diff --git a/src/hooks/session-recovery/detect-error-type.test.ts b/src/hooks/session-recovery/detect-error-type.test.ts
new file mode 100644
index 00000000..d20e7cc9
--- /dev/null
+++ b/src/hooks/session-recovery/detect-error-type.test.ts
@@ -0,0 +1,129 @@
+/// <reference types="bun-types" />
+import { describe, expect, it } from "bun:test"
+import { detectErrorType, extractMessageIndex } from "./detect-error-type"
+
+describe("detectErrorType", () => {
+  it("#given a tool_use/tool_result error #when detecting #then returns tool_result_missing", () => {
+    //#given
+    const error = { message: "tool_use block must be followed by tool_result" }
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBe("tool_result_missing")
+  })
+
+  it("#given a thinking block order error #when detecting #then returns thinking_block_order", () => {
+    //#given
+    const error = { message: "thinking must be the first block in the response" }
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBe("thinking_block_order")
+  })
+
+  it("#given a thinking disabled violation #when detecting #then returns thinking_disabled_violation", () => {
+    //#given
+    const error = { message: "thinking is disabled and cannot contain thinking blocks" }
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBe("thinking_disabled_violation")
+  })
+
+  it("#given an unrecognized error #when detecting #then returns null", () => {
+    //#given
+    const error = { message: "some random error" }
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("#given a malformed error with circular references #when detecting #then returns null without crashing", () => {
+    //#given
+    const circular: Record<string, unknown> = {}
+    circular.self = circular
+
+    //#when
+    const result = detectErrorType(circular)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("#given a proxy error with non-standard structure #when detecting #then returns null without crashing", () => {
+    //#given
+    const proxyError = {
+      data: "not-an-object",
+      error: 42,
+      nested: { deeply: { error: true } },
+    }
+
+    //#when
+    const result = detectErrorType(proxyError)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("#given a null error #when detecting #then returns null", () => {
+    //#given
+    const error = null
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("#given an error with data.error containing message #when detecting #then extracts correctly", () => {
+    //#given
+    const error = {
+      data: {
+        error: {
+          message: "tool_use block requires tool_result",
+        },
+      },
+    }
+
+    //#when
+    const result = detectErrorType(error)
+
+    //#then
+    expect(result).toBe("tool_result_missing")
+  })
+})
+
+describe("extractMessageIndex", () => {
+  it("#given an error referencing messages.5 #when extracting #then returns 5", () => {
+    //#given
+    const error = { message: "Invalid value at messages.5: tool_result is required" }
+
+    //#when
+    const result = extractMessageIndex(error)
+
+    //#then
+    expect(result).toBe(5)
+  })
+
+  it("#given a malformed error #when extracting #then returns null without crashing", () => {
+    //#given
+    const circular: Record<string, unknown> = {}
+    circular.self = circular
+
+    //#when
+    const result = extractMessageIndex(circular)
+
+    //#then
+    expect(result).toBeNull()
+  })
+})
diff --git a/src/hooks/session-recovery/detect-error-type.ts b/src/hooks/session-recovery/detect-error-type.ts
index f51c0d0d..3f2f9a1c 100644
--- a/src/hooks/session-recovery/detect-error-type.ts
+++ b/src/hooks/session-recovery/detect-error-type.ts
@@ -34,40 +34,48 @@ function getErrorMessage(error: unknown): string {
 }
 
 export function extractMessageIndex(error: unknown): number | null {
-  const message = getErrorMessage(error)
-  const match = message.match(/messages\.(\d+)/)
-  return match ? parseInt(match[1], 10) : null
+  try {
+    const message = getErrorMessage(error)
+    const match = message.match(/messages\.(\d+)/)
+    return match ? parseInt(match[1], 10) : null
+  } catch {
+    return null
+  }
 }
 
 export function detectErrorType(error: unknown): RecoveryErrorType {
-  const message = getErrorMessage(error)
+  try {
+    const message = getErrorMessage(error)
 
-  if (
-    message.includes("assistant message prefill") ||
-    message.includes("conversation must end with a user message")
-  ) {
-    return "assistant_prefill_unsupported"
+    if (
+      message.includes("assistant message prefill") ||
+      message.includes("conversation must end with a user message")
+    ) {
+      return "assistant_prefill_unsupported"
+    }
+
+    if (
+      message.includes("thinking") &&
+      (message.includes("first block") ||
+        message.includes("must start with") ||
+        message.includes("preceeding") ||
+        message.includes("final block") ||
+        message.includes("cannot be thinking") ||
+        (message.includes("expected") && message.includes("found")))
+    ) {
+      return "thinking_block_order"
+    }
+
+    if (message.includes("thinking is disabled") && message.includes("cannot contain")) {
+      return "thinking_disabled_violation"
+    }
+
+    if (message.includes("tool_use") && message.includes("tool_result")) {
+      return "tool_result_missing"
+    }
+
+    return null
+  } catch {
+    return null
   }
-
-  if (
-    message.includes("thinking") &&
-    (message.includes("first block") ||
-      message.includes("must start with") ||
-      message.includes("preceeding") ||
-      message.includes("final block") ||
-      message.includes("cannot be thinking") ||
-      (message.includes("expected") && message.includes("found")))
-  ) {
-    return "thinking_block_order"
-  }
-
-  if (message.includes("thinking is disabled") && message.includes("cannot contain")) {
-    return "thinking_disabled_violation"
-  }
-
-  if (message.includes("tool_use") && message.includes("tool_result")) {
-    return "tool_result_missing"
-  }
-
-  return null
 }