diff --git a/packages/core/src/tool/patch.ts b/packages/core/src/tool/patch.ts
index 5e90c355966..c7307c069ef 100644
--- a/packages/core/src/tool/patch.ts
+++ b/packages/core/src/tool/patch.ts
@@ -5,6 +5,7 @@ import { ToolFailure } from "@opencode-ai/ai"
 import { FileDiff } from "@opencode-ai/schema/file-diff"
 import { createTwoFilesPatch, diffLines } from "diff"
 import { Effect, Schema } from "effect"
+import { PlatformError } from "effect/PlatformError"
 import path from "path"
 import { FSUtil } from "@opencode-ai/util/fs-util"
 import { Location } from "../location"
@@ -81,12 +82,11 @@ export const Plugin = {
               output: Output,
               execute: (input, context) => {
                 const applied: Array<typeof Applied.Type> = []
-                const fail = (path: string, error?: unknown) => {
-                  const prefix =
-                    applied.length === 0
-                      ? `Unable to apply patch at ${path}`
-                      : `Patch partially applied before failing at ${path}. Applied: ${applied.map((item) => item.resource).join(", ")}`
-                  return new ToolFailure({ message: prefix, error })
+                const fail = (operation: string, error: unknown) => {
+                  const completed = applied.map((item) => item.resource).join(", ")
+                  return new ToolFailure({
+                    message: `${operation}: ${errorMessage(error)}${completed ? `. Completed before failure: ${completed}` : ""}`,
+                  })
                 }
                 return Effect.gen(function* () {
                   const source = {
@@ -101,11 +101,7 @@ export const Plugin = {
                     ),
                   )
                   if (hunks.length === 0) {
-                    const normalized = input.patchText.replace(/\r\n/g, "\n").replace(/\r/g, "\n").trim()
-                    if (normalized === "*** Begin Patch\n*** End Patch") {
-                      return yield* new ToolFailure({ message: "patch rejected: empty patch" })
-                    }
-                    return yield* new ToolFailure({ message: "patch verification failed: no hunks found" })
+                    return yield* new ToolFailure({ message: "patch rejected: empty patch" })
                   }
                   const prepared: Prepared[] = []
                   const targets: Target[] = []
@@ -145,7 +141,7 @@ export const Plugin = {
                           Effect.mapError(
                             (error) =>
                               new ToolFailure({
-                                message: `patch verification failed: ${error instanceof Error ? error.message : String(error)}`,
+                                message: `patch verification failed: Failed to delete ${target.resource}: ${errorMessage(error)}`,
                               }),
                           ),
                         )
@@ -161,7 +157,7 @@ export const Plugin = {
                             Effect.mapError(
                               (error) =>
                                 new ToolFailure({
-                                  message: `patch verification failed: Failed to read file to update ${target.canonical}: ${error instanceof Error ? error.message : String(error)}`,
+                                  message: `patch verification failed: Failed to read file to update ${target.canonical}: ${errorMessage(error)}`,
                                 }),
                             ),
                           )
@@ -175,7 +171,7 @@ export const Plugin = {
                               Effect.mapError(
                                 (error) =>
                                   new ToolFailure({
-                                    message: `patch verification failed: Failed to read file to update ${target.canonical}: ${error instanceof Error ? error.message : String(error)}`,
+                                    message: `patch verification failed: Failed to read file to update ${target.canonical}: ${errorMessage(error)}`,
                                   }),
                               ),
                             ),
@@ -184,7 +180,8 @@ export const Plugin = {
                       const before = original.replace(/^\uFEFF/, "")
                       const update = yield* Effect.try({
                         try: () => Patch.derive(hunk.path, hunk.chunks, original),
-                        catch: (error) => new ToolFailure({ message: `patch verification failed: ${String(error)}` }),
+                        catch: (error) =>
+                          new ToolFailure({ message: `patch verification failed: ${errorMessage(error)}` }),
                       })
                       const moveTarget = hunk.movePath ? resolveTarget(location, hunk.movePath) : undefined
                       if (moveTarget) targets.push(moveTarget)
@@ -211,7 +208,13 @@ export const Plugin = {
                         moveTarget,
                       })
                       if (!moveTarget) updates.set(target.canonical, Patch.joinBom(update.content, update.bom))
-                    }).pipe(Effect.mapError((error) => (error instanceof ToolFailure ? error : fail(hunk.path, error))))
+                    }).pipe(
+                      Effect.mapError((error) =>
+                        error instanceof ToolFailure
+                          ? error
+                          : new ToolFailure({ message: `Unable to prepare patch at ${hunk.path}`, error }),
+                      ),
+                    )
                   }
 
                   const patchFiles = prepared.map(patchFile)
@@ -234,12 +237,16 @@ export const Plugin = {
                     (change) =>
                       Effect.gen(function* () {
                         if (change.type === "add") {
-                          yield* fs.writeWithDirs(
-                            change.target.canonical,
-                            change.contents.endsWith("\n") || change.contents === ""
-                              ? change.contents
-                              : `${change.contents}\n`,
-                          )
+                          yield* fs
+                            .writeWithDirs(
+                              change.target.canonical,
+                              change.contents.endsWith("\n") || change.contents === ""
+                                ? change.contents
+                                : `${change.contents}\n`,
+                            )
+                            .pipe(
+                              Effect.mapError((error) => fail(`Failed to write ${change.target.resource}`, error)),
+                            )
                           applied.push({
                             type: change.type,
                             resource: change.target.resource,
@@ -248,7 +255,11 @@ export const Plugin = {
                           return
                         }
                         if (change.type === "delete") {
-                          yield* fs.remove(change.target.canonical)
+                          yield* fs
+                            .remove(change.target.canonical)
+                            .pipe(
+                              Effect.mapError((error) => fail(`Failed to delete ${change.target.resource}`, error)),
+                            )
                           applied.push({
                             type: change.type,
                             resource: change.target.resource,
@@ -257,8 +268,15 @@ export const Plugin = {
                           return
                         }
                         if (change.moveTarget) {
-                          yield* fs.writeWithDirs(change.moveTarget.canonical, change.content)
-                          yield* fs.remove(change.target.canonical)
+                          const moveTarget = change.moveTarget
+                          yield* fs
+                            .writeWithDirs(moveTarget.canonical, change.content)
+                            .pipe(Effect.mapError((error) => fail(`Failed to write ${moveTarget.resource}`, error)))
+                          yield* fs.remove(change.target.canonical).pipe(
+                            Effect.mapError((error) =>
+                              fail(`Wrote ${moveTarget.resource} but failed to remove ${change.target.resource}`, error),
+                            ),
+                          )
                           applied.push({
                             type: change.type,
                             resource: change.moveTarget.resource,
@@ -266,13 +284,15 @@ export const Plugin = {
                           })
                           return
                         }
-                        yield* fs.writeWithDirs(change.target.canonical, change.content)
+                        yield* fs
+                          .writeWithDirs(change.target.canonical, change.content)
+                          .pipe(Effect.mapError((error) => fail(`Failed to write ${change.target.resource}`, error)))
                         applied.push({
                           type: change.type,
                           resource: change.target.resource,
                           target: change.target.canonical,
                         })
-                      }).pipe(Effect.mapError((error) => fail(change.path, error))),
+                      }),
                     { discard: true },
                   )
                   return { applied, files: patchFiles }
@@ -282,7 +302,11 @@ export const Plugin = {
                     content: toModelOutput(output),
                     metadata: { files: output.files },
                   })),
-                  Effect.mapError((error) => (error instanceof ToolFailure ? error : fail("patch", error))),
+                  Effect.mapError((error) =>
+                    error instanceof ToolFailure
+                      ? error
+                      : new ToolFailure({ message: "Unable to apply patch", error }),
+                  ),
                 )
               },
             }),
@@ -306,6 +330,14 @@ export const Plugin = {
   }),
 }
 
+function errorMessage(error: unknown) {
+  if (error instanceof PlatformError) {
+    if (error.reason._tag === "NotFound") return "file does not exist"
+    return error.reason.description ?? error.reason.message
+  }
+  return error instanceof Error ? error.message : String(error)
+}
+
 function patchFile(change: Prepared): typeof FileDiff.Info.Type {
   const target = (change.type === "update" ? change.moveTarget : undefined)?.resource ?? change.target.resource
   const patch = trimDiff(
diff --git a/packages/core/test/patch.test.ts b/packages/core/test/patch.test.ts
index 18dd3695ce0..bd75b266479 100644
--- a/packages/core/test/patch.test.ts
+++ b/packages/core/test/patch.test.ts
@@ -246,6 +246,35 @@ describe("Patch", () => {
     ).toBe("line 1\nLINE 2\nline 3\nLINE 4\n")
   })
 
+  test("appends a pure-addition chunk to a nonempty file", () => {
+    expect(Patch.derive("update.txt", [{ oldLines: [], newLines: ["added 1", "added 2"] }], "line 1\nline 2\n").content).toBe(
+      "line 1\nline 2\nadded 1\nadded 2\n",
+    )
+  })
+
+  test("applies a pure-addition chunk after an earlier replacement", () => {
+    expect(
+      Patch.derive(
+        "update.txt",
+        [
+          { oldLines: [], newLines: ["after-context", "second-line"] },
+          { oldLines: ["line1", "line2", "line3"], newLines: ["line1", "line2-replacement"] },
+        ],
+        "line1\nline2\nline3\n",
+      ).content,
+    ).toBe("line1\nline2-replacement\nafter-context\nsecond-line\n")
+  })
+
+  test("applies a deletion-only update chunk", () => {
+    expect(
+      Patch.derive(
+        "update.txt",
+        [{ oldLines: ["line1", "line2", "line3"], newLines: ["line1", "line3"] }],
+        "line1\nline2\nline3\n",
+      ).content,
+    ).toBe("line1\nline3\n")
+  })
+
   test("updates empty files and adds a trailing newline", () => {
     expect(Patch.derive("empty.txt", [{ oldLines: [], newLines: ["First line"] }], "").content).toBe("First line\n")
     expect(Patch.derive("no-newline.txt", [{ oldLines: ["old"], newLines: ["new"] }], "old").content).toBe("new\n")
@@ -327,6 +356,12 @@ describe("Patch", () => {
     ).toThrow("Failed to find expected lines")
   })
 
+  test("identifies a missing blank line", () => {
+    expect(() =>
+      Patch.derive("update.txt", [{ oldLines: [""], newLines: ["added"] }], "content\n"),
+    ).toThrow("Failed to find an expected blank line in update.txt")
+  })
+
   test("parses an update without an explicit first chunk header", () => {
     expect(parse("*** Begin Patch\n*** Update File: file.txt\n import foo\n+bar\n*** End Patch")).toEqual([
       {
@@ -413,11 +448,14 @@ describe("Patch", () => {
 
   test("rejects invalid add and delete lines", () => {
     expect(() => parse("*** Begin Patch\n*** Add File: file.txt\nbad\n*** End Patch")).toThrow(
-      "Invalid hunk at line 3: 'bad' is not a valid hunk header",
+      "Invalid hunk at line 3: Invalid Add File line for 'file.txt': expected a line starting with '+', got 'bad'",
     )
     expect(() => parse("*** Begin Patch\n*** Delete File: file.txt\nbad\n*** End Patch")).toThrow(
-      "Invalid hunk at line 3: 'bad' is not a valid hunk header",
+      "Invalid hunk at line 3: Unexpected line after Delete File 'file.txt': 'bad'. Delete hunks do not contain body lines",
     )
+    expect(() =>
+      parse("*** Begin Patch\n*** Delete File: file.txt\n*** Frobnicate File: next.txt\n*** End Patch"),
+    ).toThrow("Invalid hunk at line 3: '*** Frobnicate File: next.txt' is not a valid hunk header")
   })
 
   test("rejects an empty update hunk", () => {
@@ -478,6 +516,6 @@ describe("Patch", () => {
     }
     expect(() =>
       parse("*** Begin Patch\n*** Update File: old.txt\n*** Move to: \n@@\n-old\n+new\n*** End Patch"),
-    ).toThrow("Invalid hunk at line 3: '*** Move to:' is not a valid hunk header")
+    ).toThrow("Invalid hunk at line 3: Move destination for 'old.txt' must not be empty")
   })
 })
diff --git a/packages/core/test/tool-patch.test.ts b/packages/core/test/tool-patch.test.ts
index cfae81eef86..64616e7d2ab 100644
--- a/packages/core/test/tool-patch.test.ts
+++ b/packages/core/test/tool-patch.test.ts
@@ -2,6 +2,7 @@ import fs from "fs/promises"
 import path from "path"
 import { describe, expect } from "bun:test"
 import { Effect, Exit, Layer, Schema } from "effect"
+import { systemError } from "effect/PlatformError"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { FSUtil } from "@opencode-ai/util/fs-util"
@@ -28,6 +29,8 @@ const sessionID = SessionV2.ID.make("ses_patch_tool_test")
 const assertions: PermissionV2.AssertInput[] = []
 let denyAction: string | undefined
 let failRemoveTarget: string | undefined
+let failRemoveErrorTarget: string | undefined
+let failWriteTarget: string | undefined
 let readsBeforeEditApproval = 0
 let editApproved = false
 let afterEditApproval = (): Effect.Effect<void> => Effect.void
@@ -65,6 +68,8 @@ const reset = () => {
   assertions.length = 0
   denyAction = undefined
   failRemoveTarget = undefined
+  failRemoveErrorTarget = undefined
+  failWriteTarget = undefined
   readsBeforeEditApproval = 0
   editApproved = false
   afterEditApproval = () => Effect.void
@@ -82,8 +87,33 @@ const filesystem = Layer.effect(
         }).pipe(Effect.andThen(fs.readFile(target))),
       remove: (target, options) => {
         if (failRemoveTarget && path.basename(target) === failRemoveTarget) return Effect.die("forced remove failure")
+        if (failRemoveErrorTarget && path.basename(target) === failRemoveErrorTarget) {
+          return Effect.fail(
+            systemError({
+              _tag: "Unknown",
+              module: "FileSystem",
+              method: "remove",
+              description: "forced remove failure",
+              pathOrDescriptor: target,
+            }),
+          )
+        }
         return fs.remove(target, options)
       },
+      writeWithDirs: (target, content, mode) => {
+        if (failWriteTarget && path.basename(target) === failWriteTarget) {
+          return Effect.fail(
+            systemError({
+              _tag: "Unknown",
+              module: "FileSystem",
+              method: "writeWithDirs",
+              description: "forced write failure",
+              pathOrDescriptor: target,
+            }),
+          )
+        }
+        return fs.writeWithDirs(target, content, mode)
+      },
     })
   }),
 ).pipe(Layer.provide(LayerNode.compile(FSUtil.node)))
@@ -302,6 +332,27 @@ describe("PatchTool", () => {
     ),
   )
 
+  it.live("moves a file without changing its contents", () =>
+    withTempTool((directory, registry) =>
+      Effect.gen(function* () {
+        const source = path.join(directory, "old.txt")
+        const destination = path.join(directory, "moved.txt")
+        yield* Effect.promise(() => fs.writeFile(source, "same\n"))
+        expect(
+          yield* executeTool(
+            registry,
+            call("*** Begin Patch\n*** Update File: old.txt\n*** Move to: moved.txt\n@@\n same\n*** End Patch"),
+          ),
+        ).toMatchObject({
+          status: "completed",
+          content: [{ type: "text", text: "Success. Updated the following files:\nM moved.txt" }],
+        })
+        expect(yield* exists(source)).toBe(false)
+        expect(yield* Effect.promise(() => fs.readFile(destination, "utf8"))).toBe("same\n")
+      }),
+    ),
+  )
+
   it.live("moves a symlink without deleting its target", () =>
     withTempTool((directory, registry) =>
       Effect.gen(function* () {
@@ -451,10 +502,17 @@ describe("PatchTool", () => {
   it.live("rejects an empty patch", () =>
     withTempTool((_directory, registry) =>
       Effect.gen(function* () {
-        expect(yield* executeTool(registry, call("*** Begin Patch\n*** End Patch"))).toEqual({
-          status: "error",
-          error: { type: "tool.execution", message: "patch rejected: empty patch" },
-        })
+        for (const patchText of [
+          "*** Begin Patch\n*** End Patch",
+          " *** Begin Patch \n *** End Patch ",
+          "<<EOF\n*** Begin Patch\n*** End Patch\nEOF",
+          "*** Begin Patch\n*** Environment ID: remote\n*** End Patch",
+        ]) {
+          expect(yield* executeTool(registry, call(patchText))).toEqual({
+            status: "error",
+            error: { type: "tool.execution", message: "patch rejected: empty patch" },
+          })
+        }
       }),
     ),
   )
@@ -525,7 +583,10 @@ describe("PatchTool", () => {
           ),
         ).toMatchObject({
           status: "error",
-          error: { message: expect.stringContaining("Failed to find expected lines") },
+          error: {
+            type: "tool.execution",
+            message: "patch verification failed: Failed to find expected lines in unchanged.txt:\nmissing",
+          },
         })
         expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("line1\nline2\n")
       }),
@@ -569,12 +630,83 @@ describe("PatchTool", () => {
     ),
   )
 
-  it.live("rejects a delete when the target file is missing", () =>
+  it.live("identifies a missing delete target", () =>
     withTempTool((_directory, registry) =>
       Effect.gen(function* () {
         expect(
           yield* executeTool(registry, call("*** Begin Patch\n*** Delete File: missing.txt\n*** End Patch")),
-        ).toMatchObject({ status: "error", error: { message: expect.stringContaining("patch verification failed") } })
+        ).toEqual({
+          status: "error",
+          error: {
+            type: "tool.execution",
+            message: "patch verification failed: Failed to delete missing.txt: file does not exist",
+          },
+        })
+      }),
+    ),
+  )
+
+  it.live("reports the failing destination and filesystem error", () =>
+    withTempTool((directory, registry) =>
+      Effect.gen(function* () {
+        yield* Effect.promise(() => fs.writeFile(path.join(directory, "old.txt"), "before\n"))
+        failWriteTarget = "new.txt"
+        expect(
+          yield* executeTool(
+            registry,
+            call("*** Begin Patch\n*** Update File: old.txt\n*** Move to: new.txt\n@@\n-before\n+after\n*** End Patch"),
+          ),
+        ).toEqual({
+          status: "error",
+          error: { type: "tool.execution", message: "Failed to write new.txt: forced write failure" },
+        })
+        expect(yield* Effect.promise(() => fs.readFile(path.join(directory, "old.txt"), "utf8"))).toBe("before\n")
+        expect(yield* exists(path.join(directory, "new.txt"))).toBe(false)
+      }),
+    ),
+  )
+
+  it.live("reports the successful prefix and filesystem error", () =>
+    withTempTool((directory, registry) =>
+      Effect.gen(function* () {
+        failWriteTarget = "second.txt"
+        expect(
+          yield* executeTool(
+            registry,
+            call("*** Begin Patch\n*** Add File: first.txt\n+first\n*** Add File: second.txt\n+second\n*** End Patch"),
+          ),
+        ).toEqual({
+          status: "error",
+          error: {
+            type: "tool.execution",
+            message: "Failed to write second.txt: forced write failure. Completed before failure: first.txt",
+          },
+        })
+        expect(yield* Effect.promise(() => fs.readFile(path.join(directory, "first.txt"), "utf8"))).toBe("first\n")
+        expect(yield* exists(path.join(directory, "second.txt"))).toBe(false)
+      }),
+    ),
+  )
+
+  it.live("reports a destination written before move removal fails", () =>
+    withTempTool((directory, registry) =>
+      Effect.gen(function* () {
+        yield* Effect.promise(() => fs.writeFile(path.join(directory, "old.txt"), "before\n"))
+        failRemoveErrorTarget = "old.txt"
+        expect(
+          yield* executeTool(
+            registry,
+            call("*** Begin Patch\n*** Update File: old.txt\n*** Move to: new.txt\n@@\n-before\n+after\n*** End Patch"),
+          ),
+        ).toEqual({
+          status: "error",
+          error: {
+            type: "tool.execution",
+            message: "Wrote new.txt but failed to remove old.txt: forced remove failure",
+          },
+        })
+        expect(yield* Effect.promise(() => fs.readFile(path.join(directory, "old.txt"), "utf8"))).toBe("before\n")
+        expect(yield* Effect.promise(() => fs.readFile(path.join(directory, "new.txt"), "utf8"))).toBe("after\n")
       }),
     ),
   )
@@ -628,7 +760,7 @@ describe("PatchTool", () => {
                       registry,
                       call(`*** Begin Patch\n*** Update File: ${target}\n@@\n-before\n+after\n*** End Patch`),
                     ),
-                  ).toMatchObject({ status: "error" })
+                  ).toMatchObject({ status: "error", error: { type: "permission.rejected" } })
                   expect(assertions.map((input) => input.action)).toEqual(["external_directory"])
                   expect(readsBeforeEditApproval).toBe(0)
                   expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("before\n")
@@ -645,6 +777,24 @@ describe("PatchTool", () => {
     ),
   )
 
+  it.live("preserves edit permission rejection", () =>
+    withTempTool((directory, registry) =>
+      Effect.gen(function* () {
+        const target = path.join(directory, "target.txt")
+        yield* Effect.promise(() => fs.writeFile(target, "before\n"))
+        denyAction = "edit"
+        expect(
+          yield* executeTool(
+            registry,
+            call("*** Begin Patch\n*** Update File: target.txt\n@@\n-before\n+after\n*** End Patch"),
+          ),
+        ).toMatchObject({ status: "error", error: { type: "permission.rejected" } })
+        expect(assertions.map((input) => input.action)).toEqual(["edit"])
+        expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("before\n")
+      }),
+    ),
+  )
+
   it.live("treats a sibling path inside the project worktree as internal", () =>
     Effect.acquireUseRelease(
       Effect.promise(() => tmpdir()),
diff --git a/packages/util/src/patch.ts b/packages/util/src/patch.ts
index 843e14efa14..54766c5a61e 100644
--- a/packages/util/src/patch.ts
+++ b/packages/util/src/patch.ts
@@ -69,7 +69,7 @@ export function parse(patchText: string): Result.Result<ReadonlyArray<Hunk>, Par
     }
     if (header.startsWith("*** Add File: ")) {
       const path = header.slice("*** Add File: ".length).trim()
-      const parsed = parseAdd(lines, index + 1, end)
+      const parsed = parseAdd(lines, index + 1, end, path)
       if ("error" in parsed) return Result.fail(parsed.error)
       hunks.push({ type: "add", path, contents: parsed.content })
       index = parsed.next
@@ -77,6 +77,19 @@ export function parse(patchText: string): Result.Result<ReadonlyArray<Hunk>, Par
     }
     if (header.startsWith("*** Delete File: ")) {
       const path = header.slice("*** Delete File: ".length).trim()
+      const next = lines[index + 1]?.trim()
+      if (index + 1 < end && next !== undefined && !isBoundary(next)) {
+        if (next.startsWith("*** ")) {
+          return Result.fail(new InvalidHunkError({ line: next, lineNumber: index + 2 }))
+        }
+        return Result.fail(
+          new InvalidHunkError({
+            line: next,
+            lineNumber: index + 2,
+            reason: `Unexpected line after Delete File '${path}': '${next}'. Delete hunks do not contain body lines`,
+          }),
+        )
+      }
       hunks.push({ type: "delete", path })
       index++
       continue
@@ -90,7 +103,13 @@ export function parse(patchText: string): Result.Result<ReadonlyArray<Hunk>, Par
       if (move === "*** Move to:" || move?.startsWith("*** Move to: ")) {
         movePath = move.slice("*** Move to: ".length).trim()
         if (!movePath) {
-          return Result.fail(new InvalidHunkError({ line: lines[next]!.trim(), lineNumber: next + 1 }))
+          return Result.fail(
+            new InvalidHunkError({
+              line: lines[next]!.trim(),
+              lineNumber: next + 1,
+              reason: `Move destination for '${path}' must not be empty`,
+            }),
+          )
         }
         next++
       }
@@ -126,12 +145,20 @@ function parseAdd(
   lines: ReadonlyArray<string>,
   start: number,
   end: number,
+  path: string,
 ): { content: string; next: number } | { error: InvalidHunkError } {
   const content: string[] = []
   let index = start
   while (index < end && !isBoundary(lines[index]!.trim())) {
     if (!lines[index]!.startsWith("+")) {
-      return { error: new InvalidHunkError({ line: lines[index]!.trim(), lineNumber: index + 1 }) }
+      const line = lines[index]!.trim()
+      return {
+        error: new InvalidHunkError({
+          line,
+          lineNumber: index + 1,
+          reason: `Invalid Add File line for '${path}': expected a line starting with '+', got '${line}'`,
+        }),
+      }
     }
     content.push(lines[index]!.slice(1))
     index++
@@ -303,6 +330,11 @@ function computeReplacements(lines: ReadonlyArray<string>, path: string, chunks:
       if (newLines.at(-1) === "") newLines = newLines.slice(0, -1)
       found = seek(lines, oldLines, lineIndex, chunk.endOfFile)
     }
+    if (found === -1 && chunk.oldLines.every((line) => line === "")) {
+      const expected =
+        chunk.oldLines.length === 1 ? "an expected blank line" : `${chunk.oldLines.length} consecutive blank lines`
+      throw new Error(`Failed to find ${expected} in ${path}`)
+    }
     if (found === -1) throw new Error(`Failed to find expected lines in ${path}:\n${chunk.oldLines.join("\n")}`)
     replacements.push([found, oldLines.length, newLines])
     lineIndex = found + oldLines.length