diff --git a/src/app/api/agent/history/[threadId]/schema.ts b/src/app/api/agent/history/[threadId]/schema.ts
index 4c318cb..78d5c70 100644
--- a/src/app/api/agent/history/[threadId]/schema.ts
+++ b/src/app/api/agent/history/[threadId]/schema.ts
@@ -1,13 +1,28 @@
 import { z } from "@/lib/api/openapi/zod";
 import { registry } from "@/lib/api/openapi/registry";
 
-// LangGraph checkpoint messages are loosely typed; documented as an open object
-// array so the docs convey the shape without over-constraining it.
-const HistoryMessage = z
-  .looseObject({
+// Projected to the fields the UI renders (see `projectHistory`); `data` varies by message type.
+const HistoryMessageData = z
+  .object({
     id: z.string().optional(),
-    type: z.string().optional(),
     content: z.unknown().optional(),
+    tool_calls: z.array(z.unknown()).optional().openapi({ description: "AI messages only" }),
+    pendingToolCallIds: z
+      .array(z.string())
+      .optional()
+      .openapi({ description: "Tool calls the approval gate paused" }),
+    tool_call_id: z.string().optional().openapi({ description: "Tool messages only" }),
+    name: z.string().optional().openapi({ description: "Tool messages only" }),
+    status: z.string().optional().openapi({ description: "Tool messages only" }),
+    artifact: z.unknown().optional().openapi({ description: "Client-only tool output (charts)" }),
+    attachments: z.array(z.unknown()).optional().openapi({ description: "Human messages only" }),
+  })
+  .openapi("HistoryMessageData");
+
+const HistoryMessage = z
+  .object({
+    type: z.enum(["human", "ai", "tool", "error"]),
+    data: HistoryMessageData,
   })
   .openapi("HistoryMessage");
 
diff --git a/src/services/agentService.ts b/src/services/agentService.ts
index 83b72a5..1effe72 100644
--- a/src/services/agentService.ts
+++ b/src/services/agentService.ts
@@ -3,11 +3,12 @@ import { ensureThread } from "@/lib/thread";
 import type { MessageOptions, MessageResponse } from "@/types/message";
 import * as threadRepo from "@/lib/repositories/threadRepository";
 import { getHistory } from "@/lib/agent/memory";
-import { BaseMessage, HumanMessage } from "@langchain/core/messages";
+import { HumanMessage } from "@langchain/core/messages";
 import { Command } from "@langchain/langgraph";
 import type { HITLRequest, HITLResponse, Decision } from "langchain";
 import { processAttachmentsForAI } from "@/lib/storage/content";
 import { streamMessages, type AgentRun } from "./messageStream";
+import { projectHistory } from "./historyProjection";
 import { CallbackHandler } from "@langfuse/langchain";
 
 // Only instantiate when tracing is enabled; avoids errors when Langfuse credentials are absent.
@@ -98,7 +99,7 @@ export async function fetchThreadHistory(threadId: string): Promise<MessageRespo
   if (!thread) return [];
   try {
     const history = await getHistory(threadId);
-    return history.map((msg: BaseMessage) => msg.toDict() as MessageResponse);
+    return projectHistory(history);
   } catch (e) {
     console.error("fetchThreadHistory error", e);
     return [];
diff --git a/src/services/historyProjection.test.ts b/src/services/historyProjection.test.ts
new file mode 100644
index 0000000..813514e
--- /dev/null
+++ b/src/services/historyProjection.test.ts
@@ -0,0 +1,123 @@
+import { describe, it, expect } from "vitest";
+import { __projectMessage } from "./historyProjection";
+import type { AIMessageData, BasicMessageData, ToolMessageData } from "@/types/message";
+
+// `toDict()` output as the checkpoint really returns it — provider fields included.
+const aiWithToolCall = {
+  type: "ai",
+  data: {
+    id: "ai-1",
+    content: [{ type: "tool_call", id: "call-1", name: "log_expense", args: { amount: 12 } }],
+    content_blocks: [
+      { type: "tool_call", id: "call-1", name: "log_expense", args: { amount: 12 } },
+    ],
+    tool_calls: [{ type: "tool_call", id: "call-1", name: "log_expense", args: { amount: 12 } }],
+    usage_metadata: { input_tokens: 6807, output_tokens: 107, total_tokens: 6914 },
+    response_metadata: { finish_reason: "tool_use", output_version: "v1" },
+    invalid_tool_calls: [],
+    additional_kwargs: {},
+    name: "model",
+  },
+};
+
+describe("projectMessage", () => {
+  it("drops LangChain fields no component reads", () => {
+    const out = __projectMessage(aiWithToolCall);
+    for (const key of [
+      "content_blocks",
+      "usage_metadata",
+      "response_metadata",
+      "invalid_tool_calls",
+      "additional_kwargs",
+      "name",
+    ]) {
+      expect(out.data).not.toHaveProperty(key);
+    }
+  });
+
+  it("drops a content array that only restates tool_calls", () => {
+    const out = __projectMessage(aiWithToolCall);
+    const data = out.data as AIMessageData;
+    expect(data.content).toBeUndefined();
+    // The call itself must survive — it is what the UI renders.
+    expect(data.tool_calls).toEqual(aiWithToolCall.data.tool_calls);
+  });
+
+  it("keeps content when it mixes prose with a tool call", () => {
+    const out = __projectMessage({
+      type: "ai",
+      data: {
+        id: "ai-2",
+        content: [
+          { type: "text", text: "Logging that now." },
+          { type: "tool_call", id: "call-2", name: "log_expense", args: {} },
+        ],
+        tool_calls: [{ type: "tool_call", id: "call-2", name: "log_expense", args: {} }],
+      },
+    });
+    const data = out.data as AIMessageData;
+    expect(data.content).toHaveLength(2);
+  });
+
+  it("keeps plain assistant prose", () => {
+    const out = __projectMessage({
+      type: "ai",
+      data: { id: "ai-3", content: "You spent 42 EUR on dining.", usage_metadata: { a: 1 } },
+    });
+    expect((out.data as AIMessageData).content).toBe("You spent 42 EUR on dining.");
+  });
+
+  it("preserves pendingToolCallIds — the only approval-gate signal", () => {
+    const out = __projectMessage({
+      type: "ai",
+      data: { id: "ai-4", content: "", pendingToolCallIds: ["call-9"] },
+    });
+    expect((out.data as AIMessageData).pendingToolCallIds).toEqual(["call-9"]);
+  });
+
+  it("keeps the fields a tool result is paired and rendered by", () => {
+    const out = __projectMessage({
+      type: "tool",
+      data: {
+        id: "tool-1",
+        content: '{"ok":true}',
+        tool_call_id: "call-1",
+        name: "log_expense",
+        status: "success",
+        metadata: { versions: { "@langchain/core": "1.2.1" } },
+        additional_kwargs: {},
+      },
+    });
+    const data = out.data as ToolMessageData;
+    expect(data).toEqual({
+      id: "tool-1",
+      content: '{"ok":true}',
+      tool_call_id: "call-1",
+      name: "log_expense",
+      status: "success",
+    });
+  });
+
+  it("keeps a chart artifact — the chart renders from it, not from content", () => {
+    const artifact = { chartType: "line", rows: [{ month: "2026-01", total: 12 }] };
+    const out = __projectMessage({
+      type: "tool",
+      data: { id: "t", content: "{}", tool_call_id: "c", name: "render_chart", artifact },
+    });
+    expect((out.data as ToolMessageData).artifact).toEqual(artifact);
+  });
+
+  it("passes human content through so checkpoint attachments survive", () => {
+    const content = [
+      { type: "text", text: "load these" },
+      {
+        type: "text",
+        text: "...",
+        file_metadata: { name: "releve.csv", key: "k", url: "u", type: "text/csv", size: 10 },
+      },
+    ];
+    const out = __projectMessage({ type: "human", data: { id: "h-1", content } });
+    expect(out.type).toBe("human");
+    expect((out.data as BasicMessageData).content).toEqual(content);
+  });
+});
diff --git a/src/services/historyProjection.ts b/src/services/historyProjection.ts
new file mode 100644
index 0000000..6d049b6
--- /dev/null
+++ b/src/services/historyProjection.ts
@@ -0,0 +1,79 @@
+import type { BaseMessage } from "@langchain/core/messages";
+import type {
+  AIMessageData,
+  BasicMessageData,
+  MessageResponse,
+  ToolCall,
+  ToolMessageData,
+} from "@/types/message";
+
+// `toDict()` is LangChain's serialization, not our wire contract — casting it shipped every
+// field it carries. Projecting explicitly keeps `MessageResponse` the actual shape.
+
+/** Keep a key only when it carries information. */
+function put<T extends object>(target: T, key: keyof T, value: unknown): void {
+  if (value === undefined || value === null) return;
+  if (Array.isArray(value) && value.length === 0) return;
+  if (typeof value === "object" && !Array.isArray(value) && Object.keys(value).length === 0) return;
+  (target as Record<string, unknown>)[key as string] = value;
+}
+
+type ContentBlock = { type?: string; id?: string };
+
+/** True when `content` only restates `tool_calls`. Mixed content keeps its prose. */
+function isRedundantToolCallContent(content: unknown, toolCalls: ToolCall[] | undefined): boolean {
+  if (!Array.isArray(content) || content.length === 0) return false;
+  if (!toolCalls || toolCalls.length === 0) return false;
+  const ids = new Set(toolCalls.map((call) => call.id));
+  return (content as ContentBlock[]).every(
+    (block) => block?.type === "tool_call" && !!block.id && ids.has(block.id),
+  );
+}
+
+/** `toDict()` output — `data` carries provider fields beyond LangChain's declared type. */
+type CheckpointMessage = {
+  type?: string;
+  data?: Record<string, unknown>;
+};
+
+function projectMessage(raw: CheckpointMessage): MessageResponse {
+  const type = raw.type;
+  const d = raw.data ?? {};
+
+  if (type === "ai") {
+    const data = {} as AIMessageData;
+    const toolCalls = d.tool_calls as ToolCall[] | undefined;
+    put(data, "id", d.id);
+    if (!isRedundantToolCallContent(d.content, toolCalls)) put(data, "content", d.content);
+    put(data, "tool_calls", toolCalls);
+    put(data, "pendingToolCallIds", d.pendingToolCallIds);
+    return { type: "ai", data };
+  }
+
+  if (type === "tool") {
+    const data = {} as ToolMessageData;
+    put(data, "id", d.id);
+    put(data, "content", d.content);
+    put(data, "tool_call_id", d.tool_call_id);
+    put(data, "name", d.name);
+    put(data, "status", d.status);
+    // A chart renders from its artifact, not from content — dropping it blanks the chart.
+    put(data, "artifact", d.artifact);
+    return { type: "tool", data };
+  }
+
+  // Human: attachments ride inside the content array, so it passes through untouched.
+  const data = {} as BasicMessageData;
+  put(data, "id", d.id);
+  put(data, "content", d.content);
+  put(data, "attachments", d.attachments);
+  return { type: type === "error" ? "error" : "human", data };
+}
+
+/** Project a checkpoint's messages onto the fields the UI reads. */
+export function projectHistory(messages: BaseMessage[]): MessageResponse[] {
+  return messages.map((msg) => projectMessage(msg.toDict() as unknown as CheckpointMessage));
+}
+
+/** Exported for tests, which feed plain `toDict()` output. */
+export const __projectMessage = projectMessage;