diff --git a/packages/app/src/components/prompt-input/build-request-parts.test.ts b/packages/app/src/components/prompt-input/build-request-parts.test.ts
index b284c3884..b0fd3a050 100644
--- a/packages/app/src/components/prompt-input/build-request-parts.test.ts
+++ b/packages/app/src/components/prompt-input/build-request-parts.test.ts
@@ -64,4 +64,214 @@ describe("buildRequestParts", () => {
     expect(fooFiles).toHaveLength(2)
     expect(synthetic).toHaveLength(1)
   })
+
+  test("handles Windows paths correctly (simulated on macOS)", () => {
+    const prompt: Prompt = [{ type: "file", path: "src\\foo.ts", content: "@src\\foo.ts", start: 0, end: 11 }]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@src\\foo.ts",
+      messageID: "msg_win_1",
+      sessionID: "ses_win_1",
+      sessionDirectory: "D:\\projects\\myapp", // Windows path
+    })
+
+    // Should create valid file URLs
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // URL should be parseable
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Should not have encoded backslashes in wrong place
+      expect(filePart.url).not.toContain("%5C")
+      // Should have normalized to forward slashes
+      expect(filePart.url).toContain("/src/foo.ts")
+    }
+  })
+
+  test("handles Windows absolute path with special characters", () => {
+    const prompt: Prompt = [{ type: "file", path: "file#name.txt", content: "@file#name.txt", start: 0, end: 14 }]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@file#name.txt",
+      messageID: "msg_win_2",
+      sessionID: "ses_win_2",
+      sessionDirectory: "C:\\Users\\test\\Documents", // Windows path
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // URL should be parseable
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Special chars should be encoded
+      expect(filePart.url).toContain("file%23name.txt")
+      // Should have Windows drive letter properly encoded
+      expect(filePart.url).toMatch(/file:\/\/\/[A-Z]%3A/)
+    }
+  })
+
+  test("handles Linux absolute paths correctly", () => {
+    const prompt: Prompt = [{ type: "file", path: "src/app.ts", content: "@src/app.ts", start: 0, end: 10 }]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@src/app.ts",
+      messageID: "msg_linux_1",
+      sessionID: "ses_linux_1",
+      sessionDirectory: "/home/user/project",
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // URL should be parseable
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Should be a normal Unix path
+      expect(filePart.url).toBe("file:///home/user/project/src/app.ts")
+    }
+  })
+
+  test("handles macOS paths correctly", () => {
+    const prompt: Prompt = [{ type: "file", path: "README.md", content: "@README.md", start: 0, end: 9 }]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@README.md",
+      messageID: "msg_mac_1",
+      sessionID: "ses_mac_1",
+      sessionDirectory: "/Users/kelvin/Projects/opencode",
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // URL should be parseable
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Should be a normal Unix path
+      expect(filePart.url).toBe("file:///Users/kelvin/Projects/opencode/README.md")
+    }
+  })
+
+  test("handles context files with Windows paths", () => {
+    const prompt: Prompt = []
+
+    const result = buildRequestParts({
+      prompt,
+      context: [
+        { key: "ctx:1", type: "file", path: "src\\utils\\helper.ts" },
+        { key: "ctx:2", type: "file", path: "test\\unit.test.ts", comment: "check tests" },
+      ],
+      images: [],
+      text: "test",
+      messageID: "msg_win_ctx",
+      sessionID: "ses_win_ctx",
+      sessionDirectory: "D:\\workspace\\app",
+    })
+
+    const fileParts = result.requestParts.filter((part) => part.type === "file")
+    expect(fileParts).toHaveLength(2)
+
+    // All file URLs should be valid
+    fileParts.forEach((part) => {
+      if (part.type === "file") {
+        expect(() => new URL(part.url)).not.toThrow()
+        expect(part.url).not.toContain("%5C") // No encoded backslashes
+      }
+    })
+  })
+
+  test("handles absolute Windows paths (user manually specifies full path)", () => {
+    const prompt: Prompt = [
+      { type: "file", path: "D:\\other\\project\\file.ts", content: "@D:\\other\\project\\file.ts", start: 0, end: 25 },
+    ]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@D:\\other\\project\\file.ts",
+      messageID: "msg_abs",
+      sessionID: "ses_abs",
+      sessionDirectory: "C:\\current\\project",
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // Should handle absolute path that differs from sessionDirectory
+      expect(() => new URL(filePart.url)).not.toThrow()
+      expect(filePart.url).toContain("/D%3A/other/project/file.ts")
+    }
+  })
+
+  test("handles selection with query parameters on Windows", () => {
+    const prompt: Prompt = [
+      {
+        type: "file",
+        path: "src\\App.tsx",
+        content: "@src\\App.tsx",
+        start: 0,
+        end: 11,
+        selection: { startLine: 10, startChar: 0, endLine: 20, endChar: 5 },
+      },
+    ]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@src\\App.tsx",
+      messageID: "msg_sel",
+      sessionID: "ses_sel",
+      sessionDirectory: "C:\\project",
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // Should have query parameters
+      expect(filePart.url).toContain("?start=10&end=20")
+      // Should be valid URL
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Query params should parse correctly
+      const url = new URL(filePart.url)
+      expect(url.searchParams.get("start")).toBe("10")
+      expect(url.searchParams.get("end")).toBe("20")
+    }
+  })
+
+  test("handles file paths with dots and special segments on Windows", () => {
+    const prompt: Prompt = [
+      { type: "file", path: "..\\..\\shared\\util.ts", content: "@..\\..\\shared\\util.ts", start: 0, end: 21 },
+    ]
+
+    const result = buildRequestParts({
+      prompt,
+      context: [],
+      images: [],
+      text: "@..\\..\\shared\\util.ts",
+      messageID: "msg_dots",
+      sessionID: "ses_dots",
+      sessionDirectory: "C:\\projects\\myapp\\src",
+    })
+
+    const filePart = result.requestParts.find((part) => part.type === "file")
+    expect(filePart).toBeDefined()
+    if (filePart?.type === "file") {
+      // Should be valid URL
+      expect(() => new URL(filePart.url)).not.toThrow()
+      // Should preserve .. segments (backend normalizes)
+      expect(filePart.url).toContain("/..")
+    }
+  })
 })
diff --git a/packages/app/src/components/prompt-input/build-request-parts.ts b/packages/app/src/components/prompt-input/build-request-parts.ts
index 7010a1fd8..11aec9631 100644
--- a/packages/app/src/components/prompt-input/build-request-parts.ts
+++ b/packages/app/src/components/prompt-input/build-request-parts.ts
@@ -30,11 +30,21 @@ type BuildRequestPartsInput = {
 const absolute = (directory: string, path: string) =>
   path.startsWith("/") ? path : (directory + "/" + path).replace("//", "/")
 
-const encodeFilePath = (filepath: string): string =>
-  filepath
+const encodeFilePath = (filepath: string): string => {
+  // Normalize Windows paths: convert backslashes to forward slashes
+  let normalized = filepath.replace(/\\/g, "/")
+
+  // Handle Windows absolute paths (D:/path -> /D:/path for proper file:// URLs)
+  if (/^[A-Za-z]:/.test(normalized)) {
+    normalized = "/" + normalized
+  }
+
+  // Encode each path segment (preserving forward slashes as path separators)
+  return normalized
     .split("/")
     .map((segment) => encodeURIComponent(segment))
     .join("/")
+}
 
 const fileQuery = (selection: FileSelection | undefined) =>
   selection ? `?start=${selection.startLine}&end=${selection.endLine}` : ""
diff --git a/packages/app/src/context/file/path.test.ts b/packages/app/src/context/file/path.test.ts
index dba9ae06d..95247c08b 100644
--- a/packages/app/src/context/file/path.test.ts
+++ b/packages/app/src/context/file/path.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, test } from "bun:test"
-import { createPathHelpers, stripQueryAndHash, unquoteGitPath } from "./path"
+import { createPathHelpers, stripQueryAndHash, unquoteGitPath, encodeFilePath } from "./path"
 
 describe("file path helpers", () => {
   test("normalizes file inputs against workspace root", () => {
@@ -25,3 +25,328 @@ describe("file path helpers", () => {
     expect(unquoteGitPath("a/b/c.ts")).toBe("a/b/c.ts")
   })
 })
+
+describe("encodeFilePath", () => {
+  describe("Linux/Unix paths", () => {
+    test("should handle Linux absolute path", () => {
+      const linuxPath = "/home/user/project/README.md"
+      const result = encodeFilePath(linuxPath)
+      const fileUrl = `file://${result}`
+
+      // Should create a valid URL
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/home/user/project/README.md")
+
+      const url = new URL(fileUrl)
+      expect(url.protocol).toBe("file:")
+      expect(url.pathname).toBe("/home/user/project/README.md")
+    })
+
+    test("should handle Linux path with special characters", () => {
+      const linuxPath = "/home/user/file#name with spaces.txt"
+      const result = encodeFilePath(linuxPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/home/user/file%23name%20with%20spaces.txt")
+    })
+
+    test("should handle Linux relative path", () => {
+      const relativePath = "src/components/App.tsx"
+      const result = encodeFilePath(relativePath)
+
+      expect(result).toBe("src/components/App.tsx")
+    })
+
+    test("should handle Linux root directory", () => {
+      const result = encodeFilePath("/")
+      expect(result).toBe("/")
+    })
+
+    test("should handle Linux path with all special chars", () => {
+      const path = "/path/to/file#with?special%chars&more.txt"
+      const result = encodeFilePath(path)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toContain("%23") // #
+      expect(result).toContain("%3F") // ?
+      expect(result).toContain("%25") // %
+      expect(result).toContain("%26") // &
+    })
+  })
+
+  describe("macOS paths", () => {
+    test("should handle macOS absolute path", () => {
+      const macPath = "/Users/kelvin/Projects/opencode/README.md"
+      const result = encodeFilePath(macPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/Users/kelvin/Projects/opencode/README.md")
+    })
+
+    test("should handle macOS path with spaces", () => {
+      const macPath = "/Users/kelvin/My Documents/file.txt"
+      const result = encodeFilePath(macPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toContain("My%20Documents")
+    })
+  })
+
+  describe("Windows paths", () => {
+    test("should handle Windows absolute path with backslashes", () => {
+      const windowsPath = "D:\\dev\\projects\\opencode\\README.bs.md"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      // Should create a valid, parseable URL
+      expect(() => new URL(fileUrl)).not.toThrow()
+
+      const url = new URL(fileUrl)
+      expect(url.protocol).toBe("file:")
+      expect(url.pathname).toContain("README.bs.md")
+      expect(result).toBe("/D%3A/dev/projects/opencode/README.bs.md")
+    })
+
+    test("should handle mixed separator path (Windows + Unix)", () => {
+      // This is what happens in build-request-parts.ts when concatenating paths
+      const mixedPath = "D:\\dev\\projects\\opencode/README.bs.md"
+      const result = encodeFilePath(mixedPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/D%3A/dev/projects/opencode/README.bs.md")
+    })
+
+    test("should handle Windows path with spaces", () => {
+      const windowsPath = "C:\\Program Files\\MyApp\\file with spaces.txt"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toContain("Program%20Files")
+      expect(result).toContain("file%20with%20spaces.txt")
+    })
+
+    test("should handle Windows path with special chars in filename", () => {
+      const windowsPath = "D:\\projects\\file#name with ?marks.txt"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toContain("file%23name%20with%20%3Fmarks.txt")
+    })
+
+    test("should handle Windows root directory", () => {
+      const windowsPath = "C:\\"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/C%3A/")
+    })
+
+    test("should handle Windows relative path with backslashes", () => {
+      const windowsPath = "src\\components\\App.tsx"
+      const result = encodeFilePath(windowsPath)
+
+      // Relative paths shouldn't get the leading slash
+      expect(result).toBe("src/components/App.tsx")
+    })
+
+    test("should NOT create invalid URL like the bug report", () => {
+      // This is the exact scenario from bug report by @alexyaroshuk
+      const windowsPath = "D:\\dev\\projects\\opencode\\README.bs.md"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      // The bug was creating: file://D%3A%5Cdev%5Cprojects%5Copencode/README.bs.md
+      expect(result).not.toContain("%5C") // Should not have encoded backslashes
+      expect(result).not.toBe("D%3A%5Cdev%5Cprojects%5Copencode/README.bs.md")
+
+      // Should be valid
+      expect(() => new URL(fileUrl)).not.toThrow()
+    })
+
+    test("should handle lowercase drive letters", () => {
+      const windowsPath = "c:\\users\\test\\file.txt"
+      const result = encodeFilePath(windowsPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/c%3A/users/test/file.txt")
+    })
+  })
+
+  describe("Cross-platform compatibility", () => {
+    test("should preserve Unix paths unchanged (except encoding)", () => {
+      const unixPath = "/usr/local/bin/app"
+      const result = encodeFilePath(unixPath)
+      expect(result).toBe("/usr/local/bin/app")
+    })
+
+    test("should normalize Windows paths for cross-platform use", () => {
+      const windowsPath = "C:\\Users\\test\\file.txt"
+      const result = encodeFilePath(windowsPath)
+      // Should convert to forward slashes and add leading /
+      expect(result).not.toContain("\\")
+      expect(result).toMatch(/^\/[A-Za-z]%3A\//)
+    })
+
+    test("should handle relative paths the same on all platforms", () => {
+      const unixRelative = "src/app.ts"
+      const windowsRelative = "src\\app.ts"
+
+      const unixResult = encodeFilePath(unixRelative)
+      const windowsResult = encodeFilePath(windowsRelative)
+
+      // Both should normalize to forward slashes
+      expect(unixResult).toBe("src/app.ts")
+      expect(windowsResult).toBe("src/app.ts")
+    })
+  })
+
+  describe("Edge cases", () => {
+    test("should handle empty path", () => {
+      const result = encodeFilePath("")
+      expect(result).toBe("")
+    })
+
+    test("should handle path with multiple consecutive slashes", () => {
+      const result = encodeFilePath("//path//to///file.txt")
+      // Multiple slashes should be preserved (backend handles normalization)
+      expect(result).toBe("//path//to///file.txt")
+    })
+
+    test("should encode Unicode characters", () => {
+      const unicodePath = "/home/user/文档/README.md"
+      const result = encodeFilePath(unicodePath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      // Unicode should be encoded
+      expect(result).toContain("%E6%96%87%E6%A1%A3")
+    })
+
+    test("should handle already normalized Windows path", () => {
+      // Path that's already been normalized (has / before drive letter)
+      const alreadyNormalized = "/D:/path/file.txt"
+      const result = encodeFilePath(alreadyNormalized)
+
+      // Should not add another leading slash
+      expect(result).toBe("/D%3A/path/file.txt")
+      expect(result).not.toContain("//D")
+    })
+
+    test("should handle just drive letter", () => {
+      const justDrive = "D:"
+      const result = encodeFilePath(justDrive)
+      const fileUrl = `file://${result}`
+
+      expect(result).toBe("/D%3A")
+      expect(() => new URL(fileUrl)).not.toThrow()
+    })
+
+    test("should handle Windows path with trailing backslash", () => {
+      const trailingBackslash = "C:\\Users\\test\\"
+      const result = encodeFilePath(trailingBackslash)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/C%3A/Users/test/")
+    })
+
+    test("should handle very long paths", () => {
+      const longPath = "C:\\Users\\test\\" + "verylongdirectoryname\\".repeat(20) + "file.txt"
+      const result = encodeFilePath(longPath)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).not.toContain("\\")
+    })
+
+    test("should handle paths with dots", () => {
+      const pathWithDots = "C:\\Users\\..\\test\\.\\file.txt"
+      const result = encodeFilePath(pathWithDots)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      // Dots should be preserved (backend normalizes)
+      expect(result).toContain("..")
+      expect(result).toContain("/./")
+    })
+  })
+
+  describe("Regression tests for PR #12424", () => {
+    test("should handle file with # in name", () => {
+      const path = "/path/to/file#name.txt"
+      const result = encodeFilePath(path)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/path/to/file%23name.txt")
+    })
+
+    test("should handle file with ? in name", () => {
+      const path = "/path/to/file?name.txt"
+      const result = encodeFilePath(path)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/path/to/file%3Fname.txt")
+    })
+
+    test("should handle file with % in name", () => {
+      const path = "/path/to/file%name.txt"
+      const result = encodeFilePath(path)
+      const fileUrl = `file://${result}`
+
+      expect(() => new URL(fileUrl)).not.toThrow()
+      expect(result).toBe("/path/to/file%25name.txt")
+    })
+  })
+
+  describe("Integration with file:// URL construction", () => {
+    test("should work with query parameters (Linux)", () => {
+      const path = "/home/user/file.txt"
+      const encoded = encodeFilePath(path)
+      const fileUrl = `file://${encoded}?start=10&end=20`
+
+      const url = new URL(fileUrl)
+      expect(url.searchParams.get("start")).toBe("10")
+      expect(url.searchParams.get("end")).toBe("20")
+      expect(url.pathname).toBe("/home/user/file.txt")
+    })
+
+    test("should work with query parameters (Windows)", () => {
+      const path = "C:\\Users\\test\\file.txt"
+      const encoded = encodeFilePath(path)
+      const fileUrl = `file://${encoded}?start=10&end=20`
+
+      const url = new URL(fileUrl)
+      expect(url.searchParams.get("start")).toBe("10")
+      expect(url.searchParams.get("end")).toBe("20")
+    })
+
+    test("should parse correctly in URL constructor (Linux)", () => {
+      const path = "/var/log/app.log"
+      const fileUrl = `file://${encodeFilePath(path)}`
+      const url = new URL(fileUrl)
+
+      expect(url.protocol).toBe("file:")
+      expect(url.pathname).toBe("/var/log/app.log")
+    })
+
+    test("should parse correctly in URL constructor (Windows)", () => {
+      const path = "D:\\logs\\app.log"
+      const fileUrl = `file://${encodeFilePath(path)}`
+      const url = new URL(fileUrl)
+
+      expect(url.protocol).toBe("file:")
+      expect(url.pathname).toContain("app.log")
+    })
+  })
+})
diff --git a/packages/app/src/context/file/path.ts b/packages/app/src/context/file/path.ts
index 155f05aaf..e1d47c644 100644
--- a/packages/app/src/context/file/path.ts
+++ b/packages/app/src/context/file/path.ts
@@ -81,7 +81,16 @@ export function decodeFilePath(input: string) {
 }
 
 export function encodeFilePath(filepath: string): string {
-  return filepath
+  // Normalize Windows paths: convert backslashes to forward slashes
+  let normalized = filepath.replace(/\\/g, "/")
+
+  // Handle Windows absolute paths (D:/path -> /D:/path for proper file:// URLs)
+  if (/^[A-Za-z]:/.test(normalized)) {
+    normalized = "/" + normalized
+  }
+
+  // Encode each path segment (preserving forward slashes as path separators)
+  return normalized
     .split("/")
     .map((segment) => encodeURIComponent(segment))
     .join("/")