diff --git a/packages/server/src/server/agent/providers/acp-agent.ts b/packages/server/src/server/agent/providers/acp-agent.ts
index 4944cb9e6..214b2f613 100644
--- a/packages/server/src/server/agent/providers/acp-agent.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.ts
@@ -86,6 +86,7 @@ import {
   type ProviderRuntimeSettings,
 } from "../provider-launch-config.js";
 import { renderPromptAttachmentAsText } from "../prompt-attachments.js";
+import { appendOrReplaceGrowingAssistantMessage, runProviderTurn } from "./provider-runner.js";
 import { findExecutable } from "../../../utils/executable.js";
 import { spawnProcess } from "../../../utils/spawn.js";
 
@@ -857,84 +858,20 @@ export class ACPAgentSession implements AgentSession, ACPClient {
   }
 
   async run(prompt: AgentPromptInput, options?: AgentRunOptions): Promise<AgentRunResult> {
-    const timeline: AgentTimelineItem[] = [];
-    let finalText = "";
-    let usage: AgentUsage | undefined;
-    let turnId: string | null = null;
-    let settled = false;
-    let resolveCompletion!: () => void;
-    let rejectCompletion!: (error: Error) => void;
-    const buffered: AgentStreamEvent[] = [];
-
-    const completion = new Promise<void>((resolve, reject) => {
-      resolveCompletion = resolve;
-      rejectCompletion = reject;
+    const result = await runProviderTurn({
+      prompt,
+      runOptions: options,
+      startTurn: (p, o) => this.startTurn(p, o),
+      subscribe: (callback) => this.subscribe(callback),
+      getSessionId: () => this.sessionId ?? "",
+      reduceFinalText: appendOrReplaceGrowingAssistantMessage,
     });
 
-    const processEvent = (event: AgentStreamEvent) => {
-      if (settled) {
-        return;
-      }
-      if (turnId && "turnId" in event && event.turnId && event.turnId !== turnId) {
-        return;
-      }
-      if (event.type === "timeline") {
-        timeline.push(event.item);
-        if (event.item.type === "assistant_message") {
-          finalText = event.item.text.startsWith(finalText)
-            ? event.item.text
-            : `${finalText}${event.item.text}`;
-        }
-        return;
-      }
-      if (event.type === "turn_completed") {
-        usage = event.usage;
-        settled = true;
-        resolveCompletion();
-        return;
-      }
-      if (event.type === "turn_failed") {
-        settled = true;
-        rejectCompletion(new Error(event.error));
-        return;
-      }
-      if (event.type === "turn_canceled") {
-        settled = true;
-        resolveCompletion();
-      }
-    };
-
-    const unsubscribe = this.subscribe((event) => {
-      if (!turnId) {
-        buffered.push(event);
-        return;
-      }
-      processEvent(event);
-    });
-
-    try {
-      const started = await this.startTurn(prompt, options);
-      turnId = started.turnId;
-      for (const event of buffered) {
-        processEvent(event);
-      }
-      if (!settled) {
-        await completion;
-      }
-    } finally {
-      unsubscribe();
-    }
-
     if (!this.sessionId) {
       throw new Error("ACP session did not expose a session id");
     }
 
-    return {
-      sessionId: this.sessionId,
-      finalText,
-      usage,
-      timeline,
-    };
+    return result;
   }
 
   async startTurn(
diff --git a/packages/server/src/server/agent/providers/claude-agent.ts b/packages/server/src/server/agent/providers/claude-agent.ts
index 8776881bd..6dbb0059d 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -42,6 +42,7 @@ import {
   formatProviderDiagnosticError,
   toDiagnosticErrorMessage,
 } from "./diagnostic-utils.js";
+import { appendOrReplaceGrowingAssistantMessage, runProviderTurn } from "./provider-runner.js";
 import { renderPromptAttachmentAsText } from "../prompt-attachments.js";
 
 import type {
@@ -1543,77 +1544,14 @@ class ClaudeAgentSession implements AgentSession {
   }
 
   async run(prompt: AgentPromptInput, options?: AgentRunOptions): Promise<AgentRunResult> {
-    const timeline: AgentTimelineItem[] = [];
-    let finalText = "";
-    let usage: AgentUsage | undefined;
-    let turnId: string | null = null;
-    const bufferedEvents: AgentStreamEvent[] = [];
-    let settled = false;
-    let resolveCompletion!: () => void;
-    let rejectCompletion!: (error: Error) => void;
-
-    const processEvent = (event: AgentStreamEvent) => {
-      if (settled) {
-        return;
-      }
-      const eventTurnId = (event as { turnId?: string }).turnId;
-      if (turnId && eventTurnId && eventTurnId !== turnId) {
-        return;
-      }
-      if (event.type === "timeline") {
-        timeline.push(event.item);
-        if (event.item.type === "assistant_message") {
-          if (!finalText) {
-            finalText = event.item.text;
-          } else if (event.item.text.startsWith(finalText)) {
-            finalText = event.item.text;
-          } else {
-            finalText += event.item.text;
-          }
-        }
-        return;
-      }
-      if (event.type === "turn_completed") {
-        usage = event.usage;
-        settled = true;
-        resolveCompletion();
-        return;
-      }
-      if (event.type === "turn_failed") {
-        settled = true;
-        rejectCompletion(new Error(event.error));
-        return;
-      }
-      if (event.type === "turn_canceled") {
-        settled = true;
-        resolveCompletion();
-      }
-    };
-
-    const completion = new Promise<void>((resolve, reject) => {
-      resolveCompletion = resolve;
-      rejectCompletion = reject;
+    const result = await runProviderTurn({
+      prompt,
+      runOptions: options,
+      startTurn: (p, o) => this.startTurn(p, o),
+      subscribe: (callback) => this.subscribe(callback),
+      getSessionId: () => this.claudeSessionId ?? "",
+      reduceFinalText: appendOrReplaceGrowingAssistantMessage,
     });
-    const unsubscribe = this.subscribe((event) => {
-      if (!turnId) {
-        bufferedEvents.push(event);
-        return;
-      }
-      processEvent(event);
-    });
-
-    try {
-      const result = await this.startTurn(prompt, options);
-      turnId = result.turnId;
-      for (const event of bufferedEvents) {
-        processEvent(event);
-      }
-      if (!settled) {
-        await completion;
-      }
-    } finally {
-      unsubscribe();
-    }
 
     this.cachedRuntimeInfo = {
       provider: "claude",
@@ -1626,12 +1564,7 @@ class ClaudeAgentSession implements AgentSession {
       throw new Error("Session ID not set after run completed");
     }
 
-    return {
-      sessionId: this.claudeSessionId,
-      finalText,
-      usage,
-      timeline,
-    };
+    return result;
   }
 
   async startTurn(
diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
index ace2484d4..a4fc1352c 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
@@ -60,6 +60,7 @@ import {
   resolveBinaryVersion,
   toDiagnosticErrorMessage,
 } from "./diagnostic-utils.js";
+import { runProviderTurn } from "./provider-runner.js";
 import type { WorkspaceGitService } from "../../workspace-git-service.js";
 
 const DEFAULT_TIMEOUT_MS = 14 * 24 * 60 * 60 * 1000;
@@ -2962,81 +2963,22 @@ class CodexAppServerAgentSession implements AgentSession {
   }
 
   async run(prompt: AgentPromptInput, options?: AgentRunOptions): Promise<AgentRunResult> {
-    const timeline: AgentTimelineItem[] = [];
-    let finalText = "";
-    let usage: AgentUsage | undefined;
-    let turnId: string | null = null;
-    const bufferedEvents: AgentStreamEvent[] = [];
-    let settled = false;
-    let resolveCompletion!: () => void;
-    let rejectCompletion!: (error: Error) => void;
-
-    const processEvent = (event: AgentStreamEvent) => {
-      if (settled) {
-        return;
-      }
-      const eventTurnId = (event as { turnId?: string }).turnId;
-      if (turnId && eventTurnId && eventTurnId !== turnId) {
-        return;
-      }
-      if (event.type === "timeline") {
-        timeline.push(event.item);
-        if (event.item.type === "assistant_message") {
-          finalText = event.item.text;
-        } else if (event.item.type === "tool_call" && event.item.detail.type === "plan") {
-          finalText = event.item.detail.text;
+    return runProviderTurn({
+      prompt,
+      runOptions: options,
+      startTurn: (p, o) => this.startTurn(p, o),
+      subscribe: (callback) => this.subscribe(callback),
+      getSessionId: async () => (await this.getRuntimeInfo()).sessionId ?? "",
+      reduceFinalText: ({ current, item }) => {
+        if (item.type === "assistant_message") {
+          return item.text;
         }
-        return;
-      }
-      if (event.type === "turn_completed") {
-        usage = event.usage;
-        settled = true;
-        resolveCompletion();
-        return;
-      }
-      if (event.type === "turn_failed") {
-        settled = true;
-        rejectCompletion(new Error(event.error));
-        return;
-      }
-      if (event.type === "turn_canceled") {
-        settled = true;
-        resolveCompletion();
-      }
-    };
-
-    const completion = new Promise<void>((resolve, reject) => {
-      resolveCompletion = resolve;
-      rejectCompletion = reject;
+        if (item.type === "tool_call" && item.detail.type === "plan") {
+          return item.detail.text;
+        }
+        return current;
+      },
     });
-    const unsubscribe = this.subscribe((event) => {
-      if (!turnId) {
-        bufferedEvents.push(event);
-        return;
-      }
-      processEvent(event);
-    });
-
-    try {
-      const result = await this.startTurn(prompt, options);
-      turnId = result.turnId;
-      for (const event of bufferedEvents) {
-        processEvent(event);
-      }
-      if (!settled) {
-        await completion;
-      }
-    } finally {
-      unsubscribe();
-    }
-
-    const info = await this.getRuntimeInfo();
-    return {
-      sessionId: info.sessionId ?? "",
-      finalText,
-      usage,
-      timeline,
-    };
   }
 
   async startTurn(
diff --git a/packages/server/src/server/agent/providers/opencode-agent.ts b/packages/server/src/server/agent/providers/opencode-agent.ts
index d6050ba89..6c84936b1 100644
--- a/packages/server/src/server/agent/providers/opencode-agent.ts
+++ b/packages/server/src/server/agent/providers/opencode-agent.ts
@@ -57,6 +57,7 @@ import {
   resolveBinaryVersion,
   toDiagnosticErrorMessage,
 } from "./diagnostic-utils.js";
+import { runProviderTurn } from "./provider-runner.js";
 import { renderPromptAttachmentAsText } from "../prompt-attachments.js";
 
 const OPENCODE_CAPABILITIES: AgentCapabilityFlags = {
@@ -2271,78 +2272,13 @@ class OpenCodeAgentSession implements AgentSession {
   }
 
   async run(prompt: AgentPromptInput, options?: AgentRunOptions): Promise<AgentRunResult> {
-    const timeline: AgentTimelineItem[] = [];
-    let finalText = "";
-    let usage: AgentUsage | undefined;
-    let turnId: string | null = null;
-    const bufferedEvents: AgentStreamEvent[] = [];
-    let settled = false;
-    let resolveCompletion!: () => void;
-    let rejectCompletion!: (error: Error) => void;
-
-    const processEvent = (event: AgentStreamEvent) => {
-      if (settled) {
-        return;
-      }
-      const eventTurnId = (event as { turnId?: string }).turnId;
-      if (turnId && eventTurnId && eventTurnId !== turnId) {
-        return;
-      }
-      if (event.type === "timeline") {
-        timeline.push(event.item);
-        if (event.item.type === "assistant_message") {
-          finalText = event.item.text;
-        }
-        return;
-      }
-      if (event.type === "turn_completed") {
-        usage = event.usage;
-        settled = true;
-        resolveCompletion();
-        return;
-      }
-      if (event.type === "turn_failed") {
-        settled = true;
-        rejectCompletion(new Error(event.error));
-        return;
-      }
-      if (event.type === "turn_canceled") {
-        settled = true;
-        resolveCompletion();
-      }
-    };
-
-    const completion = new Promise<void>((resolve, reject) => {
-      resolveCompletion = resolve;
-      rejectCompletion = reject;
+    return runProviderTurn({
+      prompt,
+      runOptions: options,
+      startTurn: (p, o) => this.startTurn(p, o),
+      subscribe: (callback) => this.subscribe(callback),
+      getSessionId: () => this.sessionId,
     });
-    const unsubscribe = this.subscribe((event) => {
-      if (!turnId) {
-        bufferedEvents.push(event);
-        return;
-      }
-      processEvent(event);
-    });
-
-    try {
-      const result = await this.startTurn(prompt, options);
-      turnId = result.turnId;
-      for (const event of bufferedEvents) {
-        processEvent(event);
-      }
-      if (!settled) {
-        await completion;
-      }
-    } finally {
-      unsubscribe();
-    }
-
-    return {
-      sessionId: this.sessionId,
-      finalText,
-      usage,
-      timeline,
-    };
   }
 
   async interrupt(): Promise<void> {
diff --git a/packages/server/src/server/agent/providers/provider-runner.test.ts b/packages/server/src/server/agent/providers/provider-runner.test.ts
new file mode 100644
index 000000000..a2b646507
--- /dev/null
+++ b/packages/server/src/server/agent/providers/provider-runner.test.ts
@@ -0,0 +1,154 @@
+import { describe, expect, test } from "vitest";
+import type { AgentPromptInput, AgentRunOptions, AgentStreamEvent } from "../agent-sdk-types.js";
+import { appendOrReplaceGrowingAssistantMessage, runProviderTurn } from "./provider-runner.js";
+
+class FakeTurnRunner {
+  readonly events: AgentStreamEvent[] = [];
+  private subscriber: ((event: AgentStreamEvent) => void) | null = null;
+
+  constructor(
+    private readonly turnId: string,
+    private readonly sessionId: string,
+    private readonly onStart?: () => void,
+  ) {}
+
+  async startTurn(
+    _prompt: AgentPromptInput,
+    _options?: AgentRunOptions,
+  ): Promise<{ turnId: string }> {
+    this.onStart?.();
+    return { turnId: this.turnId };
+  }
+
+  subscribe(callback: (event: AgentStreamEvent) => void): () => void {
+    this.subscriber = callback;
+    return () => {
+      this.subscriber = null;
+    };
+  }
+
+  getSessionId(): string {
+    return this.sessionId;
+  }
+
+  emit(event: AgentStreamEvent): void {
+    this.events.push(event);
+    this.subscriber?.(event);
+  }
+}
+
+describe("runProviderTurn", () => {
+  test("buffers events emitted before startTurn returns", async () => {
+    const runner = new FakeTurnRunner("turn-1", "session-1", () => {
+      runner.emit({
+        type: "timeline",
+        provider: "codex",
+        turnId: "turn-1",
+        item: { type: "assistant_message", text: "hello" },
+      });
+      runner.emit({
+        type: "turn_completed",
+        provider: "codex",
+        turnId: "turn-1",
+        usage: { inputTokens: 1 },
+      });
+    });
+
+    const result = await runProviderTurn({
+      prompt: "hi",
+      startTurn: (prompt, options) => runner.startTurn(prompt, options),
+      subscribe: (callback) => runner.subscribe(callback),
+      getSessionId: () => runner.getSessionId(),
+    });
+
+    expect(result).toEqual({
+      sessionId: "session-1",
+      finalText: "hello",
+      usage: { inputTokens: 1 },
+      timeline: [{ type: "assistant_message", text: "hello" }],
+    });
+  });
+
+  test("ignores events from other turns after the turn id is known", async () => {
+    const runner = new FakeTurnRunner("turn-1", "session-1");
+    const resultPromise = runProviderTurn({
+      prompt: "hi",
+      startTurn: (prompt, options) => runner.startTurn(prompt, options),
+      subscribe: (callback) => runner.subscribe(callback),
+      getSessionId: () => runner.getSessionId(),
+    });
+
+    runner.emit({
+      type: "timeline",
+      provider: "opencode",
+      turnId: "other-turn",
+      item: { type: "assistant_message", text: "wrong" },
+    });
+    runner.emit({
+      type: "timeline",
+      provider: "opencode",
+      turnId: "turn-1",
+      item: { type: "assistant_message", text: "right" },
+    });
+    runner.emit({ type: "turn_completed", provider: "opencode", turnId: "turn-1" });
+
+    await expect(resultPromise).resolves.toMatchObject({
+      finalText: "right",
+      timeline: [{ type: "assistant_message", text: "right" }],
+    });
+  });
+
+  test("rejects when the turn fails", async () => {
+    const runner = new FakeTurnRunner("turn-1", "session-1");
+    const resultPromise = runProviderTurn({
+      prompt: "hi",
+      startTurn: (prompt, options) => runner.startTurn(prompt, options),
+      subscribe: (callback) => runner.subscribe(callback),
+      getSessionId: () => runner.getSessionId(),
+    });
+
+    runner.emit({
+      type: "turn_failed",
+      provider: "claude",
+      turnId: "turn-1",
+      error: "provider failed",
+    });
+
+    await expect(resultPromise).rejects.toThrow("provider failed");
+  });
+
+  test("supports growing assistant message reducers", async () => {
+    const runner = new FakeTurnRunner("turn-1", "session-1");
+    const resultPromise = runProviderTurn({
+      prompt: "hi",
+      startTurn: (prompt, options) => runner.startTurn(prompt, options),
+      subscribe: (callback) => runner.subscribe(callback),
+      getSessionId: () => runner.getSessionId(),
+      reduceFinalText: appendOrReplaceGrowingAssistantMessage,
+    });
+
+    runner.emit({
+      type: "timeline",
+      provider: "acp",
+      turnId: "turn-1",
+      item: { type: "assistant_message", text: "hello" },
+    });
+    runner.emit({
+      type: "timeline",
+      provider: "acp",
+      turnId: "turn-1",
+      item: { type: "assistant_message", text: "hello world" },
+    });
+    runner.emit({
+      type: "timeline",
+      provider: "acp",
+      turnId: "turn-1",
+      item: { type: "assistant_message", text: "!" },
+    });
+    runner.emit({ type: "turn_completed", provider: "acp", turnId: "turn-1" });
+
+    await expect(resultPromise).resolves.toMatchObject({
+      finalText: "hello world!",
+    });
+  });
+});
diff --git a/packages/server/src/server/agent/providers/provider-runner.ts b/packages/server/src/server/agent/providers/provider-runner.ts
new file mode 100644
index 000000000..d13d171b6
--- /dev/null
+++ b/packages/server/src/server/agent/providers/provider-runner.ts
@@ -0,0 +1,128 @@
+import type {
+  AgentPromptInput,
+  AgentRunOptions,
+  AgentRunResult,
+  AgentStreamEvent,
+  AgentTimelineItem,
+} from "../agent-sdk-types.js";
+
+export type ProviderFinalTextReducer = (params: {
+  current: string;
+  item: AgentTimelineItem;
+}) => string;
+
+export interface ProviderTurnRunner {
+  startTurn(prompt: AgentPromptInput, options?: AgentRunOptions): Promise<{ turnId: string }>;
+  subscribe(callback: (event: AgentStreamEvent) => void): () => void;
+  getSessionId(): string | Promise<string>;
+}
+
+export interface RunProviderTurnOptions extends ProviderTurnRunner {
+  prompt: AgentPromptInput;
+  runOptions?: AgentRunOptions;
+  reduceFinalText?: ProviderFinalTextReducer;
+}
+
+export async function runProviderTurn({
+  prompt,
+  runOptions,
+  startTurn,
+  subscribe,
+  getSessionId,
+  reduceFinalText = replaceFinalTextWithAssistantMessage,
+}: RunProviderTurnOptions): Promise<AgentRunResult> {
+  const timeline: AgentTimelineItem[] = [];
+  let finalText = "";
+  let usage: AgentRunResult["usage"];
+  let turnId: string | null = null;
+  const bufferedEvents: AgentStreamEvent[] = [];
+  let settled = false;
+  let resolveCompletion!: () => void;
+  let rejectCompletion!: (error: Error) => void;
+
+  const processEvent = (event: AgentStreamEvent) => {
+    if (settled) {
+      return;
+    }
+    const eventTurnId = "turnId" in event ? event.turnId : undefined;
+    if (turnId && eventTurnId && eventTurnId !== turnId) {
+      return;
+    }
+    if (event.type === "timeline") {
+      timeline.push(event.item);
+      finalText = reduceFinalText({ current: finalText, item: event.item });
+      return;
+    }
+    if (event.type === "turn_completed") {
+      usage = event.usage;
+      settled = true;
+      resolveCompletion();
+      return;
+    }
+    if (event.type === "turn_failed") {
+      settled = true;
+      rejectCompletion(new Error(event.error));
+      return;
+    }
+    if (event.type === "turn_canceled") {
+      settled = true;
+      resolveCompletion();
+    }
+  };
+
+  const completion = new Promise<void>((resolve, reject) => {
+    resolveCompletion = resolve;
+    rejectCompletion = reject;
+  });
+  const unsubscribe = subscribe((event) => {
+    if (!turnId) {
+      bufferedEvents.push(event);
+      return;
+    }
+    processEvent(event);
+  });
+
+  try {
+    const result = await startTurn(prompt, runOptions);
+    turnId = result.turnId;
+    for (const event of bufferedEvents) {
+      processEvent(event);
+    }
+    await completion;
+  } finally {
+    unsubscribe();
+  }
+
+  return {
+    sessionId: await getSessionId(),
+    finalText,
+    usage,
+    timeline,
+  };
+}
+
+export function replaceFinalTextWithAssistantMessage({
+  current,
+  item,
+}: {
+  current: string;
+  item: AgentTimelineItem;
+}): string {
+  return item.type === "assistant_message" ? item.text : current;
+}
+
+export function appendOrReplaceGrowingAssistantMessage({
+  current,
+  item,
+}: {
+  current: string;
+  item: AgentTimelineItem;
+}): string {
+  if (item.type !== "assistant_message") {
+    return current;
+  }
+  if (!current) {
+    return item.text;
+  }
+  return item.text.startsWith(current) ? item.text : `${current}${item.text}`;
+}