diff --git a/src/tools/AgentTool/AgentTool.tsx b/src/tools/AgentTool/AgentTool.tsx
index 16ef0c5f..44bc8e6f 100644
--- a/src/tools/AgentTool/AgentTool.tsx
+++ b/src/tools/AgentTool/AgentTool.tsx
@@ -80,8 +80,16 @@ function getAutoBackgroundMs(): number {
 
 // Base input schema without multi-agent parameters
 const baseInputSchema = lazySchema(() => z.object({
-  description: z.string().describe('A short (3-5 word) description of the task'),
-  prompt: z.string().describe('The task for the agent to perform'),
+  description: z.string().describe(
+    'REQUIRED. A short (3-5 word) description of the task. ' +
+    'This parameter must always be provided — never omit it. ' +
+    'Example: "search for files" or "fix the bug in login".'
+  ),
+  prompt: z.string().describe(
+    'REQUIRED. The full task description for the agent to perform. ' +
+    'This parameter must always be provided — never omit it. ' +
+    'Example: "Find all TypeScript files that import from react and list their paths."'
+  ),
   subagent_type: z.string().optional().describe('The type of specialized agent to use for this task'),
   model: z.enum(['sonnet', 'opus', 'haiku']).optional().describe("Optional model override for this agent. Takes precedence over the agent definition's model frontmatter. If omitted, uses the agent definition's model, or inherits from the parent."),
   run_in_background: z.boolean().optional().describe('Set to true to run this agent in the background. You will be notified when it completes.')
diff --git a/src/tools/BashTool/BashTool.tsx b/src/tools/BashTool/BashTool.tsx
index 4ad8bcf3..c0526b6b 100644
--- a/src/tools/BashTool/BashTool.tsx
+++ b/src/tools/BashTool/BashTool.tsx
@@ -225,7 +225,11 @@ const isBackgroundTasksDisabled =
 // eslint-disable-next-line custom-rules/no-process-env-top-level -- Intentional: schema must be defined at module load
 isEnvTruthy(process.env.CLAUDE_CODE_DISABLE_BACKGROUND_TASKS);
 const fullInputSchema = lazySchema(() => z.strictObject({
-  command: z.string().describe('The command to execute'),
+  command: z.string().describe(
+    'REQUIRED. The shell command to execute. ' +
+    'This parameter must always be provided — never omit it. ' +
+    'Example: "ls -la" or "npm install" or "git status".'
+  ),
   timeout: semanticNumber(z.number().optional()).describe(`Optional timeout in milliseconds (max ${getMaxTimeoutMs()})`),
   description: z.string().optional().describe(`Clear, concise description of what this command does in active voice. Never use words like "complex" or "risk" in the description - just describe what it does.
 
diff --git a/src/tools/FileEditTool/types.ts b/src/tools/FileEditTool/types.ts
index 86be2683..0e6d26f3 100644
--- a/src/tools/FileEditTool/types.ts
+++ b/src/tools/FileEditTool/types.ts
@@ -5,12 +5,20 @@ import { semanticBoolean } from '../../utils/semanticBoolean.js'
 // The input schema with optional replace_all
 const inputSchema = lazySchema(() =>
   z.strictObject({
-    file_path: z.string().describe('The absolute path to the file to modify'),
-    old_string: z.string().describe('The text to replace'),
+    file_path: z.string().describe(
+      'REQUIRED. The absolute path to the file to modify (must be absolute, not relative). ' +
+      'Example: "/home/user/project/src/index.ts" or "C:\\\\Users\\\\user\\\\project\\\\index.ts". ' +
+      'This parameter must always be provided — never omit it.'
+    ),
+    old_string: z.string().describe(
+      'REQUIRED. The exact text to find and replace in the file. ' +
+      'This parameter must always be provided — never omit it.'
+    ),
     new_string: z
       .string()
       .describe(
-        'The text to replace it with (must be different from old_string)',
+        'REQUIRED. The text to replace old_string with (must be different from old_string). ' +
+        'This parameter must always be provided — never omit it.',
       ),
     replace_all: semanticBoolean(
       z.boolean().default(false).optional(),
diff --git a/src/tools/FileReadTool/FileReadTool.ts b/src/tools/FileReadTool/FileReadTool.ts
index 1ceed46d..d7e6f34e 100644
--- a/src/tools/FileReadTool/FileReadTool.ts
+++ b/src/tools/FileReadTool/FileReadTool.ts
@@ -226,7 +226,11 @@ function detectSessionFileType(
 
 const inputSchema = lazySchema(() =>
   z.strictObject({
-    file_path: z.string().describe('The absolute path to the file to read'),
+    file_path: z.string().describe(
+      'REQUIRED. The absolute path to the file to read (must be absolute, not relative). ' +
+      'Example: "/home/user/project/src/index.ts" or "C:\\\\Users\\\\user\\\\project\\\\index.ts". ' +
+      'This parameter must always be provided — never omit it.'
+    ),
     offset: semanticNumber(z.number().int().nonnegative().optional()).describe(
       'The line number to start reading from. Only provide if the file is too large to read at once',
     ),
diff --git a/src/tools/FileWriteTool/FileWriteTool.ts b/src/tools/FileWriteTool/FileWriteTool.ts
index 6da0afc8..e04f9bb5 100644
--- a/src/tools/FileWriteTool/FileWriteTool.ts
+++ b/src/tools/FileWriteTool/FileWriteTool.ts
@@ -58,9 +58,16 @@ const inputSchema = lazySchema(() =>
     file_path: z
       .string()
       .describe(
-        'The absolute path to the file to write (must be absolute, not relative)',
+        'REQUIRED. The absolute path to the file to write (must be absolute, not relative). ' +
+          'Example: "/home/user/project/src/index.ts" or "C:\\\\Users\\\\user\\\\project\\\\index.ts". ' +
+          'This parameter must always be provided — never omit it.',
+      ),
+    content: z
+      .string()
+      .describe(
+        'REQUIRED. The full content to write to the file. ' +
+          'This parameter must always be provided — never omit it.',
       ),
-    content: z.string().describe('The content to write to the file'),
   }),
 )
 type InputSchema = ReturnType<typeof inputSchema>
diff --git a/src/tools/GlobTool/GlobTool.ts b/src/tools/GlobTool/GlobTool.ts
index 7b24554d..745f57f2 100644
--- a/src/tools/GlobTool/GlobTool.ts
+++ b/src/tools/GlobTool/GlobTool.ts
@@ -25,7 +25,11 @@ import {
 
 const inputSchema = lazySchema(() =>
   z.strictObject({
-    pattern: z.string().describe('The glob pattern to match files against'),
+    pattern: z.string().describe(
+      'REQUIRED. The glob pattern to match files against. ' +
+      'This parameter must always be provided — never omit it. ' +
+      'Example: "**/*.ts" or "src/**/*.tsx" or "*.json".'
+    ),
     path: z
       .string()
       .optional()
diff --git a/src/tools/GrepTool/GrepTool.ts b/src/tools/GrepTool/GrepTool.ts
index 9bdd024a..4acab7b4 100644
--- a/src/tools/GrepTool/GrepTool.ts
+++ b/src/tools/GrepTool/GrepTool.ts
@@ -35,7 +35,9 @@ const inputSchema = lazySchema(() =>
     pattern: z
       .string()
       .describe(
-        'The regular expression pattern to search for in file contents',
+        'REQUIRED. The regular expression pattern to search for in file contents. ' +
+        'This parameter must always be provided — never omit it. ' +
+        'Example: "function\\\\s+\\\\w+" or "import.*from" or "TODO".',
       ),
     path: z
       .string()
diff --git a/src/tools/SkillTool/SkillTool.ts b/src/tools/SkillTool/SkillTool.ts
index befde568..eff0e0bd 100644
--- a/src/tools/SkillTool/SkillTool.ts
+++ b/src/tools/SkillTool/SkillTool.ts
@@ -292,7 +292,14 @@ export const inputSchema = lazySchema(() =>
   z.object({
     skill: z
       .string()
-      .describe('The skill name. E.g., "commit", "review-pr", or "pdf"'),
+      .describe(
+        'The skill name WITHOUT the leading slash. ' +
+          'When the user types /skill-name, pass skill: "skill-name" (strip the slash). ' +
+          'Examples: user types "/commit" → skill: "commit", ' +
+          'user types "/review-pr" → skill: "review-pr", ' +
+          'user types "/cufa-equity-report" → skill: "cufa-equity-report". ' +
+          'For namespaced skills use a colon: "ms-office-suite:pdf".',
+      ),
     args: z.string().optional().describe('Optional arguments for the skill'),
   }),
 )
@@ -352,6 +359,18 @@ export const SkillTool: Tool<InputSchema, Output, Progress> = buildTool({
   toAutoClassifierInput: ({ skill }) => skill ?? '',
 
   async validateInput({ skill }, context): Promise<ValidationResult> {
+    // Guard against null/undefined from non-Claude function calling (e.g., GPT-5/Codex).
+    // OpenAI function calling may omit required parameters that Claude tool use always provides.
+    if (!skill || typeof skill !== 'string') {
+      return {
+        result: false,
+        message:
+          'Missing skill name. Pass the slash command name as the skill parameter ' +
+          '(e.g., skill: "commit" for /commit, skill: "review-pr" for /review-pr).',
+        errorCode: 1,
+      }
+    }
+
     // Skills are just skill names, no arguments
     const trimmed = skill.trim()
     if (!trimmed) {
diff --git a/src/tools/SkillTool/prompt.ts b/src/tools/SkillTool/prompt.ts
index b05a58f3..b4bbedb2 100644
--- a/src/tools/SkillTool/prompt.ts
+++ b/src/tools/SkillTool/prompt.ts
@@ -192,6 +192,11 @@ Important:
 - Do not invoke a skill that is already running
 - Do not use this tool for built-in CLI commands (like /help, /clear, etc.)
 - If you see a <${COMMAND_NAME_TAG}> tag in the current conversation turn, the skill has ALREADY been loaded - follow the instructions directly instead of calling this tool again
+
+IMPORTANT for OpenAI/GPT function calling:
+- The \`skill\` parameter is REQUIRED — never omit it or pass null/undefined
+- Strip the leading \`/\`: "/commit" → skill: "commit", "/review-pr" → skill: "review-pr"
+- When the user types \`/skill-name\`, always call this tool with skill: "skill-name"
 `
 })
 
diff --git a/src/tools/WebFetchTool/WebFetchTool.ts b/src/tools/WebFetchTool/WebFetchTool.ts
index 258e1c2b..1aa7b525 100644
--- a/src/tools/WebFetchTool/WebFetchTool.ts
+++ b/src/tools/WebFetchTool/WebFetchTool.ts
@@ -35,8 +35,16 @@ async function scrapeWithFirecrawl(url: string): Promise<{ markdown: string; byt
 
 const inputSchema = lazySchema(() =>
   z.strictObject({
-    url: z.string().url().describe('The URL to fetch content from'),
-    prompt: z.string().describe('The prompt to run on the fetched content'),
+    url: z.string().url().describe(
+      'REQUIRED. The URL to fetch content from. ' +
+      'This parameter must always be provided — never omit it. ' +
+      'Example: "https://example.com/page" or "https://docs.example.com/api".'
+    ),
+    prompt: z.string().describe(
+      'REQUIRED. The prompt/question to answer using the fetched content. ' +
+      'This parameter must always be provided — never omit it. ' +
+      'Example: "What are the main features described on this page?" or "Summarize the API documentation."'
+    ),
   }),
 )
 type InputSchema = ReturnType<typeof inputSchema>
diff --git a/src/tools/WebSearchTool/WebSearchTool.ts b/src/tools/WebSearchTool/WebSearchTool.ts
index fa9b5360..b2a85af4 100644
--- a/src/tools/WebSearchTool/WebSearchTool.ts
+++ b/src/tools/WebSearchTool/WebSearchTool.ts
@@ -30,7 +30,11 @@ import {
 
 const inputSchema = lazySchema(() =>
   z.strictObject({
-    query: z.string().min(2).describe('The search query to use'),
+    query: z.string().min(2).describe(
+      'REQUIRED. The search query to use (minimum 2 characters). ' +
+      'This parameter must always be provided — never omit it. ' +
+      'Example: "TypeScript best practices 2024" or "how to fix CORS error".'
+    ),
     allowed_domains: z
       .array(z.string())
       .optional()