diff --git a/src/config/schema/hooks.ts b/src/config/schema/hooks.ts
index e3bad81a8..fea9c6371 100644
--- a/src/config/schema/hooks.ts
+++ b/src/config/schema/hooks.ts
@@ -25,6 +25,7 @@ export const HookNameSchema = z.enum([
   "interactive-bash-session",
 
   "thinking-block-validator",
+  "tool-pair-validator",
   "ralph-loop",
   "category-skill-reminder",
 
diff --git a/src/hooks/index.ts b/src/hooks/index.ts
index 3966a5bea..051cbd12a 100644
--- a/src/hooks/index.ts
+++ b/src/hooks/index.ts
@@ -26,6 +26,7 @@ export { createNonInteractiveEnvHook } from "./non-interactive-env";
 export { createInteractiveBashSessionHook } from "./interactive-bash-session";
 
 export { createThinkingBlockValidatorHook } from "./thinking-block-validator";
+export { createToolPairValidatorHook } from "./tool-pair-validator";
 export { createCategorySkillReminderHook } from "./category-skill-reminder";
 export { createRalphLoopHook, type RalphLoopHook } from "./ralph-loop";
 export { createNoSisyphusGptHook } from "./no-sisyphus-gpt";
diff --git a/src/hooks/tool-pair-validator/hook.test.ts b/src/hooks/tool-pair-validator/hook.test.ts
new file mode 100644
index 000000000..6b18f15f0
--- /dev/null
+++ b/src/hooks/tool-pair-validator/hook.test.ts
@@ -0,0 +1,156 @@
+declare const describe: (name: string, fn: () => void) => void
+declare const it: (name: string, fn: () => void | Promise<void>) => void
+declare const expect: <T>(value: T) => {
+  toEqual(expected: unknown): void
+  toHaveLength(expected: number): void
+}
+
+import { createToolPairValidatorHook } from "./hook"
+
+const TOOL_RESULT_PLACEHOLDER = "Tool output unavailable (context compacted)"
+
+type TestPart = {
+  type: string
+  id?: string
+  callID?: string
+  tool_use_id?: string
+  content?: string
+  text?: string
+}
+
+type TestMessage = {
+  info: { role: "assistant" | "user" }
+  parts: TestPart[]
+}
+
+async function runTransform(messages: TestMessage[]): Promise<void> {
+  const hook = createToolPairValidatorHook()
+  const transform = hook["experimental.chat.messages.transform"]
+
+  if (!transform) {
+    throw new Error("missing tool pair validator transform")
+  }
+
+  await transform({}, { messages: messages as never })
+}
+
+describe("createToolPairValidatorHook", () => {
+  it("leaves matching tool pairs unchanged", async () => {
+    //#given
+    const messages = [
+      { info: { role: "assistant" }, parts: [{ type: "tool", callID: "call_1" }] },
+      { info: { role: "user" }, parts: [{ type: "tool_result", tool_use_id: "call_1", content: "done" }] },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#then
+    expect(messages).toEqual([
+      { info: { role: "assistant" }, parts: [{ type: "tool", callID: "call_1" }] },
+      { info: { role: "user" }, parts: [{ type: "tool_result", tool_use_id: "call_1", content: "done" }] },
+    ])
+  })
+
+  it("injects a missing tool_result into the next user message", async () => {
+    //#given
+    const messages = [
+      { info: { role: "assistant" }, parts: [{ type: "tool_use", id: "toolu_1" }] },
+      { info: { role: "user" }, parts: [{ type: "text", text: "continue" }] },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#then
+    expect(messages[1]?.parts).toEqual([
+      { type: "tool_result", tool_use_id: "toolu_1", content: TOOL_RESULT_PLACEHOLDER },
+      { type: "text", text: "continue" },
+    ])
+  })
+
+  it("injects a synthetic user message when the next user message is missing", async () => {
+    //#given
+    const messages = [
+      {
+        info: { role: "assistant" },
+        parts: [
+          { type: "tool_use", id: "toolu_1" },
+          { type: "text", text: "working" },
+          { type: "tool_use", id: "toolu_2" },
+        ],
+      },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#then
+    expect(messages).toEqual([
+      {
+        info: { role: "assistant" },
+        parts: [
+          { type: "tool_use", id: "toolu_1" },
+          { type: "text", text: "working" },
+          { type: "tool_use", id: "toolu_2" },
+        ],
+      },
+      {
+        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 },
+        ],
+      },
+    ])
+  })
+
+  it("injects a synthetic user message before a non-user next message", async () => {
+    //#given
+    const messages = [
+      { info: { role: "assistant" }, parts: [{ type: "tool_use", id: "toolu_1" }] },
+      { info: { role: "assistant" }, parts: [{ type: "text", text: "follow-up" }] },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#then
+    expect(messages).toHaveLength(3)
+    expect(messages).toEqual([
+      { 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 }],
+      },
+      { info: { role: "assistant" }, parts: [{ type: "text", text: "follow-up" }] },
+    ])
+  })
+
+  it("injects only the missing tool_results for partial matches", async () => {
+    //#given
+    const messages = [
+      {
+        info: { role: "assistant" },
+        parts: [{ type: "tool_use", id: "toolu_1" }, { type: "tool", callID: "call_2" }],
+      },
+      {
+        info: { role: "user" },
+        parts: [
+          { type: "tool_result", tool_use_id: "toolu_1", content: "done" },
+          { type: "text", text: "continue" },
+        ],
+      },
+    ] satisfies TestMessage[]
+
+    //#when
+    await runTransform(messages)
+
+    //#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: "text", text: "continue" },
+    ])
+  })
+})
diff --git a/src/hooks/tool-pair-validator/hook.ts b/src/hooks/tool-pair-validator/hook.ts
new file mode 100644
index 000000000..89a76e701
--- /dev/null
+++ b/src/hooks/tool-pair-validator/hook.ts
@@ -0,0 +1,184 @@
+import type { Message, Part } from "@opencode-ai/sdk"
+
+import { log } from "../../shared/logger"
+
+const TOOL_RESULT_PLACEHOLDER = "Tool output unavailable (context compacted)"
+
+type ToolUsePart = {
+  type: "tool_use"
+  id: string
+  [key: string]: unknown
+}
+
+type ToolResultPart = {
+  type: "tool_result"
+  tool_use_id: string
+  content: string
+  [key: string]: unknown
+}
+
+type TransformPart = Part | ToolUsePart | ToolResultPart
+
+type TransformMessageInfo = Message | {
+  role: "user"
+  sessionID?: string
+}
+
+interface MessageWithParts {
+  info: TransformMessageInfo
+  parts: TransformPart[]
+}
+
+type MessagesTransformHook = {
+  "experimental.chat.messages.transform"?: (
+    input: Record<string, never>,
+    output: { messages: MessageWithParts[] }
+  ) => Promise<void>
+}
+
+function getToolUseID(part: TransformPart): string | null {
+  const candidate = part as { type?: unknown; id?: unknown; callID?: unknown }
+
+  if (candidate.type === "tool_use" && typeof candidate.id === "string" && candidate.id.length > 0) {
+    return candidate.id
+  }
+
+  if (candidate.type === "tool" && typeof candidate.callID === "string" && candidate.callID.length > 0) {
+    return candidate.callID
+  }
+
+  return null
+}
+
+function getToolResultID(part: TransformPart): string | null {
+  const candidate = part as { type?: unknown; tool_use_id?: unknown }
+
+  if (candidate.type === "tool_result" && typeof candidate.tool_use_id === "string" && candidate.tool_use_id.length > 0) {
+    return candidate.tool_use_id
+  }
+
+  return null
+}
+
+function extractUniqueToolUseIDs(parts: TransformPart[]): string[] {
+  const seen = new Set<string>()
+  const toolUseIDs: string[] = []
+
+  for (const part of parts) {
+    const toolUseID = getToolUseID(part)
+    if (!toolUseID || seen.has(toolUseID)) {
+      continue
+    }
+
+    seen.add(toolUseID)
+    toolUseIDs.push(toolUseID)
+  }
+
+  return toolUseIDs
+}
+
+function extractToolResultIDs(parts: TransformPart[]): Set<string> {
+  const toolResultIDs = new Set<string>()
+
+  for (const part of parts) {
+    const toolResultID = getToolResultID(part)
+    if (toolResultID) {
+      toolResultIDs.add(toolResultID)
+    }
+  }
+
+  return toolResultIDs
+}
+
+function createToolResultPart(toolUseID: string): ToolResultPart {
+  return {
+    type: "tool_result",
+    tool_use_id: toolUseID,
+    content: TOOL_RESULT_PLACEHOLDER,
+  }
+}
+
+function findToolResultInsertIndex(parts: TransformPart[]): number {
+  let lastToolResultIndex = -1
+
+  for (let i = 0; i < parts.length; i++) {
+    if (getToolResultID(parts[i])) {
+      lastToolResultIndex = i
+    }
+  }
+
+  return lastToolResultIndex === -1 ? 0 : lastToolResultIndex + 1
+}
+
+function insertMissingToolResults(message: MessageWithParts, missingToolUseIDs: string[]): void {
+  const toolResultParts = missingToolUseIDs.map((toolUseID) => createToolResultPart(toolUseID))
+  const insertIndex = findToolResultInsertIndex(message.parts)
+  message.parts.splice(insertIndex, 0, ...toolResultParts)
+}
+
+function createSyntheticUserMessage(assistantMessage: MessageWithParts, missingToolUseIDs: string[]): MessageWithParts {
+  const assistantInfo = assistantMessage.info as { sessionID?: unknown }
+  const sessionID = typeof assistantInfo.sessionID === "string" ? assistantInfo.sessionID : undefined
+
+  return {
+    info: {
+      role: "user",
+      ...(sessionID ? { sessionID } : {}),
+    },
+    parts: missingToolUseIDs.map((toolUseID) => createToolResultPart(toolUseID)),
+  }
+}
+
+function getMessageID(message: TransformMessageInfo): string | undefined {
+  const candidate = message as { id?: unknown }
+  return typeof candidate.id === "string" ? candidate.id : undefined
+}
+
+function repairMissingToolResults(messages: MessageWithParts[], assistantIndex: number): void {
+  const assistantMessage = messages[assistantIndex]
+  const toolUseIDs = extractUniqueToolUseIDs(assistantMessage.parts)
+
+  if (toolUseIDs.length === 0) {
+    return
+  }
+
+  const nextMessage = messages[assistantIndex + 1]
+
+  if (nextMessage?.info.role !== "user") {
+    messages.splice(assistantIndex + 1, 0, createSyntheticUserMessage(assistantMessage, toolUseIDs))
+    log("[tool-pair-validator] Repaired missing tool_result blocks", {
+      assistantMessageID: getMessageID(assistantMessage.info),
+      syntheticUserMessageInserted: true,
+      repairedToolUseIDs: toolUseIDs,
+    })
+    return
+  }
+
+  const existingToolResultIDs = extractToolResultIDs(nextMessage.parts)
+  const missingToolUseIDs = toolUseIDs.filter((toolUseID) => !existingToolResultIDs.has(toolUseID))
+
+  if (missingToolUseIDs.length === 0) {
+    return
+  }
+
+  insertMissingToolResults(nextMessage, missingToolUseIDs)
+  log("[tool-pair-validator] Repaired missing tool_result blocks", {
+    assistantMessageID: getMessageID(assistantMessage.info),
+    syntheticUserMessageInserted: false,
+    repairedToolUseIDs: missingToolUseIDs,
+  })
+}
+
+export function createToolPairValidatorHook(): MessagesTransformHook {
+  return {
+    "experimental.chat.messages.transform": async (_input, output) => {
+      for (let i = 0; i < output.messages.length; i++) {
+        if (output.messages[i].info.role !== "assistant") {
+          continue
+        }
+
+        repairMissingToolResults(output.messages, i)
+      }
+    },
+  }
+}
diff --git a/src/hooks/tool-pair-validator/index.ts b/src/hooks/tool-pair-validator/index.ts
new file mode 100644
index 000000000..717bede96
--- /dev/null
+++ b/src/hooks/tool-pair-validator/index.ts
@@ -0,0 +1 @@
+export { createToolPairValidatorHook } from "./hook"
diff --git a/src/plugin/hooks/create-transform-hooks.ts b/src/plugin/hooks/create-transform-hooks.ts
index d593efae7..c57a959bb 100644
--- a/src/plugin/hooks/create-transform-hooks.ts
+++ b/src/plugin/hooks/create-transform-hooks.ts
@@ -5,6 +5,7 @@ import {
   createClaudeCodeHooksHook,
   createKeywordDetectorHook,
   createThinkingBlockValidatorHook,
+  createToolPairValidatorHook,
 } from "../../hooks"
 import {
   contextCollector,
@@ -17,6 +18,7 @@ export type TransformHooks = {
   keywordDetector: ReturnType<typeof createKeywordDetectorHook> | null
   contextInjectorMessagesTransform: ReturnType<typeof createContextInjectorMessagesTransformHook>
   thinkingBlockValidator: ReturnType<typeof createThinkingBlockValidatorHook> | null
+  toolPairValidator: ReturnType<typeof createToolPairValidatorHook> | null
 }
 
 export function createTransformHooks(args: {
@@ -63,10 +65,19 @@ export function createTransformHooks(args: {
       )
     : null
 
+  const toolPairValidator = isHookEnabled("tool-pair-validator")
+    ? safeCreateHook(
+        "tool-pair-validator",
+        () => createToolPairValidatorHook(),
+        { enabled: safeHookEnabled },
+      )
+    : null
+
   return {
     claudeCodeHooks,
     keywordDetector,
     contextInjectorMessagesTransform,
     thinkingBlockValidator,
+    toolPairValidator,
   }
 }
diff --git a/src/plugin/messages-transform.ts b/src/plugin/messages-transform.ts
index 6ea674d8a..cd28b3832 100644
--- a/src/plugin/messages-transform.ts
+++ b/src/plugin/messages-transform.ts
@@ -20,5 +20,9 @@ export function createMessagesTransformHandler(args: {
     await args.hooks.thinkingBlockValidator?.[
       "experimental.chat.messages.transform"
     ]?.(input, output)
+
+    await args.hooks.toolPairValidator?.[
+      "experimental.chat.messages.transform"
+    ]?.(input, output)
   }
 }