diff --git a/plugins/summarize/plugin.toml b/plugins/summarize/plugin.toml
index 7f633cd92..d9db261d1 100644
--- a/plugins/summarize/plugin.toml
+++ b/plugins/summarize/plugin.toml
@@ -19,6 +19,7 @@ default = "gemini"
 
 [options.model]
 type = "string"
+"x-reset-by" = "provider"
 title = "Model"
 description = "The model name sent to the provider. Unset uses the provider's default."
 
@@ -35,11 +36,13 @@ default = 20
 
 [options.api_key]
 type = "string"
+"x-reset-by" = "provider"
 title = "API key"
 description = "Sent to the selected provider. The provider's variable in the environment is used when this is unset. Optional for `openai` with a custom `endpoint`."
 
 [options.endpoint]
 type = "string"
+"x-reset-by" = "provider"
 title = "Endpoint URL"
 description = "Override the provider's base URL, for proxies, OpenAI-compatible servers (for example `https://openrouter.ai/api/v1` or `http://localhost:11434/v1`) and tests. Empty uses the provider's default."
 
@@ -68,7 +71,7 @@ default = 3
 [options.system_prompt]
 type = "string"
 title = "System prompt"
-description = "The system instruction sent with every request; diffr builds each request's message from the file's numbered lines and the folds to summarize. Unset uses the default: https://github.com/devdotfast/diffr/blob/main/plugins/summarize/plugin.toml#L72-L78"
+description = "The system instruction sent with every request; diffr builds each request's message from the file's numbered lines and the folds to summarize. Unset uses the default: https://github.com/devdotfast/diffr/blob/main/plugins/summarize/plugin.toml#L75-L81"
 default = """\
 For each listed fold, rewrite that function body as short \
 pseudocode. Keep the names. No prose, no comments, no code fences. Use as few lines as \
@@ -94,6 +97,7 @@ default = 20
 # screens do not edit an object option, so this is a file-only override.
 [options.provider_details]
 type = "object"
+"x-reset-by" = "provider"
 title = "Provider details"
 description = "The provider's default endpoint, the environment variables that hold its key, and whether a custom endpoint may go without a key."
 required = ["endpoint", "key_variables", "keyless_custom_endpoint"]
diff --git a/src/config/prune.rs b/src/config/prune.rs
index d69cd55c6..9c790f44e 100644
--- a/src/config/prune.rs
+++ b/src/config/prune.rs
@@ -154,7 +154,7 @@ fn get<'a>(table: &'a dyn TableLike, path: &[&str]) -> Option<&'a Item> {
     }
 }
 
-fn remove(table: &mut dyn TableLike, path: &[&str]) {
+pub(crate) fn remove(table: &mut dyn TableLike, path: &[&str]) {
     match path {
         [key] => {
             table.remove(key);
diff --git a/src/config/store.rs b/src/config/store.rs
index f7d377c8d..5716d59e1 100644
--- a/src/config/store.rs
+++ b/src/config/store.rs
@@ -44,6 +44,7 @@ pub(crate) fn set(path: &Path, key: &str, value: &str) -> Result<(), ConfigError
     let mut document: toml_edit::DocumentMut = existing
         .parse()
         .map_err(|error| ConfigError(format!("{}: {error}", path.display())))?;
+    reset(&mut document, key, &typed, &existing, directory)?;
     assign(&mut document, key, &typed)?;
     prune(&mut document, directory)?;
     write(path, document)
@@ -57,6 +58,34 @@ fn read(path: &Path) -> Result<String, ConfigError> {
     }
 }
 
+/// When `key` takes a new value, clear the options of its plugin that
+/// belong to it: a provider's key, endpoint and model go with the provider.
+fn reset(
+    document: &mut toml_edit::DocumentMut,
+    key: &str,
+    value: &toml::Value,
+    existing: &str,
+    directory: &Path,
+) -> Result<(), ConfigError> {
+    let ["plugins", namespace, name, field] = key.split('.').collect::<Vec<_>>()[..] else {
+        return Ok(());
+    };
+    let config = Config::from_toml_in(existing, directory)?;
+    let Some(entry) = config.plugins.entries.get(&format!("{namespace}.{name}")) else {
+        return Ok(());
+    };
+    if entry.options.get(field) == serde_json::to_value(value).ok().as_ref() {
+        return Ok(());
+    }
+    for option in entry.folder().manifest.reset_by(field) {
+        super::prune::remove(
+            document.as_table_mut(),
+            &["plugins", namespace, name, option],
+        );
+    }
+    Ok(())
+}
+
 /// Validate the document, then keep only what differs from the defaults,
 /// and the file's `version`.
 fn prune(document: &mut toml_edit::DocumentMut, directory: &Path) -> Result<(), ConfigError> {
@@ -609,6 +638,35 @@ mod sparse_tests {
         );
     }
 
+    #[test]
+    fn a_new_provider_clears_the_old_ones_settings_but_keeps_the_prompt() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("config.toml");
+        let summarize = |key: &str, value: &str| {
+            set(&path, &format!("plugins.bundled.summarize.{key}"), value).unwrap()
+        };
+        summarize("provider", "anthropic");
+        summarize("api_key", "anthropic-key");
+        summarize("endpoint", "https://proxy.example/anthropic");
+        summarize("model", "claude-sonnet-5-5");
+        summarize("system_prompt", "Be terse.");
+        summarize("min_lines", "7");
+        // The same provider again changes nothing.
+        summarize("provider", "anthropic");
+        let raw = read_toml(&path);
+        assert_eq!(
+            raw["plugins"]["bundled"]["summarize"]["api_key"].as_str(),
+            Some("anthropic-key")
+        );
+        summarize("provider", "gemini");
+        let raw = read_toml(&path);
+        let entry = raw["plugins"]["bundled"]["summarize"].as_table().unwrap();
+        let mut keys: Vec<&str> = entry.keys().map(String::as_str).collect();
+        keys.sort_unstable();
+        // Gemini is the default provider, so it is pruned too.
+        assert_eq!(keys, ["min_lines", "system_prompt"], "{raw}");
+    }
+
     #[test]
     fn an_explicit_plugin_list_still_pins_membership() {
         let dir = tempfile::tempdir().unwrap();
diff --git a/src/plugin/config.rs b/src/plugin/config.rs
index c184a55c6..c4203a3c3 100644
--- a/src/plugin/config.rs
+++ b/src/plugin/config.rs
@@ -29,6 +29,8 @@ pub(crate) const MANIFEST_FILE: &str = "plugin.toml";
 /// An option's default that depends on another option's value:
 /// `{ key = "<option>", values = { <value> = <default>, ... } }`.
 const DEFAULT_BY: &str = "x-default-by";
+/// The option whose change clears this one, such as a provider's key.
+const RESET_BY: &str = "x-reset-by";
 pub(crate) const COMPONENT_FILE: &str = "plugin.wasm";
 
 /// A plugin's `plugin.toml`.
@@ -112,9 +114,31 @@ impl Manifest {
         self.check_defaults_by()
     }
 
-    /// Every `x-default-by` names an option with choices, gives a default
-    /// for each choice, and every choice's defaults satisfy the schema.
+    /// The options a change of `key` clears.
+    pub(crate) fn reset_by<'a>(&'a self, key: &'a str) -> impl Iterator<Item = &'a String> + 'a {
+        self.options
+            .iter()
+            .filter(move |(_, option)| option.get(RESET_BY).and_then(Value::as_str) == Some(key))
+            .map(|(name, _)| name)
+    }
+
+    /// Every `x-reset-by` names another option. Every `x-default-by` names an
+    /// option with choices, gives a default for each choice, and every
+    /// choice's defaults satisfy the schema.
     fn check_defaults_by(&self) -> Result<(), String> {
+        for (name, option) in &self.options {
+            if let Some(by) = option.get(RESET_BY) {
+                if !by
+                    .as_str()
+                    .is_some_and(|key| key != name && self.options.contains_key(key))
+                {
+                    return Err(format!(
+                        "{}: option {name:?}: {RESET_BY} must name another option",
+                        self.name
+                    ));
+                }
+            }
+        }
         for (name, option) in &self.options {
             let Some(by) = option.get(DEFAULT_BY) else {
                 continue;
@@ -712,6 +736,14 @@ mod tests {
         )
         .unwrap_err();
         assert!(cycle.contains("cycle"), "{cycle}");
+        let reset = super::Manifest::parse(
+            "name = 'p'\ntitle = 'P'\n[options.key]\ntitle = 'Key'\ntype = 'string'\n\"x-reset-by\" = 'missing'\n",
+        )
+        .unwrap_err();
+        assert!(
+            reset.contains("x-reset-by must name another option"),
+            "{reset}"
+        );
     }
 
     #[test]
diff --git a/tui/packages/hunk/src/diffr/config.ts b/tui/packages/hunk/src/diffr/config.ts
index a899870b2..8e95a74b7 100644
--- a/tui/packages/hunk/src/diffr/config.ts
+++ b/tui/packages/hunk/src/diffr/config.ts
@@ -96,6 +96,15 @@ export function flattenSchema(rawSchema: unknown, rawValues: unknown): Setting[]
   walk(root, rawValues, "");
   return settings;
 }
+/** Each setting's value as `values`, the output of `config show`, has it. */
+export function withValues(settings: Setting[], values: unknown): Setting[] {
+  return settings.map((setting) => ({
+    ...setting,
+    value: setting.key
+      .split(".")
+      .reduce<unknown>((value, part) => (value && typeof value === "object" ? (value as Record<string, unknown>)[part] : undefined), values),
+  }));
+}
 /** Subsequence match: every query character in order, scored by tightness and word starts. */
 export function fuzzyScore(query: string, text: string): number | null {
   const q = query.toLowerCase(), t = text.toLowerCase();
diff --git a/tui/packages/hunk/src/ui/Settings.test.tsx b/tui/packages/hunk/src/ui/Settings.test.tsx
index 1e755375a..f5daf9cfa 100644
--- a/tui/packages/hunk/src/ui/Settings.test.tsx
+++ b/tui/packages/hunk/src/ui/Settings.test.tsx
@@ -3,7 +3,7 @@ import { testRender } from "@opentui/react/test-utils";
 import { act } from "react";
 import { Settings, displayValue, isSecret, nextValue } from "./Settings";
 import { schemaFixture, valuesFixture } from "../diffr/config.test";
-import { flattenSchema, type ConfigClient } from "../diffr/config";
+import { flattenSchema, parseValue, type ConfigClient } from "../diffr/config";
 
 test("values show as not set, secrets only as stored, and toggles flip or cycle", () => {
   const [minLines, collapse, , provider, apiKey] = flattenSchema(schemaFixture, valuesFixture);
@@ -20,11 +20,17 @@ test("values show as not set, secrets only as stored, and toggles flip or cycle"
 
 test("rows lead with titles under group headings; toggles change in place and typed values open a prompt", async () => {
   const writes: [string, string][] = [];
+  const values = structuredClone(valuesFixture);
+  const settings = flattenSchema(schemaFixture, valuesFixture);
   const client: ConfigClient = {
     schema: () => schemaFixture,
-    show: () => structuredClone(valuesFixture),
+    show: () => structuredClone(values),
     set: (key, value) => {
       writes.push([key, value]);
+      // Store it as diffr would, typed by the setting.
+      const parts = key.split(".");
+      const parent = parts.slice(0, -1).reduce<Record<string, unknown>>((object, part) => object[part] as Record<string, unknown>, values);
+      parent[parts.at(-1)!] = parseValue(settings.find((setting) => setting.key === key)!, value);
     },
   };
   let quit = false;
@@ -135,3 +141,37 @@ test("rows lead with titles under group headings; toggles change in place and ty
     await act(async () => { t.renderer.destroy(); });
   }
 });
+
+test("a change shows every value it moves, such as a default that follows the provider", async () => {
+  const values = structuredClone(valuesFixture);
+  const client: ConfigClient = {
+    schema: () => schemaFixture,
+    show: () => structuredClone(values),
+    set: (key, value) => {
+      if (key === "plugins.bundled.summarize.provider") {
+        values.plugins.bundled.summarize.provider = value;
+        values.plugins.bundled.summarize.model = `${value}-model`;
+      }
+    },
+  };
+  const t = await testRender(
+    <Settings client={client} initial={flattenSchema(schemaFixture, values)} onQuit={() => {}} />,
+    { width: 100, height: 24 },
+  );
+  const press = async (key: string) => {
+    await act(async () => { t.mockInput.pressKey(key); });
+    await act(async () => { await t.renderOnce(); });
+  };
+  const line = (text: string) => t.captureCharFrame().split("\n").find((l) => l.includes(text)) ?? "";
+  try {
+    await act(async () => { await t.renderOnce(); });
+    const query = "provider";
+    for (const char of query) await press(char);
+    await press("RETURN");
+    for (const _ of query) await press("BACKSPACE");
+    expect(line("Provider")).toContain("none");
+    expect(line("Model")).toContain("none-model");
+  } finally {
+    t.renderer.destroy();
+  }
+});
diff --git a/tui/packages/hunk/src/ui/Settings.tsx b/tui/packages/hunk/src/ui/Settings.tsx
index 74aaa4be9..6b59b0b23 100644
--- a/tui/packages/hunk/src/ui/Settings.tsx
+++ b/tui/packages/hunk/src/ui/Settings.tsx
@@ -7,6 +7,7 @@ import {
   formatValue,
   isDefault,
   parseValue,
+  withValues,
   type ConfigClient,
   type Setting,
 } from "../diffr/config";
@@ -99,12 +100,13 @@ export function Settings({
   const current = visible[selected];
   const theme = dark;
 
-  /** Write through the CLI first; the row changes only once the write succeeded. */
+  /** Write through the CLI first, then reread every value: a change can move
+   *  others, such as a default that follows the provider. */
   const save = (setting: Setting, text: string) => {
     const value = parseValue(setting, text);
     client.set(setting.key, text);
     update({
-      settings: state.current.settings.map((s) => (s.key === setting.key ? { ...s, value } : s)),
+      settings: withValues(state.current.settings, client.show()),
       status: { text: `${setting.title}: ${displayValue({ ...setting, value })}`, error: false },
     });
   };