diff --git a/docs/v2-native-core.md b/docs/v2-native-core.md
index 1585f56c..c74373eb 100644
--- a/docs/v2-native-core.md
+++ b/docs/v2-native-core.md
@@ -201,7 +201,11 @@ the history cannot place stays unexposed, as the RFD forbids guessing a parent.
   that does not support it. A model switch that the adapter did not make (`PostModelSwitch`, such
   as a typed `/model`, or a refusal fallback) updates the options. Claude Code's
   `fast_mode_state` (on `init` and on results of prompts) updates the Fast mode option, with a
-  notice when it turns off a Fast mode the user had on.
+  notice when it turns off a Fast mode the user had on. A value that names no entry is rejected
+  with -32602 ("Invalid value for config option model: not-a-model") and changes nothing, as on
+  the bridge and v1, which share `configOptionValue`. A model alias names an entry only by whole
+  words (`resolveModelPreference`): `opus`, `opusplan`, `claude-sonnet-5[1m]` and provider ids
+  such as `us.anthropic.claude-sonnet-5-v1:0` do, `not-a-model`, `son` and `foo-sonnet` do not.
 - `src/v2-native/clear-context.ts`: ExitPlanMode's "Yes, clear context" option, offered to the
   root's plan as on the bridge. The answer interrupts the cycle, and the turn stays open: the
   session sends Claude Code `/clear`, then sets the chosen mode and sends "Implement the following
diff --git a/src/session-model.ts b/src/session-model.ts
index f338afad..b2fdbf5d 100644
--- a/src/session-model.ts
+++ b/src/session-model.ts
@@ -84,35 +84,54 @@ function versionedModelDisplayName(model: ModelInfo): string {
   return `${withoutContext} ${version}${contextSuffix[0]}`;
 }
 
-function tokenizeModelPreference(model: string): { tokens: string[]; contextHint?: string } {
-  const lower = model.trim().toLowerCase();
-  const contextHint = lower.match(MODEL_CONTEXT_HINT_PATTERN)?.[1]?.toLowerCase();
-  const tokens = lower
-    .replace(MODEL_CONTEXT_HINT_PATTERN, " $1 ")
+function modelWords(value: string): string[] {
+  return value
+    .toLowerCase()
     .split(/[^a-z0-9]+/)
-    .filter(Boolean)
-    .map((token) => {
-      if (token === "opusplan") return "opus";
-      if (token === "best" || token === "default") return "";
-      return token;
-    })
-    .filter((token) => token && token !== "claude")
-    .filter((token) => /[a-z]/.test(token) || token.endsWith("m"));
-  return { tokens, contextHint };
+    .filter(Boolean);
 }
 
-function scoreModelMatch(model: ModelInfo, tokens: string[], contextHint?: string): number {
-  const haystack = `${model.value} ${model.displayName}`.toLowerCase();
-  let score = 0;
-  let nonHintMatched = false;
-  for (const token of tokens) {
-    if (!haystack.includes(token)) continue;
-    if (token !== contextHint) nonHintMatched = true;
-    score += token === contextHint ? 3 : 1;
+/** Whether `needle`'s words appear in `haystack`'s words, adjacent and in
+ *  order: "opus" in "claude-opus-4-5", but not "son" in "sonnet" or "a" in
+ *  "Default (recommended)". */
+function containsWords(haystack: string, needle: string): boolean {
+  const words = modelWords(haystack);
+  const wanted = modelWords(needle);
+  if (wanted.length === 0) return false;
+  for (let start = 0; start + wanted.length <= words.length; start++) {
+    if (wanted.every((word, offset) => words[start + offset] === word)) return true;
   }
-  return contextHint && !nonHintMatched ? 0 : score;
+  return false;
 }
 
+/** The words of `preference` that name a model: without what
+ *  `normalizeModelId` drops (provider prefixes, dates, `-v1:0`, `-fast`,
+ *  context hints), version numbers, "claude", and "best"/"default".
+ *  "us.anthropic.claude-sonnet-4-5-v1:0" gives ["sonnet"], "opusplan" gives
+ *  ["opus"]. */
+function modelPreferenceTokens(preference: string): string[] {
+  return modelWords(normalizeModelId(preference))
+    .map((token) => (token === "opusplan" ? "opus" : token))
+    .filter((token) => /[a-z]/.test(token) && !["claude", "best", "default"].includes(token));
+}
+
+/** Whether every one of `tokens` is a word of `model`'s value or display name. */
+function modelCoversTokens(model: ModelInfo, tokens: string[]): boolean {
+  const words = new Set(modelWords(`${model.value} ${model.displayName}`));
+  return tokens.length > 0 && tokens.every((token) => words.has(token));
+}
+
+/**
+ * The entry of `models` that `preference` names, or null when it names none.
+ * In order: an entry whose value, display name or `resolvedModel` equals it
+ * (in either context-hint spelling); an entry whose value or display name
+ * contains its words ("opus" for "claude-opus-4-5"), or whose value it
+ * contains while every model word of the preference is the entry's
+ * ("sonnet" for "us.anthropic.claude-sonnet-5-v1:0"); then any entry with all
+ * of its model words, preferring one with the same context hint ("sonnet[1m]"
+ * for "claude-sonnet-5-1m"). The fuzzy tiers need a compatible version, and
+ * words match whole: "not-a-model", "son" and "foo-sonnet" name no entry.
+ */
 export function resolveModelPreference(models: ModelInfo[], preference: string): ModelInfo | null {
   const trimmed = preference.trim();
   if (!trimmed) return null;
@@ -134,28 +153,26 @@ export function resolveModelPreference(models: ModelInfo[], preference: string):
   if (resolvedMatch) return resolvedMatch;
 
   const preferenceHint = contextHintOf(trimmed);
+  const tokens = modelPreferenceTokens(trimmed);
   const includesMatch = models.find((model) => {
     if (!modelVersionsCompatible(trimmed, model)) return false;
     if (contextHintOf(model.value) !== preferenceHint) return false;
-    const value = model.value.toLowerCase();
-    const display = model.displayName.toLowerCase();
-    return value.includes(lower) || display.includes(lower) || lower.includes(value);
+    return (
+      containsWords(model.value, trimmed) ||
+      containsWords(model.displayName, trimmed) ||
+      (containsWords(trimmed, model.value) && modelCoversTokens(model, tokens))
+    );
   });
   if (includesMatch) return includesMatch;
 
-  const { tokens, contextHint } = tokenizeModelPreference(trimmed);
-  if (tokens.length === 0) return null;
-  let bestMatch: ModelInfo | null = null;
-  let bestScore = 0;
-  for (const model of models) {
-    if (!modelVersionsCompatible(trimmed, model)) continue;
-    const score = scoreModelMatch(model, tokens, contextHint);
-    if (score > 0 && (!bestMatch || score > bestScore)) {
-      bestMatch = model;
-      bestScore = score;
-    }
-  }
-  return bestMatch;
+  const candidates = models.filter(
+    (model) => modelVersionsCompatible(trimmed, model) && modelCoversTokens(model, tokens),
+  );
+  return (
+    candidates.find((model) => contextHintOf(model.value) === preferenceHint) ??
+    candidates[0] ??
+    null
+  );
 }
 
 export function matchResumedModel(models: ModelInfo[], liveModel: string): ModelInfo {
diff --git a/src/tests/acp-v2.test.ts b/src/tests/acp-v2.test.ts
index d8f4278c..15c73243 100644
--- a/src/tests/acp-v2.test.ts
+++ b/src/tests/acp-v2.test.ts
@@ -963,21 +963,67 @@ describe("ACP v2 sessions", () => {
     });
 
     it.each([
-      ["a select value for a boolean option", { configId: "fast", type: "id", value: "on" }],
-      ["a boolean for a select option", { configId: "effort", type: "boolean", value: true }],
-      ["a level the model does not offer", { configId: "effort", type: "id", value: "max" }],
-    ] as const)("rejects %s", async (_case, request) => {
-      const client = v2Client();
-      await client.app.connectWith(connectRouter(), async (agent) => {
-        await initializeV2(agent);
-        const { sessionId } = await agent.request(v2.methods.agent.session.new, { cwd });
-        await expect(
-          agent.request(v2.methods.agent.session.setConfigOption, { sessionId, ...request }),
-        ).rejects.toMatchObject({ code: -32602 });
-        await client.authUpdate(1);
-      });
-      expect(sdk.flagSettings).toEqual([]);
-    });
+      [
+        "a select value for a boolean option",
+        { configId: "fast", type: "id", value: "on" },
+        'Config option fast takes a boolean value, but received: "on"',
+      ],
+      [
+        "a boolean for a select option",
+        { configId: "effort", type: "boolean", value: true },
+        "Config option effort takes a string value, but received: true",
+      ],
+      [
+        "a level the model does not offer",
+        { configId: "effort", type: "id", value: "max" },
+        "Invalid value for config option effort: max",
+      ],
+      // "a" is a substring of "claude": a substring match would pick Opus.
+      [
+        "a model the session does not offer",
+        { configId: "model", type: "id", value: "not-a-model" },
+        "Invalid value for config option model: not-a-model",
+      ],
+      [
+        "a mode the session does not offer",
+        { configId: "mode", type: "id", value: "not-a-mode" },
+        "Invalid value for config option mode: not-a-mode",
+      ],
+      [
+        "an agent the session does not offer",
+        { configId: "agent", type: "id", value: "not-an-agent" },
+        "Invalid value for config option agent: not-an-agent",
+      ],
+    ] as const)(
+      "rejects %s and leaves the options as they were",
+      async (_case, request, message) => {
+        const models: Array<string | undefined> = [];
+        sdk.setModel = async (model) => {
+          models.push(model);
+        };
+        const client = v2Client();
+        const { created, listed } = await client.app.connectWith(connectRouter(), async (agent) => {
+          await initializeV2(agent);
+          const created = await agent.request(v2.methods.agent.session.new, { cwd });
+          const { sessionId } = created;
+          await expect(
+            agent.request(v2.methods.agent.session.setConfigOption, { sessionId, ...request }),
+          ).rejects.toMatchObject({ code: -32602, message: expect.stringContaining(message) });
+          // Setting a valid value again returns every option's current value.
+          const listed = await agent.request(v2.methods.agent.session.setConfigOption, {
+            sessionId,
+            configId: "agent",
+            type: "id",
+            value: "default",
+          });
+          await client.authUpdate(1);
+          return { created, listed };
+        });
+        expect(sdk.flagSettings).toEqual([{ agent: null }]);
+        expect(models).toEqual([]);
+        expect(values(listed.configOptions)).toEqual(values(created.configOptions));
+      },
+    );
 
     it("rebuilds the options for a model switch, and clears an effort the new model lacks", async () => {
       const client = v2Client();
diff --git a/src/tests/model-resolution.test.ts b/src/tests/model-resolution.test.ts
index 95556df2..088585f6 100644
--- a/src/tests/model-resolution.test.ts
+++ b/src/tests/model-resolution.test.ts
@@ -73,8 +73,26 @@ describe("resolveModelPreference - resolvedModel matching", () => {
     expect(resolveModelPreference(LIVE_SHAPED_MODELS, "claude-opus-4-8")?.value).toBe("opus[1m]");
   });
 
-  it("returns null for a preference that matches nothing", () => {
-    expect(resolveModelPreference(LIVE_SHAPED_MODELS, "claude-gpt-99")).toBeNull();
+  // Words match whole: "a" is inside "Default", "son" inside "sonnet".
+  it.each(["claude-gpt-99", "not-a-model", "a", "son", "foo-sonnet", "not-default-model", "best"])(
+    "returns null for %s, which names no model",
+    (preference) => {
+      expect(resolveModelPreference(LIVE_SHAPED_MODELS, preference)).toBeNull();
+    },
+  );
+
+  it.each([
+    ["us.anthropic.claude-sonnet-5-v1:0", "sonnet"],
+    ["global.anthropic.claude-haiku-4-5-20251001-v1:0", "haiku"],
+    ["anthropic/claude-sonnet-5:batch", "sonnet"],
+    ["claude-opus-4-8@20260101", "opus[1m]"],
+    ["claude-sonnet-5-fast", "sonnet"],
+    ["claude-haiku-4-5", "haiku"],
+    ["opusplan", "opus[1m]"],
+    ["opus", "opus[1m]"],
+    ["Sonnet 5", "sonnet"],
+  ])("resolves the spelling %s to %s", (preference, value) => {
+    expect(resolveModelPreference(LIVE_SHAPED_MODELS, preference)?.value).toBe(value);
   });
 
   it("matches a '-1m' id-suffix preference against a '[1m]'-spelled resolvedModel", () => {
diff --git a/src/tests/session-config-options.test.ts b/src/tests/session-config-options.test.ts
index 71795041..9c48623e 100644
--- a/src/tests/session-config-options.test.ts
+++ b/src/tests/session-config-options.test.ts
@@ -330,18 +330,28 @@ describe("session config options", () => {
       expect(modelOption?.currentValue).toBe("claude-sonnet-4-6");
     });
 
-    it("throws for completely invalid model value", async () => {
-      await expect(
-        agent.setSessionConfigOption({
-          sessionId: SESSION_ID,
-          configId: "model",
-          value: "gpt-4",
-        }),
-      ).rejects.toMatchObject({
-        code: -32602,
-        message: expect.stringContaining("Invalid value for config option model: gpt-4"),
-      });
-    });
+    // "a" is a substring of "Claude", "son" of "sonnet": only whole words match.
+    it.each(["gpt-4", "not-a-model", "son", "foo-sonnet"])(
+      "rejects the model %s and leaves the model as it was",
+      async (value) => {
+        await expect(
+          agent.setSessionConfigOption({ sessionId: SESSION_ID, configId: "model", value }),
+        ).rejects.toMatchObject({
+          code: -32602,
+          message: expect.stringContaining(`Invalid value for config option model: ${value}`),
+        });
+
+        expect(setModelSpy).not.toHaveBeenCalled();
+        const session = (
+          agent as unknown as {
+            sessions: Record<string, { configOptions: typeof MOCK_CONFIG_OPTIONS }>;
+          }
+        ).sessions[SESSION_ID];
+        expect(session.configOptions.find((o) => o.id === "model")?.currentValue).toBe(
+          "claude-opus-4-5",
+        );
+      },
+    );
 
     it("rejects a non-string value for a select option with invalid params", async () => {
       await expect(