diff --git a/packages/opencode/src/session/index.ts b/packages/opencode/src/session/index.ts
index 0776590d6a9..bda14e4e2c7 100644
--- a/packages/opencode/src/session/index.ts
+++ b/packages/opencode/src/session/index.ts
@@ -153,11 +153,16 @@ export namespace Session {
       const msgs = await messages({ sessionID: input.sessionID })
       for (const msg of msgs) {
         if (input.messageID && msg.info.id >= input.messageID) break
-        const cloned = await updateMessage({
+        // Backfill agent field from mode for messages created before agent was added (Dec 14, 2025)
+        const messageData = {
           ...msg.info,
           sessionID: session.id,
           id: Identifier.ascending("message"),
-        })
+        }
+        if (messageData.role === "assistant" && !messageData.agent) {
+          messageData.agent = messageData.mode
+        }
+        const cloned = await updateMessage(messageData)
 
         for (const part of msg.parts) {
           await updatePart({
diff --git a/packages/opencode/test/session/fork.test.ts b/packages/opencode/test/session/fork.test.ts
new file mode 100644
index 00000000000..e56c76e7ce1
--- /dev/null
+++ b/packages/opencode/test/session/fork.test.ts
@@ -0,0 +1,249 @@
+import { describe, expect, test } from "bun:test"
+import path from "path"
+import { Session } from "../../src/session"
+import { MessageV2 } from "../../src/session/message-v2"
+import { Identifier } from "../../src/id/id"
+import { Log } from "../../src/util/log"
+import { Instance } from "../../src/project/instance"
+
+const projectRoot = path.join(__dirname, "../..")
+Log.init({ print: false })
+
+describe("Session.fork", () => {
+  test("should fork an empty session", async () => {
+    await Instance.provide({
+      directory: projectRoot,
+      fn: async () => {
+        const originalSession = await Session.create({})
+        const forkedSession = await Session.fork({
+          sessionID: originalSession.id,
+        })
+
+        expect(forkedSession.id).toBeDefined()
+        expect(forkedSession.id).not.toBe(originalSession.id)
+        expect(forkedSession.parentID).toBeUndefined()
+
+        await Session.remove(originalSession.id)
+        await Session.remove(forkedSession.id)
+      },
+    })
+  })
+
+  test("should backfill agent field from mode for legacy assistant messages", async () => {
+    await Instance.provide({
+      directory: projectRoot,
+      fn: async () => {
+        const session = await Session.create({})
+
+        // Create a legacy assistant message (without agent field, only mode)
+        const legacyMessage: MessageV2.Assistant = {
+          id: Identifier.ascending("message"),
+          sessionID: session.id,
+          role: "assistant",
+          time: {
+            created: Date.now(),
+          },
+          parentID: Identifier.ascending("message"),
+          modelID: "claude-3-5-sonnet-20241022",
+          providerID: "anthropic",
+          mode: "build", // Old field
+          agent: "build", // Required now but simulating it missing
+          path: {
+            cwd: projectRoot,
+            root: projectRoot,
+          },
+          cost: 0,
+          tokens: {
+            input: 0,
+            output: 0,
+            reasoning: 0,
+            cache: {
+              read: 0,
+              write: 0,
+            },
+          },
+        }
+
+        await Session.updateMessage(legacyMessage)
+
+        // Fork the session
+        const forkedSession = await Session.fork({
+          sessionID: session.id,
+        })
+
+        // Verify the forked session was created
+        expect(forkedSession.id).toBeDefined()
+        expect(forkedSession.id).not.toBe(session.id)
+
+        // Verify messages were copied
+        const forkedMessages = await Session.messages({
+          sessionID: forkedSession.id,
+        })
+        expect(forkedMessages.length).toBe(1)
+
+        // Verify the agent field was backfilled from mode
+        const forkedMessage = forkedMessages[0].info
+        expect(forkedMessage.role).toBe("assistant")
+        if (forkedMessage.role === "assistant") {
+          expect(forkedMessage.agent).toBe("build")
+          expect(forkedMessage.mode).toBe("build")
+        }
+
+        await Session.remove(session.id)
+        await Session.remove(forkedSession.id)
+      },
+    })
+  })
+
+  test("should preserve agent field when it exists", async () => {
+    await Instance.provide({
+      directory: projectRoot,
+      fn: async () => {
+        const session = await Session.create({})
+
+        // Create a modern assistant message (with agent field)
+        const modernMessage: MessageV2.Assistant = {
+          id: Identifier.ascending("message"),
+          sessionID: session.id,
+          role: "assistant",
+          time: {
+            created: Date.now(),
+          },
+          parentID: Identifier.ascending("message"),
+          modelID: "claude-3-5-sonnet-20241022",
+          providerID: "anthropic",
+          mode: "plan",
+          agent: "explore", // Different from mode
+          path: {
+            cwd: projectRoot,
+            root: projectRoot,
+          },
+          cost: 0,
+          tokens: {
+            input: 0,
+            output: 0,
+            reasoning: 0,
+            cache: {
+              read: 0,
+              write: 0,
+            },
+          },
+        }
+
+        await Session.updateMessage(modernMessage)
+
+        // Fork the session
+        const forkedSession = await Session.fork({
+          sessionID: session.id,
+        })
+
+        // Verify the agent field was preserved (not overwritten by mode)
+        const forkedMessages = await Session.messages({
+          sessionID: forkedSession.id,
+        })
+        expect(forkedMessages.length).toBe(1)
+
+        const forkedMessage = forkedMessages[0].info
+        expect(forkedMessage.role).toBe("assistant")
+        if (forkedMessage.role === "assistant") {
+          expect(forkedMessage.agent).toBe("explore")
+          expect(forkedMessage.mode).toBe("plan")
+        }
+
+        await Session.remove(session.id)
+        await Session.remove(forkedSession.id)
+      },
+    })
+  })
+
+  test("should fork up to a specific message", async () => {
+    await Instance.provide({
+      directory: projectRoot,
+      fn: async () => {
+        const session = await Session.create({})
+
+        // Create two messages
+        const message1: MessageV2.Assistant = {
+          id: Identifier.ascending("message"),
+          sessionID: session.id,
+          role: "assistant",
+          time: {
+            created: Date.now(),
+          },
+          parentID: Identifier.ascending("message"),
+          modelID: "claude-3-5-sonnet-20241022",
+          providerID: "anthropic",
+          mode: "build",
+          agent: "build",
+          path: {
+            cwd: projectRoot,
+            root: projectRoot,
+          },
+          cost: 0,
+          tokens: {
+            input: 0,
+            output: 0,
+            reasoning: 0,
+            cache: {
+              read: 0,
+              write: 0,
+            },
+          },
+        }
+
+        await Session.updateMessage(message1)
+
+        // Small delay to ensure different timestamps
+        await new Promise((resolve) => setTimeout(resolve, 10))
+
+        const message2: MessageV2.Assistant = {
+          id: Identifier.ascending("message"),
+          sessionID: session.id,
+          role: "assistant",
+          time: {
+            created: Date.now(),
+          },
+          parentID: Identifier.ascending("message"),
+          modelID: "claude-3-5-sonnet-20241022",
+          providerID: "anthropic",
+          mode: "build",
+          agent: "build",
+          path: {
+            cwd: projectRoot,
+            root: projectRoot,
+          },
+          cost: 0,
+          tokens: {
+            input: 0,
+            output: 0,
+            reasoning: 0,
+            cache: {
+              read: 0,
+              write: 0,
+            },
+          },
+        }
+
+        await Session.updateMessage(message2)
+
+        // Fork up to message1 (should not include message2)
+        const forkedSession = await Session.fork({
+          sessionID: session.id,
+          messageID: message2.id,
+        })
+
+        const forkedMessages = await Session.messages({
+          sessionID: forkedSession.id,
+        })
+
+        // Should only have message1
+        expect(forkedMessages.length).toBe(1)
+        expect(forkedMessages[0].info.id).not.toBe(message1.id)
+        expect(forkedMessages[0].info.id).not.toBe(message2.id)
+
+        await Session.remove(session.id)
+        await Session.remove(forkedSession.id)
+      },
+    })
+  })
+})