diff --git a/docs/auto-think.md b/docs/auto-think.md
new file mode 100644
index 000000000..4ad3154ca
--- /dev/null
+++ b/docs/auto-think.md
@@ -0,0 +1,124 @@
+---
+summary: "Auto-think: Automatic thinking level classification based on message content"
+read_when:
+  - You want to enable automatic thinking level selection
+  - You want to configure auto-think heuristics or rules
+---
+# Auto-Think
+
+Auto-think automatically classifies incoming messages and selects an appropriate thinking level without requiring explicit `/think` directives from users.
+
+## Overview
+
+When enabled, auto-think analyzes each incoming message using heuristics to determine complexity:
+
+- **High complexity**: Debug requests, security reviews, architecture discussions, large code blocks
+- **Medium complexity**: How-to questions, implementation requests, step-by-step guides, comparisons
+- **Low complexity**: Simple lookups, definitions, format/translation requests
+- **Minimal/Off**: Short messages, greetings, unclassified content
+
+## Configuration
+
+Enable auto-think in your agent config:
+
+```yaml
+agents:
+  defaults:
+    autoThink:
+      enabled: true
+```
+
+### Full options
+
+```yaml
+agents:
+  defaults:
+    autoThink:
+      enabled: true
+      floor: "off"        # Never go below this level
+      ceiling: "high"     # Never go above this level
+      rules:              # Custom patterns (optional)
+        - match: "newsletter"
+          level: "medium"
+        - match: "security|audit"
+          level: "high"
+```
+
+### Config reference
+
+| Option | Type | Default | Description |
+|--------|------|---------|-------------|
+| `enabled` | boolean | `false` | Enable auto-think classification |
+| `floor` | ThinkLevel | `"off"` | Minimum thinking level |
+| `ceiling` | ThinkLevel | `"high"` | Maximum thinking level |
+| `rules` | Array | `[]` | Custom pattern rules |
+
+### Custom rules
+
+Rules are evaluated in order; the first match wins. Each rule has:
+
+- `match`: A regex pattern (case-insensitive) or plain string
+- `level`: The thinking level to use when matched
+
+Example:
+
+```yaml
+rules:
+  - match: "urgent|asap"
+    level: "high"
+  - match: "quick question"
+    level: "off"
+```
+
+## Override behavior
+
+Auto-think respects the existing directive precedence:
+
+1. **Inline directive** (`/t high` in the message) — always wins
+2. **Session sticky** (previous `/think:high` directive-only message)
+3. **Auto-think classification** — when enabled and no directive present
+4. **Default level** (`thinkingDefault` config)
+
+Users can always override auto-think with explicit `/think` directives.
+
+## Built-in heuristics
+
+### High complexity signals
+
+- Debug/debugging requests
+- Error messages, stack traces, exceptions
+- Security, vulnerability, audit keywords
+- Architecture, design pattern discussions
+- Refactoring, optimization requests
+- Code blocks > 500 characters
+
+### Medium complexity signals
+
+- "How do/would/should/can" questions
+- "Explain", "analyze", "compare" requests
+- Step-by-step guides
+- Implementation/build/create requests
+- Planning, strategy discussions
+- Code blocks > 100 characters
+- Messages > 2000 characters
+
+### Low complexity signals
+
+- Simple "What is X?" questions
+- Translation/conversion requests
+- List/enumerate requests
+- Definition lookups
+
+### Fallbacks
+
+- Very short messages (< 50 chars): `off`
+- Unclassified messages: `minimal`
+
+## Performance
+
+Auto-think uses pure regex heuristics with no additional API calls. Classification adds negligible latency (< 1ms).
+
+## Related
+
+- [Thinking levels](/tools/thinking) — Manual thinking control via `/think` directives
+- [Token use](/token-use) — Cost implications of thinking levels
diff --git a/src/auto-reply/auto-think.test.ts b/src/auto-reply/auto-think.test.ts
new file mode 100644
index 000000000..b1e4fad0e
--- /dev/null
+++ b/src/auto-reply/auto-think.test.ts
@@ -0,0 +1,184 @@
+import { describe, expect, it } from "vitest";
+import { classifyThinkLevel, type AutoThinkConfig } from "./auto-think.js";
+
+describe("auto-think", () => {
+  describe("classifyThinkLevel", () => {
+    it("returns undefined when disabled", () => {
+      expect(classifyThinkLevel("debug this code", { enabled: false })).toBeUndefined();
+      expect(classifyThinkLevel("debug this code", undefined)).toBeUndefined();
+      expect(classifyThinkLevel("debug this code", {})).toBeUndefined();
+    });
+
+    it("returns undefined for empty messages", () => {
+      const config: AutoThinkConfig = { enabled: true };
+      expect(classifyThinkLevel("", config)).toBeUndefined();
+      expect(classifyThinkLevel("   ", config)).toBeUndefined();
+    });
+
+    describe("high complexity detection", () => {
+      const config: AutoThinkConfig = { enabled: true };
+
+      it("detects debug-related messages", () => {
+        expect(classifyThinkLevel("Can you debug this code?", config)).toBe("high");
+        expect(classifyThinkLevel("I'm debugging an issue", config)).toBe("high");
+      });
+
+      it("detects error-related messages", () => {
+        expect(classifyThinkLevel("Getting an error: TypeError", config)).toBe("high");
+        expect(classifyThinkLevel("Here's the stack trace", config)).toBe("high");
+        expect(classifyThinkLevel("Exception thrown at line 42", config)).toBe("high");
+      });
+
+      it("detects security-related messages", () => {
+        expect(classifyThinkLevel("Review for security vulnerabilities", config)).toBe("high");
+        expect(classifyThinkLevel("Is this code vulnerable to SQL injection?", config)).toBe(
+          "high",
+        );
+        expect(classifyThinkLevel("Security audit needed", config)).toBe("high");
+      });
+
+      it("detects architecture-related messages", () => {
+        expect(classifyThinkLevel("Help me architect this system", config)).toBe("high");
+        expect(classifyThinkLevel("What design pattern should I use?", config)).toBe("high");
+        expect(classifyThinkLevel("System design for a chat app", config)).toBe("high");
+      });
+
+      it("detects large code blocks", () => {
+        const largeCode = "```\n" + "x".repeat(600) + "\n```";
+        expect(classifyThinkLevel(largeCode, config)).toBe("high");
+      });
+    });
+
+    describe("medium complexity detection", () => {
+      const config: AutoThinkConfig = { enabled: true };
+
+      it("detects how-to questions", () => {
+        expect(classifyThinkLevel("How do I implement a linked list?", config)).toBe("medium");
+        expect(classifyThinkLevel("How should I structure this?", config)).toBe("medium");
+        expect(classifyThinkLevel("How can I improve performance?", config)).toBe("medium");
+      });
+
+      it("detects analysis requests", () => {
+        expect(classifyThinkLevel("Explain how this algorithm works", config)).toBe("medium");
+        expect(classifyThinkLevel("Analyze this data structure", config)).toBe("medium");
+        expect(classifyThinkLevel("Compare these two approaches", config)).toBe("medium");
+      });
+
+      it("detects step-by-step requests", () => {
+        expect(classifyThinkLevel("Walk me through this step by step", config)).toBe("medium");
+        expect(classifyThinkLevel("Give me a step-by-step guide", config)).toBe("medium");
+      });
+
+      it("detects implementation requests", () => {
+        expect(classifyThinkLevel("Implement a binary search tree", config)).toBe("medium");
+        expect(classifyThinkLevel("Build a REST API for this", config)).toBe("medium");
+        expect(classifyThinkLevel("Create a function that does X", config)).toBe("medium");
+      });
+
+      it("detects medium code blocks", () => {
+        const mediumCode = "```\n" + "x".repeat(150) + "\n```";
+        expect(classifyThinkLevel(mediumCode, config)).toBe("medium");
+      });
+    });
+
+    describe("low complexity detection", () => {
+      const config: AutoThinkConfig = { enabled: true };
+
+      it("detects simple what/when/where questions", () => {
+        expect(classifyThinkLevel("What is a closure?", config)).toBe("low");
+        expect(classifyThinkLevel("When was Python released?", config)).toBe("low");
+        expect(classifyThinkLevel("Where is the config file?", config)).toBe("low");
+      });
+
+      it("detects translation/conversion requests", () => {
+        expect(classifyThinkLevel("Translate this to Spanish", config)).toBe("low");
+        expect(classifyThinkLevel("Convert this to JSON", config)).toBe("low");
+        expect(classifyThinkLevel("Reformat this code", config)).toBe("low");
+      });
+
+      it("detects simple list requests", () => {
+        expect(classifyThinkLevel("List the programming languages", config)).toBe("low");
+        expect(classifyThinkLevel("Name all the HTTP methods", config)).toBe("low");
+      });
+    });
+
+    describe("length-based fallbacks", () => {
+      const config: AutoThinkConfig = { enabled: true };
+
+      it("returns off for very short messages", () => {
+        expect(classifyThinkLevel("hi", config)).toBe("off");
+        expect(classifyThinkLevel("thanks", config)).toBe("off");
+      });
+
+      it("returns medium for very long messages", () => {
+        const longMessage = "a".repeat(2500);
+        expect(classifyThinkLevel(longMessage, config)).toBe("medium");
+      });
+
+      it("returns minimal for unclassified messages", () => {
+        expect(
+          classifyThinkLevel("Here's some random text that doesn't match patterns", config),
+        ).toBe("minimal");
+      });
+    });
+
+    describe("floor and ceiling", () => {
+      it("respects floor setting", () => {
+        const config: AutoThinkConfig = { enabled: true, floor: "low" };
+        expect(classifyThinkLevel("hi", config)).toBe("low"); // Would be "off" without floor
+      });
+
+      it("respects ceiling setting", () => {
+        const config: AutoThinkConfig = { enabled: true, ceiling: "medium" };
+        expect(classifyThinkLevel("debug this security vulnerability", config)).toBe("medium"); // Would be "high" without ceiling
+      });
+
+      it("respects both floor and ceiling", () => {
+        const config: AutoThinkConfig = { enabled: true, floor: "low", ceiling: "medium" };
+        expect(classifyThinkLevel("hi", config)).toBe("low");
+        expect(classifyThinkLevel("debug this", config)).toBe("medium");
+      });
+    });
+
+    describe("custom rules", () => {
+      it("applies custom rules before built-in patterns", () => {
+        const config: AutoThinkConfig = {
+          enabled: true,
+          rules: [{ match: "newsletter", level: "medium" }],
+        };
+        expect(classifyThinkLevel("Write the newsletter", config)).toBe("medium");
+      });
+
+      it("supports regex patterns in rules", () => {
+        const config: AutoThinkConfig = {
+          enabled: true,
+          rules: [{ match: "urgent|asap|immediately", level: "high" }],
+        };
+        expect(classifyThinkLevel("Fix this ASAP", config)).toBe("high");
+        expect(classifyThinkLevel("This is urgent", config)).toBe("high");
+      });
+
+      it("skips invalid regex patterns", () => {
+        const config: AutoThinkConfig = {
+          enabled: true,
+          rules: [
+            { match: "[invalid(regex", level: "high" },
+            { match: "valid", level: "medium" },
+          ],
+        };
+        expect(classifyThinkLevel("This is valid", config)).toBe("medium");
+      });
+
+      it("first matching rule wins", () => {
+        const config: AutoThinkConfig = {
+          enabled: true,
+          rules: [
+            { match: "first", level: "low" },
+            { match: "first", level: "high" },
+          ],
+        };
+        expect(classifyThinkLevel("first match wins", config)).toBe("low");
+      });
+    });
+  });
+});
diff --git a/src/auto-reply/auto-think.ts b/src/auto-reply/auto-think.ts
new file mode 100644
index 000000000..461087b64
--- /dev/null
+++ b/src/auto-reply/auto-think.ts
@@ -0,0 +1,143 @@
+/**
+ * Auto-think: Heuristic-based thinking level classification.
+ *
+ * Analyzes incoming message content and determines an appropriate thinking level
+ * without requiring explicit user directives.
+ */
+
+import type { ThinkLevel } from "./thinking.js";
+
+export interface AutoThinkConfig {
+  /** Enable auto-think classification */
+  enabled?: boolean;
+  /** Minimum thinking level (floor) */
+  floor?: ThinkLevel;
+  /** Maximum thinking level (ceiling) */
+  ceiling?: ThinkLevel;
+  /** Custom pattern rules (evaluated in order, first match wins) */
+  rules?: AutoThinkRule[];
+}
+
+export interface AutoThinkRule {
+  /** Regex pattern or string to match (case-insensitive) */
+  match: string;
+  /** Thinking level to use when matched */
+  level: ThinkLevel;
+}
+
+const THINK_LEVEL_ORDER: ThinkLevel[] = ["off", "minimal", "low", "medium", "high", "xhigh"];
+
+function clampLevel(level: ThinkLevel, floor?: ThinkLevel, ceiling?: ThinkLevel): ThinkLevel {
+  const levelIdx = THINK_LEVEL_ORDER.indexOf(level);
+  const floorIdx = floor ? THINK_LEVEL_ORDER.indexOf(floor) : 0;
+  const ceilingIdx = ceiling ? THINK_LEVEL_ORDER.indexOf(ceiling) : THINK_LEVEL_ORDER.length - 1;
+
+  if (levelIdx < floorIdx) return floor!;
+  if (levelIdx > ceilingIdx) return ceiling!;
+  return level;
+}
+
+/**
+ * High-complexity patterns that warrant deeper thinking.
+ * Debug, security, architecture, multi-step problems.
+ */
+const HIGH_PATTERNS = [
+  /\b(debug|debugging|debugger)\b/i,
+  /\b(error|exception|traceback|stack\s*trace)\b/i,
+  /\b(security|vulnerable|vulnerability|exploit|cve|audit)\b/i,
+  /\b(architect|architecture|design\s+pattern|system\s+design)\b/i,
+  /\b(refactor|rewrite|restructure)\b/i,
+  /\b(optimize|optimization|performance\s+issue)\b/i,
+  /\b(race\s+condition|deadlock|memory\s+leak)\b/i,
+  /\b(review|code\s+review|pr\s+review)\b/i,
+  /```[\s\S]{500,}/i, // Large code blocks (500+ chars)
+];
+
+/**
+ * Medium-complexity patterns that benefit from structured thinking.
+ * Multi-step tasks, comparisons, analysis, implementation.
+ */
+const MEDIUM_PATTERNS = [
+  /\b(how\s+(do|would|should|can|to))\b/i,
+  /\b(explain|analyze|compare|contrast|evaluate)\b/i,
+  /\b(step[\s-]?by[\s-]?step|walkthrough|guide\s+me)\b/i,
+  /\b(implement|build|create|develop|write)\b/i,
+  /\b(plan|strategy|approach|roadmap)\b/i,
+  /\b(trade[\s-]?off|pros?\s+and\s+cons?|advantages?\s+and\s+disadvantages?)\b/i,
+  /\b(multiple|several|various|different)\s+(ways?|options?|approaches?|methods?)\b/i,
+  /```[\s\S]{100,}/i, // Medium code blocks (100+ chars)
+];
+
+/**
+ * Low-complexity patterns that need minimal thinking.
+ * Simple lookups, translations, formatting.
+ */
+const LOW_PATTERNS = [
+  /^(what|when|where|who|which)\s+(is|are|was|were)\b/i,
+  /\b(translate|convert|format|reformat)\b/i,
+  /\b(list|enumerate|name)\s+(the|all|some)\b/i,
+  /\b(define|definition\s+of)\b/i,
+];
+
+/**
+ * Classify the thinking level for a message using heuristics.
+ *
+ * @param message - The user's message content
+ * @param config - Optional auto-think configuration
+ * @returns The classified thinking level, or undefined if auto-think is disabled
+ */
+export function classifyThinkLevel(
+  message: string,
+  config?: AutoThinkConfig,
+): ThinkLevel | undefined {
+  if (!config?.enabled) return undefined;
+  if (!message?.trim()) return undefined;
+
+  const text = message.trim();
+
+  // Check custom rules first (if any)
+  if (config.rules?.length) {
+    for (const rule of config.rules) {
+      try {
+        const pattern = new RegExp(rule.match, "i");
+        if (pattern.test(text)) {
+          return clampLevel(rule.level, config.floor, config.ceiling);
+        }
+      } catch {
+        // Invalid regex, skip
+      }
+    }
+  }
+
+  // Check for high complexity signals
+  for (const pattern of HIGH_PATTERNS) {
+    if (pattern.test(text)) {
+      return clampLevel("high", config.floor, config.ceiling);
+    }
+  }
+
+  // Check for medium complexity signals
+  for (const pattern of MEDIUM_PATTERNS) {
+    if (pattern.test(text)) {
+      return clampLevel("medium", config.floor, config.ceiling);
+    }
+  }
+
+  // Check for low complexity signals
+  for (const pattern of LOW_PATTERNS) {
+    if (pattern.test(text)) {
+      return clampLevel("low", config.floor, config.ceiling);
+    }
+  }
+
+  // Length-based heuristics as fallback
+  if (text.length > 2000) {
+    return clampLevel("medium", config.floor, config.ceiling);
+  }
+  if (text.length < 50) {
+    return clampLevel("off", config.floor, config.ceiling);
+  }
+
+  // Default: minimal thinking for unclassified messages
+  return clampLevel("minimal", config.floor, config.ceiling);
+}
diff --git a/src/auto-reply/reply/get-reply-run.ts b/src/auto-reply/reply/get-reply-run.ts
index ba96023ce..d112cc250 100644
--- a/src/auto-reply/reply/get-reply-run.ts
+++ b/src/auto-reply/reply/get-reply-run.ts
@@ -21,6 +21,7 @@ import { isReasoningTagProvider } from "../../utils/provider-utils.js";
 import { hasControlCommand } from "../command-detection.js";
 import { buildInboundMediaNote } from "../media-note.js";
 import type { MsgContext, TemplateContext } from "../templating.js";
+import { classifyThinkLevel } from "../auto-think.js";
 import {
   type ElevatedLevel,
   formatXHighModelHint,
@@ -261,6 +262,14 @@ export async function runPreparedReply(
       prefixedCommandBody = parts.slice(1).join(" ").trim();
     }
   }
+  // Auto-think: classify thinking level based on message content
+  if (!resolvedThinkLevel && agentCfg?.autoThink?.enabled) {
+    const autoLevel = classifyThinkLevel(prefixedCommandBody, agentCfg.autoThink);
+    if (autoLevel) {
+      resolvedThinkLevel = autoLevel;
+      logVerbose(`Auto-think classified message as "${autoLevel}"`);
+    }
+  }
   if (!resolvedThinkLevel) {
     resolvedThinkLevel = await modelState.resolveDefaultThinkingLevel();
   }
diff --git a/src/config/types.agent-defaults.ts b/src/config/types.agent-defaults.ts
index 9c6ce0211..3508a4614 100644
--- a/src/config/types.agent-defaults.ts
+++ b/src/config/types.agent-defaults.ts
@@ -134,6 +134,26 @@ export type AgentDefaultsConfig = {
   memorySearch?: MemorySearchConfig;
   /** Default thinking level when no /think directive is present. */
   thinkingDefault?: "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
+  /**
+   * Auto-think: Automatically classify thinking level based on message content.
+   * When enabled, analyzes the incoming message using heuristics to determine
+   * the appropriate thinking level without requiring explicit /think directives.
+   */
+  autoThink?: {
+    /** Enable auto-think classification. */
+    enabled?: boolean;
+    /** Minimum thinking level (floor). */
+    floor?: "off" | "minimal" | "low" | "medium" | "high";
+    /** Maximum thinking level (ceiling). */
+    ceiling?: "off" | "minimal" | "low" | "medium" | "high";
+    /** Custom pattern rules (evaluated in order, first match wins). */
+    rules?: Array<{
+      /** Regex pattern or string to match (case-insensitive). */
+      match: string;
+      /** Thinking level to use when matched. */
+      level: "off" | "minimal" | "low" | "medium" | "high";
+    }>;
+  };
   /** Default verbose level when no /verbose directive is present. */
   verboseDefault?: "off" | "on" | "full";
   /** Default elevated level when no /elevated directive is present. */
diff --git a/src/config/zod-schema.agent-defaults.ts b/src/config/zod-schema.agent-defaults.ts
index a849078ed..f17f4ea04 100644
--- a/src/config/zod-schema.agent-defaults.ts
+++ b/src/config/zod-schema.agent-defaults.ts
@@ -113,6 +113,46 @@ export const AgentDefaultsSchema = z
         z.literal("xhigh"),
       ])
       .optional(),
+    autoThink: z
+      .object({
+        enabled: z.boolean().optional(),
+        floor: z
+          .union([
+            z.literal("off"),
+            z.literal("minimal"),
+            z.literal("low"),
+            z.literal("medium"),
+            z.literal("high"),
+          ])
+          .optional(),
+        ceiling: z
+          .union([
+            z.literal("off"),
+            z.literal("minimal"),
+            z.literal("low"),
+            z.literal("medium"),
+            z.literal("high"),
+          ])
+          .optional(),
+        rules: z
+          .array(
+            z
+              .object({
+                match: z.string(),
+                level: z.union([
+                  z.literal("off"),
+                  z.literal("minimal"),
+                  z.literal("low"),
+                  z.literal("medium"),
+                  z.literal("high"),
+                ]),
+              })
+              .strict(),
+          )
+          .optional(),
+      })
+      .strict()
+      .optional(),
     verboseDefault: z.union([z.literal("off"), z.literal("on"), z.literal("full")]).optional(),
     elevatedDefault: z
       .union([z.literal("off"), z.literal("on"), z.literal("ask"), z.literal("full")])