diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 8a2d352a51e5..e509cb71c482 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -712,7 +712,7 @@ function providerMeta(metadata: Record<string, any> | undefined) {
 export const toModelMessagesEffect = Effect.fnUntraced(function* (
   input: WithParts[],
   model: Provider.Model,
-  options?: { stripMedia?: boolean; toolOutputMaxChars?: number },
+  options?: { stripMedia?: boolean; stripPlanModeReminders?: boolean; toolOutputMaxChars?: number },
 ) {
   const result: UIMessage[] = []
   const toolNames = new Set<string>()
@@ -771,6 +771,15 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
     return { type: "json", value: output as never }
   }
 
+  const skipText = (part: TextPart) => {
+    if (part.ignored) return true
+    if (!options?.stripPlanModeReminders) return false
+    if (!part.synthetic) return false
+    if (part.metadata?.["kind"] === "plan_reminder") return true
+    // Fallback for sessions persisted before reminders were tagged with metadata.
+    return part.text.includes("Plan mode is active") || part.text.includes("Plan mode ACTIVE")
+  }
+
   for (const msg of input) {
     if (msg.parts.length === 0) continue
 
@@ -782,7 +791,7 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
       }
       result.push(userMessage)
       for (const part of msg.parts) {
-        if (part.type === "text" && !part.ignored)
+        if (part.type === "text" && !skipText(part))
           userMessage.parts.push({
             type: "text",
             text: part.text,
@@ -969,7 +978,7 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
 export function toModelMessages(
   input: WithParts[],
   model: Provider.Model,
-  options?: { stripMedia?: boolean; toolOutputMaxChars?: number },
+  options?: { stripMedia?: boolean; stripPlanModeReminders?: boolean; toolOutputMaxChars?: number },
 ): Promise<ModelMessage[]> {
   return Effect.runPromise(toModelMessagesEffect(input, model, options).pipe(Effect.provide(EffectLogger.layer)))
 }
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 600eb42f795e..48cb7ecaabae 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -236,6 +236,7 @@ export const layer = Layer.effect(
             type: "text",
             text: PROMPT_PLAN,
             synthetic: true,
+            metadata: { kind: "plan_reminder" },
           })
         }
         const wasPlan = input.messages.some((msg) => msg.info.role === "assistant" && msg.info.agent === "plan")
@@ -349,6 +350,7 @@ This is critical - your turn should only end with either asking the user a quest
 NOTE: At any point in time through this workflow you should feel free to ask the user questions or clarifications. Don't make large assumptions about user intent. The goal is to present a well researched plan to the user, and tie any loose ends before implementation begins.
 </system-reminder>`,
         synthetic: true,
+        metadata: { kind: "plan_reminder" },
       })
       userMessage.parts.push(part)
       return input.messages
@@ -1479,7 +1481,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
               sys.skills(agent),
               Effect.sync(() => sys.environment(model)),
               instruction.system().pipe(Effect.orDie),
-              MessageV2.toModelMessagesEffect(msgs, model),
+              MessageV2.toModelMessagesEffect(msgs, model, { stripPlanModeReminders: agent.name !== "plan" }),
             ])
             const system = [...env, ...(skills ? [skills] : []), ...instructions]
             const format = lastUser.format ?? { type: "text" as const }
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index 9591a5d6230f..ef7612f9e452 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -195,6 +195,100 @@ describe("session.message-v2.toModelMessage", () => {
     ])
   })
 
+  test("can strip stale synthetic plan mode reminders", async () => {
+    const messageID = "m-user"
+    const planTurn: MessageV2.WithParts = {
+      info: userInfo("m-plan"),
+      parts: [
+        {
+          ...basePart("m-plan", "p0"),
+          type: "text",
+          text: "<system-reminder>\nPlan mode ACTIVE - older session without metadata.\n</system-reminder>",
+          synthetic: true,
+        },
+        {
+          ...basePart("m-plan", "p0b"),
+          type: "text",
+          text: "draft a plan",
+        },
+      ] as MessageV2.Part[],
+    }
+    const buildTurn: MessageV2.WithParts = {
+      info: userInfo(messageID),
+      parts: [
+        {
+          ...basePart(messageID, "p1"),
+          type: "text",
+          text: "<system-reminder>\nPlan mode is active. Do not edit files.\n</system-reminder>",
+          synthetic: true,
+          metadata: { kind: "plan_reminder" },
+        },
+        {
+          ...basePart(messageID, "p2"),
+          type: "text",
+          text: "<system-reminder>\nYour operational mode has changed from plan to build.\n</system-reminder>",
+          synthetic: true,
+        },
+        {
+          ...basePart(messageID, "p3"),
+          type: "text",
+          text: "Please implement the plan.",
+        },
+      ] as MessageV2.Part[],
+    }
+    const input: MessageV2.WithParts[] = [planTurn, buildTurn]
+
+    expect(await MessageV2.toModelMessages(input, model, { stripPlanModeReminders: true })).toStrictEqual([
+      {
+        role: "user",
+        content: [{ type: "text", text: "draft a plan" }],
+      },
+      {
+        role: "user",
+        content: [
+          {
+            type: "text",
+            text: "<system-reminder>\nYour operational mode has changed from plan to build.\n</system-reminder>",
+          },
+          { type: "text", text: "Please implement the plan." },
+        ],
+      },
+    ])
+  })
+
+  test("keeps plan reminders when not stripping", async () => {
+    const messageID = "m-user"
+    const input: MessageV2.WithParts[] = [
+      {
+        info: userInfo(messageID),
+        parts: [
+          {
+            ...basePart(messageID, "p1"),
+            type: "text",
+            text: "<system-reminder>\nPlan mode is active.\n</system-reminder>",
+            synthetic: true,
+            metadata: { kind: "plan_reminder" },
+          },
+          {
+            ...basePart(messageID, "p2"),
+            type: "text",
+            text: "draft a plan",
+          },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    expect(await MessageV2.toModelMessages(input, model)).toStrictEqual([
+      {
+        role: "user",
+        content: [
+          { type: "text", text: "<system-reminder>\nPlan mode is active.\n</system-reminder>" },
+          { type: "text", text: "draft a plan" },
+        ],
+      },
+    ])
+  })
+
   test("converts user text/file parts and injects compaction/subtask prompts", async () => {
     const messageID = "m-user"