diff --git a/src/agents/multimodal-looker.test.ts b/src/agents/multimodal-looker.test.ts
new file mode 100644
index 000000000..f2f282644
--- /dev/null
+++ b/src/agents/multimodal-looker.test.ts
@@ -0,0 +1,27 @@
+import { describe, test, expect } from "bun:test"
+import { createMultimodalLookerAgent } from "./multimodal-looker"
+
+describe("createMultimodalLookerAgent", () => {
+  test("prompt explicitly enumerates the agent's available tools to prevent death loop on small VL models", () => {
+    // given
+    const agent = createMultimodalLookerAgent("openai/gpt-5-nano")
+
+    // when
+    const prompt = typeof agent.prompt === "string" ? agent.prompt : ""
+
+    // then
+    expect(prompt).toMatch(/available tools/i)
+    expect(prompt).toContain("read")
+  })
+
+  test("prompt instructs the agent never to call other tools", () => {
+    // given
+    const agent = createMultimodalLookerAgent("openai/gpt-5-nano")
+
+    // when
+    const prompt = typeof agent.prompt === "string" ? agent.prompt : ""
+
+    // then
+    expect(prompt.toLowerCase()).toContain("never")
+  })
+})
diff --git a/src/agents/multimodal-looker.ts b/src/agents/multimodal-looker.ts
index 2d89e422b..2a4b80431 100644
--- a/src/agents/multimodal-looker.ts
+++ b/src/agents/multimodal-looker.ts
@@ -23,6 +23,8 @@ export function createMultimodalLookerAgent(model: string): AgentConfig {
     ...restrictions,
     prompt: `You interpret media files that cannot be read as plain text.
 
+Your only available tools are 'read' and 'call_omo_agent'. Always use 'read' to load the file first, then analyze the returned content. Never attempt to call any other tool.
+
 Your job: examine the attached file and extract ONLY what was requested.
 
 When to use you:
diff --git a/src/features/claude-code-agent-loader/claude-model-mapper.ts b/src/features/claude-code-agent-loader/claude-model-mapper.ts
index 7737de5f7..f0c0f9a86 100644
--- a/src/features/claude-code-agent-loader/claude-model-mapper.ts
+++ b/src/features/claude-code-agent-loader/claude-model-mapper.ts
@@ -11,6 +11,7 @@ const CLAUDE_CODE_ALIAS_MAP = new Map<string, string>([
 
 function mapClaudeModelString(model: string | undefined): string | undefined {
   if (!model) return undefined
+  if (typeof model !== "string") return undefined
 
   const trimmed = model.trim()
   if (trimmed.length === 0) return undefined
diff --git a/src/plugin-interface.ts b/src/plugin-interface.ts
index 5bcc0c364..9aa982808 100644
--- a/src/plugin-interface.ts
+++ b/src/plugin-interface.ts
@@ -8,6 +8,7 @@ import { createCommandExecuteBeforeHandler } from "./plugin/command-execute-befo
 import { createMessagesTransformHandler } from "./plugin/messages-transform"
 import { createSystemTransformHandler } from "./plugin/system-transform"
 import { createEventHandler } from "./plugin/event"
+import { createToolDefinitionHandler } from "./plugin/tool-definition"
 import { createToolExecuteAfterHandler } from "./plugin/tool-execute-after"
 import { createToolExecuteBeforeHandler } from "./plugin/tool-execute-before"
 
@@ -70,6 +71,10 @@ export function createPluginInterface(args: {
       hooks,
     }),
 
+    "tool.definition": createToolDefinitionHandler({
+      hooks,
+    }),
+
     "tool.execute.before": createToolExecuteBeforeHandler({
       ctx,
       hooks,
diff --git a/src/plugin/tool-definition.test.ts b/src/plugin/tool-definition.test.ts
new file mode 100644
index 000000000..23c5167c6
--- /dev/null
+++ b/src/plugin/tool-definition.test.ts
@@ -0,0 +1,63 @@
+import { describe, it, expect } from "bun:test"
+import { createToolDefinitionHandler } from "./tool-definition"
+import { createTodoDescriptionOverrideHook } from "../hooks/todo-description-override/hook"
+import { TODOWRITE_DESCRIPTION } from "../hooks/todo-description-override/description"
+import type { CreatedHooks } from "../create-hooks"
+
+function buildHooks(overrides: Partial<CreatedHooks> = {}): CreatedHooks {
+  return overrides as CreatedHooks
+}
+
+describe("createToolDefinitionHandler (regression for #3705)", () => {
+  describe("#given todoDescriptionOverride hook is registered", () => {
+    describe("#when the tool.definition handler runs for the todowrite tool", () => {
+      it("#then forwards to the hook and rewrites the description", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: createTodoDescriptionOverrideHook() }),
+        })
+        const output = { description: "opencode core default", parameters: {} }
+
+        //#when
+        await handler({ toolID: "todowrite" }, output)
+
+        //#then
+        expect(output.description).toBe(TODOWRITE_DESCRIPTION)
+      })
+    })
+
+    describe("#when the tool.definition handler runs for any other tool", () => {
+      it("#then leaves the description untouched", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: createTodoDescriptionOverrideHook() }),
+        })
+        const output = { description: "bash native description", parameters: {} }
+
+        //#when
+        await handler({ toolID: "bash" }, output)
+
+        //#then
+        expect(output.description).toBe("bash native description")
+      })
+    })
+  })
+
+  describe("#given todoDescriptionOverride hook is disabled (null)", () => {
+    describe("#when the tool.definition handler runs for todowrite", () => {
+      it("#then is a no-op", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: null }),
+        })
+        const output = { description: "opencode default kept", parameters: {} }
+
+        //#when
+        await handler({ toolID: "todowrite" }, output)
+
+        //#then
+        expect(output.description).toBe("opencode default kept")
+      })
+    })
+  })
+})
diff --git a/src/plugin/tool-definition.ts b/src/plugin/tool-definition.ts
new file mode 100644
index 000000000..59f794eff
--- /dev/null
+++ b/src/plugin/tool-definition.ts
@@ -0,0 +1,16 @@
+import type { CreatedHooks } from "../create-hooks"
+
+export function createToolDefinitionHandler(args: {
+  hooks: CreatedHooks
+}): (
+  input: { toolID: string },
+  output: { description: string; parameters: unknown },
+) => Promise<void> {
+  const { hooks } = args
+  return async (input, output) => {
+    const overrideHook = hooks.todoDescriptionOverride
+    if (overrideHook) {
+      await overrideHook["tool.definition"](input, output)
+    }
+  }
+}
diff --git a/src/shared/fallback-chain-from-models.test.ts b/src/shared/fallback-chain-from-models.test.ts
index 16690d12c..94c6851ee 100644
--- a/src/shared/fallback-chain-from-models.test.ts
+++ b/src/shared/fallback-chain-from-models.test.ts
@@ -395,3 +395,65 @@ describe("findMostSpecificFallbackEntry", () => {
     })
   })
 })
+
+// Regression: type-guard against non-string model field (issue #4145).
+// Crash signature was `model.trim is not a function` aborting session.processor
+// for every provider when a caller forwarded a FallbackModelObject where a
+// plain string was expected.
+describe("parseFallbackModelEntry: non-string input (issue #4145)", () => {
+  test("returns undefined when caller forwards an object instead of a string", () => {
+    //#given
+    const wrong = { model: "anthropic/claude-opus-4-7", variant: "high" } as unknown as string
+
+    //#when
+    const parsed = parseFallbackModelEntry(wrong, "anthropic")
+
+    //#then: must not throw, must reject the malformed entry
+    expect(parsed).toBeUndefined()
+  })
+
+  test("returns undefined for null and undefined", () => {
+    //#given
+    const nullInput = null as unknown as string
+    const undefinedInput = undefined as unknown as string
+
+    //#when / #then
+    expect(parseFallbackModelEntry(nullInput, "anthropic")).toBeUndefined()
+    expect(parseFallbackModelEntry(undefinedInput, "anthropic")).toBeUndefined()
+  })
+
+  test("parseFallbackModelObjectEntry returns undefined when nested model field is non-string", () => {
+    //#given: FallbackModelObject whose .model was somehow forwarded as a nested
+    //object instead of a flat string (issue #4145 reproduction).
+    const malformedObject = {
+      model: ({ model: "claude-opus-4-7" } as unknown) as string,
+      variant: "high",
+    }
+
+    //#when
+    const parsed = parseFallbackModelObjectEntry(malformedObject, "anthropic")
+
+    //#then: must not throw, must reject the malformed entry
+    expect(parsed).toBeUndefined()
+  })
+
+  test("buildFallbackChainFromModels skips entries whose model is a number", () => {
+    //#given
+    const fallbackModels = [
+      "openai/gpt-5.5",
+      (42 as unknown) as string,
+    ]
+
+    //#when
+    const chain = buildFallbackChainFromModels(fallbackModels, "openai")
+
+    //#then: only the valid string survives, the number is dropped without crashing
+    expect(chain).toEqual([
+      {
+        providers: ["openai"],
+        model: "gpt-5.5",
+        variant: undefined,
+      },
+    ])
+  })
+})
diff --git a/src/shared/fallback-chain-from-models.ts b/src/shared/fallback-chain-from-models.ts
index 12e615d76..248250e22 100644
--- a/src/shared/fallback-chain-from-models.ts
+++ b/src/shared/fallback-chain-from-models.ts
@@ -4,6 +4,9 @@ import { normalizeFallbackModels } from "./model-resolver"
 import { KNOWN_VARIANTS } from "./known-variants"
 
 function parseVariantFromModel(rawModel: string): { modelID: string; variant?: string } {
+  if (typeof rawModel !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModel = rawModel.trim()
   if (!trimmedModel) {
     return { modelID: "" }
@@ -33,6 +36,7 @@ export function parseFallbackModelEntry(
   contextProviderID: string | undefined,
   defaultProviderID = "opencode",
 ): FallbackEntry | undefined {
+  if (typeof model !== "string") return undefined
   const trimmed = model.trim()
   if (!trimmed) return undefined
 
diff --git a/src/shared/model-string-parser.ts b/src/shared/model-string-parser.ts
index 220bbd880..b10cd989d 100644
--- a/src/shared/model-string-parser.ts
+++ b/src/shared/model-string-parser.ts
@@ -11,6 +11,9 @@ const KNOWN_VARIANTS = new Set([
 ])
 
 export function parseVariantFromModelID(rawModelID: string): { modelID: string; variant?: string } {
+  if (typeof rawModelID !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModelID = rawModelID.trim()
   if (!trimmedModelID) {
     return { modelID: "" }
@@ -38,6 +41,7 @@ export function parseVariantFromModelID(rawModelID: string): { modelID: string;
 export function parseModelString(
   model: string,
 ): { providerID: string; modelID: string; variant?: string } | undefined {
+  if (typeof model !== "string") return undefined
   const trimmedModel = model.trim()
   if (!trimmedModel) return undefined
 
diff --git a/src/tools/delegate-task/model-string-parser.ts b/src/tools/delegate-task/model-string-parser.ts
index 820bb3cc3..a932ccd50 100644
--- a/src/tools/delegate-task/model-string-parser.ts
+++ b/src/tools/delegate-task/model-string-parser.ts
@@ -11,6 +11,9 @@ const KNOWN_VARIANTS = new Set([
 ])
 
 export function parseVariantFromModelID(rawModelID: string): { modelID: string; variant?: string } {
+  if (typeof rawModelID !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModelID = rawModelID.trim()
   if (!trimmedModelID) {
     return { modelID: "" }
@@ -38,6 +41,7 @@ export function parseVariantFromModelID(rawModelID: string): { modelID: string;
 export function parseModelString(
   model: string,
 ): { providerID: string; modelID: string; variant?: string } | undefined {
+  if (typeof model !== "string") return undefined
   const trimmedModel = model.trim()
   if (!trimmedModel) return undefined