7 +97,25 @@ describe("FakeSessionAdapter", () => {
     expect(chunks.every((chunk, index) => chunk.seq === BigInt(index + 1))).toBe(true);
   });
 
-  it("refuses duplicate prompt dedup keys for the same session", async () => {
+  it("classifies unsupported backend as human-required backend_unavailable", async () => {
+    const fixtureRoot = makeFixtureRoot();
+    tempRoots.push(fixtureRoot);
+    const adapter = new FakeSessionAdapter({ fixtureRoot, writeDelayMs: 0 });
+
+    await expect(
+      adapter.start({
+        runId,
+        roleId: "implementer",
+        backend: "codex",
+        cwd: fixtureRoot,
+      }),
+    ).rejects.toMatchObject({
+      class: "human_required",
+      code: "backend_unavailable",
+    });
+  });
+
+  it("treats duplicate prompt dedup keys as idempotent success without reprocessing", async () => {
     const fixtureRoot = makeFixtureRoot();
     tempRoots.push(fixtureRoot);
     const adapter = new FakeSessionAdapter({ fixtureRoot, writeDelayMs: 0 });
@@ -116,13 +134,12 @@ describe("FakeSessionAdapter", () => {
 
     await adapter.sendPrompt(handle, first);
 
-    await expect(adapter.sendPrompt(handle, duplicate)).rejects.toMatchObject({
-      code: "duplicate_prompt_dedup_key",
-    });
+    await expect(adapter.sendPrompt(handle, duplicate)).resolves.toEqual({ promptId: dedupKey });
     await waitForFile(first.expectedArtifact);
+    expect(existsSync(duplicate.expectedArtifact)).toBe(false);
   });
 
-  it("preserves prompt dedup history across crash and rebootstrap recovery", async () => {
+  it("records dedup history only after a fake prompt is accepted", async () => {
     const fixtureRoot = makeFixtureRoot();
     tempRoots.push(fixtureRoot);
     const adapter = new FakeSessionAdapter({ fixtureRoot, writeDelayMs: 0 });
@@ -141,7 +158,7 @@ describe("FakeSessionAdapter", () => {
       code: "prompt_send_transient",
     });
     await expect(adapter.sendPrompt(handle, crash)).rejects.toMatchObject({
-      code: "duplicate_prompt_dedup_key",
+      code: "prompt_send_transient",
     });
 
     const ok = envelope({ dedupKey: "d".repeat(64) });
@@ -150,9 +167,7 @@ describe("FakeSessionAdapter", () => {
     await waitForFile(ok.expectedArtifact);
     await adapter.rebootstrap(handle);
 
-    await expect(adapter.sendPrompt(handle, ok)).rejects.toMatchObject({
-      code: "duplicate_prompt_dedup_key",
-    });
+    await expect(adapter.sendPrompt(handle, ok)).resolves.toEqual({ promptId: "d".repeat(64) });
   });
 
   it("rejects prompts whose run or role do not match the session", async () => {
@@ -207,6 +222,26 @@ describe("FakeSessionAdapter", () => {
     expect(existsSync(prompt.expectedArtifact)).toBe(false);
   });
 
+  it("does not record dedup history when fixture resolution fails", async () => {
+    const fixtureRoot = mkdtempSync(join(tmpdir(), "devflow-empty-fake-fixtures-"));
+    tempRoots.push(fixtureRoot);
+    const adapter = new FakeSessionAdapter({ fixtureRoot, writeDelayMs: 0 });
+    const handle = await adapter.start({
+      runId,
+      roleId: "implementer",
+      backend: "fake",
+      cwd: fixtureRoot,
+    });
+    const missing = envelope({ dedupKey: "f".repeat(64) });
+
+    await expect(adapter.sendPrompt(handle, missing)).rejects.toMatchObject({
+      code: "fake_fixture_missing",
+    });
+    await expect(adapter.sendPrompt(handle, missing)).rejects.toMatchObject({
+      code: "fake_fixture_missing",
+    });
+  });
+
   it("supports invalid, timeout, and crash sentinel scenarios", async () => {
     const fixtureRoot = makeFixtureRoot();
     tempRoots.push(fixtureRoot);

packages/session/src/fake.ts

@@ -21,7 +21,7 @@ import type {
 
 export interface FakeSessionAdapterOptions {
   fixtureRoot?: string;
-  writeDelayMs?: number;
+  writeDelayMs?: number | ((envelope: PromptEnvelope) => number);
   sessionIdFactory?: () => string;
   now?: () => Date;
 }
@@ -40,14 +40,16 @@ interface FakeSessionRecord {
 
 export class FakeSessionAdapter implements SessionAdapter {
   private readonly fixtureRoot: string;
-  private readonly writeDelayMs: number;
+  private readonly writeDelayMs: (envelope: PromptEnvelope) => number;
   private readonly sessionIdFactory: () => string;
   private readonly now: () => Date;
   private readonly sessions = new Map<string, FakeSessionRecord>();
 
   constructor(options: FakeSessionAdapterOptions = {}) {
     this.fixtureRoot = options.fixtureRoot ?? defaultFixtureRoot();
-    this.writeDelayMs = options.writeDelayMs ?? 50;
+    const writeDelayMs = options.writeDelayMs;
+    this.writeDelayMs =
+      typeof writeDelayMs === "function" ? writeDelayMs : () => writeDelayMs ?? 50;
     this.sessionIdFactory = options.sessionIdFactory ?? randomUUID;
     this.now = options.now ?? (() => new Date());
   }
@@ -55,7 +57,7 @@ export class FakeSessionAdapter implements SessionAdapter {
   async start(input: StartInput): Promise<SessionHandle> {
     if (input.backend !== "fake") {
       throw new DevflowError("FakeSessionAdapter only supports the fake backend", {
-        class: "fatal",
+        class: "human_required",
         code: "backend_unavailable",
         runId: input.runId,
       });
@@ -87,15 +89,10 @@ export class FakeSessionAdapter implements SessionAdapter {
       });
     }
     if (record.sentDedupKeys.has(envelope.dedupKey)) {
-      throw new DevflowError("Duplicate prompt dedup key refused by fake session", {
-        class: "recoverable",
-        code: "duplicate_prompt_dedup_key",
-        runId: record.runId,
-      });
+      return { promptId: envelope.dedupKey };
     }
 
     const scenarioName = scenarioFromInstructions(envelope.instructions);
-    record.sentDedupKeys.add(envelope.dedupKey);
 
     if (scenarioName === "crash") {
       this.appendTranscript(record, `[fake] received prompt ${envelope.uuid}; crashing`);
@@ -107,6 +104,7 @@ export class FakeSessionAdapter implements SessionAdapter {
     }
 
     if (scenarioName === "timeout") {
+      record.sentDedupKeys.add(envelope.dedupKey);
       this.appendTranscript(record, `[fake] received prompt ${envelope.uuid}; timeout`);
       return { promptId: envelope.dedupKey };
     }
@@ -121,9 +119,12 @@ export class FakeSessionAdapter implements SessionAdapter {
             envelope.runId,
           );
 
+    record.sentDedupKeys.add(envelope.dedupKey);
+
+    const writeDelayMs = this.writeDelayMs(envelope);
     this.appendTranscript(
       record,
-      `[fake] received prompt ${envelope.uuid}; will write ${envelope.expectedArtifact} in ${this.writeDelayMs}ms`,
+      `[fake] received prompt ${envelope.uuid}; will write ${envelope.expectedArtifact} in ${writeDelayMs}ms`,
     );
     const timer = setTimeout(() => {
       record.timers.delete(timer);
@@ -145,7 +146,7 @@ export class FakeSessionAdapter implements SessionAdapter {
         return;
       }
       this.appendTranscript(record, `[fake] wrote artifact ${envelope.expectedArtifact}`);
-    }, this.writeDelayMs);
+    }, writeDelayMs);
     record.timers.add(timer);
 
     return { promptId: envelope.dedupKey };