fix(shared,delegate-task,claude-code-agent-loader): guard model parsers against non-string input (fixes #4145)

After the 4.2.0 unified-dispatch refactor (a42f894f / df198d8b / fee515c5 / 989ab717 / dd3fecaf / 1bbe065c / 12bd6580), at least one caller in the new prompt-async-gate path forwards a FallbackModelObject (or some other non-string shape) into parsers that statically claim 'model: string'. The downstream .trim() call then throws 'model.trim is not a function', which rejects the session.processor promise and surfaces as 'Aborted process' + UI 'interrupted'. The issue (#4145) reports this aborts 90% of subagent dispatches across every provider on 4.2.0 + opencode 1.15.4.

This patch adds a 'typeof x !== "string"' runtime guard at the four parser entrypoints called from the dispatch path:

- src/shared/fallback-chain-from-models.ts :: parseVariantFromModel, parseFallbackModelEntry

- src/tools/delegate-task/model-string-parser.ts :: parseVariantFromModelID, parseModelString

- src/shared/model-string-parser.ts (duplicate file with same API) :: parseVariantFromModelID, parseModelString

- src/features/claude-code-agent-loader/claude-model-mapper.ts :: mapClaudeModelString

Each parser now returns undefined / { modelID: "" } for non-string input instead of throwing. This unblocks subagent dispatch and leaves the underlying caller bug for a follow-up.

Regression coverage: three new tests in src/shared/fallback-chain-from-models.test.ts pin the non-string behavior (object, null/undefined, number). Existing 38 tests still pass. Total: 41/41 green, typecheck clean.

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

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,
+      },
+    ])
+  })
+})

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
 

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
 

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