diff --git a/.changeset/calm-dolphins-buffer.md b/.changeset/calm-dolphins-buffer.md
new file mode 100644
index 0000000000..dfbf559bc3
--- /dev/null
+++ b/.changeset/calm-dolphins-buffer.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-opencode': patch
+---
+
+fix(ai-opencode): bound memory while buffering parts before their message role is known
diff --git a/packages/ai-opencode/src/stream/translate.ts b/packages/ai-opencode/src/stream/translate.ts
index 8889bd0240..1d3094e9c9 100644
--- a/packages/ai-opencode/src/stream/translate.ts
+++ b/packages/ai-opencode/src/stream/translate.ts
@@ -375,7 +375,13 @@ export async function* translateOpencodeStream(
       const role = messageRoles.get(part.messageID)
       if (role === undefined) {
         const parts = pendingParts.get(part.messageID) ?? []
-        parts.push({ part, delta })
+        // A text or reasoning part carries its full text, so keep only its
+        // latest update. Keeping each update holds every snapshot in memory.
+        const index = isToolPart(part)
+          ? -1
+          : parts.findIndex((pending) => pending.part.id === part.id)
+        if (index === -1) parts.push({ part, delta })
+        else parts[index] = { part }
         pendingParts.set(part.messageID, parts)
       } else if (role === 'assistant') {
         yield* handlePart(part, delta)
diff --git a/packages/ai-opencode/tests/translate.test.ts b/packages/ai-opencode/tests/translate.test.ts
index 5ec94c31f6..a62b71ce68 100644
--- a/packages/ai-opencode/tests/translate.test.ts
+++ b/packages/ai-opencode/tests/translate.test.ts
@@ -146,6 +146,7 @@ describe('translateOpencodeStream', () => {
   it('accumulates incremental text deltas', async () => {
     const chunks = await collect([
       session,
+      messageUpdated('msg-1', 'assistant'),
       textPart('part-1', 'Hel', 'Hel'),
       textPart('part-1', 'Hello', 'lo'),
       done(),
@@ -164,6 +165,7 @@ describe('translateOpencodeStream', () => {
   it('derives the delta from full-text snapshots when no delta is given', async () => {
     const chunks = await collect([
       session,
+      messageUpdated('msg-1', 'assistant'),
       textPart('part-1', 'Hel'),
       textPart('part-1', 'Hello'),
       done(),
@@ -173,6 +175,23 @@ describe('translateOpencodeStream', () => {
     expect(contents[1]).toMatchObject({ delta: 'lo', content: 'Hello' })
   })
 
+  it('keeps only the latest update of a part while its role is unknown', async () => {
+    const chunks = await collect([
+      session,
+      textPart('part-1', 'The', 'The'),
+      textPart('part-1', 'The answer', ' answer'),
+      textPart('part-1', 'The answer is 42', ' is 42'),
+      done(),
+    ])
+    const contents = chunks.filter((c) => c.type === 'TEXT_MESSAGE_CONTENT')
+
+    expect(contents).toHaveLength(1)
+    expect(contents[0]).toMatchObject({
+      delta: 'The answer is 42',
+      content: 'The answer is 42',
+    })
+  })
+
   it('reports usage with cache and reasoning details', async () => {
     const chunks = await collect([
       session,