diff --git a/src/hooks/session-recovery/recover-tool-result-missing.test.ts b/src/hooks/session-recovery/recover-tool-result-missing.test.ts
index 430f4e719..add36d9a3 100644
--- a/src/hooks/session-recovery/recover-tool-result-missing.test.ts
+++ b/src/hooks/session-recovery/recover-tool-result-missing.test.ts
@@ -82,8 +82,10 @@ describe("recoverToolResultMissing", () => {
       body: {
         parts: [{
           type: "tool_result",
+          toolUseId: "call_recovered",
           tool_use_id: "call_recovered",
-          content: "Operation cancelled by user (ESC pressed)",
+          isError: true,
+          content: [{ type: "text", text: "Operation cancelled by user (ESC pressed)" }],
         }],
       },
     })
@@ -123,8 +125,10 @@ describe("recoverToolResultMissing", () => {
       body: {
         parts: [{
           type: "tool_result",
+          toolUseId: "toolu_recovered",
           tool_use_id: "toolu_recovered",
-          content: "Operation cancelled by user (ESC pressed)",
+          isError: true,
+          content: [{ type: "text", text: "Operation cancelled by user (ESC pressed)" }],
         }],
       },
     })
diff --git a/src/hooks/session-recovery/recover-tool-result-missing.ts b/src/hooks/session-recovery/recover-tool-result-missing.ts
index 60d5f7aec..b01c926ec 100644
--- a/src/hooks/session-recovery/recover-tool-result-missing.ts
+++ b/src/hooks/session-recovery/recover-tool-result-missing.ts
@@ -6,6 +6,14 @@ import { normalizeSDKResponse } from "../../shared"
 import { promptAsyncAfterSessionIdle } from "../shared/prompt-async-gate"
 
 type Client = ReturnType<typeof createOpencodeClient>
+type ToolResultContent = { type: "text"; text: string }
+type ToolResultPart = {
+  type: "tool_result"
+  toolUseId: string
+  tool_use_id?: string
+  isError?: boolean
+  content: ToolResultContent[]
+}
 type ClientWithPromptAsync = {
   session: {
     promptAsync: (opts: { path: { id: string }; body: Record<string, unknown> }) => Promise<unknown>
@@ -96,8 +104,10 @@ export async function recoverToolResultMissing(
 
   const toolResultParts = toolUseIds.map((id) => ({
     type: "tool_result" as const,
+    toolUseId: id,
     tool_use_id: id,
-    content: "Operation cancelled by user (ESC pressed)",
+    isError: true,
+    content: [{ type: "text" as const, text: "Operation cancelled by user (ESC pressed)" }],
   }))
 
   const launchAgent = resumeConfig?.agent
diff --git a/src/hooks/session-recovery/recover-unavailable-tool.test.ts b/src/hooks/session-recovery/recover-unavailable-tool.test.ts
new file mode 100644
index 000000000..4076283f5
--- /dev/null
+++ b/src/hooks/session-recovery/recover-unavailable-tool.test.ts
@@ -0,0 +1,105 @@
+import { afterEach, beforeEach, describe, expect, it, mock } from "bun:test"
+
+import type { MessageData } from "./types"
+
+let sqliteBackend = false
+let storedParts: Array<{ type: string; id?: string; callID?: string; name?: string; tool?: string; [key: string]: unknown }> = []
+
+mock.module("../../shared/opencode-storage-detection", () => ({
+  isSqliteBackend: () => sqliteBackend,
+}))
+
+mock.module("./storage", () => ({
+  readParts: () => storedParts,
+}))
+
+const { recoverUnavailableTool } = await import("./recover-unavailable-tool")
+
+const failedAssistantMsg: MessageData = {
+  info: { id: "msg_failed", role: "assistant", error: 'No such tool: bash' },
+  parts: [],
+}
+
+function createMockClient(messages: MessageData[] = []) {
+  const promptAsync = mock(() => Promise.resolve({}))
+
+  return {
+    client: {
+      session: {
+        messages: mock(() => Promise.resolve({ data: messages })),
+        promptAsync,
+      },
+    } as never,
+    promptAsync,
+  }
+}
+
+describe("recoverUnavailableTool", () => {
+  beforeEach(() => {
+    sqliteBackend = false
+    storedParts = []
+  })
+
+  afterEach(() => {
+    mock.restore()
+  })
+
+  it("sends a schema-compatible recovered tool result for sqlite fallback", async () => {
+    //#given
+    sqliteBackend = true
+    const { client, promptAsync } = createMockClient([
+      {
+        info: { id: "msg_failed", role: "assistant" },
+        parts: [{ type: "tool", id: "prt_valid_call", callID: "call_recovered", name: "bash", input: {} }],
+      },
+    ])
+
+    //#when
+    const result = await recoverUnavailableTool(client, "ses_1", failedAssistantMsg)
+
+    //#then
+    expect(result).toBe(true)
+    expect(promptAsync).toHaveBeenCalledWith({
+      path: { id: "ses_1" },
+      body: {
+        parts: [{
+          type: "tool_result",
+          toolUseId: "call_recovered",
+          tool_use_id: "call_recovered",
+          isError: true,
+          content: [{ type: "text", text: '{"status":"error","error":"Tool not available. Please continue without this tool."}' }],
+        }],
+      },
+    })
+  })
+
+  it("sends a schema-compatible recovered tool result for stored parts fallback", async () => {
+    //#given
+    storedParts = [{
+      type: "tool",
+      id: "prt_stored_valid_call",
+      callID: "toolu_recovered",
+      tool: "bash",
+      state: { input: {} },
+    }]
+    const { client, promptAsync } = createMockClient()
+
+    //#when
+    const result = await recoverUnavailableTool(client, "ses_2", failedAssistantMsg)
+
+    //#then
+    expect(result).toBe(true)
+    expect(promptAsync).toHaveBeenCalledWith({
+      path: { id: "ses_2" },
+      body: {
+        parts: [{
+          type: "tool_result",
+          toolUseId: "toolu_recovered",
+          tool_use_id: "toolu_recovered",
+          isError: true,
+          content: [{ type: "text", text: '{"status":"error","error":"Tool not available. Please continue without this tool."}' }],
+        }],
+      },
+    })
+  })
+})
diff --git a/src/hooks/session-recovery/recover-unavailable-tool.ts b/src/hooks/session-recovery/recover-unavailable-tool.ts
index b45a4f7ff..01c6660c9 100644
--- a/src/hooks/session-recovery/recover-unavailable-tool.ts
+++ b/src/hooks/session-recovery/recover-unavailable-tool.ts
@@ -10,8 +10,10 @@ type Client = ReturnType<typeof createOpencodeClient>
 
 interface ToolResultPart {
   type: "tool_result"
-  tool_use_id: string
-  content: string
+  toolUseId: string
+  tool_use_id?: string
+  isError?: boolean
+  content: Array<{ type: "text"; text: string }>
 }
 
 interface PromptWithToolResultInput {
@@ -91,8 +93,10 @@ export async function recoverUnavailableTool(
 
   const toolResultParts = targetToolUses.map((part) => ({
     type: "tool_result" as const,
+    toolUseId: part.id,
     tool_use_id: part.id,
-    content: '{"status":"error","error":"Tool not available. Please continue without this tool."}',
+    isError: true,
+    content: [{ type: "text" as const, text: '{"status":"error","error":"Tool not available. Please continue without this tool."}' }],
   }))
 
   try {
diff --git a/src/hooks/tool-pair-validator/hook.test.ts b/src/hooks/tool-pair-validator/hook.test.ts
index a6449da76..af97fa76a 100644
--- a/src/hooks/tool-pair-validator/hook.test.ts
+++ b/src/hooks/tool-pair-validator/hook.test.ts
@@ -14,8 +14,10 @@ type TestPart = {
   type: string
   id?: string
   callID?: string
+  toolUseId?: string
   tool_use_id?: string
-  content?: string
+  isError?: boolean
+  content?: string | Array<{ type: "text"; text: string }>
   text?: string
 }
 
@@ -65,7 +67,13 @@ describe("createToolPairValidatorHook", () => {
 
     //#then
     expect(messages[1]?.parts).toEqual([
-      { type: "tool_result", tool_use_id: "toolu_1", content: TOOL_RESULT_PLACEHOLDER },
+      {
+        type: "tool_result",
+        toolUseId: "toolu_1",
+        tool_use_id: "toolu_1",
+        isError: true,
+        content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+      },
       { type: "text", text: "continue" },
     ])
   })
@@ -99,8 +107,20 @@ describe("createToolPairValidatorHook", () => {
       {
         info: { role: "user" },
         parts: [
-          { type: "tool_result", tool_use_id: "toolu_1", content: TOOL_RESULT_PLACEHOLDER },
-          { type: "tool_result", tool_use_id: "toolu_2", content: TOOL_RESULT_PLACEHOLDER },
+          {
+            type: "tool_result",
+            toolUseId: "toolu_1",
+            tool_use_id: "toolu_1",
+            isError: true,
+            content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+          },
+          {
+            type: "tool_result",
+            toolUseId: "toolu_2",
+            tool_use_id: "toolu_2",
+            isError: true,
+            content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+          },
         ],
       },
     ])
@@ -122,7 +142,13 @@ describe("createToolPairValidatorHook", () => {
       { info: { role: "assistant" }, parts: [{ type: "tool_use", id: "toolu_1" }] },
       {
         info: { role: "user" },
-        parts: [{ type: "tool_result", tool_use_id: "toolu_1", content: TOOL_RESULT_PLACEHOLDER }],
+        parts: [{
+          type: "tool_result",
+          toolUseId: "toolu_1",
+          tool_use_id: "toolu_1",
+          isError: true,
+          content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+        }],
       },
       { info: { role: "assistant" }, parts: [{ type: "text", text: "follow-up" }] },
     ])
@@ -150,7 +176,13 @@ describe("createToolPairValidatorHook", () => {
     //#then
     expect(messages[1]?.parts).toEqual([
       { type: "tool_result", tool_use_id: "toolu_1", content: "done" },
-      { type: "tool_result", tool_use_id: "call_2", content: TOOL_RESULT_PLACEHOLDER },
+      {
+        type: "tool_result",
+        toolUseId: "call_2",
+        tool_use_id: "call_2",
+        isError: true,
+        content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+      },
       { type: "text", text: "continue" },
     ])
   })
@@ -189,11 +221,34 @@ describe("createToolPairValidatorHook", () => {
       //#then
       expect(backgroundMessages).toEqual(originalBackgroundMessages)
       expect(mainMessages[1]?.parts).toEqual([
-        { type: "tool_result", tool_use_id: "toolu_main_1", content: TOOL_RESULT_PLACEHOLDER },
+        {
+          type: "tool_result",
+          tool_use_id: "toolu_main_1",
+          toolUseId: "toolu_main_1",
+          isError: true,
+          content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
+        },
         { type: "text", text: "continue main session" },
       ])
     } finally {
       _resetForTesting()
     }
   })
+
+  it("treats existing camelCase toolUseId results as already paired", async () => {
+    //#given
+    const messages = [
+      { info: { role: "assistant" }, parts: [{ type: "tool_use", id: "toolu_1" }] },
+      { info: { role: "user" }, parts: [{ type: "tool_result", toolUseId: "toolu_1", content: [{ type: "text", text: "done" }] }] },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#then
+    expect(messages).toEqual([
+      { info: { role: "assistant" }, parts: [{ type: "tool_use", id: "toolu_1" }] },
+      { info: { role: "user" }, parts: [{ type: "tool_result", toolUseId: "toolu_1", content: [{ type: "text", text: "done" }] }] },
+    ])
+  })
 })
diff --git a/src/hooks/tool-pair-validator/hook.ts b/src/hooks/tool-pair-validator/hook.ts
index 72d53fee0..9a4107810 100644
--- a/src/hooks/tool-pair-validator/hook.ts
+++ b/src/hooks/tool-pair-validator/hook.ts
@@ -13,8 +13,10 @@ type ToolUsePart = {
 
 type ToolResultPart = {
   type: "tool_result"
-  tool_use_id: string
-  content: string
+  toolUseId: string
+  tool_use_id?: string
+  isError?: boolean
+  content: Array<{ type: "text"; text: string }>
   [key: string]: unknown
 }
 
@@ -52,9 +54,17 @@ function getToolUseID(part: TransformPart): string | null {
 }
 
 function getToolResultID(part: TransformPart): string | null {
-  const candidate = part as { type?: unknown; tool_use_id?: unknown }
+  const candidate = part as { type?: unknown; toolUseId?: unknown; tool_use_id?: unknown }
 
-  if (candidate.type === "tool_result" && typeof candidate.tool_use_id === "string" && candidate.tool_use_id.length > 0) {
+  if (candidate.type !== "tool_result") {
+    return null
+  }
+
+  if (typeof candidate.toolUseId === "string" && candidate.toolUseId.length > 0) {
+    return candidate.toolUseId
+  }
+
+  if (typeof candidate.tool_use_id === "string" && candidate.tool_use_id.length > 0) {
     return candidate.tool_use_id
   }
 
@@ -94,8 +104,10 @@ function extractToolResultIDs(parts: TransformPart[]): Set<string> {
 function createToolResultPart(toolUseID: string): ToolResultPart {
   return {
     type: "tool_result",
+    toolUseId: toolUseID,
     tool_use_id: toolUseID,
-    content: TOOL_RESULT_PLACEHOLDER,
+    isError: true,
+    content: [{ type: "text", text: TOOL_RESULT_PLACEHOLDER }],
   }
 }
 
diff --git a/src/plugin/messages-transform.test.ts b/src/plugin/messages-transform.test.ts
index d4cb0637d..1620d1c3b 100644
--- a/src/plugin/messages-transform.test.ts
+++ b/src/plugin/messages-transform.test.ts
@@ -139,12 +139,20 @@ describe("createMessagesTransformHandler", () => {
     expect(messages).toHaveLength(5)
     expect(messages[2]).toEqual({
       info: { role: "user" },
-      parts: [{ type: "tool_result", tool_use_id: "toolu_01SRMQs3DUtVKWoSxC8bxxVA", content: "Tool output unavailable (context compacted)" }],
+      parts: [{
+        type: "tool_result",
+        toolUseId: "toolu_01SRMQs3DUtVKWoSxC8bxxVA",
+        tool_use_id: "toolu_01SRMQs3DUtVKWoSxC8bxxVA",
+        isError: true,
+        content: [{ type: "text", text: "Tool output unavailable (context compacted)" }],
+      }],
     })
     expect(messages[4]?.parts[0]).toEqual({
       type: "tool_result",
+      toolUseId: "toolu_01Lu5cHvRtEvzoifP1UVBVRb",
       tool_use_id: "toolu_01Lu5cHvRtEvzoifP1UVBVRb",
-      content: "Tool output unavailable (context compacted)",
+      isError: true,
+      content: [{ type: "text", text: "Tool output unavailable (context compacted)" }],
     })
     expect(messages[4]?.parts[1]).toEqual({ type: "text", text: "next" })
   })